Originally Posted by
megan
As the aircraft is certified to FAR 25 I'd have thought all take offs would be planned to be able to stop with failure at the EF point and the first stopping action taken at V1 (§25.109 Accelerate-stop distance), or be able to continue the take off with one engine out if the failure occurs at a point where the first stopping action would be post V1 (§25.111 Takeoff path).
The issue is that all takeoff calculations assume a fully functional aircraft up to the point of failure (identification). So yes, failure AT the EF point is fine. But this wasn't a case of a failure at the EF point, this appears to have been an aircraft grossly underperforming from the start of the TO run. Consequently, even if you should have plenty of runway to stop from V1 if everything is normal to that point, if you aren't accelerating fast enough you'll eat up a lot more runway geting to V1, and there won't be close to enough runway left to stop (indeed in an extreme case, you might not ever get to V1 before the end of the runway).
The part 25 runway performance criteria represent a standard which has been shown in practice to generally provide adequate safety margins for most failures or eventualities. They do NOT provide a guarantee for anything except the precise cases to which they apply.