34 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. the proper speed, and gently lowers the nose wheel 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, close the throttle and adjust the pitch attitude  smoothly but firmly to the proper landing attitude.  The corrective action for a porpoise is the same as for a  bounce and similarly depends on its severity. When it is very  slight and there is no extreme change in the airplane’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  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. Attempts to correct a severe porpoise  with flight control and power inputs is most likely untimely  and out of sequence with the oscillations and only make the  situation worse. Do not attempt to salvage the landing. Apply  full power while simultaneously maintaining directional  control and lowering the nose to a safe climb attitude. 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  gear, 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 fpm descent. Within a fraction of a  second, the airplane must be slowed from this rate of vertical  descent to zero without damage. When a pilot permits the airplane weight to become  concentrated about the nose wheel during the takeoff or  landing roll, a condition known as wheel barrowing occurs.  Wheel barrowing 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 nose wheel,  particularly in crosswind conditions. One of the most  common causes of wheel barrowing during the landing roll  is a simultaneous touchdown of the main and nose wheel  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. 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 is 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. If wheel barrowing is encountered and runway and other  conditions permit, it is advisable to promptly initiate a go- around. Wheel barrowing does not occur if the pilot achieves  and maintains the correct landing attitude, touches down at  8-33 Wheel Barrowing Hard Landing