the power required curve represents the speed at which the  lowest brake horsepower sustains level flight. This is termed  the best endurance airspeed.  An airplane performing a low airspeed, high pitch attitude  power approach for a short-field landing is an example  of operating in the region of reversed command. If an  unacceptably high sink rate should develop, it may be  possible for the pilot to reduce or stop the descent by applying  power. But without further use of power, the airplane would  probably stall or be incapable of flaring for the landing.  Merely lowering the nose of the airplane to regain flying  speed in this situation, without the use of power, would  result in a rapid sink rate and corresponding loss of altitude.  If during a soft-field takeoff and climb, for example, the pilot  attempts to climb out of ground effect without first attaining  normal climb pitch attitude and airspeed, the airplane may  inadvertently enter the region of reversed command at a  dangerously low altitude. Even with full power, the airplane  may be incapable of climbing or even maintaining altitude.  The pilot’s only recourse in this situation is to lower the pitch  attitude in order to increase airspeed, which inevitably results  in a loss of altitude.  Airplane pilots must give particular attention to precise  control of airspeed when operating in the low flight speeds  of the region of reversed command.  The majority of pilot-caused aircraft accidents occur during  the takeoff and landing phase of flight. Because of this fact,  the pilot must be familiar with all the variables that influence  the takeoff and landing performance of an aircraft and must  strive for exacting, professional procedures of operation  during these phases of flight.  The amount of power that is applied to the brakes without  skidding the tires is referred to as braking effectiveness.  Ensure that runways are adequate in length for takeoff  acceleration and landing deceleration when less than ideal  surface conditions are being reported.  Runway surfaces vary widely from one airport to another.  The runway surface encountered may be concrete, asphalt,  gravel, dirt, or grass. The runway surface for a specific  airport is noted in the Chart Supplement U.S. (formerly  Airport/Facility Directory). Any surface that is not hard  and smooth increases the ground roll during takeoff. This  is due to the inability of the tires to roll smoothly along the  runway. Tires can sink into soft, grassy, or muddy runways.  Potholes or other ruts in the pavement can be the cause of  poor tire movement along the runway. Obstructions such  as mud, snow, or standing water reduce the airplane’s  acceleration down the runway. Although muddy and wet  surface conditions can reduce friction between the runway  and the tires, they can also act as obstructions and reduce  the landing distance.  [Figure 11-16]  Braking effectiveness  is another consideration when dealing with various runway  types. The condition of the surface affects the braking ability  of the aircraft.  Runway conditions affect takeoff and landing performance.  Typically, performance chart information assumes paved,  level, smooth, and dry runway surfaces. Since no two  runways are alike, the runway surface differs from one  runway to another, as does the runway gradient or slope.  [Figure 11-15]  Takeoff and landing performance is a condition of  accelerated and decelerated motion. For instance, during  takeoff an aircraft starts at zero speed and accelerates to  the takeoff speed to become airborne. During landing, the  aircraft touches down at the landing speed and decelerates  to zero speed. The important factors of takeoff or landing  performance are:  • The takeoff or landing speed is generally a function  of the stall speed or minimum flying speed.  • The rate of acceleration/deceleration during the  takeoff or landing roll. The speed (acceleration and  deceleration) experienced by any object varies directly  with the imbalance of force and inversely with the  mass of the object. An airplane on the runway moving  at 75 knots has four times the energy it has traveling  at 37 knots. Thus, an airplane requires four times as  much distance to stop as required at half the speed.  • The takeoff or landing roll distance is a function of  both acceleration/deceleration and speed.  Takeoff and Landing Performance  Runway Surface and Gradient  Figure 11-14.  Power required curve.  Power setting  Speed  Best endurance speed  P o w e r e q u i r e d Region of  reversed  command  Ma xi mu m  p o w e r a v a il a b l e  Excess power  11-12 