!
PB- 245 581
AIRCRAFT ACCIDENT REFOXT.
NORTHWEST A I R L I N E S ,
INCORPORATED, B O E I N G 7 2 7 - 2 5 , N26 4 U S , NEAR T H I E L L S ,
NEW Y O R K .
DECEKBER 1 , 1 9 7 4
National Transportation Safecy Board
W a s h i n g t o n , D. C.
13 August 1975
DISTRIBUTED BY:
National Technical Information Service
U. S. DEPARTRENT OF COMMERCE
NTSB/AAR-75-13
r
!
NSTB-AAR-75-13
I
-
TECHNICAL REPORT DOCUMENTATION PAPJE
1 . Report No.
T2.Governrrcnt Accession No.
hTSS-AAK-75-13
1
I . T i t l e and S u b t i t l e A i r c r a f t Accident Report
Norchwrst A i r l i n e s . l n c . , Boeiw 727-151, N274US,
near 'L'hiells, liew York, Decedcr 1, 1974
-
1.
Author(s)
j.
Performing Organization Name acd Aadress
NaKional ' T r a w p o r t a t i o n Safety h a r d
Bureau 0 : Aviation S a f e t y
Washitlgton. D. C.
205%
lZ.Spon5orlng Lge.lcy Name and Address
NATIONAL TRANSPORTATION SAFETY BOARD
Washington, 0. C. 205%
).Recipient's
Catalog k o .
5.Report Oate
AuRUSt 1 3 , 1975
6.Performing Organization
C o d e
B.Perforrning o r g a n i z a t i o n
Report No.
IO.Uork Unit.No,
1401-A
II.Contract o r Grant No.
II).Tvp'eport
and
Per'
Covered
A'
-
Accident Report
iceher 1, 1974
14.Sponscring Agency Code
5.supplementary Notes
%.Abstract
A',out 1926 e.9.t. on December 1. 1974, Northvest A i r l i n e s P l i g h t 6231, a Boeing
127-251, crashed about 3.2 mi w e s t of l h i e l l s , New York. The a c c i d e n t occurredatmu:
12 n i n u t e s a f t e r t h e f l i g h t had departed John F. Kennedy I n t e r n a t i o n a l A i r p o r t ,
Jaouica. Kew i o r k , and w h i l e on a f e r r y f l i g h t t o Buffalo, !:ew York. Three crmr e d x r s . t h e only persons aboard t h e a i r c r a f t , died i n t h e crnL.1. The a i r c r a f t was
iestroyed.
The a i r c r a f t s t a l l e d a t 24,800 f e e t rn.s.1. and entered an uncontrolled spiralli
descent i n t o che ground. ?hroughout t h e s t a l l and d e s c e n t , t h e f l i g h t c r e w did not
recognize t h e a c t u a l c o n d i t i o n of t h e a i r c r a f t and d i d not take t h e correcc measure8
necessary t o r e t u r n tllc a i r c r a f t t o l e v e l f l i g h t . Near 3,500 feet m . s . l . , a large
p o r t i o n of t h c l e f t h o r i z o n t a l s t a b i l i z e r separnted from t h e a i r c r a f t , which =de
c o n t r o l of t h e a i r c r a f t f n p o s s i h l e .
The 1;atioMl R a n s p o r t a t i o n Safecy Board determines t h a t t h e probable c a u s t of
t h i s accident w a s t h e loss of c o n t r o l of t h e m r c r a f t because t h e f l i g h t c r e u f n i l c d I
recognize and c o r r e c t t h e a i r c r a f t ' s high- angle- of- attack, low-speed a t a l l and i t s
descending S p i r a l . Thc a c a l l was p r e c i p i t a t e d by t h e E l i p h t c r w ' s i q r o p e r reaction
t o erroneous airspeed and t a c h i n d i c a t i o n s which had r e s u l t e d from a blockage of t h e
p i c o t he.>ds by a r m s p h e r i c i c i w . Contrary t o standard o p e r a t i o n n l procedures. t h e
f l t g h t c r c u had :IOC activnred t h e p i t d t head h e a t e r s .
17.Key Words
Scheduled a i r c a r r l e r s , p i t o t system errors; a t m s pheric icing ; s t n l l a ; n t t i t d e i n s t r m n t f l y i n g .
?'his document Is n v n l l n b l e
throw11 t h e National 'Technical Informtion Service,
&f&f~JE#8iei~Hd~l
1 9 . S c c u r i t y Clabrification
( o f t h i s report!
UIKLASSIFIEO
:ITCU form 1765.2
(Rev. 9 / 7 4 )
2O.Security C l a s s i f i c a t i o n
( G f this page;
UNCLASSIFIEO
k=d.*-*b*
NATIONAL TECHNICAL
INFO?(MATION StRVICE
21,:io. o f Page,
32
J;
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22.Price
,A
r
T U L E OF CONTENTS
.
1.1
1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
1.10
1.11
1.12
1.13
1.14
1.15
1.16
1.16.1
1.16.2
1.17
1.17.1
1.17.2
1.17.3
.
2.1
2
.
2 2
. . .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. ..
.. ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ...
. . ....... .. .. .. .. ....... .. .. .. ....... . . .
. . . .. .. .. .. .. .. .. .. .. .. .. .. .. ..
. . ....... .. .. .. ............. ... ... ... ......... .. ..
.. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. ..
. . . . . . . . . . . . . . . . . . .. .. .. .. ...... . .
Synepsls
Investigation
tlistory of t h e F l i g h t
I n j u r i e s to Persons
Darmge t o Aircraft
Othcr Danuge
C r w Infornution
A i r c r a f t Infornution
P!cteorological Infornution
A i d s to Navigation
Cormnrnications
Aerodrome and Ground F'ncflities
F l i g h t Rccorders
A i r c r a f t Wreckage
P~edlcaland Pathological 1nf;rnutiou
Fire
Survival Aspects
Tests and Rescarch
P i t o t Head mamination and I c i n g Tests
Aircraft Perforcunce W a l y s i s
Othcr I n f o r m t i o n
Prctakeoff Checklist
Airspeed Ncasuring Sgstcm
B-727 Stall C h a r a c t e r i s t i c s
Analysis and Conclusions
Analysis
Conclusions
. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. ..
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......................
t'idlngs . . . . . . . . . . . . . . . . . . . . .
(bl Probable Causc . . . . . . . . . . . . . . . . . .
?.ccomrrndatlons . . . . . . . . . . . . . . . . . . . .
(a)
3.
1
1
2
5
5
7
8
a
9
9
9
10
11
11
12
13
15
15
19
19
21
21
Appenlixcs
.I n v e s t i g a t i o n and Ilearing . . . . . . .
Crew I n f o r m t i o n . . . . . . . . . . . .
Appendix C - Aircraft Z n f o m w i o n . . . . . . . . . .
Appendix D .Rcconr.xndattons A-75 25-27 t o FAA . . .
Appendix A
Appendix B
t
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I
II
i
ti
23
24
26
27
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F i l e No. i-0031
XATICIW. TRANSPORTATION SAFETY h0AF.D
WAS1IIt;r~TUN. D.C.
20594
AIRCRAFT ACCIDEhT REPORT
. Adopted August 13, 1975
-MRT?lk'EST AIRLIhZS, INC.
BOEING 727-251. N274US
NEAR RiIEZLS, Nk?vf YORK
DEEMBER 1. 1974
SYrnPSIS
About 1926 e.8.t.
on Decem5er 1, 1974. Northwest A i r l i n r s F l i g h t
6231. a Boeing 727-251, crashed about 3.2 nmi west of T h i c l l s , Sew York.
F l i g h t 6231 was a f e r r y f l i g h t t o B u f f a l o , h'ew 'ark. The accident occurred about 12 minutes a f t e r t h e f l i g h t had departed .John F. tiennedy I n t e r n a t i o n a l A i r p o r t , J a m i c a . Iiw York. Three c r c m c b e r s . the only persons
aboard the a i r c r a f t , died i n t h e c r a s h . The a i r c r a C t was destroyed.
The a i r c r a f t s t a l l e d n t 24.800 f e e t m.s.1.
and entered a n uncantrolled. s p i r n l l i n d descent t o t h e ground. Throughout t h e s t a l l and descent
t h e f l i g h t c r o v d i d not recognize the a c t u a l c o n d i t i o n of t h e a i r c r a f t and
d i d not take t h e c o r r e c t 1.ca8urcs necessary t o r e t u r n t h e a i r c r a f t t o
l e v e l f l i g h t . Near 3,500 f e e t m.s.l.,
a l a r g e p o r t i o n of t h e l e f t horiz o n t a l s t a b i l i z e r separated from t h e a i r c r a f t , which made c o n t r o l of t h e
a i r c r a f t impossible.
The Notionnl Transportation S a f e t y Hoard determine8 t h a t t h e probable
cause of t h i s accident was t h e l o s s of c o n t r o l of th.; a i r c r a f t because t h e
f l i g h t c r w f a i l e d t o reccgnlze and c o r r e c t t h e a i r c r a f t ' s high-angle-ofa t t a c k . lov-speed s t a l l and i t s descending s p i r a l . The s t a l l was precipit a t e d by t h e flightcrew's improper r e a c t i o n t o erroneous airspeed and
Mach i n d i c a t i o n s which had r e s u l t e d from a blockage of t h e p i t o t heads by
atnospheric iclng. Contrary t o standard o p e r a t i o n a l procedures. t h e
f l i g h t c r e w hnd not a c t i v a t e d t h e pito: head h e a t c r s .
1.
IXITSTIGATIfiN
1.1 lliatory of F l i g h t
On Dccenber 1, 1971,. Northwest Airlinns, I n c . , F l i g h t 6231, a Roeing
727-251, h'27LUS, was a f e r r y f l i g h t from John F. Kennedy I n t e r n a t i o n a l
A i r p o r t (JFK), Jarmica. N e w York. t o Buffalo, K e w York. Three c r w members were t h e only persons almard t h e a i r c r a f t .
I . ' l i g l , t 6231 d c p r t e d JFi: ;&bout1 9 1 1 1 / on a st3ndard i n s t r u e n s d e p a r t u r e .
After t a k e o f f , Kenncdy dcpxrture c o n t r o l c l e a r e d t h c f l i g h t t o c l i * t u
14,000 f c e t . 21 A t 1920:21, Xew York a l r r o u t e t r a f f i c c o n t r o l c e n t c r
(%IS)assumed r a d a r c o n t r o l of t h e f l i g h t , and a t 1921:07, %?N cleared
t h e € l i g h t to climb to f l i g h t lcvcl 310. 21
F l i g h t 6231 7rocecded without reported d i f f i c u l t y u n t i l 13?4:42,
" on ZWi frequency.
when a crewmember t r a n s m i t t e d , "kyday. myday
The ZhY c o n t r o l l e r rcsponded, 'I.. go ahead
and che crewmember s a i d ,
"Roger. we're out of c o n t r o l , descending through 20,000 f e e t . "
.
...
,I'
After giving interim a l t i e u d e c l e a r a n c c s , a t 1925:21, t h e ZW cont r o l l e r askcd FliEht 6231 what thcir problem was, and a crehmcmber responded, 'We're desccnding through 12, we're i n a s t a l l . " ?he sound uf
ap. a c t i v e r a d i o t r a n s m i t t e r was recorded a t 1925:39.
There were nc
f u r t h e r transmissiocs from F l i g h t 6231.
A?. 1925:57, F l i g h t 6231 crashed i n a f o r e s t i n t h e IlarriEun S t a t e
Park, about 3 . 2 n m i west of T h i e l l s , New York. No one witncssed i h e crash.
ihe accident occurred during hours o f darkness.
