NAVWEPS OO-ROLRO 
AIRPlANE PERFORMANCE 
effect on takeoff distance and is approximated 
by the following relationship: 
$2 v 2 ..-.= Sl c 1 13 
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 
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: 
TAS 1 
E-33=5 
where 
TAS= true airspeed 
EAS= equivalent airspeed 
a=altitude density ratio 
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. 
To evaluate the effect of density altitude on 
landing distance, the following relationships 
are used : 
since an increase in altitude does not alter 
acceleration, the effect would be due to 
the greater TAS 
where 
S1= standard sea level landing dis- 
tance 
Sa=Ianding distance at altitude 
c=altitude density ratio 
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