As discussed in Chapter 6, engine pressure ratio (EPR) is the  ratio between exhaust pressure (jet blast) and inlet (static)  pressure on a turbo jet or turbo fan engine. An EPR gauge  tells the pilot how much power the engines are generating.  The higher the EPR, the higher the engine thrust. EPR is  used to avoid over-boosting an engine and to set takeoff and  go around power if needed. This information is important to  know before taking off as it helps determine the performance  of the aircraft.  In addition to the important factors of proper procedures,  many other variables affect the takeoff performance of an  aircraft. Any item that alters the takeoff speed or acceleration  rate during the takeoff roll affects the takeoff distance.  ratio, the increase in takeoff distance would be approximately  25 to 30 percent. Such a powerful effect requires proper  consideration of gross weight in predicting takeoff distance.  The effect of wind on takeoff distance is large, and proper  consideration must also be provided when predicting takeoff  distance. The effect of a headwind is to allow the aircraft to  reach the lift-off speed at a lower groundspeed, while the  effect of a tailwind is to require the aircraft to achieve a  greater groundspeed to attain the lift-off speed.  A headwind that is 10 percent of the takeoff airspeed reduces  the takeoff distance approximately 19 percent. However, a  tailwind that is 10 percent of the takeoff airspeed increases  the takeoff distance approximately 21 percent. In the case  where the headwind speed is 50 percent of the takeoff speed,  the takeoff distance would be approximately 25 percent of  the zero wind takeoff distance (75 percent reduction).  The effect of wind on landing distance is identical to its  effect on takeoff distance.  Figure 11-19  illustrates the general  effect of wind by the percent change in takeoff or landing  distance as a function of the ratio of wind velocity to takeoff  or landing speed.  For example, the effect of gross weight on takeoff distance  is significant, and proper consideration of this item must be  made in predicting the aircraft’s takeoff distance. Increased  gross weight can be considered to produce a threefold effect  on takeoff performance:  If the gross weight increases, a greater speed is necessary to  produce the greater lift necessary to get the aircraft airborne  at the takeoff lift coefficient. As an example of the effect of  a change in gross weight, a 21 percent increase in takeoff  weight requires a 10 percent increase in lift-off speed to  support the greater weight.  The effect of proper takeoff speed is especially important  when runway lengths and takeoff distances are critical. The  takeoff speeds specified in the AFM/POH are generally  the minimum safe speeds at which the aircraft can become  airborne. Any attempt to take off below the recommended  speed means that the aircraft could stall, be difficult to  control, or have a very low initial ROC. In some cases, an  A change in gross weight changes the net accelerating force  and changes the mass that is being accelerated. If the aircraft  has a relatively high thrust-to-weight ratio, the change in the  net accelerating force is slight and the principal effect on  acceleration is due to the change in mass.  For example, a 10 percent increase in takeoff gross weight  would cause:  With ISA conditions, increasing the takeoff weight of the  average Cessna 182 from 2,400 pounds to 2,700 pounds (11  percent increase) results in an increased takeoff distance from  440 feet to 575 feet (23 percent increase).  For the aircraft with a high thrust-to-weight ratio, the increase  in takeoff distance might be approximately 21 to 22 percent,  but for the aircraft with a relatively low thrust-to-weight  Figure 11-19.  Effect of wind on takeoff and landing.  11-15  •  A 5 percent increase in takeoff velocity  •  At least a 9 percent decrease in rate of acceleration  •  At least a 21 percent increase in takeoff distance  3.  Increased retarding force (drag and ground friction)  2.  Greater mass to accelerate  1.  Higher lift-off speed  Percent increase  in takeoff or  landing distance  Percent decrease  in takeoff or  landing distance  Ratio of wind  velocity to takeoff  or landing speed  Headwind  Tailwind  Reference line 80  70  60  50  40  30  20  10  30 %  20%  10%  10%  20%  30% 10  20  30  40  50  60  Ratio of wind  velocity to takeoff  or landing speed 