Technical Question

[Mach75]

Hi guys, I'm still a little confused about what the correct answer is for the following question. I've seen a few different answers ... Any input?

- What would be better: To experience an engine failure 10 kts below V1 and continue the takeoff or to experience an engine failure 10 kts above V1 and reject the takeoff?

Can anyone tell me what the correct answer is and the theory behind it?

It would seem to me that either situation would result in an unbalanced field length. V1+10kts & rejecting the t/o would possibly result in a failure to stop the a/c in the stopping distance available. While V1-10kts and continuing the t/o would most likely result in achieving a reduced screen height (ie. like V1 wet) but an otherwise safe t/o provided net climb gradient performance is met.

Does my theory hold true or does anyone out there have anything to add?

[Drunknsailor]

There is a lot of discussion on this thread on the tech log forum. The short answer is that NEITHER of these options are a valid answer. V1 exists for a reason. If an interviewer phrased this question like you did, it would be wise to tread very lightly about it. You could discuss the possible outcomes of either choice and then you'd better finish it off with some way that you are NOT going to be in that situation!!

Happy hunting

[mrfox]

There are some padding in the calcs to allow time to recognize and react to the engine failure. Guess it depends on your definition of when the "experience" of engine failure actually occurs.

[JonaLX]

Hi,

if you consider V1 on a wet runway, it's often V1 dry reduced by a few knots (to be sure sufficient runway is available in case of RTO), and if an engine failure happens at V1, you still would have enough acceleration to reach a screen height of 15 ft.

So if you get an engine failure 10 kts before V1 (dry) and continue the take-off, there might be possibilities you'd still be safe.

That's why personnaly, I'd go with the 10 kts before V1 answer.

But you might as well think of the other.
On my ATPL books (might not be correct) take-off roll required is based on an engine failure at V1, accelaration for 2 seconds with one engine failed from V1, and then the distance required to stop the airplane. So maybe if it takes less than 2 seconds to reach V1+10kts with one engine failed, it still may be safe enough to abort.

That would be my general way of thinking about the problem.

[VVJM265]

http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/c2614e27b49bf38686256ba300696689/$FILE/AC%2025-7A%20Change%201.pdf

There are some good charts and somewhat better explanations on the FAA specs in AC 25-7; see sections 13 & 14 relavent to the discussion at hand.

[luvmuhud]

I would go for the V1 - 10 and continue for the same reasons as JonaLX. (May still give 15' clearance at the screen)

The other reason that I'd take this option is that to stop a moving aircraft, we need to convert it's kinetic energy into heat energy (brakes) or noise (reverse thrust) etc etc. Remember that kinetic energy is proportional to the square of velocity. The difference in energy between 130kts and 120kts is much larger than the difference between 120kts and 110kts. So, trying to stop from 130kts is a disproportionately bigger ask than trying to stop from 120kts.

[Mullah Lite]

There are some padding in the calcs to allow time to recognize and react to the engine failure. Guess it depends on your definition of when the "experience" of engine failure actually occurs.


Fox

The powers that calculate give us one second to recognise the failure. The question is: how much of a life review can one fit in a single flash?? :eek:

[Sal-e]

I'm risking being simple here but this is what I would do (if in fact I would ever face that situation....of course one shouldn't).
I would rather abort 10kts above, knowing I would go beyond the remaining runway but at least decelerating whilst doing so. The other option has too many unknowns......biggest one being "how much screen height is left if any at all?"....all this whilst accelerating to a much higher speed and possibly hitting something at that higher speed. There's other considerations with the first option as well, you wouldn't do it going off the end of a cliff!!!
So there you have it.... simple, might even fail you an aural exam with someone who thinks having the title of TRI or TRE makes them God but can never say they've experienced the above scenario for real.......
But one thing is for sure, if the worst case occurred on both scenarios, I know which one would likely kill more people!

[SMOC]

Here you go.

A 747, HVY weight, balanced field, dry, no reverse, shows that if you decide to abort at 1kt over V1 you will go off the end of the runway at 20kts and 4kts over V1, it's up to 60kts don't have the graph in front of me but 10kts is probably over 120kts.

(see pics of a recent abort http://www.pprune.org/forums/showthread.php?t=229589 )

Now if you 'GO' at 20kts below V1 you will cross the end of the runway @ 15', 10kts below 20', 5kts below 25' and @V1 as you'd expect 35'.

So if you ask me the benifits of going below V1 are obvious assuming the A/C is flyable.
(10kt < V1 = 20' and 10kt > V1 = 120+kts off the end).

Then you can dump or not, come back and use full length with all the services notified.

You obviously have to look out for Vmcg which must be at least 10kts below V1 for the question to be valid.


SMOC

[Wizofoz]

Option 1 if the T/O is runway limited, option 2 if there is heaps of runway and the T/O is structural or climb limited.