Landing: Part IV, Touchdown
Alexander Burton, CFI
Pacific Rim Aviation Academy Inc.
Pitt Meadows Regional
Airport
“There is a chord in every heart that has a sigh in it if touched aright.”
--Ouida--
In the
last couple of months, we have taken a look at the first stages of the
landing process: the approach, the visual information flow affecting our
performance as a pilot, and the flair.
With the
flair, we’ve brought ourselves down to inches above the landing surface: the
moment when the aircraft’s wheels touch. This is the moment! We are,
however, still in flight; we can still change our mind and abort the landing
should that be necessary. We are in control of the aircraft, have power
available, if necessary; we are not a victim of this moment; let’s always
keep that firmly in mind.
If all is
going well, we can choose to allow the aircraft to make contact with the
surface.
The
essential function of the flair, the manoeuvre immediately preceding
touchdown, is to give the aircraft time for its speed and kinetic energy to
decay sufficiently to allow safe contact with the landing surface. Generally
speaking, we always choose to make contact with the ground, that unforgiving
surface, at as slow an airspeed as possible, consistent with safety in the
prevailing conditions.
Rushing
through the flair phase of the landing process is not an excellent plan.
Touching down with excess speed, excess kinetic energy, can potentially lead
to several potentially exciting and expensive adventures. We want to
transition from flight to parked as gently and seamlessly as possible. Take
your time; patience is a great virtue.
There are
a number of factors to be alert for and correct, as necessary, prior to
allowing an aircraft to touch the landing surface. The longitudinal axis of
the machine must be lined up with the axis of our landing path. Our primary
method of directing the aircraft’s longitudinal axis is appropriate rudder
input.
Remember,
when you fly an aircraft with side-by-side seating, you sit on a line
parallel with the aircraft’s longitudinal axis rather than directly on top
of it. Many ab initio students make the simple error of sighting
across the centre of the cowling or across the spinner as they attempt to
manoeuvre the aircraft straight down the runway and end up touching down
with the aircraft aligned five to ten degrees left of the axis of landing.
If you think of the centre of your breastbone and the control yoke’s shaft
as defining the longitudinal axis, it will make your perceptual task much
simpler.
Crosswind
drift must be compensated for so the aircraft’s flight path is also aligned
with the axis of landing. We do not want the wheels to touch the landing
surface while the aircraft is drifting sideways in relation to the axis of
landing. Our primary method of dealing with crosswind drift is appropriate
aileron input coupled with appropriate rudder input to prevent inducing a
turn.
The
typical error for pilots of all experience levels is to neutralize aileron
input and level the wings immediately prior to touchdown. If there is any
crosswind blowing, the moment we remove compensatory aileron input, the wind
begins to work its way with the aircraft and drift across the axis of
landing begins.
The
lighter the aircraft, the more quickly it will respond to the effects of
crosswind drift. For a light aircraft, say a J-3 Cub with a gross weight of
1200 pounds flying in a 15 mph. crosswind, within 6 seconds following the
removal of crosswind compensation the aircraft will have picked up 7 mph. of
sideways drift (1). This condition is neither desirable nor safe and can,
potentially, lead to disaster.
Keep in
mind, as well, a change in one control input alters how the other inputs
affect the aircraft’s flight path and orientation. In a crosswind situation,
if I increase aileron input, I must compensate for the aircraft’s now
increased urge to turn by increasing opposite rudder input to maintain
longitudinal alignment.
So here we
are, inches above the landing surface, compensated for crosswind and gusts,
ready to allow the wheels to make ground contact. The specific type of
landing we have chosen will dictate just how we proceed. For simplicity,
however, let’s stick to the normal landing.
With a
tricycle gear aircraft, we would like to touch down on the main gear in a
slightly nose-high attitude at as slow an airspeed as possible consistent
with ambient conditions. In still air it would be very nice to hear that
stall warning sounding in the background. If the wind is a bit more brisk or
gusty, if there is some crosswind component to be accommodated, or we are
operating in poor light conditions—at night, for example—a bit faster
touchdown is a better bet.
