Discussion of the Takeoff Analysis

Increasing the flap setting improves the BFL capability at the expense of a loss in climb gradient. The next section verifies the gradient requirements. With one engine inoperative, the loss of thrust percentage for a two-engine aircraft is the highest (i. e., 50%). With one engine failed, the aircraft acceleration suffers severely and the ground run from Vi to liftoff is high.

Table 13.16 summarizes the takeoff performance and associated speed sched­ules for the two flap settings and provides an example of the procedure. The ratio of speed schedules can be varied for pilot ease, as long as it satisfies FAR requirements.

At a lower flap setting of 8 deg, the decision speed Vi is close to the rotation speed Vi = 0.93VR. The situation improves with a higher flap setting of 20 deg when Vi = 0.9 Vr.

Higher flap settings provide more time between the decision speed Vi and the rotation speed VR. However, it is not problematic if V1 is close to VR. If one engine fails close to the decision speed, then the rotation speed VR is reached very quickly; that is, even if a pilot’s reaction is slow, the aircraft will still take off if there is suf­ficient runway length available (the BFL can be considerably lower than the avail­able airfield length). Also, Vmu is close to VR; hence, tail dragging is not likely. If an engine fails early enough, then a pilot has sufficient time to recognize the failure and abort the takeoff.

With more than two engines, the decision speed Vi is farther from the rotation speed VR. A pilot must remain alert as the aircraft speed approaches the decision speed Vi and must react quickly if an engine fails.

Figure 13.11. Balanced field length