The geographic c o o r d i n a t e s of t h e eccident s i t e are 41° 12' 53" N.
l a t i t u d e and 74O 5 ' 40" W. longitude.
1.2
1.4
I n j u r i e s t o Persons
Injuries
(:rew
-
Fatal
lionfatal
Xone
3
0
0
Pdssewers
0
0
0
0
0
Damage t o A i r c r a f t
The a i r c r a f t was destroyed.
1.4
Other Damas
Trees and bushes were e i t h e r damaged or destroyed.
1.5
Crew I n f o r m t i o n
The crewmcmbers were q u a l i f i e d and c e r t i f i c a t e d for t h e f l i g h t . The
'three crewmembers had off- duty pcriods of 15 hours 31 minutes during t h e
24-hour period preceding t h e f l i g h t . (See Appendlx B.)
-:I
-2 1
-3 1
A l l times h e r e i n a r e e a s t e r n s t a n d a r d , based on t h e 24-hour c l o c k .
A l l a l t i t u d e s h e r e i n a r e mean sea l e v e l , u n l e s s otherwise i n d i c a t e d .
An a l t i t u d e of 31,000 f e e t which is rmintnincd r r i t h a n altimeter
s e t t i n g of 2 9 . 9 2 inches.
- 3 I n October 1974, t h e f i r s t o f f i c e r advanced from second o f f i c e r i n
B-707 a i r c r a f t t o f i r s t o i c i c e r i n 8-727 a i r c r a f t ; he had flown about 46
hours I n the l a t t e r c a p a c i t y .
1.6
Aircrafc I n f o r m t i o n
N271ruS vas owned and operated by Northwest A i r l i n e s , Inc.
I t was
c e r t i f i c a t e d and maintatned i n accordance wirh Federal Avlation Administ r a t i o n (FAA) r e g u l a t i o n s and requirements. (See Appendix C.)
KZ74US was loaded w i t h 48.500 lbs. of Jet A f u e l . Tha groas weight
a t takeoff was about,147,000 l b s . The weight and center >f g r a v i t y (c.g.)
were w f t h i n prescribed l i m i t s . The a i r c r a f t was i n compliance w i t h a l l
p e r t i n e n t airworthiness d i r e c t i v e s .
I n t h e Boeing 727 a i r c r a f t . t h e p i t o t - s t a t i c iwtruments on t h e capt a i n ' s panel, t h e P i t o t - s t a t i c instruments on t h e f i r s t o f f i c e r ' s panel,
and the p i c o t - o t a r i c instrumentation i n t h e f l i g h t d a t a recorder (FUR) are
connected t o s e p a r a t e p i t o t and s t a t i c sourcea. The t h r e e p i t o t systems
have no conaon elements and a r e c o n u l e t e l y independent. The t h r e e s t a t i c
system8 are a l s o independent except for m n u a l s e l e c t o r valves i n b a t h t h e
c a p t a i n ' s and f i r s t o f f i c e r ' s systerra which provide for selec'-ior: of t h e
FDR s t a t i c system as an al.ternarc pressure Bource if e i t h e r primary EoUrcz
malfunctions.
The f i r s t o f f i c e r ' s n i t m a m s t a t i c systems are connected t o a Mach
airspeed warning switch. The switch activates a warning horn when i t
aenses a d1fferent:al p r c w u r e which i n d i c a t e s t l m t t h e airc r a f t ' s speed 19 sxceedfny: Vm or
21 arpending on t h e a i r c r a f t ' s
a l t i t u d e . A redurd.Jnt Ukch airspeed warning system is incorporated i n
systems.
t h e FDR p i t o t and szat:..
?&,.
The p i t o t head for the c a p t a i n ' s p i t o t system is located on t h e l e f t
s i d e of t h e a i r c r a f t ' s fuselage; the p i t o t heads for the f i r s t o f f i c e r ' s
system and t h e FUR system ar? located on t h e r i g h t s i d e of t h e fcselage.
Each of t h e w heads incorporates a heating element and a s m l l d r a i n hole,
for exhaustire moisture, a f t of the t o t a l prcssure sensing i n l e t . The
t h r e e s t a t f c s y s t e m each have a s t a t i c pore located on e i t h e r side of t h e
fuselage. The l e f t s t a t i c port ie connected t o t h e r i g h t s t a t i c p o r t t G
o f f s e t s i d e s l i p e:fzcts by balancing the pressures w f t h i n t h e systems.
Each of the p o r t s is equipped with J heating element.
I n a d d i t i o n t o t h e abovn oysrrms, two independent p i t o t - s t a t i c
systems a r e connected t o a n*,chaniam i n t h e a i r c r a f t ' s l o n g i t u d i n a l cont r o l system. 'fie force wh1c.n the pilo: nust exert to m v e t h e e l e v a t o r
c o n t r o l s u ? f a c e a varJ?s as II function of t h e dynamic p r e s s u r e measured
by these ayetern.
two p t t o t heads for t h e s e s y s t e m are uuunted one
on each aid.? of t h e v e r t i c u l s t a b i l i z e r , and t h e i r denign i s s i m i l a r to
t h e o t h e r pi'.trt h e a h .
?I.@
-,..-----.__
-.%ximu::
ngera:.ing
1 r L t speed or maxin~noperating limit Mach.
i
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- 4 1.7
F:cc?orological I n f o r m t i o n
Sorthwest A i r l i n e s ' nlctcurology department sl:pplied t h e weather inCwcntion f o r F l i g h t 6231. This information included a synopsis of surf.wc c o n d i t i o n s , terminal f o r e c a s t s , a tropopause and ;rind f o r e c a s t f o r
the 300-rLllibnr l e v e l , a p p r o p r i a t e sur face observations, and turbulence
p i u t s . For t h e period 1700 t o 2300, Northwest meteorologists folecasted
mdcrqte t o heavy snowshowers from Lake Xichigan t o the Appalachian
E!ountains and m d e r a t e t o hcavy rainshowers and s c ~ t t c r e dtlmuierstorrms
e a s t sf the Appalachians.
h'orthwest's turhuiencc p l o t ITP) No. East 2 was i n e f f e c t and a v a i l : h l c t u the flightcrew on the day of the accident. TP East 2 was a t r i angular area defined by l i n e s connecting P i t t s b u r g h , Pennsylvania, N e w
'iork C l t y , Sew York, and Richmnd, Virginia. Thunderstorm c e l l s with
m-~ximuntops t o 28,300 f e e t were located i n t h i s area.
SIGE3:T ?/ Delta 2 , issued n t 1755 an3 v a l i d 1755 t o 2200, predicted
frcqucnt & c r a t e k i n g in clouds, l o c a l l y severe ic p r e c i p i t a t i o n above
t h c i r c e z i w l e v e l , which was a t the surface i n soutlnrestern N e w York and
w!lich sloped t o 6,000 feet eastward t o the A t l a n t i c c o a s t .
llle s u r f a c e weather observations a t Ncwburgh, N e w York, aborit 17
n i l c s north of the accident s i t e , werc:
- cEstinuted
c e i l i n g -- 2,500 f e e t brokeu, 5,000 f e e t overa s t , v i s i b i l i t y -- 12 miles, temperature -- 340.". , d m
point -- 2 Z o F . , i i i n d - - 070O a t 14 kn, g u s t s -- 24 kn,
a1timetr)r s e t t i n g -- 29.98 i n .
2000 - S i r d l a r conditions t o tlrose reporred a t 1900 except
t h a t very l i g h t
p e l l z t s were fa11iIG.
1900
ict,
Another Korthwest f l i g h t was on a similar r o u t e behind F l i g h t 6231.
The c a p t a i n of t h a t f l i g h t s t a t e d t!mt he encountered i c i n g and l i g h t
turhulcnce i n h i s c l i c b . Ile was i n instrulnent conditions fro4 1,500 f e e t
to ?3,1;00 f e e t , cxcept f o r a few minutes between cloud l a y e r s a t an
inrerrledinte a l t i t u d e .
I.?!
. . \ i d s t o :;,tvi.X:tti<>n
fiere were no prohlcna v i t h navigational a i d s .
1.9
Comnicatioz
Thcrc were no probl.em with air-to-ground c o m n i c a t i o n s .
-51
i s an advisory of weather severe enough t o be p o t e n t i a l l y
hazardous t o a11 a i r c r a f t . I t i s broadcast on n a v i g a t i o n a l and voice
frcqucncics and by f l i g h t s e r v i c e s t a t i o n s . I t is a l s o transmitted on
Service-A weather tele:ypc c i r c u i t s .
h SIL::E'T
- 5 1.10
Aerodrome and Ground F a c i l i t i e s
Not applicable.
1.11 F l i g h t Recorders
N274US was eqllipped with a Fairct.ild Elodel 5424 f l t z h t d a t a recorder
(FDR), s e r i a l No. 5146, and a F a i r c h i l d A-100 cockpit voice recorder
(CVR), s e r i a l No. 1640. Both recorders sustained s u p e r f i c i a l mechanical
damage, but the recording tapes were i n t a c t and undamaged. A l l of t h e
FDR t r a c e s and t h e CVR c h a m e l s were c l e a r l y recorded.
The readout of t h e FDR t r a c e s involved 11 minutes 54.6 seconds of
f l i g h t , beginning 15 seconds before l i f t o f f .
Pertinent portions of the CVR tape w x e t r a n s c r i b e d , b e g i n n i q with
t h e f l i g h t c r e w ' s execution of thc pretakeoff c h e z k l i s t and ending with t h e
sounds of impact. The following t r a n s c r i p t was made of t h e f l i g h t c r e w ' s
a c t i v l t i e s between 1906:36 and 1906:51:
F i r s t Officer:
Zero, zero and thirty- one, f l f t e e n , : i f t e e n
Second O f f i c e r :
Eug.
Second Officer:
P i t o t heat.
F i r s t Officer:
Captain:
,... b l u e .
Off and on.
One forty-two i s t h e bug.
... do you want
F i r s t Officer:
Or
F i r s t Officer:
Huh!
the engine heat on?
Sound.of f i v e c l i c k s .
Air-to-ground c o m n i c a t i o n s , cockpit conversations, and other sounds
recorded on t h e CVR were c o r r e l a t e d t o t h e FDR a l t i t u d e , a i r s p e r d , heading, and v e r t i c a l a c c e l e r a t i o n t r a c e s by matching t h s r a d i o transmission
time i n d i c a t i o n s on both the CVR and FDR.
The FDR t o CVR c o r r e l a t i o n shared t h a t a f t e r t a k e o f f , the a i r c r a f t
clirobtd t o 13,500 f e e t and remained a t t h a t a l t i t u d e f o r about 50 seconds,
during which time t h e airepeed $1 increased from 264 kn t o 304 kn. During
t h a t 50 seconds, t h e airspeed t r a c e showed two a b e r r a t i o n s i n a 27-second
period; each a b e r r a t i o n was characterized by a sudden reduction i n airspeed.
These reductions were 40 kn and 140 kn and l a s t e d f o r 7 and 5 seconds,
respectively.
-6/
A l l a i r s p e t d s a r e indicated a i r s p e e d s , unless otherwise noted.
The a i r c r a f t then began t o C l i d 2,500 feet per minute while mainraining an airspeed of about 305 kn. A s t h e a l t i t u d e increaded above
16,000 f e c t , t h e recorded airspeed began t o increase. Subsequently. both
t h e r a t e of clinb and t h e rate of change ia airspeed increased. About
t h i s same time, t h e f i r s t o f f i c e r c o m n t e d , "LIo you r e a l i z e we're & o i q
340 kn and I'm climbing 5,000 f e e t a minute?"
