Takeoff Distance Determinations

Concepts and Terms

Balanced Field

Special Case: Accelerate-Go adjustment

Light Aircraft Considerations

For light twin aircraft, VR is generally VMC, and, in the absence of a computed V1, V1 would be VR. On a runway of typical length at a major air-carrier utilized airport, any accelerate-stop decision speed would be quite high, but, in any case, it would not be greater than VR.

Once airborne, V1 will no longer exist and in-flight procedures become governing; however, in such a circumstance, it may still be possible to abort the departure to the runway remaining, landing and safely stopping; and while the acceleration from V1 (VR) to V2 (presumably VXSE) to clear an obstacle would continue to be relevant, it now exists in the realm of flight maneuvers (i.e., gear retracted, flaps to recommended climb setting, secure failed engine, emergency climb-out checklist).

Pathological and Other Forms

It is assumed that under optimal conditions (i.e., no system failures) a takeoff may be accomplished; that is, the existence of a practical element one (above, accelerate-go) is presupposed (so even if taking the 15% margin somewhat flexibly, assume that the airplane can get out of the airport).

There will always be a decision speed with respect to element three (above, accelerate-stop); that is, unless a runway of an exceptional length is utilized, at some point the aircraft will have accelerated to a point where it may no longer reject and stop on the runway (given twelve, ten, or even eight thousand foot runways, this might not be applicable).

• If the runway is so short as to not allow a safe RTO under any but the most limited speed conditions, the aircraft may be committed to takeoff but not have an ability to clear an obstacle given a system failure (e.g., an engine failure after V1);
• If the runway is so long as to preclude a meaningful V1, V1 would become VR (thus initiating the aircraft becoming airborne in this case commits to the aircraft becoming airborne), and any engine failure past this point is an in-flight emergency, requiring either an emergency landing (landing and stopping on the runway remaining) or emergency climbout (certainly requiring immediate feathering if the inoperative engine is completely inoperative and windmilling).

Given the former of the foregoing points,

• It may not be possible to assure that element two (above, obstacle clearance following critical engine failure) is satisfied (thus knowing that an accelerate-stop may be mandatory even after it is no longer possible).
• It is possible that an aircraft (e.g., a light twin) may be beyond its accelerate-stop decision point, thus effectively committed to takeoff, but still be below VMC, which may preclude it ever getting airborne.

If the runway is so inadequate that a meaningful V1 cannot be computed, the takeoff should not be attempted and action should be taken to change the circumstances of the takeoff.

Element two is most readily addressed by lowering the takeoff weight, thus facilitating climb-out (it may be generally assumed that with all engines operative, element one takes care of itself, but that still needs to be determined as well). This is because for any given aircraft in any given conditions (weight, maximum available thrust, wind, density altitude), the accelerate-go distance is either there or it is not: the only thing that can be varied is the weight of the aircraft (although, in the case of a cross-wind runway of substantially greater length than the active runway, changing runways for departure may have some effect).

Extrapolating these principals to single-engine aircraft

Single-engine aircraft face analogous decision points.

At some point in the takeoff roll, the aircraft may be committed to takeoff (i.e., insufficient runway remaining to abort):

• Beyond this point, does the pilot continue to takeoff even if there is an indication of a system failure (albeit one that does not force an immediate abort)?

At some point in the climb-out, the aircraft may be out of position for either an emergency on-airport landing (straight-ahead or with a shallow turn), an off-airport landing, or a return to the airport.

• What options exist during the time period, and what preflight planning can be done to mitigate this?

To answer these considerations, it may be that a safe single-engine take-off requires, and "should" only be attempted, when the following elements are met:

1. The runway is long enough for the aircraft to
   1. accelerate to VR and then stop on the paved surfaces remaining;
   2. abort to the runway in the event of an engine failure
      1. immediately following liftoff;
      2. prior to being in a position to maneuver in order to accomplish a safe on- or off-airport landing;
2. The aircraft should remain in position to abort to the airport or to a safe off-airport site prior to leaving the airport environment and proceeding on-course.

While the traditional "conventional wisdom" that one goes straight ahead with an engine failure on take-off must not be discounted, there has to be someplace to go straight ahead, and that itself requires some degree of pre-flight planning.

Finally, if the decision is made to conduct an emergency descent — or an engine failure forces an emergency descent — to a forced landing, the engine must be secured, with at least the throttle retarded, fuel off, and, with a constant-speed propeller, the prop control in the minimum-drag position, coarsest pitch [control full out]). If nothing else, these steps will preclude the engine unexpectedly coming to life at an inopportune moment, reduce the likelihood of spilled fuel, and lengthen the glide.

Conclusion

Just as with any other phase of flight, takeoffs present management issues that are best resolved prior to the flight. Although accident data consistently indicate accidents are most likely during the approach and landing phases, the aircraft itself is most vulnerable (i.e., high speed on the ground, low airspeed in flight, low altitude) on takeoff and climb out. This should require every bit as much preflight preparation as, for example, computing weight & balance, fuel management, and navigation.

Notes-References