Takeoff Distance Determinations

Concepts and Terms

Pilots are required to determine takeoff performance prior to flight (14 CFR 91.103(b)). Takeoff performance becomes more involved as the aircraft become larger and have higher performance. For multiengine aircraft, a principal computation concerns accelerate-stop and accelerate-go distances and the associated V1 speed. The computed distances are based on the takeoff and climb performance in the Aircraft Flight Manual.

Accelerate Stop ⇐ Total Accelerate-Stop Distance

(to end of runway or, if available, to the end of the runway stopway)

V1 In this case, V1 means the maximum speed in the takeoff at which the pilot must take the first action (e.g., apply brakes, reduce thrust, deploy speed brakes) to stop the airplane within the accelerate-stop distance (i.e., in the available remaining runway/stopway). Above this speed, the aircraft is committed to takeoff.

Decision Speed for a given runway length

Accelerate Go ⇐ Total Accelerate-Go Distance

V2 (over end of runway, stopway, and clearway to over an obstacle*)

V1 In this case, V1 means the minimum speed in the takeoff, following a failure of the critical engine at VEF, at which the pilot can continue the takeoff and achieve the required height above the takeoff surface within the takeoff distance. Below this speed, the aircraft is committed to abort the takeoff.

Decision Speed for a given takeoff profile

*Over 35 ft obstacle at V2 for Part 121 (14 CFR 23.53(a)(1), 121.189(d)(2))

*Over 50 ft obstacle otherwise (14 CFR 23.53(b))

Specifically, the FAA poses the following three criteria for V1 (Part 121):

1) Will achieve 35-ft altitude with all engines operating and a margin of 15% of the actual distance used remaining;

2) Will achieve 35-ft altitude with the critical engine failed one second prior to V1;

3) Will stop with an engine failure or other event prior to V1 and the rejected takeoff (RTO) initiated at V1;

… all within the existing runway length available.

Balanced Field

A field is balanced when the computed decision speed for the accelerate-stop distance is greater than or equal to the computed decision speed for the accelerate-go distance. The lower of these two computed speeds will be V1. If, alternatively, the computed decision speed for accelerate-stop is less than the computed decision speed for accelerate-go, the aircraft is committed to the takeoff before it can successfully accomplish the takeoff. In such a situation, V1 has no meaning.

Special Case: Accelerate-Go adjustment

The the accelerate-go distance may be greater than the accelerate-stop distance considering the availability of a clearway beyond the end of the runway/stopway (paved surface[s]); however, V1 with respect to stopping the aircraft will be the same regardless of the go-length available. Therefore, consider the following:

Accelerate Stop ⇐ To end of paved surfaces

Go ⇐ To over obstacle (35 ft @ V2 [Part 121] or 50 ft [Part 91])

In this example, if at V1 the takeoff can be either rejected or accomplished, the field is "balanced" and the takeoff roll can be initiated with a general assurance of safety, even if the aircraft passes over the end of the paved surfaces without meeting the obstacle-clearance requirements (it will, or at least it should, before it reaches the obstacle).

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

Pilot Guide to Takeoff Safety

https://www.faa.gov/other_visit/aviation_industry/airline_operators/training/media/takeoff_safety.pdf
Retrieved 2014年11月29日