Safety COLUMn
NOVEMBER 2015
BY BILL COOPER
FROM THRESHOLD TO TAXI SPEED
Some pilots seem to think that
when the destination is in sight,
the flight’s over. Well, it isn’t.
There may be as much as 40% of
the total risk management for that
flight still to go. Much of it in the
approach, the rest starting after
overflying the threshold.
En-route, aircraft have substantial energy, in the forms
of speed and height (Kinetic and Potential Energy,
respectively). On descent, much of this is dissipated by
drag, turns etc., until final, when the aircraft is relatively
low and slow, though substantial residual energy remains
as the aircraft crosses the runway threshold for landing.
Alas, there is still plenty of opportunity to make a mess
of things before taxi speed is achieved.
A good landing
A good landing is one where the aircraft touches down
•
•
•
•
On the CentreLine (CL),
In the Touchdown Zone, at the
Correct Configuration and
Correct Airspeed: Vref (1.3Vso). (e.g. 65kt for PA28-161)
Vref is the target threshold speed, at 50’ above the runway,
so the aircraft will normally touchdown slower than Vref.
Note that the definition does not qualify smoothness,
alignment with CL, etc., suggesting that it is more
important to touchdown firmly on target rather than
neatly, smoothly but further down the runway. Smooth
landings are satisfying, but you can’t start stopping ‘til
it’s on the ground.
After this, the pilot still has to keep the aircraft aligned
(within +/-1m of the centre line (CL)) and slowed to taxi
speed. Steering and braking is required throughout and
these are not easy to coordinate.
STEERING
Like birds, for aircraft to fly well, they need to be light for their
size and, again like birds, are built for flight and can be clumsy
on the ground. Aircraft don’t handle as well as dedicated
ground vehicles.
www.sfcaero.com.au
Aircraft are wide, with narrow track, affected by winds
and intolerant of rough or swampy surfaces. The
conventional tricycle (3-wheel) undercarriage layout of
braked mainwheels and steerable or castoring nosewheel
does not handle taxiway turns all that well either, so taxi
speed should not exceed 15kt and 5kt is more appropriate
for parking. The gear does better in a straight-line (more
or less) for TKOF and LDG, assisted over about 30kt by
the rudder’s aerodynamic steering.
On landing, both braking and steering have to be smoothly
merged, to maintain the runway CL until slowed to taxi
speed. The CL is important in this multi-tasking skill
(slowing and steering), which is, I suspect, not helped by
road experience, where we drive on one side or other of
the road. When learning to land, some land down the left
side of the runway, others of different Nationality down
the right side. The only correct alignment, for runways
and taxiways, is down the centre, except when passing
other traffic on taxiways, move right, with the left wing
tip clear of the CL. Be alert for right wing clearance from
signs, parked aircraft etc.
When turning on the ground, the turn radius should be
such that all wheels continue to rotate. To swing the
aircraft in a small space, the inner wheel should not stop
rotating altogether. NEVER lock up a wheel, i.e. brake
hard on one wheel, as pivoting about a stationary wheel
applies a large load to both the undercarriage leg and
the tyre, perhaps enough to upset wheel alignment
(degrading taxi, TKOF and LDG steering) and possibly
separating the tyre tread from the case. This may not be
visible on inspection but may cause a tyre failure on a
subsequent TKOF or LDG.
Castoring nosewheels, e.g. Diamond and Cirrus, need
special care, as they lack shimmy dampers (apart from
friction) and rely for ground steering on differential
mainwheel braking. On TKOF, when you are adding energy
to get off the ground, gentle differential braking may
be necessary to maintain the centreline, especially in a
crosswind. Also, landing requires a special flat technique,
as conventional, nose-high, minimum touchdown speed
LDGs may cause spat-damaging shimmy.
BRAKING
A disc brake is a machine that efficiently converts aircraft
speed into heat. To do this, the wheel must continue to
spin. The brake pads, in contact with the spinning brake
disc, continue to convert energy to heat until released
at taxi speed.
60 Birch Street, Bankstown Airport NSW 2200
SAFETY column • November 2015
However, if the brakes are applied too hard, they ‘lock up’ (i.e., the wheel stops spinning), braking is confined to the
rubber tyre tread sliding along the ground, a very poor energy converter and a likely prelude to an off-runway excursion,
especially if the tyre blows. Even if it doesn’t blow, it puts flats on the tyre. Always report lockups.
Light aircraft brakes are controlled by brake pedals above the
rudder pedals. The rudder pedals are of course moving the rudder,
but in most training aircraft can also steer the aircraft on the
ground, either by a mechanical connection to the nosewheel (as in
Cessna and Piper) or by differential braking (Diamond and Cirrus).
The dual function of the rudder pedals: steering and braking, can
cause confusion and even loss of control if, for example, during a
touch and go, you want to steer a little left and inadvertently apply
the left brake as well, the aircraft may respond with strong left
yaw, due to the differential braking. Several off-runway excursions
have occurred on the training runway for this very reason. Seating
position is important. You should slide the seat back so that you
can just apply full rudder. Sitting further forward increases the risk
of inadvertent braking, as your knees are bent and the foot angle is too acute on the pedals. Sitting back also improves
the instrument scan.
The landing technique diagram in the previous page does not mention braking until the nosewheel is on the ground.
This is for the good reason that the wings still bear much of the aircraft weight after touchdown because flaps are
down and we’re still speeding through the air at a large angle of attack. Aerodynamic (drag) braking is occurring, but
the angle of attack must be reduced to unload the wings and shift the aircraft weight to the gear, so that wheel braking
can commence in earnest. Light aircraft lack the sophisticated RPT/MIL equipment used to stop landed aircraft on the
runway, such as spoilers, reverse thrust and parachutes, but light aircraft land much slower anyway.
Components such as brakes are weight-critical and hence not robust. They need only to be strong and durable enough
to:
• Slow the landed aircraft to taxi speed within the available runway;
• Manoeuvre and stop on the apron;
• Hold the aircraft still during engine run-up.
But bad pilot techniques such as:
• Using the brakes to control taxi speed, or;
• Smoking the tyres (applying maximum braking) consistently after landing is abusive of vital equipment and
expensive of maintenance (which increases your aircraft rental cost, by the way!).
Just about the worst no no (except for landing with the gear up) is to land with the brakes on! A pilot, possibly a little
fatigued after a long flight, anxious to stop in the available runway length, may be tempted to get on the brakes early.
But if you land at the:
• Right Speed, in the;
• Touchdown Zone, wait until the;
• Nosewheel is on the ground, then;
• Brake Smoothly and Evenly;
• Steering on the CL,
Then that is the best that you can do. If you look like landing long on a short strip, go around.
Back before the flight, you will have calculated the landing distance required vs available distance, checked the weather
and if it’s OK when you get there, you can be confident of a successful arrival.
By the way, a pre-landing checklist item is PARK
BRAKE……OFF. That may puzzle some people; ‘Why
should the park brake be on?’ Well, mysteriously, it
has happened and not just to beginners. On the first
flight of a prototype British V-bomber (Victor), at the
beginning of an intensive development program, it
was landed by the test crew with the park brake on.
The damage took 3 months to fix. The Company was
not happy. Every checklist item is there for a reason.
‘The flight is not over until the
aircraft is put to bed’
www.sfcaero.com.au
60 Birch Street, Bankstown Airport NSW 2200