PPRuNe Forums - View Single Post - Rejecting A Takeoff After V1…why Does It (still) Happen?
Old 7th Dec 2010, 08:58
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homebuilt
 
Join Date: Mar 2005
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Ah yes the failing brake argument....that brakes are perfectly reliable for landings, but during RTOs your brake disks turn into marshmellows...but not on landings...only RTOs.
Somehow yes, because you have to take into account the huge difference between TOW (MTOW) and LW (MLW) on an heavy transport airplane. Speaking of the B744 for example you have a MTOW close to 400 metric tons (396800 kgs), vs a MLW of 285700 kgs (with actual LW around 250 - 260 metric tons in the average).

If you can compare the brake energy required to stop a close to 400-ton airplane starting at ~170 kts (~V1 at MTOW) - the case of the RTO -, and the brake energy required to stop the same airplane but at 260 tons, starting at ~145 kts (~Vref at that weight), with the cinetic energy formula 1/2 M V2, that gives you a balance of a little more than 2 against 1.

Roughly speaking, that means that on such an airplane, in order to be able to reject the take-off brakes must deliver a little bit more than twice the energy that they have to give for a max breaking landing (most times I perform landings using the autobrake set to 3 on a range that shows 1,2,3,4, MAX, and .... RTO).

That's all the difference and that's why a MTOW RTO at V1 is likely to finish in the grass with the landing gear burning...

But as I told earlier with the example of the MD83 colliding the Shorts 330, it's airmanship management and indeed sometimes you won't have any other choice but rejecting T/O whichever the speed... But in that case one has to be aware of the potential consequences.
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