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Comment by damiankennedy | Hacker News Reader

When a plane reaches V1 (which depends on the model, weight, runway length, temperature etc) the pilot has 2 seconds to abort takeoff or they are too fast to stop before the end of the runway. At V1 a multi-engine plane will take-off even with one engine out. The pilot will normally wait until Vr or V2 to rotate as this ensures a climb rate of 2.4 degrees. If the runway is long enough V1 will be higher than V2 but commercial aircraft will de-rate the engines and make use of the available runway to avoid running the engines at maximum.

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I know. Except that V1 by definition cannot be higher than Vr. You can't decide to abort (which you have to do for an engine failure below V1) after you've started rotation. Same for V2, it cannot be lower than V1 or Vr. So if that would theoretically be the case, you get a lower V1.

But that's all in the high speed regime. What I was commenting is that from the radio calls you can deduce that this bird did not go through the engine as claimed. They just hit the bird.

This two second number is not found in Part 25 of the Federal Aviation Regulations, what is your source? Is this a non-US definition, or is it specific to a carrier?

Also in Part 25, V2 cannot be less than Vr which cannot be less than V1. They call all be equal, but V2 cannot be less than V1 by definition.

Pilots will wait until at least Vr but not before making sure the airplane is tracking straight down the runway before lifting the nose wheel off the runway. At least that's how my airline teaches it...

They may be thinking about CS 25.109 in EASA? It specifies how the accelerate stop distance is calculated. That calculation assumes: "the pilot takes the first action to reject the take-off at the V1" so there is no 2 seconds to decide later. However it does add a distance equal to 2 seconds at V1 speed to the total.

That's a safety margin in the calculation, because you still need to move the thrustlevers and in some cases manually hit the brakes. Can't do that in a millisecond.

That doesn't sound exactly correct per the definitions I'm familiar with -- a multi-engine plane with a failure will not safely climb at V1, that's what V2 defines (the speed at which an engine-out aircraft can climb at 200ft/s, with at least 35ft of altitude at the end of the runway). Vr should be when the pilot begins rotation unless an engine is out; I don't believe there's any guidance to wait until V2.

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