The flightcrew diecussed t h e i n p l i c a t i o n s of t h e high airspeed and
high r a t e of climb. ?he second o f f t c e r coc.ented, ' T h a t ' s because we're
l i g h t , " a f t e r which t h e cap'tain s a i d , "IC g i v e s IJP r e a l f a s t , " and "I
wish I had my shoulder harness on, i t ' s going t o g i v e up p r e t t y soon.''
The r a t e of climb eventually cxceeded 6,500 f e e t per minute.
The sound of an overspeed warning horn was recorded as t h e a l t i t u d e
reached 23,000 f e e t . A t t h a t time, the recorded airspeed was 405 kn and
t h e following conversation took place:
Captain:
'Would you b e l i e v e t h a t {I."
First Officer:
Captain:
"I b e l i e v e i t , I j u s t c a n ' t do anything about it."
"No, j u s t p u l l her back, let her climb."
This l a s t c o m n t was followed by t h e sound of a second overspeed warning
horn.
The sound of t h e s t a l l warning s t i c k s w k e r was recorded intermfttently l e s s thzn 10 seconds a f t e r t h e onset of t h e overspeed warning. Five
seconds l a t e r , v e r t i c a l a c c e l e r a t i o n reduced t o 0.8g., and t h e a l t i t u d e
leveled a t 24,800 f e e t . The recorded airspeed was 420 kn.
The s t a l l warning began again and continued while the f i r s t o f f i c e r
comoented, "There's t h a t Mach b u f f e t , ?/ guess we'll have t o p u l l it up."
followed by the c a p t a i n ' s comnand, " Pull i t up," and t h e sound of t h e
lalxlir& gear warning horn. The FDR readbut shows t h e P o l l w i n g :
-71
Two seconds l a t e r (about 13 seconds a f t e r t h e a i r c r a f t a r r i v e d
a t 24,800 f e e t ) , the v e r t i c a l a c c e l e r a t i o n t r a c e again declined t o
0.82 and t h e a l t i t u d e t r a c e began t o descend a t 2 r a t e of 15,000
f e e t per mfnute. The airspeed t r a c e decreased simultaneously a t a
r a t e of I* kn per second and t h e magnetic h e a d i w t r a c e changed from
290' t o 080° w i t h i n 10 seconds, which i n d i c a t e d that t h e a i r c r a f t
was turning r a p i d l y t o the r i g h t .
A s l i g h t b u f f e t t h a t occurs when an a i r c r a f t exceeds i t a c r i t i c a l
?hch number. The b u f f e t i s caused by the :ormition of a chock wave
an t h e airfoil s u r f a c e s and a separation of a i r f l o w a f t of the shock
wave. The change from lamfnar flow t o turbulent flow a f t of t h e
shock wave causes a high frequency v i b r a t i o n i:, t h e c o n t r o l s u r f a c e s
which is described as " buffet" o r "buzz,"
- 7 As the a i r c r a f t continued to descend, t h e v e r t i c a l a c c e l e r a t i o n
The a i r c r a f t ' s magnetic heading trace
f l u c t u a t e d , b u t moved b a s i c a l l j j t o t h e r i g h t . About 10 seconds
a f t e r chc descent began, t h e "Mayday" was transmitted.
trace increased t o 1.58.
Thirty-t!iree seconds l a t e r t h e crew reported. 'Ve're d e s c e d i w
through 12, we're i n a stall." About 5 seconds a f t e r t h a t transndssfon, t h e c a p t a i n comrunded. 'Tlaps two....,"
& n d a sound aimilar t o
movement of t h e f l a p haruile was recorded. There was no apparent
change i n t h e r s t e of descent; however. t h e v e r t i c a l a c c e l e r a t i o n
t r a c e increased i m e d i a t e l y , w i t h peaks t o +3g. The recorded a i r speed decrzased t o zero, and t h e sound of t h e s t a l l warnir4 became
intermittent.
Five secondn a f t e r t h e copcnin's eomrund for f l a p , t h e f i r s t
o f f i c e r s a i d , "pI?11 now
p u l l , t h a t ' s i t . " Ten seconds l a t e r .
the peak values for vertical a c c e l e r a t i o n increasa: t o +5g. The
r a t e of descent decreased s l i g h t l y ; hovever, t h e a l t i t u d e continued
to decrease to i,O90 f e e t
the e l e v a t i o n of t h e t e r r a i n a t t h e accident n i t e . The a i r c r a f t had descended from 24,800 f e e t i n 83
seconds.
...
--
1.12
A i r c r a f t Wreckaxe
The a i r c r a f t s t r u c k t h e ground i n n s l i g h t l y nosedoVn and r i g h t W i n g d m a t t i t u d e in an area where the t c r r a i n sloped damward about 100. The
a t r c r a f t s t r u c t u r e hmJ d i s i n t c g r a t e d and ruptured and was d i s t o r t e d
tensively. >'here was no evidencc of a p r e e x i s t i n g u a l f u n c t i o n in any of
--
the a i r c r a f t ' s s y s t e m .
Except f o r both e l e v a t o r t i p s , the l e f t h o r i z o n t a l s t a b i l i z e r , srd
three pieces of l i g h t s t r u c t u r e from t h e l e f t s t a b i l i z e r , t h e e n t i r e airc r a f t was located w i t h i n an area 180 f e e t long and 100 f e e t wide. The
above colcponents were located bctweon 375 feet and 4,200 fcet from the
main wreckage.
The h o r i z o n t a l s t a b i l i z e r trim n e t t i n g was 1.2 o n i t s of trim a i r c r a f t noseup. 'The landing gear and s p o i l e r s were r e t r a c t e d . The w i n g
t r a i l i n g edge f l a p s were extended t o t h e 2 O p o s i t i o n , and t h e Nos. 2 . 3 ,
6 , and 7 leading edge s l a t s were f u l l y extended, which corresponded to a
t r a i l i n g edge f l a p s e l e c t i o n of 2'.
The No. 1 and W J . 3 engines were separated from t h e i r r e s p e c t i v e
pylons. 'fie No. 2 cnglne renuincd i n i t s m u n t i n g in t h e empennage. The
engines exilibited i!qmct damgo but l i t t l e r o t a t i o n a l d,amage. The speed
servo cams i n a l l t h r e c f u e l c o n t r o l u n i t s were a t o r near t h e i r high
speed d e t e n t s .
The outboard s e c t i o n of the l e f t h o r i z o n t a l s t i l b l l i z e r had separnted
between s t a t i o n s 50 ?:rd 6 0 . The inboard s e c t i o n rernlined attached to t h e
- 8 -
v e r t i c a l s t a b i l i z e r . The l e f t e l e v a t o r between s t a t i o n s 78 and 223 r%m i n e d attached t o t h e separated s e c t i o n . The r i g h t h o r i z o n t a l s t a b i l i z e r
vas attached t o t h e v e r t i c a l s t a b i l i z e r except f o r t h e t i p s e c t i o n from
s t a t i o n 188 outboard. The r i g h t e l e v a t o r , from s t a t i o n 188 inboard, reotzFncd a t t a c h & t o t h e h o r i z o n t a l s t a b i l i z e r .
The three a t t i t u d e i n d i c a t o r s were damaged on impact. '-:.a
indicators
showed s i m i l a r a t t i t u d e i n f o r m t i o n
Xi" nosedam, with the w i n g s
level.
--
The two p i c o t head h e a t e r swicches were In t h e "off" p o s i t i o n and t h e
switches' toggle l e v e r s were bent .aft. The damage :o t h e switch levers
and t h e d e b r i s deposited on them was t h a t which would b e expected i f they
had been i n t h e "off" p o s i t i o n a t impact. A new switch with i t 8 toggle
lever i n the "off" p o s i t i o n , when s t r u c k with a heavy o b j e c t , exhibited
internal damage s i m i l a r t o the damge found i n t h e internal p o r t i o n s of
t h e r i g h t p i t o t h e a t e r switch.
f o u r of t h e f i v e p i t o t head h e a t e r c i r c u i t breakeru were operable and
were e l e c t r i c a l l y closed. The a m i l l a r y p i t o t head h e a t e r c i r c u i t breaker
vas jacmed i n t o i t s m u n t i n g n t r u c t u r e , and i t was e l e c t r i c a l l y open.
h c l e f t e l e v a t o r p i t o t head was lying on t h e frozen ground; when ret r i e v e d , a t l e a s t e i g h t dreps of water drfpped from t h e p r e s s u r e i n l e t
p a t . After exposure t o s u n l i g h t , nure water drained from t h e p o r t . The
The presc a p t a i n ' s p i t o r h a d v a s r e t r i e v e d and c l e n r e d , o f frozen
sure i n i e t port was f i l l e d w i t h dry wood f i b e r s . A f t e r exposure t o sunl i g h t , wet wood f i b e r s were remved from t h e i n t e r i m of t h e i n l e t p o r t ,
srd w i s t u r e was present on t h e inner sur Tee of the port. The c o p i l o t ' s
picot head and t h e uuxilary p i t o t head were crushed and damaged severely;
they could not be checked f o r v a t e r c o n t e n t , The r i g h t e l e v a t o r p i t o t
head r e m i n e d a t r x h e d t o t h e v e r t i c a l s t a b i l i z e r . The head was i n good
conditl'on and contained no water o r ice.
mud.
The engine a n t i - i c e switches for t h e N O S . 1 and 2 engines were i n t h e
"open" p o s l t i o n . The switch f o r t h e No. 3 engine was in t h e "closed"
p o s i t i o n and t h e switch handle was bent a f t . T e s t s of the bulb filaments
of the engtne a n t i - i c e i n d i c a t o r l i g h t s showed t h a t a l l three l i g h t s were
01, a t %act.
1.13
Xedicill and Pathological I n f o r m t i o n
The three crewmembers were k i l l e d in t h e crash. Toxicological t e s t s
disclosed no evidence of carbon monoxide, hydrogen cyanfde, a l c o h o l , or
drugs i n any of t h e crewmembers.
1.14
There vas no f i r e , e i t h e r during f l i g h t or a f t e r impact.
- 9 1.15
Survival AsDects
The accident w a s not survivnblc.
1.16
1.16.1
Tests and P.esearch
P i t o t Head Examfnation and Icing Tests
A metallurgical examination of the separated h e a t e r conductor wire i n
the p i t o t head from the f i r s t o f f i c e r ' s p i t o t system showed t h a t t h e circumference of t1.e wire was reduced before the wire broke. The metal i n
the wire had not melted, and t h e r e were no s i g n s of e l e c t r i c a l c u r r e n t
arcing or s h o r t i n g .
A p i t o t head of t h e same type t h a t provided p i t o t pressu-e t o t h e
f i r s t o f f i c e r ' s airspeedlllach i n d i c a t o r was exposed t o i c i n g conditions
i n a wi:d tunnel. With the p i t o t heater i n o p e r a t i v e , 1 t o 2 inches of
i c e f o r d over t h e pressure i n l e t port. During t h e exposure, a t h i n
film of water flowed i n t o t h e pressure p o r t , some of which flowed out of
the d r a i n hole.
Blockq3e of t h e d r a i n hole by ice seemed t o depend on t h e length of
time r e q u i r d f o r i c e t o € o m and block t h e t o t a l pressure i n l e t p o r t .
The lo?lger i t took f o r i c e t o form and block t h e t o t a l pressure p o r t , t h e
mre 11ke.ly i
t became t h a t t h e d r a i n h o l e would b e blocked by i c e . A l s o ,
the greqter the snglc between the longitudinal exis of t h e p i t o t head and
the r e l a t i v e wind, t h e g r e a t e r t h e likelihood t h a t t h e d r a i n hole would
become blocked with ice.
