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