Bouncing During Touchdown
When the aircraft contacts the ground with a sharp impact as the result of an improper attitude or an excessive rate of sink, it can bounce back into the air. The severity of the bounce depends on the airspeed at the moment of contact and the rebound attitude the WSC aircraft. It can increase the AOA and, in addition to bouncing, be lifted. It can rebound in a yawed condition and/or nose up or down. Design and situational factors create their own unique scenarios.
The corrective action for a bounce is the same as for ballooning and similarly depends on its severity. When the bounce is very slight and there is not an extreme change in the aircraft’s pitch attitude, a follow-up landing may be executed by applying sufficient power to cushion the subsequent touchdown and smoothly adjusting the pitch to the proper touchdown attitude.
Extreme caution and attention must be exercised any time a bounce occurs, but particularly when there is a crosswind. During the bounce, the wind causes the aircraft to roll with the wind, thus exposing even more surface to the crosswind and drifting the aircraft more rapidly.
When a bounce is severe, the safest procedure is to execute a go-around immediately. 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. The go-around procedure should be continued even though the aircraft may descend and another bounce may be encountered. It would be extremely foolish to attempt a landing from a bad bounce since airspeed diminishes very rapidly in the nose-high attitude, and a stall may occur before a subsequent touchdown could be made.
In a bounced landing that is improperly recovered, the aircraft comes in nose first, setting off a series of motions that imitate the jumps and dives of a porpoise—hence the name. The problem is improper aircraft attitude at touchdown, sometimes caused by inattention, not knowing where the ground is, or forcing the aircraft onto the runway at an exceedingly high descent rate.
Porpoising can also be caused by improper airspeed control. Usually, if an approach is too fast, the aircraft floats and the pilot tries to force it on the runway when the aircraft still tends to fly. A gust of wind, a bump in the runway, or even a slight push on the control bar sends the aircraft aloft again.
The corrective action for a porpoise is the same as for a bounce, and similarly depends on its severity. When it is very slight with no extreme change in the aircraft’s pitch attitude, a follow-up landing may be executed by applying sufficient power to cushion the subsequent touchdown, and smoothly adjusting the pitch to the proper touchdown attitude.
When a porpoise is severe, the safest procedure is to execute an immediate go-around. In a severe porpoise, the aircraft’s pitch oscillations can become progressively worse until the aircraft 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, only making 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.
Wing Rising After Touchdown
In all the proper landing techniques except the soft field, the nose is lowered after the front wheel touches to put a negative AOA on the wing and keep the WSC aircraft on the ground. However, there may be instances when landing in a crosswind that a wing wants to rise during the after-landing roll. This may occur whether or not there is a loss of directional control depending on the amount of crosswind and the degree of corrective action.
Any time an aircraft is rolling on the ground in a crosswind condition, the upwind wing is receiving a greater force from the wind than the downwind wing. This causes a lift differential. Also, as the upwind wing rises, there is an increase in the AOA which increases lift on the upwind wing rolling the aircraft downwind.
When the effects of these two factors are great enough, the upwind wing may rise even though directional control is maintained. If no correction is applied, it is possible that the upwind wing rises sufficiently to cause the downwind wing to strike the ground.
In a crosswind, the windward wing should be lowered slightly as a preventive measure to avoid it from lifting. But in the event a wing starts to rise during the landing roll, the pilot should immediately lower the nose while lowering the wing. The wing should be lowered as soon as possible. The further a wing is allowed to rise before taking corrective action, the more wing surface is exposed to the force of the crosswind.
When the aircraft 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 aircraft. 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 aircraft’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 descent of 340 feet per minute. Within a fraction of a second, the aircraft 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 aircraft’s inertia and weight. The lift decreases rapidly as the aircraft’s forward speed is decreased and the force on the landing gear increases by the impact of touchdown. When the descent stops, the lift is almost zero leaving only the landing gear to carry both aircraft weight and inertia force. The load imposed at the instant of touchdown may easily be three or four times the actual weight of the aircraft, depending on the severity of contact. After a hard landing, the WSC carriage and wing should be inspected by qualified personnel for airworthiness.