Constant a l t i t u d e pressure measurements showed t h a t when t h e t o t a l
pressure i n l e t port was blocked by ice and t h e d r a i n hole r e m i n e d open,
pressure changes occurred t h a t would cause a reduction cf indicated a i r speed. However, when both the t o t a l pressure port and d r a i n hole were
blocked, t h e t o t a l pressure r c m i n e d c o n s t a n t , which would cause indicated
airspeed t o remain fixed. Also, abrupt and m a l l pressure f l u c t u a t i o n s
occurred s h u r t l y before e i t h e r the pressure porc o r d r a i n hole became
blocked by ice.
I n an e f f o r t t o reproduce t h e apparent inconsistencies between the
airspeed and a l t i t u d e values on t h e FDR t r a c e s , t e s t s were conducted with
an airspeed i n d i c a t o r end an a?timeter connected t o vacuum and pressure
sourcen. By a l t e r i n g the vacuum t o t h e a l t i m e t e r and to t h e airspeed
i n d i c a t o r , t h e a l t i t u d e t r a c e could b e reproduced. However, following
ascent above 16,000 f e e t , t h e FDR airspeed and a l t i t u d e values could be
Rimultaneously duplicated only when t h e t o t a l pressure t o t h e airspeed
indicator was fixed a t i t s FUR value f o r an a l t i m e t e r reading of about
15,675 f e e t and an indicated airspeed of about 302 kn.
- 10 1.1.6.2
A i r c r a f t Performance Analysis
Following t h e accident, t h e Safety Board requested t h a t t h e a i r c r a f t
marlufacturer analyze t h e d a t a from t h e CVR and FDR t o determine:
(1) The
consistency of these d a t a , p a r t i c u l a r l y t h e a i r s p e d and a l t i t u d e values,
w i t h t h e t h s x e e i c a l performance of t h e a i r c r a f t ; (2) t h e s i g n i f i c a q e and
possible reason f o r a sirmltaneous a c t i v a t i o n of t h e overspeed and stall
warning systems; and (3) t h e body a t t i t u d e of the a i r c r a f t during i t s
f i n a l ascent and descent. The following a r e some results of t h e manuf a c t u r e r ' s performance a n a l y s i s :
The airspeed and a l t i t u d e values which were recorded were c o n s i s t e n t
with t h e a i r c r a f t ' s predicted c l i d performance u n t i l t h e a i r c r a f t reached
16,000 f e e t . The sioultaneous i n c r e a s e s in both airspeed and r a t e of ascent which were recorded t h e r e a f t e r exceeded t h e t h e o r e t i c a l performance
c a p a b i l f t y of a B-727-200 series a i r c r a f t of t h e same weight a s N27+US.
Conv:quently, the recorded airspeed values were suspected t o be erroneous,
and i t appeared that they varied d i r e c t l y with t h e change i n recorded a l t i tude. The recorded airspeeds c o r r e l a t e d w i t h i n 5 percent with t h e theor e t i c a l airspeeds which would be expected i f t h e pressure measured i n t h e
p i t o t system had remained constant a f t e r t h e a i r c r a f t ' s climb through
16,000 f e e t .
The indicated airspeed of t h e a i r c r a f t when t h e s t i c k shaker was
f i r n t a c t i v a t e d wa8 calculated t o be 165 kn a s compared t o t h e 412 kn
recorded by t h e FDR. The decrease i n airspeed from 305 kn t o 165 kn a s
the a i r c r a f t climbed from 16,000 f e e t t o 24,000 f e e t (within 116 seconds)
is w i t h i n t h e a i r c r a f t ' s theoret?:?? pliub power performance. The a i r c r a f t ' s p i t c h L t t i t u d e would hme been about 30° noseup a s s t i c k shaker
speed was approached. The s t a l l warning s t i c k shaker i s a c t i v a t e d by
angle of a t t a c k instrumentation which i d completely independent o f , and
t h e r e f o r e not a f f e c t e d by e r o r s in, t h e a i r c r a f t ' s airspeed measuring
systems.
V e r t i c a l a c c e l e r a t i o n reduced s l i g h t l y a s the a i r c r a f t leveled ac
24,800 feet probably because the p i l o t relaxed t h e back pressure being
applied t o t h e c o n t r o l column. The s t i c k shaker ceased m m e n t a r i l y ; however, t h e a i r c r a f t continued t o d e c e l e r a t e because of t h e drag induced by
the high body
- i t u d e , and t h e s t i c k shaker r e a c t i v a t e d . Boeing personnel i n t e r p r e t e d th* sound of t h e landing gear warning horn on t h e CVR t o
i n d i c a t e t h a t the t h r u s t l e v e r s had been retarded t o i d l e . The second reduction in v e r t i c a l a c c e l e r a t i o n
t o 0.8g which was coincident w i t h a
sudden descent and a rapid magnetic heading change
was probably caused
by an aercdynamic s t a l l w i t h a probable l o s s of l a t e r a l control.
--
--
Theoretical r e l a t i o n s h i p s of angle of a t t a c k , v e l o c i t y , and drag were
compared t o the recorded r a t e of descent and load fa,.tor t o determine the
a t t i t u d e of t h e a i r c r a f t a f t e r the s t a l l . The comparison showed that the
a i r c r a i t a t t a i n e d an angle of a t t a c k of 22O, o r g r e a t e r , during t h e
- 11
descent. Transient nosedam a t t i t u d e s of mre than 6
0
' would have been
r e q u i r e d t o achieve the measured descent r a t e with an angle of a t t a c k of
2 2 O . The v a r i a t i o n s i n load Factors, which averaged about +1.5g, were
a t t r i b u t e d t o v a r i a t i o n s i n t h e a i r c r a f t ' s angle of bank.
The a i r c r a f t was probably exceeding 230 k n , with a nosedam a t t i t u d e
of about 50° a s i t descended below 11.000 feet, when t h e f l a p s were extended t o Z0. The momentary c e s s a t i o n of t h e s t i c k shaker i n d i c a t e d that
the angle of a t t a c k had been reduced t o less than 13O. The i n c r e a s e i n
v e r t i c a l a c c e l e r a t i o n t o 2.5g was a t t r i b u t e d t o t h e a i r c r a f t ' s being i n
a t i g h t nosedam s p i r a l w i t h a bank angle between 70° and BOo.
With a norlrully operating e l e v a t o r f e e l system, and a stabi'*--- trim
s e t t i n g of 1.2 u n i t s a i r c r a f t noseup, t h e p i l o t would have t o exert a p u l l
f o r c e of between 4 5 and 50 l b s . t o achieve a 2.56 load f a c t o r a t 5,000
f e e t and 250 kn. If, however, t h e e l e v a t o r p i t o t system was blocked so
that the system sensed a zero indicated a i r s p e e d , a p u l l f o r c e of l e s s
than 30 i b s . woqld have produced t h e same load f a c t o r , A f t e r the a i r c r a f t
had descended through 5,000 f e e t , t h e load f a c t o r reached peak values of
+5g.
The manufacturer's engineers s t a t e d t h a t the a i r c r a f t ' s s t r u c t u r a l
limits would have been exceeded a t high angles of s i d e s l i p and load fact o r s approaching +5g. They s t a t e d t h a t a consequent f a i l u r e of t h e
elevator assemblies could have produced an aerodynamic f l u t t e r which
could have, i n t u r n , caused t h e e l e v a t o r spar t o f a i l and t h e l e f t horizontal s t a b i l i z e r t o s e p a r a t e . With t h e a i r c r a f t a t a s t a l l angle of
a t t a c k when tne h o r i z o n t a l s t a b i l i z e r separated, an uncontrollable noseup
pitching moment would have been produced, which could have r e s u l t e d i n
an angle of a t t a c k of 40° o r mre.
1.17
1.17.1
Other Information
Pretakeoff Checklist
Northwest A i r l i n e s ' operational procedures r e q u i r e t h a t t h e f l i g h t crew make a pretakeoff check of c e r t a i n items. The second o f f i c e r i s required t o read the c h e c k l i s t items, and t h e f i r s t o f f i c e r mst check t h e
item and respond t o the second o f f i c e r ' s challenge. Included on t h e
c h e c k l i s t are:
second Officer
Flaps
l'arked Bug
I c e Protection
PI.tot Heat
Pressurnation
F i r s t Officer
1 5 , 15 (25,25) Blue
K
(C, FO) h m b e r s Set
OFT (@N)
OK
(C, FO) Zero,
Norm1 Flags
0,
- 12
Company p i l o t s s t a t e d that t h e c h e c k l i s t i s used only t o check that
the required a c t i o n has already been performed; i t i s not used as a l i s t
of items t o b e accomplished. With regard t o t h e a c t i v a t i o n of p i t o t head
h e a t e r s , i t was the f i r s t o f f i c e r ' s duty Eo t u r n t h e two switches t o the
"on" p o s i t i o n s h o r t l y a f t e r the engines h
a
d been s t a r t e d and t o check t h e
anmeter readings on t h e various h e a t e r s t o confirm t h e i r proper operation.
After checking these items, he was supposed t o leave t h e p i t o t h e a t e r
switches on and t o check t h a t they were on during t h e pratakeoff check.
1.17.2
Airspeed Meas.!ring
System
When an a i r c r a f t mov& through an a i r mass, pressure i s created ahead of
the a i r c r a f t , which adds t o t h e m i s t i n g static pressure w i t h i n t h e a i r
mass. The added pressure, dynamic p r e s s u r e , i s d i r e c t l y proportional t o
t h e v e l o c i t y of t h e a i r c r a f t . When a symnetrically shaped o b j e c r , such a s
a p i t o t head, i s placed i n t o the rnoving a i r s t r e a m , t h e flow of a i r w i l l
separate around t h e ?os@ of the o b j e c t so t h a t t h e l o c a l v e l o c i t y a t t h e
nose is zero. A t t h e zero v e l o c i t y p o i n t , t h e a i r s t r e a m dynamic p r e s s u r e
i s converted i n t o an i n c r e a s e i n t h e l o c a l s t a t i c pressure. Thus, t h e
pressure measuredat t h e n o s e o f t h e o b j e c t i s c a l l e d t o t a l p r e s s u r e , and i t
i s e q u a l t o t h e sum o f t h e dynamic pressureand t h e ambient s t a t i c pressure.
I n an a i r c r a f t airspeed measuring system, t h e t o t a l pressure i s
measured by t h e p i t o t head and is transmitted through t h e p i t o t system
plumbing t o one s i d e of a d i f f e r e n t i a l pressure measuring instrument ( a i r speed i n d i c a t o r ) . The ambient s t a t i c pressure i s measured a t s t a t i c
p o r t s which are w u n t e d in an a r e a t h a t ' i s not s i g n i f i c a n t l y influenced
by the moving airstream. The s t a t i c pressure measured a t t h e s e p o r t s i s
transmitted t o t h e opposite s i d e of the d i f f e r e n t i a l pressure measuring
insirrrment. In e f f e c t , t h e d i f f e r e n t i a l pressure instrument (whether i t
be an airspeed i n d i c a t o r gage, a f l i g h t d a t a recorder p r e s s u r e transmitt e r . o r a component w i t h i n an a i r d a t a computer) s u b t r a c t s the ambient
s t a t i c pressure measured by t h e s t a t i c system from t h e t o t a l pressure
measured by t h e p i t o t system. The r e s u l t a n t dynamic p r e s s u r e is a
d i r e c t measurerent o f i d i c a t e d airspeed.
