text: sufficient to cause deceleration of the airplane 
  it can be used in deference to the brakes in the 
  i.e., brakes 
  early stages of the landing roll, 
  and tires suffer from continuous, hard use but 
  airplane aerodynamic drag is free and does not 1 
  wear out with use. The use of aerodynamic 
  drag is applicable only for deceleration to 60 
  ot 70 percent of the touchdown speed. At 
  speeds less than 60 to 70 percent of the touch- 
  down speed, aerodynamic drag is so slight as 
  to be of little use and braking must be utilized 
  to produce continued deceleration of 
  the 
  airplane. 
  
  illustrated on 
  Powerplant 
  there are so many possible 
  figure 2.34 for 
  the 
  variations. Since the objective during 
  landing toll 
  is to decelerate, the powerplant 
  thrust should be the smallest possible positive 
  In 
  value or largest possible negative value. 
  the case of 
  idle 
  the 
  thrust of the engine is nearly constant with 
  speed throughout 
  the landing roll. The idle 
  thrust is of significant magnitude on cold days 1 
  because of the low compressor inlet air temper- 
  ature and low density altitude. Unfortu- 
  nately, such atmospheric conditions usually 
  have the corollary of poor braking action be- 
  cause of ice or water on the runway. The 
  the 
  thrust from a windmilling propeller with 
  engine at idle can produce large negative thrust 
  early in the landing roll but the negative force 
  decreases with 
  speed. The .large negative 
  thrust at high speed is valuable in adding to 
  drag and braking friction 
  to increase the net 
  retarding force. 
  
  thrust  is not 