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FLIGHT
DECEMBER I 6 T H , 1948
ALTIMETER SETTINGS
Arguments Favouring Introduction of Standard Setting (QNH)
T
HE nature of the earth's surface is such that the only
pression of the most valuable kind. I t gives the instructor
suitable datum for the measurement of heights is mean
an immediate opportunity to explain the limitations of the
sea level. For this reason virtually all maps and charts
pressure altimeter, the need for the sea-level datum and the
in use throughout the world show height measurements
fact that indicated height only rarely equals actual height
above its level. Therefore it is generally agreed that for all above the ground and that the altimeter must be interpreted
cross-country flying the altimeter is best set to sea level as this
intelligently and with a certain measure of reserve at all times.
offers the closest correlation between published heights of
The third reason supporting the decision to standardize on
ground features and the aircraft's altitude. With this setting
one altimeter setting is to expedite and simplify air traffic on
pilot or navigator can, by reference to the altimeter and a cross-country flying and in the vicinity of an airfield, and to
map, determine without difficulty or complication the terrain
avoid last-minute changes of altimeter setting under adverse
clearance or height above ground.
conditions. It is generally agreed that for traffic separation,
navigation, and all aspects of air traffic safety, a standard,
While agreeing with the above, quite a number of pilots see
setting should be used until the aircraft is about to land.
no good reason why zero setting (QFE) should not be used for
However, when number one to land where QFE is used, a pilot
landings. On the face of it there seems very little reason
why it shouldn't, and it can be quite sensibly proposed that must adjust his altimeter to a new setting, which adjustment
it is simpler, easier and safer for
may a t times be considerable,
use during the approach and land~~^~~~~~~————
thereby introducing a risk of error.
ing. Some pilots will go so far as
HE purpose of this Papsr, the full text of which Also, in the event of a mis-landing,
to say, not without some logic, that
the pilot has to change his altimeter
written by W/C. Shelfoon, Assistant Director
a QFE setting is safer even on a of was
back to the standard setting before
Training (All-weather), Air Ministry, and which
cross-country flight because it places is being distributed through the R.A.F. under the integration with other traffic or conyou higher for a given indicated
tinuing to another airfield.
heading "Altimeter Setting—QNH—Why?", is to
height than the mean sea-level qualify
From the standpoint of simplicity,
as far as possible the reasons leading up to
setting.
elimination of unnecessary adjustan Air Ministry decision to use the standard setting
The main purpose in the choice for all flying, and to explain why in the long run ment and maximum freedom from
of the standard setting is to any advantages of the zero setting (QFE) are best error, the use of the standard setting
eliminate the inevitable confusion
for landing effects an overall imdispensed with.
which arises when more than one
provement on the air traffic side
type of setting is allowed. Until •
and deletes an item from the landvery recently there was available a
ing drill. Any small revision to the
QFE, Q F F / Q F E (Alt.), regional QFF and QNH. These are
setting used on the last leg of the flight can be made at any
denned as follows : —
convenient time before becoming number one to land.
QFE. Prevailing station pressure in millibars.
The arguments in favour of the zero setting for landing
QFF.
Station pressure reduced to mean sea level by a
may be summarized as follows: —
formula using as its basis prevailing temperatures and
(a) The zero setting requires no calculations, mental or
pressures at point of observation.
otherwise, because the altimeter indicates within its
QFE (Alt.). A special setting introduced as a substitute
limits of accuracy the actual height of the aircraft above
for QFE at high-altitude airfields when the subscale did
the runway.
not have sufficient range to set QFE. The subscale was
(b) The zero setting on a BABS approach means that the
set to 1013.2 millibars and the aircraft was advised as to
height factor applied to the distance scale is always
what height its altimeter would read when it touched down
constant.
on the airfield.
(c) That on any approach minimum height will be the
QFF (Regional).
The lowest pressure forecast for the
same, e.g., 200ft, when using BABS.
whole of the flight information region for the ensuing hour,
These arguments are not unreasonable, and if there were no
used to effect some measure of uniformity for air traffic
complications would probably be quite acceptable, and while
within an area when flying IFR between certain heights.
it is the intention of the ensuing paragraph to disprove as much
QNH. The standard setting Prevailing station pressure
as possible their validity, it is constantly borne in mind that
reduced to mean sea level by the I.C.A.N. formula. When
there is no ideal solution, and that minor sacrifices, mostly
the aircraft touches down the altimeter reads the height
inconvenient, are necessary in the overall interest.
of the airfield.
Blind Landing with Zero Setting
The first reason, therefore, and one which should be kept
continually in mind, for the exclusive use of the standard
The argument that the zero setting requires no calculations
setting is to eliminate confusion and possibility of error by
for landing and simplifies the approach, being the main and
dispensing with the multiple choice system of altimeter settings.
most important one for its retention, is considered first. A fact
This will do away with " Q" code variants, and eliminate
which is often overlooked is that under instrument conditions t f i
almost completely the risk of setting errors.
descent is never continued until the altimeter reads zero.
The second reason is a more subtle one. Throughout the
Being realistic, it must be appreciated that for some years to
history of the R.A.F., with few exceptions, it has been
come the practical limitation to bad-weather landings will be
standard practice to use the zero setting (QFE) for flying
ceilings of the order of 150ft. Therefore, unless the lighting
training and, in fact, for flying throughout the Service of the or runway is seen at some predetermined height, the descent
"circuit and b u m p " variety.
Unfortunately, to the new will have to be abandoned and the aircraft climbed to a safe
pupil, nothing is more logical than that the altimeter should
height from which to attempt another approach or flv to
read zero when the aircraft is on the ground. Consequently,
another airdrome. In practice, this break-off height or the
he accepts this situation without question and, in effect, starts
point at which over-shoot action will commence is not a
off under a misconception about the pressure altimeter which
constant factor but will vary according to the airdrome,
no amount of subsequent training completely dispels. So* length of the runways, the approach aid used, type of airfrom the beginning, the young pilot is impressed with the
craft, the load carried and the prevailing weather. " Therefore,
idea that indicated height is actual height above the ground.
the break-off height must be determined by the pilot before
The number of accidents caused by collision with high
the approach is started.
ground under conditions of poor visibility gives ample proof
An accurate determination of the indicated height at which
that the use of the zero setting (QFE) results in an incorrect
the
descent will be abandoned must take into consideration all
appreciation of altimeter limitations. Accident records prove
that in time of stress, sub-conscious ideas inculcated by the the factors enumerated above, which requires reference to maps
of the surrounding area and the instrument approach diagram
use of the zero setting are prone to come to the fore and
when available.
lead the unwary to destruction
An appreciation of the aircraft height in relation to the
On the other hand, the reaction of a new pupil to an
surrounding area and the bottom limit of the descent can best
altimeter reading of some height value, e.g., 350ft, when the
be made while the altimeter is set to mean sea level because
aircraft is at rest on the ground, is that something is wrong.
spot height and altimeter correlation is still valid. In effect,
This apparently ridiculous situation will have a lasting imthe runway constitutes one of the elevations above sea level
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