NAVWEPS OO-ROLRO  AIRPlANE PERFORMANCE  effect on takeoff distance and is approximated  by the following relationship:  $2  v 2  ..-.=  Sl  c  1  13  TAS  1  E-33=5  where  where  Si= zero wind landing distance  Sa=landing distance into a headwind  I ’ , = headwind velocity  Vi=landing  ground velocity with zero  wind or, simply, the landing airspeed  TAS= true airspeed  EAS= equivalent airspeed  a=altitude density ratio  As a result of this relationship, a headwind  which is 10 percent of the landing airspeed will  reduce the landing distance 19 percent but a  tailwind (or  ‘ negative headwind) which is 10  percent of the landing speed will increase the  landing distance 21 percent. Figure 2.33 illus-  trates this general effect.  The effect  of  ranway slope  on landing distance  is due to the component of weight along the  inclined path of the airplane.  The relation-  ship is identical to the case of takeoff per-  formance but the magnitude of the effect is  not as great. While account must be made  for the effect, the ordinary values of runway  slope do not contribute a large effect  on landing  distance. For this reason, the selection of the  landing runway will ordinarily favor the direc-  tion with a downslope and ’ headwind rather  than an upslope and tailwind.  The effect  of pressure  altitude and ambient tem-  perature  is to define density  altitude and its effect  on landing performance. An increase in dens-  ity altitude will increase the landing velocity  but will not alter the net retarding force. If  a given weight and configuration of airplane  is taken to altitude above standard sea level,  the airplane will still require the same 4 to  provide lift equal to weight at the landing C,.  Thus, the airplane at altitude will land at the  same equivalent airspeed (EAS) as at sea level  but, because  of the reduced density, the true  airspeed (TM)  will be greater. The relation-  ship between true airspeed and equivalent air-  speed is as follows:  From this relationship, the minimum land-  ing distance at 5,OCO  ft. (u=O.8617) would be  16 percent greater than the minimum landing  distance at sea level. The approximate increase  in landing distance with altitude is approxi-  mately 3% percent for each 1,ooO  ft. of altitude.  Proper accounting of density altitude is neces-  sary to accurately predict landing distance.  The effect of proper  landing velocity  is impor-  tant when runway lengths and landing dis-  tances are critical. The landing speeds  specified  in the flight handbook ate generally the mini-  mum safe speeds  at which the airplane can be  landed.  Any attempt to land at below the  where  since an increase in altitude does not alter  acceleration, the effect would be due to  the greater TAS  To evaluate the effect of density altitude on  landing distance, the following relationships  are used :  Since the airplane lands at altitude with the  same weight and dynamic pressure, the drag  and braking friction throughout the landing  toll 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 un-  changed and the acceleration is the same as  with the landing at sea level.  S1= standard sea level landing dis-  tance  Sa=Ianding distance at altitude  c=altitude density ratio 