Since the ambient s t a t i c pressure i s a component p a r t of t o t a l press u r e , any change i n s t c c i c pressure would normally r e s u l t i n an equal
changc i n both t h e p i t o t and s t a t i c p r e s s u r e sy.stem8. Therefore, a change
in ambient S t a r k pressure, such a s t h a t encountered during a change i n
a l r i t u d e , would nornblly have no e f f e c t on airspeed measurement. Only a
change i n dynamic pressure produced by a change in t h e a i r c r a f t ' s v e l o c i t y
would cause a change i n t h e irulicated a i r s p e e d . I f , however, only one
s i d e of the airspeed i n d i c a t o r sensed a change in t h e ambient s t a t i c pressure, an erroneous change in ind'cated airspeed would r e s u l t , even though
the a c t u a l dynamic pressure r e m i n e d unchanged. Such a c o n d i t t o n would
occur i f eit!ler t h e p i t o t o r s t a t i c system watl hlocked o r was otherwise
rendered i n s e n s i t i v e t o external pressure changes.
- 13 I n the event of a blocked p i r o t o r s t a t i c system, the J i r e L t i o n uf
the indicated airspeed e r r o r vould depend on vhich o f the s y s t e m was
blocked and the d i r c c t i c n of change i n t h e a d i e n t s t a t i c pressure.
Under conditions vhere the pressure i n t h e static system i n c r e a s e s w i t h
respect t o the pressure i n t h e p i t o t system, the indicated rairspeed will
read low erroneously. Tor the opposite c o n d i t i o n , vhere t h e pressure i n
t h e s t a t i c system decreases with respect t o t h e pressure Lrt t h e p i t o t
system, the indicated airspeed v i 1 1 read high erroneously. The l a t t e r
vould e x i s t i f t h e p i t o t head vas blocked so t h a t a constant pressure
vas trapped i n t h e p i t o t system while the a i r c r a f t was ascending. This
i s because t h e s t a t i c system pressure vould decrease and t h e resulcnnt
d i f f e r e n t i a l pressure vould appear as an i n c r e a s e i n dyraldc p r e s s u r e .
Indicated airspeed error m y a l s o occur when the p i t o t system becomes i n s e n s i t i v e t o changes i n t o t a l pressure i n such a lunner t h a t thc
cystem vents t o an ambient s t a t i c pressure source. The p l e s s u r e nwsured
by the p i t o t system w i l l e q u a l i z e v i t h t h 2 p r e s s u r e i n t h a s t a t i c system,
and t h e dynamic pressure (Indicated airs.>eed) w i l l decrease t o zero. The
vent source i n a p i t o t head vhich can prodace t h i s kind of e r r o r is t h e
rmisture d r a i n hole vhich i s located dcunstrem frbm a blocked t o t a l
pressure sensing i n l e t .
1.17.3
0-727 S t a l l C h a r a c t e r i s t i c s
Durinp i t s type c e r t i f i c a t i o n process. t h e B-727-200 s e r i e s a i r c r a f t
demonstrated s t a l l c h a r a c t e r i s t i c s vhich met t h e rnquircoents of t h e C i v i l
A i r Regulations, p a r t s 4b. 160-162. The s i g n i f i c a n t r e q u i r e w n t s defined
therein are: (1) That. a t p.n angle of a t t a c k measurably g r e a t e r than t h a t
of nwinnua l i f t , t h e inherent f l i g h t c h a r a c t e r i s t i c s g i v e a c l e a r indicat i o n t o the p i l o t that t h e a i r c r a f t is s t a l l e d
typical indications a r e
a n o s e d m p i t c h or a rol; vhich cannot be r e a d i l y a r r e s t e d ; ( 2 ) t h a t recovery from t h e s t a l l can be e f f e c t e d by normal recovery techniques s t d r c ing a s soon a8 t h e a i r c r q f t is s t a l l e d ; (3) that t h e r e is
abnoma1
noseup pitching and t h a t t h e longitudinal c o n t r o l f o r c e be p o s i t i v e . up
t o an including t h e s t a l l ; (4) that il s a f e recovery from a s t a l l can be
effected with the c r i t i c a l engine inoperative; and (5) t h a t a clear and
d i s t i n c t i v e s t a l l varn€ng be apparent t o t h e p i l o t a t an airspeed a t
leant 7 percent above tho s t a l l i n g airspced.
--
The c e r t i f i c a t i o n s t a l l t e s t s , conducted w i t h t h e a i r c r a f t i n a l l
operating configurations and with the =st adverse vcighe and c . g . conditions, d e m n s t r a t e d that a s the a i r c r a f t v a s slowed and i t s ving a v l e of
a t t a c k vas increased, t h e b u f f e t produced by a i r f l o v separation from t h e
vir@ provided a n a t u r a l v a r n i w of impendi-g n t a l t . With thc landing
f l a p s extended, however, t h e airspeed m r g i n proviied by t h e b u f f e t warning vas considered t o be i n s u f f i c i e n t . Consequently. n s t i c k shnkcr system vas i n s t a l l e d t o provlde an a r t i f i c i a l warning f o r a l l configurations.
- 1L
I n the c l e a n c o n f i g u r a t i o n , _S/ t h e s t i c k shaker a c t i v a t c d when t h e
angle o f atcack r c x l r e d 13". When tho a i r c r a f t was S l w e d f u r t h e r ,
n a t u r a l h u f f c t i n g o w u r r c d a t an angle 3f ,ittack between 16O and Lao.
The b u f f e t was described a s "qultc heavy" when t h e speed was reduced to
w i t h f n 2 t o 3 kn of t h e speed associatcd with mximu l i f t . Whcn t h e
angle of a t t a c k f u r rmxlmul l i f t (about 22') was reached. t h e r e was ir
tendency €or t h e nose to drop if t h e p i l o t reliLvcd p r e s s u r e on t h e cont r o l colurm. Also, l a e c r a l s t a b i l i t y was reduced n o t i c e a b l y , which increased the p l l o t ' s workload in nuintaining wings- level f l i g h t .
!!urinp, c e r t i f i c a t i m f l i g h t tests, the angle of n t t a c k was increased
to 2 5 0 , a f t e r which r e c o v t r y was e f f c c t c d by relaxing t h c p u l l f o r c e on
the c o n t r o l c o l m . With t h e use of engine t h r u s t d u r i w , recovery, t h e
altLtuda l o s t was r e s t r i c t e d t o about 2,000 f e e t .
Up to the o n s e t of stall b u f f e t , t h c l o n g i t u d i n z l c o n t r o l forcce
needed t o e f f e c t s t a l l e n t r y increased as the angle oi a t t a c k incrcnsed.
A t higher an$lcs of a t t a c k . up t o and beyond t h e a n g l e for maximm l i f t ,
thc p u l l force rc+ircd to m l i n t a i n a noseup p l t c h i r s m m n t decreased.
?he f o r c e s d i d not r'cvcrse, however, and, w i t h ncrnal t r i n , a roduccion
i n p u l l force r e s u l t c d i n n decreased 3ngle of a t t a c k .
The B-727 l c ~ i t u d i n a con:rol
t
system i s capablc o f developing the noseup
pitching m n e n t s n a c d d t o o b t a i n anglcs c,f a t t a c k m c h higher than those
associated w i t h s t a l l . For an a i r c r a f t havlng t h e same weight, c.8. locat i o n , a n l s t a b i l i z e r trin scttlng as h?7qUS, t h e rmnufacturer'n a n a l y s i s
s h e d t h a t an angle of a t t a c k of appruxirnltely 37' could be a t t a i n e d i f
s continuous p u l l f o r c e wan exerted t o hold t h e c o n t r o l c o l m a f t .
L i k e other a i r c r a f t which have h o r i z o n t a l s t a b i l i z e r s located near or
on top of t h e i r v e r t i c a l s t a b i l l z c r s . t h e R-727 does pass through a range
of high angles of att:lck where l o n g i t u d i n a l i n s t a b i l i t y occurs. ?his ins t a b i l i t y causes the a i r c r a f t , when no c o n t r o l f o r c e i s applied, t o p i t c h
to even htgher angles of a t t a c k . Longitudinal i n s t a b i l i t y i s caused by
derraded horizontal s t a t i l i z c r c t f e c t i v c n e s s whcn t h c a:rcraft's a t t i t u d e
i s such that the h o r i z o n t a l s t a b i l i z e r Is enveloped by t h e lowenergy t u r bulent s i r i n the wake from t h e w i n g s . When these high a n z l e s of a t t a c k
are reachcd, a push forcc on t h e c o n t r o l colu;m is required t o reduce t h e
angle of a t t a c k . For a b-727 with an a f t c.8. l o c a t i o n and s t a b i l l z c r
trim i n t h e c r u i s e range, w i n d cunncl d a t a show thnc a noscdovn p i t c h i n g
muent w i l l dacrease the angle of a t t a c k and s t a l l recovery can be a t t a i n e d
bg applying push f o r c e s t o the c o n t r o l column.
A s t i c k pusher i s a dcvice which w i l l .apply a f o r c e to m v e t h c cont r o l c o l u m fomard whcn rlrc nnglo of act.ack f o r n u x t r m n l i f t i s cxcecded.
The usefulness of a s t i c k pusher i s c o n t r o v e r s i n l smic i t can e f f e c t p r i nury c o n t r o l of the a i r c r a f t . llwevcr, a s t i c k pushcr i s rrqufred on
B-727 and ocher a i r c r a f t r c g i s r e r e d by t h c United Kingdom. That s t i c k
pusher is designed so t h t i t s a c t i o n can be overpower& by a p u l l f o r c e
of about 80 lbs. on t h e p i l o t ' s c o n t r o l column.
-61
Without landing gear, Elapn, or s p o i l c r n extended.
...... .
I
- 15 2.
2.1
W Y S I S AND CONCLUSIONS
Analysis
The a i r c r a f t was c e r t i . f i c a t e d . equipped, and maintained i n accordance with regulations and approved procedures. The a i r c r a f t weighed subs t e n t i a l l y less than i t s authorized maximum weight f o r takeoff.
Althot?:h
t h e speed servo cams i n a l l t h r e e engine f u e l c o n t r o l l e r s
vere positioned f o r high engine revolutions per minute, t h e engines were
producing very l i t t l e t h r u s t a t impact a s evidenced by t h e absence of sign i f i c a n t r o t a t i o n a l damage t o t h e engines. Probably, t h e t h r o t t l e s had
been advanced 8hortly.before *act,
but t h e r e was e i t h e r i n s u f f i c i e n t
time for t h e engines t o a c c e l e r a t e , or a c c e l e r a t i o n was limited because
a i r f l o v i n t o t h e engine i n l e t s had been d i s t o r t e d by t h e extreme angle of
a t t a c k and probable s i d e s l i p .
The flightcrew was properly c e r t i f i c a t e d and each crewmember had received the t r a i n i n g and off- duty time prescribed by r e g u l a t i o n s . There
vaa M evidence of medical or physiological problems that might have
affected t h e i r p e r f o m n c e .
h e conversations recorded on t h e CVR revealed that, following ascent
above 13,500 f e e t , t h e flightcrew became concerned and puzzled by t h e apparent performance of the a i r c r a f t because of t h e i n d i c a t e d airspeed and
the indicated rate of ancent. The FDR airspeed and a l t i t u d e t r a c e s provided i n v e s t i g a t o r s an i n s i g h t regarding t h e s e conversations. The a i r speed t r a c e increased r a p i d l y a f t e r t h e a i r c r a f t ascended above 16,000
f e e t while the rate of climb continued t o i n c r e a s e and eventually reached
a peak value of 6,500 f e e t per minute. The Boeing Company's a n a l y s i s of
t h e airspeed and rates of climb values that r e g i s t e r e d above 16,000 f e e t
shoved t h a t these values were incoinpatible with t h e a i r c r a f t ' s p e r f o m n c e
capabilities.
