Aircraft Performance

excessive AOA may not allow the aircraft to climb out of In the prediction of takeoff distance from the AFM/POH ground effect. On the other hand, an excessive airspeed at data, the following primary considerations must be given: takeoff may improve the initial ROC and “feel” of the aircraft • Pressure altitude and temperature—to define the effect but produces an undesirable increase in takeoff distance. of density altitude on distance Assuming that the acceleration is essentially unaffected, the • Gross weight—a large effect on distance takeoff distance varies with the square of the takeoff velocity. • Wind—a large effect due to the wind or wind Thus, ten percent excess airspeed would increase the takeoff component along the runway distance 21 percent. In most critical takeoff conditions, such • Runway slope and condition—the effect of an incline an increase in takeoff distance would be prohibitive, and the and retarding effect of factors such as snow or ice pilot must adhere to the recommended takeoff speeds. Landing Performance The effect of pressure altitude and ambient temperature In many cases, the landing distance of an aircraft defines the is to define the density altitude and its effect on takeoff runway requirements for flight operations. The minimum performance. While subsequent corrections are appropriate landing distance is obtained by landing at some minimum safe for the effect of temperature on certain items of powerplant speed, that allows sufficient margin above stall and provides performance, density altitude defines specific effects on satisfactory control and capability for a go-around. Generally, takeoff performance. An increase in density altitude can the landing speed is some fixed percentage of the stall speed produce a twofold effect on takeoff performance: or minimum control speed for the aircraft in the landing 1. Greater takeoff speed configuration. As such, the landing is accomplished at some particular value of lift coefficient and AOA. The exact values 2. Decreased thrust and reduced net accelerating force depend on the aircraft characteristics but, once defined, the values are independent of weight, altitude, and wind. If an aircraft of given weight and configuration is operated at greater heights above standard sea level, the aircraft requires To obtain minimum landing distance at the specified landing the same dynamic pressure to become airborne at the takeoff speed, the forces that act on the aircraft must provide lift coefficient. Thus, the aircraft at altitude takes off at the maximum deceleration during the landing roll. The forces same indicated airspeed (IAS) as at sea level, but because of acting on the aircraft during the landing roll may require the reduced air density, the TAS is greater. various procedures to maintain landing deceleration at the peak value. The effect of density altitude on powerplant thrust depends much on the type of powerplant. An increase in altitude A distinction should be made between the procedures for above standard sea level brings an immediate decrease in minimum landing distance and an ordinary landing roll power output for the unsupercharged reciprocating engine. with considerable excess runway available. Minimum However, an increase in altitude above standard sea level does landing distance is obtained by creating a continuous peak not cause a decrease in power output for the supercharged deceleration of the aircraft; that is, extensive use of the brakes reciprocating engine until the altitude exceeds the critical for maximum deceleration. On the other hand, an ordinary operating altitude. For those powerplants that experience landing roll with considerable excess runway may allow a decay in thrust with an increase in altitude, the effect extensive use of aerodynamic drag to minimize wear and tear on the net accelerating force and acceleration rate can be on the tires and brakes. If aerodynamic drag is sufficient to approximated by assuming a direct variation with density. cause deceleration, it can be used in deference to the brakes Actually, this assumed variation would closely approximate in the early stages of the landing roll (i.e., brakes and tires the effect on aircraft with high thrust-to-weight ratios. suffer from continuous hard use, but aircraft aerodynamic drag is free and does not wear out with use). The use of Proper accounting of pressure altitude and temperature is aerodynamic drag is applicable only for deceleration to 60 mandatory for accurate prediction of takeoff roll distance. or 70 percent of the touchdown speed. At speeds less than The most critical conditions of takeoff performance are the 60 to 70 percent of the touchdown speed, aerodynamic drag result of some combination of high gross weight, altitude, is so slight as to be of little use, and braking must be utilized temperature, and unfavorable wind. In all cases, the pilot to produce continued deceleration. Since the objective during must make an accurate prediction of takeoff distance from the landing roll is to decelerate, the powerplant thrust should the performance data of the AFM/POH, regardless of the be the smallest possible positive value (or largest possible runway available, and strive for a polished, professional negative value in the case of thrust reversers). takeoff procedure. 11-16