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After calculating the following take-off figures, what would be the consequence of the pilots accidentally entering a V1 value of 124 kt.

V1: 142 kt
VR: 142 kt
V2: 145 kt

  • A
    TOD is not affected though the aircraft may not become airborne before the end of the runway in case of an engine failure.
  • B
    TOD is not affected and the aircraft will not encounter any additional challenges in becoming airborne before the end of the runway in case of an engine failure.
  • C
    TOD would decrease significantly and the aircraft might not be able to get airborne before the end of the runway in case of engine failure.
  • D
    TOD would increase significantly and the aircraft may not become airborne before the end of the runway in case of an engine failure.

Refer to figure.
V1 is the take-off decision speed. It is the point on the take-off run where the pilots will go from stopping if there is an emergency to continuing. It is created from the maximum speed at which the aircraft can get to and stop again within the ASDA (Accelerate-Stop Distance Available), and the minimum speed where the aircraft can accelerate the rest of the way to V2 speed and screen height by the end of the TODA (Take-Off Distance Available).

Sometimes there is a range of V1 speeds that can be chosen from, if the aircraft is under its FLLTOM (Field Length Limiting Take-Off Mass), but we usually choose the "balanced V1", which gives the same ASD as TOD in the event of an engine failure at VEF (just before V1).

Choosing a V1 that is much too low, such as in this question, means that the changeover point from STOP to GO (if an event happens) will happen much earlier in the take-off roll, which will cause a greatly reduced ASD (Accelerate-Stop Distance) in the event of a rejected take-off, as the aircraft would spend less runway getting up to speed, then have to stop from a lower speed.

The big problem is that, in the event of an engine failure at 124 kt, the aircraft would then have to accelerate all the way from 124 kt to 145 kt and reach the screen height (usually 35 ft) by the end of the TODA, when there is half the thrust of normal operation. As you can see from the calculated figures, the aircrat is only supposed to accelerate from 142 kt to 145 kt on one engine (and climb to 35 ft), so asking it for just over 20 kts single engine acceleration will cause the TOD to increase greatly, with the aircraft rotating very late, making it unlikely to reach the screen height in sufficient distance, also reducing obstacle clearance in the climb out.

Note: This question does not make it absolutely clear that we are considering the one engine inoperative scenario for the TOD, but our best feedback has given the correct answer we currently have, so this is the assumption we have had to make. Please send any further feedback to us in the comments or by email to info@atplquestions so we can verify this, thank you.

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