Comment by panxyh
1 day ago
Isn't there any margin? Does it calculate stopping before end of runway or before causing damage?
Surely uncertainty about the situation contributes to defaulting to committing, but what if it's a passenger plane and at V1 pilots know they've lost power? Wouldn't veering into highway at 30 mph be weighted against certain, big loss of life?
Edit: I now see that this has been partially answered by uncle comment
There is some margin in the calculations. But the training is very very clear, before V1 you must abort and after V1 you must continue. No discussion, no decision to make. You call V1, hands go off the throttles and no matter what you're going to fly.
The margin is for example that the plane must not just be able to fly, but also reach a minimum climb gradient to clear obstacles with a bit of safety margin. There is also an allowance for the time it takes from calling abort to actually hitting the brakes. And for example headwind is part of the calculation (it makes the takeoff distance shorter) but only 50% of the headwind is used in the calculations.
But all of those margins are not for the crew to use, the crew must just execute the procedure exactly as trained which means at V1 you're committed to continue the takeoff. And before V1 in case of an engine failure you have to hit the brakes to make sure you can stop before the end of the runway.
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What an intensely rude thing to say to someone who has been providing specialist knowledge in a very deep technical field up and down this page.
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Nothing about it reads as AI to me. I'm not even the commenter and I take offense when people suggest that knowledgeable, helpful HN comments are AI.
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Nope, AI would probably have written it nicer, I just typed it on my phone :)
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> Wouldn't veering into highway at 30 mph be weighted against certain, big loss of life?
V1 for this plane in those conditions is nearly 200 mph. Even if they shut down all engines and applied full brakes (and assuming the brakes/tires didn't catch fire), they'd still run off the end of the runway with enough kinetic energy to kill themselves and anyone else in the way.
V1 is the decision speed with respect to a single engine failure in a multi-engine aircraft. It's the speed below rotation speed at which the decision to abort safely can no longer be made.
Captains can make the decision to abort the takeoff in the case of absolute power loss or for 'failure to fly' (where the aircraft is clearly not going to fly, e.g. the elevator/pitch controls aren't responding). But the training is adamant: if you're uncertain what has happened after V1 you try to fly the plane away from the runway.
> abort safely
That's what I'm getting at. I want to abort unsafely. Imagine 400 meters of grass field after the end of the runway, and a water body. I'm asking wether such factors are accounted for, or if plane on ground beyond runway does-not-compute.
> I want to abort unsafely
I expect pilots are trained explicitly not to do that.
If you can't abort safely, than it follows that the safer course of action is to try to fly. I'm sure there are exceptions to that, but a pilot has barely seconds in which to decide if any of those exceptions apply, so they're not going to abandon procedure unless the situation is clear.
You should never plan to use your safety margin!
That "extra" 400m of grass? That's for all the other things that can still go wrong even when you follow procedure. e.g., you're below V1 so you abort takeoff, close throttles and hit the brakes. You should be able to safely stop on the runway.
But now your brakes fail because maybe the reason you had to abort was a fire that also managed to burn through your brake lines, or it started to rain just as you were taking off, or...
Now that's where the 400m of safety margin comes in to save your ass (hopefully). It's "extra", you don't plan on using it.
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After you reach V1, you take off.
Between V1, Rotate and V2, there’s like a 2-3kts difference (between each of them).
I am not familiar what the procedure is if you have dual-engine failure at or above V1.
A fully loaded plane is extremely likely to turn into a fireball if it hits anything on the ground, even at 30mph. It's just a thin shell of aluminum with tons of fuel sloshing inside.