I
Analysis showed t h a t t h e r e was a d i r e c t r e l a t i o n s h i p between t h e a i r speed and a l t i t u d e values. This r e l a t i o n s h i p was based on t h e assumptions
that (1) the t o t a l pressure measured by t h e FDR p i t o t system remained cons t a n t a f t e r t h e a i r c r a f t ascended above 16,000 f e e t , and (2) t h e p r e s s u r e
measured by t h e FDR s t a t i c system varied according t o the recorded a l t i tude values. These assumptions were s u b s t a n t i a t e d by t h e tests which
determined that t h e FDR airspeed and a l t i t u d e t r a c e s could be reproduced
only i f t h e t o t a l presoure t o the airspeed i n d i c a t o r was h e l d constant
during ascent above 16,000 f e e t .
Although the p i t o t systems f o r t h e c a p t a i n ' s and f i r s t o f f i c e r ' s a i r speed Mach i n d i c a t o r s and the FDR airspeed instrumentation a r e t h r e e separ a t e and completely independent systems, it i s reasonable t o conclude t h a t
a l l three systems were sensing n e a r l y i d e n t i c a l and erroneous t o t a l pressures. This can b e concluded because the flightcrew made no r e f e r e n c e t o
1
* 16
-
any d i f f e r e n c e between t h e airspeed readings on t h e c a p t a i n ' s and
first
o f f i c e r ' s i n d i c a t o r s , and t h e f i r s t o f f i c e r ' s reference t o "...going 340
kn
corresponded c l o s e l y t o t h e s i r s p e e d value recorded on the FDR a,t
thqt time. Additionally, t h e near simultaneous a c t i v a t i o n of t h e overspeed warning system8 tends t o prove t h a t t h e f i r s t o f f i c e r ' s airspeed was
close t o the value recorded on t h e FDR when t h e a i r c r a f t neared i t s peak
altitude.
..."
The erroneously high airspeed i n d i c a t i o n s were caused by a c o q l e t e
and nearly simultaneous blockage of a l l t h r e e p i t o t pressure systems.
Moreover, s i n c e t h e only c o v n elements amng t h e systems were t h e
design f e a t u r e s of t h e p i t o t heads and t h e environnrnt t o which they were
exposed, t h e Safety Board concludes that t h e p i t o t heads were blocked by
i c e which f o d around t h e heads and closed t h e d r a i n holes and t h e pressure i n l e t p o r t s . The conclusion i s supported by t h e airspeed a b e r r a t i o n s
that were recorded while t h e a i r c r a f t was f l y i n g level a t 13.500 f e e t and
by t h e m i s t u r e which was found i n t h e p i t o t heads when they were Fecovered
and examined. Additionally, i t is known t h a t i c i n g conditions e x i s t e d fn
t h e a r e a through which F l i g h t 6231 was f l y i n g , and i t is u n l i k e l y t h a t
any other type of blockage or malfunction would s i m l t a n e o u s l y a f f e c t the
three independent s y s t e m .
The formation of ice on t h e p i t o t heads should have been prevented by
e l e c t r i c a l heating elements which a r e a c t i v a t e d by t h e p i t o t h e a t e r
switches located i n t h e cockpit. The Safety Board concludes t h a t ' t h e
heating e l m n t s were never a c t i v a t e d because the p i t o t h e a t e r switches
were not in the "on" p o s i t i o n during t h e f l i g h t . This conclusion i s subs t a n t i a t e d by t h e p o s i t i o n and c o n d i t i o n of t h e switches i n t h e wreckage,
t h e i n t e r n a l damage t o t h e r i g h t switch, and t h e l a c k of evidence t h a t
e l e c t r i c a l c u r r e n t was present i n t h e h e a t e r c i r c u i t t o t h e p i t o t head i n
the f i r s t o f f i c e r ' s p i t o t system a t t h e time of impact.
The Safety Board was unable t o determine why t h e p i t o t head h e a t e r
switches were not placed i n t h e "on" p o s i t i o n b e f o r e departure. It is
c l e a r t h a t the f l i g h t c r e w p e r f o r r e d t h e pretakeoff checks required by
Nort!west's o p e r a t i o n a l procedures. However, the proper c h e c k l i s t sequence was not followed, and it is possih!e t h a t t h e f i r s t o f f i c e r positioned t h e switches improperly because of an omission i n t h e sequence
and h i s inexperience as a B-727 c o p i l o t .
While reading t h e c h e c k l i s t , t h e second o f f i c e r c a l l e d "bug" and,
before receivint: a response from e i t h e r t h e c a p t a i n or f i r s t o f f i c e r , he
omitted t h e " i c e protection" c a l l and c a l l e d " p i t o t heat." The f i r s t
o f f i c e r apparently responded t o both t h e omitted c a l l and t h e " p i t o t
heat" c a l l by saying, "off and on," b u t following t h e c a p t a i n ' s response
t o t h e "bug" c a l l , t h e f i r s t o f f i c e r asked whether t h e engine h e a t was
needed. The c a p t a i n may or w y not have responded with a nod o r hand
s i g n a l , but t h e sound of f i v e c l i c k s was recorded and t h e f i r s t o f f i c e r
returned t o t h e t a s k of setting h i s airspeed bug.
.
.-
.
,
...:
,?.
.;
.
- 17
The f i v e c l i c k s m y have been t h e uovement of t h e p i t o t h e a t e r
switch<?a t o t h e " off" p o s i t i o n and t h e mvermnt of t h e engine a n t i - i c e
switches t o the "on" p o s i t i o n
a r e v e r s a l of t h e i r nornu1 positions,.
This assumption is supported by the p o s i t i o n of t h e engine a n t i - i c e and
p i t o t heater switches i n t h e wreckage, the condition of t h e l i g h t s aseociated with t h e engine a n t i - i c e switches, and t h e lack of any r e f e r e n c e
during the f l i g h t t o the need f o r engine a n t i - i c e .
--
Because oE t h e €lightcrew's comm?nts concerning a i r c r a f t performance
and the absence of connrnts about p o s s i b l e instrumcnt e r r o r o r airspeed
system i c i n g , t h e Safety Boardconcludes t h a t t h e flightcrew a t t r i b u t e d t h e
high airspeed and t h e high r a t e of climb t o t h e a i r c r a f t ' s r e l a t i v e l y lov
gross weight and t o an encounter with unusual weather. which included
stromp updrafts. The f l i g h t c r e w ' s a n a l y s i s of t h e s i t u s t i o n m a t h a w
be,tl strongly influenced by these f a c t o r s and by t h e f a c t that both a i r speed i n s t r u n e n t s were ind1catFng e s s e n t i a l l y the same values. Hwever.
the a i r c r a f t ' s a t t i t u d e as i t neared the top of i t s ascent should have
warned them t h a t t h e a i r c r a f t ' s perfornunce was abnormal because its
nearly 30° noseup a t t i t u d e was about 2 5 O higher than t h e n o m 1 climb
a t t i t u d e , and a t such a high noseup a t t i t u d e i t would have been Lmpoasible
f o r t h e airspacd t o continue t o i n c r e a s e eve:> i f i n f l u e n c d by e x t r a y I"
d r a f t s . Becauoe t h e use of a t t i t u d e references i s a f u n d a w n t a l of insentIOent f l y i n g . which is s t r e s s e d in Northwest's f l i g h t c r e v t r a i n i n g p r o g r m ,
the Safety Bohrd concludes t h a t t h e f l i g h t c r e w improperly r e l i e d on airs p e d i n d i c a t i o n s as a means of determining a i r c r a r t perforounce.
hlthough t h e a c t i v a t i o n of t h e overspeed warning system, probably
reinforced t h e f l i g h t c r e w ' s b e l i e f that they were taking a p p r o p r i a t e
a c t i o n , t h e operation of t h e s c a l l warning s t i c k shaker should have
a l e r t e d them that t h e a i r c r a f t a c t u a l l y was aporoaching a stall. The
f i r s t o f f i c e r apparently misinterpreted t h e c o n t r o l c o l u m v i b r a t i o n produced by t h e s t i c k shaker as Mach b u f f e t because when t h e s t i c k shaker
began, he c o m n t e d , 'I... t h e r e ' s t h a t Mach buffet." The c a p t a i n apparentl y agreed with t h i s i n t e r p r e t a t i o n because he then colmunded, "Pu1l.it up."
The alnnsr sirmltaneous a c t i v a t i o n of t h e s t a l l and t h e overspeed warning
systems undoubtedly created some confusion; huuever, t h e d i f f e r e n c e s beween s t a l l L u f f e t and Mach b u f f e t a r e s u b s t a n t i a l and t h e former should
have been e a s i l y recognized. Again, though, i t appears that t h e f l i g h t crew r e l i e d alnnse exclusively on t h e airspeed i n d i c a t o r s and the:r
r e l a t e d warnire systems t o assess t h e a i r c r a f t ' s perforumnce.
Even a f t e r t h e s t a l l , as m n i f c s t e d by t h e r a p i d h a d i n g change
(banked a t t i t u d e ) and t h e suJden descent, t h e f l i g h t c r e w f a i l e d t o recogn i z e t h e problem f o r a number of seconds. They continued t o e x e r t back
pressure on t h e c o n t r o l c o l m which k e p t the a i r c r a f t a t a high angle of
a t t a c k . They probably ware having d i f f i c u l t y with l a t e r a l c o n t r o l , and
t h e a i r c r a f t entered i n t o a s p i r a l l i n g descent t o t h e r i g h t , during which
t h e actual alrspeed of t h e a i r c r a f t began t o i n c r e a s e rapidly.
- 18
?!IC
erroneous airspcm i n d i c a t i o n s , the s t e e p noscdwn a t t i t u d e , and
the p r ~ p r i o ~ c p r i vscnsations
c
a s s o c i a t e d w i t h t h e p o s i t i v c v c r t l c n l acceleration forces undoubtedly contributed t o confusion which prcventod the
f l f g h t c r n r fmm recognizing t h e t r u e c o n d i t i o n of t h e a i r c r a f t . Additiona l l y . i t i s probable t h a t t h c nosdown and banked a t t i t u d e s of t h e a i r c r a f t were so stccq chat t h c horizon references in the a t t i t u d e i n s t r u ments were nearly hidLicn. T h i s would have nude t h c l a t e r a l a t t i t u d e of
the l i r c r a f t difficult t o determine. llovever, had t h e p i l o t s concentrated
lmre on t h e n t t f t u i c i n d i c a t o r s , nnd p a r t i c u l a r l y tho p o s i t i o n of the “sky
pointers”, ?/ they probably could h a w returned t h e a i r c r d f t t o l e v e l
f l i g h t had they taken a p p r o p r i a t e c o r r e c t i v e a c t i o n w i t h i n 30 t o 40
seconds a f t e r t h e s t a l l .
rtobably because of the low airspeed i n d i c a t i o n s , t h e c a p t a i n decidGd
that the a i r c r a f t was i n a s t a l l . He Cransmicted:
‘Ve’rc descending
through 12, we’re i n a s t a l l . ” and he c a l l e d for the f l a p s to be e x t e d d
t o 2’
a ptopcr step i n the s t a l l recovery procedure. ~towcvcr, the
a c t u a l i n d i c a t e d airspccd a t that tim? was probably i n excess of 230 kn
and increasing r a p i d l y ; consequently, although tho s t i c k shaker ceased
OpCmtiOn r o n e n c a r i l y , the extcnsion of t h e f l a p s had l i t t l e favorable
--
effect.
