Aircraft Performance

In addition to the important factors of proper procedures, approximately three and one-half percent for each 1,000 feet many other variables affect the landing performance. Any of altitude. Proper accounting of density altitude is necessary item that alters the landing speed or deceleration rate during to accurately predict landing distance. the landing roll affects the landing distance. The effect of proper landing speed is important when runway The effect of gross weight on landing distance is one of the lengths and landing distances are critical. The landing speeds principal items determining the landing distance. One effect specified in the AFM/POH are generally the minimum safe of an increased gross weight is that a greater speed is required speeds at which the aircraft can be landed. Any attempt to to support the aircraft at the landing AOA and lift coefficient. land at below the specified speed may mean that the aircraft For an example of the effect of a change in gross weight, a may stall, be difficult to control, or develop high rates of 21 percent increase in landing weight requires a ten percent descent. On the other hand, an excessive speed at landing may increase in landing speed to support the greater weight. improve the controllability slightly (especially in crosswinds) but causes an undesirable increase in landing distance. When minimum landing distances are considered, braking friction forces predominate during the landing roll and, for A ten percent excess landing speed causes at least a 21 the majority of aircraft configurations, braking friction is the percent increase in landing distance. The excess speed main source of deceleration. places a greater working load on the brakes because of the additional kinetic energy to be dissipated. Also, the additional The minimum landing distance varies in direct proportion speed causes increased drag and lift in the normal ground to the gross weight. For example, a ten percent increase in attitude, and the increased lift reduces the normal force on gross weight at landing would cause a: the braking surfaces. The deceleration during this range of speed immediately after touchdown may suffer, and it is more • Five percent increase in landing velocity probable for a tire to be blown out from braking at this point. • Ten percent increase in landing distance The most critical conditions of landing performance are A contingency of this is the relationship between weight and combinations of high gross weight, high density altitude, braking friction force. and unfavorable wind. These conditions produce the greatest required landing distances and critical levels of energy The effect of wind on landing distance is large and deserves dissipation on the brakes. In all cases, it is necessary to proper consideration when predicting landing distance. Since make an accurate prediction of minimum landing distance to the aircraft lands at a particular airspeed independent of the compare with the available runway. A polished, professional wind, the principal effect of wind on landing distance is landing procedure is necessary because the landing phase of the change in the groundspeed at which the aircraft touches flight accounts for more pilot-caused aircraft accidents than down. The effect of wind on deceleration during the landing any other single phase of flight. is identical to the effect on acceleration during the takeoff. In the prediction of minimum landing distance from the The effect of pressure altitude and ambient temperature is to AFM/POH data, the following considerations must be given: define density altitude and its effect on landing performance. • Pressure altitude and temperature—to define the effect An increase in density altitude increases the landing speed of density altitude but does not alter the net retarding force. Thus, the aircraft at altitude lands at the same IAS as at sea level but, because • Gross weight—which defines the CAS for landing of the reduced density, the TAS is greater. Since the aircraft • Wind—a large effect due to wind or wind component lands at altitude with the same weight and dynamic pressure, along the runway the drag and braking friction throughout the landing roll have • Runway slope and condition—relatively small the same values as at sea level. As long as the condition is correction for ordinary values of runway slope, but a within the capability of the brakes, the net retarding force significant effect of snow, ice, or soft ground is unchanged, and the deceleration is the same as with the landing at sea level. Since an increase in altitude does not A tail wind of ten knots increases the landing distance by alter deceleration, the effect of density altitude on landing about 21 percent. An increase of landing speed by ten percent distance is due to the greater TAS. increases the landing distance by 20 percent. Hydroplaning makes braking ineffective until a decrease of speed that can The minimum landing distance at 5,000 feet is 16 percent be determined by using Figure 11-18. greater than the minimum landing distance at sea level. The approximate increase in landing distance with altitude is 11-17