We must
maintain positive control of the aircraft throughout the touchdown and
subsequent landing roll. Until the wheels are in firm contact with the
ground, we depend on rudder, aileron and elevator inputs to give us positive
control; control surfaces depend on airflow generated by airspeed for their
authority.
When the
Cessna Aircraft Corporation first came out with their delightful, little
training aircraft, the Cessna 150, in
September 1958, it was
advertised as having “Land-O-Matic” landing gear. How could a person
possibly go wrong with that?
The
significant bonus obtained by the change from conventional gear aircraft,
commonly referred to as a taildragger, to tricycle gear aircraft was the
positioning of the centre of gravity ahead of the main landing gear. This
results in the aircraft having a built-in propensity to track straight and
keep its front end toward the direction of travel. It helps us a great deal
when landing by reducing tendencies to ground loop, that sad state of
affairs when the aft end of the aircraft tries to overtake the front end at
an increasingly rapid rate.
Tricycle
gear aircraft are not entirely immune to ground looping; they are just much
less likely to initiate one than a conventional gear aircraft. It is still
critical, however, to ensure the aircraft is properly aligned with the axis
of landing and that all sideways drift has been eliminated before allowing
the wheels to touch.
With a
conventional gear aircraft, alignment and elimination of cross drift is
absolutely critical. The conventional landing gear configuration with the
centre of gravity aft of the main landing gear leaves the aircraft very
vulnerable to ground looping if the pilot gets at all casual with
eliminating drift or has not aligned the aircraft with the landing axis.
With both
tricycle and conventional gear aircraft, compensation for crosswind results
in initially touching down on the windward main wheel. Once the aircraft has
made contact with the landing surface, its speed will decay rapidly as a
result of friction with the surface. The leeward main wheel will soon make
contact with the ground so there is no need to fear the one-wheel touchdown.
Just keep flying the machine until it is done.
Once both
main wheels have contacted the ground with a tricycle gear aircraft, we
continue to hold the nose wheel off the ground as the aircraft slows. It
will come down of its own accord once speed has decayed sufficiently.
Holding if off the surface as long as practicable, consistent with safety,
helps slow us down—high angle of attack = high drag—and reduces the impact
loading on the vulnerable nose gear resulting from its contact with the
surface.
With a
conventional gear aircraft, we have some options on touchdown. We can choose
to execute a three point, a touchdown where all three wheels make contact
with the surface simultaneously, one main wheel and the tail wheel in
crosswind conditions, or we can choose a wheel landing where we make contact
with the ground on the main wheels alone, windward wheel first in the case
of crosswind condition.
The
ongoing discussion on whether a three point or a wheel landing, either tail
high or tail low, is the best choice continues so I won’t venture into that
morass. I would simply say that both types of landings have their
application and it is in a pilot’s interest to be current and competent in
executing both types so he or she can make an intelligent choice for the
given conditions and the specific aircraft he or she is operating.
Some
tailwheel aircraft seem to respond very well to one type of touchdown and
not quite so well to the other, depending on conditions. My spray pilot
friends are adamant that wheel landings, particularly in crosswind
conditions, are the only safe way to go. I find my little Citabria, which is
quite tail heavy, responds well to the three point technique, particularly
if a shorter landing-roll is of benefit although it will wheel land quite
nicely.
I make a
conscious effort to remain current on both types of touchdown so my options
are open. There is no benefit I can see from becoming dogmatic on the
subject.
As they
say, an aircraft is quite safe as long as it remains in the middle of the
air. It is at the edges of the air that difficulties often arise. The
touchdown phase of the landing process is an extremely critical moment and
deserves all the attention and focus we can give it. Learning to execute the
touchdown with skill and patience so the transfer of the aircraft’s weight
from its wings to its wheels is accomplished as gently and smoothly as
possible is a very important step in the process of becoming a proficient
and safe pilot.
Notes:
- Plourde, Harvey S., The
Complete Taildragger Pilot, Muguettte B. Plourde, Goffstown, N.H.,
1991
PRINCIPAL AIR
Main Terminal Chilliwack Airport
2-46244 Airport Road, Chilliwack, B.C. V2P 1A5
Phone: (604) 795-7861 Fax: (604) 795-7867
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