Even a f t e r the p i l o t s decidod t h a t the a i r c r a f t was s t a l l e d , t h e
Safety Roard b e l i e v e s t h a t they contfnued t o r e a c t p r i m r i l y t o t h e high
rate of desccnt i n d i c a t i o n s and proprioceptive s e n s a t i o n s because they
continucd t o e x e r t a p u l l f o r c e on t!le c o n t r o l column. This is s u b s t a n t i ated by tho increasing vertical a c c c l c r a t i o n f o r c e s as t h e descent continued. llascver, because t h e wings were not lcvcled f i r s t , t h e G i r c r a f t
continued t o descend r n p t d l y in a s p i r a l l i n g , a c c e l e r a t e d s t a l l .
Since t h e p i t o t heads for t h c e l e v a t o r f c e l system were probably
blocked by i c e . the force required of t h e p i l o t s to move the e l e v a t o r s
would have been increased while t h e a i r c r a f t was above 16.0CC, fcet.. HOWever, when the a i r c r a f t descended below t h a t a l t i t u d c , t h e f o r c e requircd
would have beendininished. As the descent continue:: below 5,000 f e e t , t h e
a c t u a l indicnted airspeed probably excecded 356 kn while t h e airspeed
sensed by t h e e l e v a t o r f c e l system was probably near zero. Consequently,
conditions were created i n which high v e r t i c a l a c c e l e r a t i o n f o r c e s could
be produced with r e l a t i v e case. As evidenced by t h e FDR a c c c l e r a t l o n
t r a c e , high w r t i c a l a c c e l e r a t i o n f o r c e s were produced below 5,400 f c e t .
A S the a i r c r a f t continued i t s descent through 3,500 f e e t , t h e high
v e r t i c a l a c c e l e r a t i o n fcrccs i d u c e d were s u f f i c i e n t t o cause the f a i l u r c o f
the l e f t h o r i z o n t a l s t a b i l i z e r . T h s r e a f t c r , t h e a i r c r a f t probably r o l l e d
t o a near wirys-Level nttitudr:, pitched up t o nn extremely high angle of
a t t a c k , and continued todescc-: i n ? . , ullcontrollable s t a l l t o :he ground.
-9/
h t r i a n g u l a r index which i a positioned above t h e movable horizon and
which m v c s i n t h e opposite d i r e c t i o n from the a i r c r a f t ’ s banked
a t t i t u d e t o lM1Cate the n u h e r of degrees of bad:.
1
i
- 19
During t h e S a f e t y Board's i n v e s t i g a t i o n , i n c i d e n t s involving p o s s i b l e
p i t o t - s t a t i c system i c i n g were reviewed. Although none of t h e s e incidents r e s u l t e d i n a c a t a s t r o p h i c accident, it became clear t h a t p i t o t o r
s t a t i c system i c i n q during f l i g h t can and does occur. Also, t h e r e s u l t a n t
e f f e c t s on pressure- operated f l i g h t instnrments can produce a t least
wmentary confusion a w n g t h e crewuembers.
While a l l of t h e f l i g h t c r e w s involved i n t h e s e i n c i d e n t s reverted t o
n t t i t u d e f l y i n g u n t i l t h e cause of t h e i c i n g could be eliminated o r i n s t r u ment f l i g h t could b e terminated, i t was apparent from t h e s e i n c i d e n t s t h a t
sore p i l o t s who understood t h e b a s i c p r i n c i p l e s of airspeed measurement
f a i l e d t o analyze t h e p o s s i b l e r e s u l t s of a blockage of the p i t o t o r
static sysrems. The pi1o:s o f t e n f a i l e d t o determine t h e proper reasons
f o r an increasing airspeed i n d i c a t i o n ; they a t t r i b u t t d such i n d i c a t i o n s
t o unusual weather phenomena.
Although unusual weather phenomena such as m u n t a i n waves, extreme
turbulence, and v e r t i c a l wind shear can produce s i g n i f i c a n t airspeed
' d e v i a t i o n s , t h e s e phenomena u s u a l l y are of s h o r t d u r a t i o n and cause erratic
o r abruptly changing airspeed i n d i c a t i o n s r a t h e r s t e a d i l y i n c r e a s i n g ,
s t e a d i l y decreasing, or f h e d airspeed I n d i c a t i o n s . Also, t h e a i r c r a f t ' s
a t t i t u d e during encounters w i t h t h e s e phenomena i s i n p o r t a n t i n determining airspeed trends and p o s s i b l e sources of e r r o r . Consequently, t h e
Safety Board b e l i e v e s that p o t e n t i a l p i t o t - s t a t i c system problems and
a t t i t u d e f l y i n g as a temporary remedy f o r t h e s e problems should b e reemphasized i n instrument f l y i n g t r a i n i n g program,, and the Safety Board has
made a recommerdation t o t h i s e f f e c t t o t h e Administrator, Federal
Aviation Administration.
2.2
Conclusions
(a)
,
Findings
1. A l l menhers of t h e f l i g h t c r e w were properly c e r t i f i c a t e d
and were q u a l i f i e d f o r t h e i r r e s p e c t i v e d u t i e s .
2.
The a i r c r a f t had been properly maintained and was airworthy f o r t h e f l i g h t ; i t s g r o s s weight and c.g. were
w i t h i n t h e prescribed 1Mts.
3.
There was no evidence of a system malfunction o r f a i l u r e o r
of a s t r u c t u r a l d e f e c t i n t h e a i r c r a f t .
4. The flightcrew had adequate weather information f o r t h e
flight.
5.
The FDR v e r t i c a l a c c e l e r a t f o n t r a c e i n d i c a t e s t h a t only
l i g h t turbulence was encountered.
- 20 6.
The weather conditions encountered during t h e f l i g h t were
conducive t o t h e fornution of d e r a t e airframe i c e .
7.
The a i r c r a f t accumul.ited s u f f i c i e n t i c e during i t s f l i g h t
t o block completely t h e d r a i n h o l e s and t o t a l pressure
i n l e t p o r t s of t h e p i t o t heads; t h e s t a t i c p o r t s were not
a f f e c t e d by t h e f c e .
8.
The p i t o t heads became hlocked a t an a l t i t u d e of about
16,000 f e e t .
9.
The ice formed on the p i t o t heads because the p i t o t head
h e a t e r switches had not been turned on b e f o r e F l i g h t 6231
departed JFK.
10.
The complete blockage of t h e p i t o t heads caused t h e cockpit
airspeed i n d i c a t o r s t o read erroneously high as t h e a i r c r a f t
climbed above 16,000 feet and t h e s t a t i c p r e s s u r e decreased.
11. The f l i g h t c r e w reacted t o 'the high airspeed i n d i c a t i o n s by
increasing t h e noseup a t t i t u d e of t h e a i r c r a f t which increased the rate of clinb. While t h i s caused t h e indicated
airspeed t o increase m r e r a p i d l y because the s t a t i c pressure decreased mre r a p i d l y with t h e increased r a t e of
climb, t h e a c t u a l airspeed was decreasing.
12.
The airspeed overspeed warning and s t a l l w a r n i q s t i c k
shaker operated simultaneously because of t h e blocked p i t o t
heads and t h e high noseup a t t i t u d e of t h e a i r c r a f t .
13. The f l i g h t c r e w misconstrued t h e operatfon of t h e s t a l l
warning s t i c k shaker as Mach b u f f e t .
14.
The flightcrew continued t o i n c r e a s e t h e noseup a t t i t u d e of
t h e a i r c r a f t following t h e o p e r a t i o n o f t h e s t a l l warning
s t i c k shaker.
15.
The a i r c r a f t s t a l l e d a t an a l t i t u d e of 24,800 f e e t w h i l e i n
a noseup a t t i t u d e of about 300.
16.
Following t h e s t a l l , t h e a i r c r a f t entered i n t o a r i g h t
s p i r a l l i n g d i v e a t a high r a t e of descent. Throughout t h e
descent, the f l i g h t c r e w reacted p r i m r i l y t o afrspeed and
r a t e of descent i n d i c a t i o n s i n s t e a d of a t t i t u d e i n d i c a t i o n s ,
and thus f a i l e d t o i n i t i a t e proper recovery techniques and
procedures.
I -
- 21
17.
(b)
I n an e f f o r t t o recover the a i r c r a f t from a high r a t e of
d e s c e n t , t h e flightcrew exerted excessive p u l l f o r c e s on
t h e c o n t r o l colunms which induced high v e r t i c a l a c i s l e r a t i o n
forces and :auseA t h e l e f t h o r i z o n t a l s t a b i l i z e r t o f a i l .
Probable Cause
The National Transportation Safety Board determfnes t h a t t h e probable
cause of t h i s accident was t h e l o s s of c o n t r o l of t h e a i r c r a f t because t h e
flightcrew f a i l e d t o recognize and c o r r e c t t h e a i r c r a f t ' s high-angle-ofa t t a c k , lcw-speed s t a l l and i t s descending s p i r a l . The s t a l l was precipated by t h e f l i g h t c r e w ' s inproper r e a c t i o n t o erroneous airspeed and
Mach indicat4ons which had r e s u l t e d f r c n a blockage of t h e p i t o t heads by
a t m s p k r i c icing. Contrary t o standard o p e r a t i o n a l procedures, t h e
flightcrew had not a c t i v a t e d t h e p i t o t head h e a t e r s .
3.
RECOMMEh'DATIONS
As a r e s u l t of t h i s a c c i d e n t , t h r e e reconmendations were made t o t h e
Administrator, Federal Aviation Administration. (See Appendlx D.)
BY THE NATIONAL TRANSPORTATION SAFETY BOARD
FRANCIS 11. "s
Member
LOUIS M. TIL4YAYER
Member
ISABEL A . BURGESS
Member
John H. Reed, Chairman. and William R . Haley, Member, d i d not p a r t i c i p a t e
i n the adoption of t h i s r e p o r t .
August 13, 1975
- 23 APPENDIX A
Investigation and Hearing
1.
Investigation
The National Transportation Safety Board w s notified of the accident
ahout 1935 on December 1 , 1974. The Safety Board inmediately dispatched
an investfgatfve team to the scene. The following wrnfng the team
established investigative groups for operations/witnesses, air traffic
control, weather, 8truCtures. parerplants. systems. flight data recorder.
nnintenance records, and cockpit voice recorder.
Parties to the investigation were: The Federal’ Aviation Administration, Northwest Airlines, Inc., The Boeing Company. Air Line Pilots
Association, International Association of Machinists and Aerospace
Workers, and the Pratt and Whitney Division of the United Aircraft
Corporation.
2.
.
Hearing
A public hearing was held at Bear Mountain, New York, on February
12 and 13, 1975. All of the parties to the investigation except the
Watt and Whitney Aircraft Dfvlsion were parties to the bearing.
-.Preceding page hlank
- 24
APPENDIX B
Aircrew Information
Captain John B , l a g o r i o
Captain Lagorio, 35, was employed by Northwest A i r l i n e s on January
17, 1966. He held A i i l i n e Transport P i l o t c e r t i f i c a t e No. 1k96609 w i t h
a i r p l a n e n u l t i e n g i n e and single- engine land r a t i n g s , c o m e r c i a 1 p r i v i leges and a type r a t i n g i n t h e B-727. He held F l i g h t Engineer certific a t e No. 1682555 and a v a l i d f i r s t - c l a s s d i c h l c e r t i f i c a t e which was
issued with no l i m l c a t i o n s on August 22, 1974.
Captain Lagorio !wd accurmiated about 7,434 flight- hours, of which
about 1,973 were i n t h e B-727. I n t h e 30-, 60-, and 90-day periods preceding t h e a c c i d e n t , he flew about 58, 122, and 185 hours, respecti\?ly,
a l l i n t h e B-727.
Captain Lagorio was advanced from f i r s t o f f i c e r t o c a p t a i n on August
He c o q l e t e d h i s last general r e f r e s h e r training on January 15,
1974, and h i s l a s t 8-727 r e f r e s h e r t r a i n i r 3 on November 15, 1974. HZ
passed a proficiency f l i g h t check i n t h e 8-727 s i m l a t o r on November 15,
1974.
