Balanced Field Length: Civil Aircraft
The rated TOFL at the MTOM is determined by the BFL in the event of an engine failure. The BFL must comply with FAR requirements. A normal takeoff with all engines operating needs a considerably shorter field length than the rated TOFL. Designers must provide the decision speed V1 for pilots that below which the takeoff must be aborted for safety reasons if an engine fails. Figure 13.9 shows the segments involved in computing the BFL.
Table 13.5. Civil aircraft first – and second-segment climb configuration
Figure 13.9. Balanced field length consideration
The figure shows that taking off with one engine inoperative (i. e., failed) has three segments to clear a 35-ft height, as follows:
Segment A: Distance covered by all-engine operating ground run until one engine fails at the decision speed V1.
Segment B: Distance covered by one-engine inoperative acceleration from V1
Segment C: Continue with the flare distance from liftoff speed VLO to clear a
35-ft obstacle height reaching aircraft speed V2.
For stopping at the decision speed Vi, there are two segments (which replace segments B and C), as follows:
Segment D: Distance covered during the reaction time for a pilot to take braking action. (Typically, 3 s is used as the pilot recognition time and braking to act, spoiler deployment, and so on. At engine failure, the thrust decay is gradual; within this reaction time before brake application, there is a minor speed gain, shown in Figure 13.9.)
Segment E: Distance to stop from VB to Vo (maximum brake effort).
The BFL is established when Segments (B + C) = Segments (D + E).
During takeoff, the aircraft accelerates. At the conceptual design phase, the average values of speed, acceleration, and thrust are taken of 0.7 of the velocity of the ground run segments. In later stages of a project, the computation is figured more accurately in smaller steps of speed increments within which average values of the variables are considered constant. CL also varies with speed changes; typical values of CL and Cd/Cl are given in Section 13.5.1.
Section 11.3.1 derives the associated governing equations to compute the TOFL. Equations 11.2 and 11.4 give:
Table 13.6. FAA second-segment climb gradient at missed approach
The landing configuration is with full flaps extended and the aircraft at landing weight. The approach segment at landing is from a 50-ft altitude to touch down. At approach, the FAR requires that an aircraft must have a minimum speed Vapp = 1.3Vstaii@iand. At touchdown, aircraft speed is Vtd = 1-15Vstaii@iand. Brakes are applied 2 s after all wheels touch down. A typical civil aircraft descent rate at touchdown is between 12 and 22 ft/s. The landing runway length should be 1.667 times the computed landing distance. Generally, this works out to be slightly less than the BFL at the MTOM (but not necessarily).
For a balked landing or missed approach at landing weight, the FAR requirements are given in Table 13.6. An aircraft is configured with full flaps, undercarriage extended, and engine in full takeoff rating. In general, this is not a problem because all engines are operational and the aircraft is lighter at the end of the mission. Military aircraft requirements are slightly different: Vapp = 1.2Vstall@land and Vtd = 1.1Vstall@land.
The approach has two segments, as follows:
• a steady, straight glide path from a 50-ft height
• flaring in a nearly circular arc to level out for touchdown, which incurs a higher g
The distances covered in these two segments depend on how steep is the glide path and how rapid is the flaring action. This book does not address these details of analysis; instead, a simplified approach is taken by computing the distance covered during the time from a 50-ft height to touchdown before the brakes are applied; it is assumed to be 6 s herein.