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When a porpoise is severe, the safest procedure is to
EXECUTE A GO-AROUND IMMEDIATELY. In a
severe porpoise, the airplane’s pitch oscillations can
become progressively worse, until the airplane strikes
the runway nose first with sufficient force to collapse
the nose gear. Pilot attempts to correct a severe porpoise
with flight control and power inputs will most
likely be untimely and out of sequence with the oscillations,
and only make the situation worse. No attempt
to salvage the landing should be made. Full power
should be applied while simultaneously maintaining
directional control, and lowering the nose to a safe
climb attitude.
WHEELBARROWING
When a pilot permits the airplane weight to become
concentrated about the nosewheel during the takeoff or
landing roll, a condition known as wheelbarrowing will
occur. Wheelbarrowing may cause loss of directional
control during the landing roll because braking action is
ineffective, and the airplane tends to swerve or pivot on
the nosewheel, particularly in crosswind conditions.
One of the most common causes of wheelbarrowing
during the landing roll is a simultaneous touchdown
of the main and nosewheel, with excessive speed,
followed by application of forward pressure on the
elevator control. Usually, the situation can be corrected
by smoothly applying back-elevator pressure.
However, if wheelbarrowing is encountered and
runway and other conditions permit, it may be advisable
to promptly initiate a go-around. Wheelbarrowing will
not occur if the pilot achieves and maintains the correct
landing attitude, touches down at the proper speed, and
gently lowers the nosewheel while losing speed on
rollout. If the pilot decides to stay on the ground rather
than attempt a go-around or if directional control is
lost, the throttle should be closed and the pitch attitude
smoothly but firmly rotated to the proper landing
attitude. Raise the flaps to reduce lift and to increase
the load on the main wheels for better braking action.
HARD LANDING
When the airplane contacts the ground during landings,
its vertical speed is instantly reduced to zero. Unless
provisions are made to slow this vertical speed and
cushion the impact of touchdown, the force of contact
with the ground may be so great it could cause
structural damage to the airplane.
The purpose of pneumatic tires, shock absorbing landing
gears, and other devices is to cushion the impact and to
increase the time in which the airplane’s vertical descent
is stopped. The importance of this cushion may be
understood from the computation that a 6-inch free fall
on landing is roughly equal, to a 340-foot-per-minute
descent. Within a fraction of a second, the airplane must
be slowed from this rate of vertical descent to zero,
without damage.
During this time, the landing gear together with some
aid from the lift of the wings must supply whatever
force is needed to counteract the force of the airplane’s
inertia and weight. The lift decreases rapidly as the
airplane’s forward speed is decreased, and the force
on the landing gear increases by the impact of
touchdown. When the descent stops, the lift will be
practically zero, leaving the landing gear alone to
carry both the airplane’s weight and inertia force.
The load imposed at the instant of touchdown may
easily be three or four times the actual weight of the
airplane depending on the severity of contact.
TOUCHDOWN IN A DRIFT OR CRAB
At times the pilot may correct for wind drift by crabbing
on the final approach. If the roundout and touchdown are
made while the airplane is drifting or in a crab, it will
contact the ground while moving sideways. This will
impose extreme side loads on the landing gear, and if
severe enough, may cause structural failure.
The most effective method to prevent drift in primary
training airplanes is the wing-low method. This technique
keeps the longitudinal axis of the airplane
aligned with both the runway and the direction of
motion throughout the approach and touchdown.
There are three factors that will cause the longitudinal
axis and the direction of motion to be misaligned
during touchdown: drifting, crabbing, or a combination
of both.
Decreasing Angle
of Attack
Decreasing Angle
of Attack
Rapid Increase in
Angle of Attack
Rapid Increase in
Angle of Attack
Normal Angle
of Attack
Normal Angle
of Attack
Figure 8-37. Porpoising.
8-32
　
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