5, 1969.
F i r s t O f f i c e r Walter A. Zadra
F i r s t O f f i c e r Zadra, 3 2 , was employed by Northwest A i r l i n e s on
January 8. 1968. He held Cormnercial P i l o t c e r t i f i r a t e No. 1624729 with
a i r p l a n e m l t i e n g i n e and single-eneine land r a t i n g s , and an instrument
r a t i n g . He held F l i g h t Engineer c e r t i f i c a t e N?. 1834609 and a v a l i d
f i r s t - c l a s s medical c e r t i f i c a t e which was issLeJ with no l i d t a t i o n s on
J u l y 9 . 197k.
f i r s t O f f i c e r Zadra had f i m about 1,550 hours a8 a p i l o t or f i r s t
o f f i c e r and about 3,152 hours as a second o f f i c e r ( f l i g h t engineer) of
which about 1,244 hours were i n t h e B-727. He upgraded from second
o f f i c e r in B-707 a i r c r a f t t o f i r s t o f f i c e r i n B-727 a i r c r a f t on October
!.6, 1974, and h e had flown about 46 hours i n the l a t t e r capacity. I n
; h e 3 0 - . 60-, and 90-days periods preceding the accident, he f l w ,
r e s p e c t i v e l y , about 46 hours as f i r s t o € f i c e r i n t h e B-727 and 23 and
76 hours a s second o f f i c e r i n t h e a-707.
First O f f i c e r Zadra completed general r e f r e s h e r t r a i n i n g on January
7, 1974, and h e passed a f i r s t o f f i c e r proficiency check i n t h e B-727
on October 16, 1974.
- 25
Appendix B
Page 2
Second Officer Jams F. Cox
Second O f f i c e r Cox, 33, was employed by Northwest A i r l i n e s on
February 2 , 1969. He held Comercia1 P i l o t c e r t i f i c a t e No. 1643627 with
multiengine land and instrmment r a t i n g s . He held F l i g h t Engineer (tarboj e t pwered) c e r t i f i c a t e No. 1920999 and a f i r s t - c l a s s medical certific a t e which was issued with no l i m i t a t i o n s on March l,, 1974.
Second Officer Cox had.acqulred about 1,938 hours of flylng time
as a second o f f i c e r wit'l Northwest A i r l i n e s , including about 1,611 hours
i n B-727 a i r c r a f t . In t h e 30-, 60-, and 90-day periods preceding the
accident, he f l m about 45, 113 and 183 hours, respectively, a l l i n
B-727 a i r c r a f t .
Second Officer Cox completed gevaral r e f r e s h e r t r a i n i n g on January
10, 1974, and he passed a second o f f i c e r proficiency check on April 10,
1974.
- 26 APPENDIX C
Aircraft Informtion
N274US was manufactured by The Boeing Company on December 2. 1969,
and i t was assigned s e r i a l No. 20295. It had a c c m l a t e d about 10,289
hours of time i n service.
N274US was powered by t h r e e P r a t t and Whitney JTBD-7 engines.
Pertinent engine data a r e a s follows:
Position
Serial
No.
--
Total Time
Time Since Heavy Maintenance
1
649153
18,641 hours
3,044 hours
2
654070
14,818 hours
2,234 hours
3
648988
17,622 hours
1,193 hours
A l l of t h e requir%d maintenance inspections and checks on t h e a i r c r a f t had been p e r f o r d i n accordance w i t h Northwest A i r l i n e s approved
directives.
-
27
-
NATIONAL TRANSPORTATION SAFETY BOARD
WASHINGTON,
DL.
APPENDIX D
ISSUED: March 2 0 , 1975
Administrator
Federal Aviation Administration
Washington, D. C. 20591
SAFETY RECOHHENDATION(S)
A-75-25 thru -27
.........................................
The !:atlono1 Transportation Safety Board is i n v e s t i g a t i n g t h e
Northwest Airlines, Inc., Boeing 727, N274US, a i r c r a f t crash which
occurred near T h i e l l e , h’ew York, on kcember 1, 1974. The Board’s
continuing investigation has revealed that ice’blocked t h e . p i t o t heads.
A p r e l i n i n a r y review of t h e evidence i n t h i s accident suggests
the poss:,.bility t h a t t h e crew concsntrated on a i r data instrumentation
t o the exclusion of a i r c r a f t a t t i t u d e !ndications. The timely use of
t h e a t t i t u d e information may have prevented t h e stall and subsequent
crash.
About 5 minutes before the rapid descent, the f l i g h t d a t a recorder
(k’DR) recsrded aberrations i n the airspeed t r a c e . These aberrations
v e x caused by t h e closure of t h e ram air i n l e t and t h e d r a i n hole of
the p i t o t mast. These aberrations were v e r i f i e d by wind-tunnel i c i n g
t e s t s of a p i t o t mast and pneumatic tests Of an a l t b e t e r and airspeer.
system. These tests produced a i r s p e e d / a l t i t u d e t r a c e s s i d l . n r t o those
recorded on the FDR.
The Safety Board is aware of other incidents i n which an aircraft
encountered d i f f i c u l t i e s while f l y i n g i n f r e e z i n g p r e c i p i t a t i o n because
of a lack of pitor. heat. I n these incidents, t h e flightcrews recognized
the problem and took c o r r e c t i v e actton.
Evidence i n thifi case i n d i c a t e s t h a t t h e pit.ot h e a t e r c o n t r o l
switches were not on, a l t h o y h t h e heaters were capable of operation.
?he a i r c r a f t had been f l y i n g i n clouds and fre?z?ng temperntures.
Recently, one air carrier reported t h a t i t is operating its p i t o t
heater system continuously and t k failure rate is minimal, l.e.,
one
element f a i l u r e per uSrcraft per year. Several o t h e r air c a r r i e r s are
a c t i v e l y connidering t h e i n s t i t u t i o n of a similar procedure, end they
b e l i e v e there would be no adverse a f f e c t on t h e life of the p i t o t heater
elements.
,~l'PESDIS 0
Honorable Alcxnnder 1'. B u t t e r f i c l d '
-
?X
-2-
-
- 29 APPENDIX D
March 13, 1975
Honorable John H. Reed
Chairman, National Transportation
Safety Board
800 Independence Avenue, S . U .
Washington. D. C. 20591
Dear M r . Chairman:
This is to acknowledge receipt of your letter of March 12 enclosing
a copy of a safety recommendation to the Federal Aviation Administrator
concerning,the Board's investigation of the Northwest Airlines. Inc.,
Boeing 7 2 7 , N274US. aircraft crash which occurred near Thielle. Sew
York, on December 1. 1974.
The rrcomuendations are receiving attention by the Department's
Assistant Secretary for Environment, Safety and Consumer Affairs,
as well as other appropriate Departmental officials,
Sincerely,
William T. Coleman, J r .
-.
I
-
30
-
APPENDIX 0
DEPARTMENT OF TRANSPORTATION
FEDERAL AVIATION ADivlINISTRATION
‘MAY 2 7 1975
Natation 1481
Honorable John H. Reed
Chairman, National Transportation Safety Board
800 Independence Avenue, S.
Washington, D. C.
20591,
U.
Dear.&. Chairman,
T h i s i s i n response t o your l e t t e r of March 1 2 which t r a n s m i t t e d
NTSB S a f e t y Recornendations A-75-25 t h r u 27.
Recommendation No. 1.
Issue an Operations Bulletin t o all a i r c a r r i e r and g e n e r f l a v i a t i o n
i n s p e c t o r s t o stress t h e need f o r p i l o t s t o use a t t i t u d e <?Cormation
when questionable information i s presented on instruments t h a t are
dependent on t h e a i r d a t a system. The information i n this B u l l e t i n
should 5 e disseminated t o all o p e r a t o r s f o r incorporation i n t o t h e i r
operations procedures and t r a i n i n g programs.
(Class 1)
Comment.
Air Carrier Operations A l e r t B u l l e t i n 75-3 dated February 13 covers
this subject. A P a r t 135, Air T a x i Bulletin, i s being prepared. He
are d l s o considering t h e issuance of a n advisory c i r c u l a r on t h e
subject.
Recommendation No. 2.
I s s u e an Airworthiness D i r e c t i v e t o r e q u i r e t h a t a warning system b e
i n s t a l l e d on t r a n s p o r t category a i r c r a f t which will i n d i c a t e , hy way
of 8 warning l i g h t , when t h e f l i g h t instrument p i t o t heatiny s y s t m
i s n o t operating. The warning l i g h t should operate d i r e c t l y from t h e
h e a t e r e l e c t r i c a i current. (Class 2)
Cament.
-We do n o t concur i n t h i s reconmendation.
Some c u r r e n t a i r c r a f t have
c y c l i n g types of p i t o t heaters. These cycle on and off as c o n t r o l l e d
by thermostats or timers. Warning l i g h t s would flash on and off with
./
,
/
-
31
APPENDIX D
2
t h e cycling. We consider this a s d i s t r a c t i n g and po3sibly ' d e t r i n c n t a l
t o safety. Other w i r c r a f t in which t h e p i t o t heat i s c o n t r o l l e d d i r e c t l y
by a s i q p l e on-off switch could be modified by adding a power r e l a y and
warning l i g h t . We do not consider this necessary or d e s i r a b l e . Operation
of p?'.tot h e a t i s on cockpit c h e c k l i s t s and i s well covered in operations
nanuals and crew t r a i n i n g .
I n a d d i t i o n , t h e e f f e c t i v e n e s s of a d d i t i o n a l
warning l i g h t s among t h e many warning l i g h t s p r e s e n t l y installed in t h e
cockpit i s of doubtful value.
Recomendation No. 3.
h e n d t h e a p p l i c a b l e Federal Air Regulations t o r e q u i r e t h e p i t o t h e a t i n g
system t o b e on any time e l e c t r i c a l power i s applied t o an a i r c r a f t .
This should a l s o be incorporated in t h e o p e r a t o r ' s operations manual.
(Class 2)
Coment.
recommendation i s considered t o apply t o dll t y p e s of a i r c r a f t in
s e r v i c e and t o f u t u r e designs. We propcse t o d e l e t e from c o n s i d e r a t i o n
those a i r c r a f t which a r e l i m i t e d t o VFR l i i g h t only s i n c e t h e y a r e n o t
required t o have any d e i c i n g c a p a b i l i t i e s .
"hi3
R e t r o f i t on e f i s t i n g a i r c r a f t p r e s e n t s m a n y problems and we do not
consider t h e recommendation p r a c t i c a l for general adoption.
Some
cyclic installations w
l
l
i
not t o l e r a t e continuous heat and w o a d have
t o be completely replaced. Continuous h e a t would be unsafe in many
cirsumstances such as extended parking with e l e c t r i c a l power
As
yoil mentioned, r e l i a b i l i t y would be reduced l e a d i n g tl more frequent
unsafe conditions in f l i g h t . We do not consider r e t r o f i t of e x i s t i n g
& c r a f t p r a c t i c a l or f e a s i b l e .
on.
For new designs t h e recommendation may be f e a s i b l e because t h e f n s t a l l a t i o n s
can Le s a f e and r e l i a b l e by design of i n t e r f a c i n g e l e c t r i c a l power systems,
p o s i t i o n i n g of p i t o t tubes, and construction of p i t o t tubes.
A regulatory
p r o j e c t l e a d i n g t o a Notice of Proposed R i l e Making and subsequently a
r u i e r e q u i r i n g an appropriately designed p i t o t heating system i s being
established.
SLxerely.
L;Actiug A d m i n i s t r a t o r