V1 question
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Interesting V1 performance conundrum
Ppruners with a penchant for performance related questions, this is one is for you...
This hypothetical V1 question is being asked by a reputable company to prospective candidates, and I am interested in knowing what you think -(I appreciate that the company in question is more likely to be interested in the factors and issues raised by the question than a definitive answer):
(Credit goes to to Ppruner 'flyingmogul' for raising it first)
'Which is better: failure 10 kts before V1 and continue T.O or failure 10 kts after V1 and abort T.O?'
No assumptions are made or given, so please feel free to elaborate on your answers and vary the variables either way (e.g wet runway, dry runway , high Vmcg, balanced field, 2/3/4 eng. aircraft etc...)
Answers on a postcard...
'round midnight.
This hypothetical V1 question is being asked by a reputable company to prospective candidates, and I am interested in knowing what you think -(I appreciate that the company in question is more likely to be interested in the factors and issues raised by the question than a definitive answer):
(Credit goes to to Ppruner 'flyingmogul' for raising it first)
'Which is better: failure 10 kts before V1 and continue T.O or failure 10 kts after V1 and abort T.O?'
No assumptions are made or given, so please feel free to elaborate on your answers and vary the variables either way (e.g wet runway, dry runway , high Vmcg, balanced field, 2/3/4 eng. aircraft etc...)
Answers on a postcard...
'round midnight.
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'Which is better: failure 10 kts before V1 and continue T.O or failure 10 kts after V1 and abort T.O?'
Good one. It would as you say, depend on a number of variables; are you obstacle limited brings to mind. However, given that lots of companies reduce V1 by 10 kts if the runway is wet, surely from a dry runway V1-10kts would get you airborne - assuming a failure at V1 - albeit reducing the screen height to 15'?
S15
Good one. It would as you say, depend on a number of variables; are you obstacle limited brings to mind. However, given that lots of companies reduce V1 by 10 kts if the runway is wet, surely from a dry runway V1-10kts would get you airborne - assuming a failure at V1 - albeit reducing the screen height to 15'?
S15
Depends. In simple terms you need to know what Vgo and Vstop are to make a decision. If say for example you are at MRTOW on a balanced field, then Vstop will equal Vgo and you will have a smjall margin in which to make your decision and neither will be correct. If taking off at a lower weight then these speeds will change - but by how much? And remember that V1 should not be below Vmcg.
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Some of the answers for the V1-10 case are in the threads:
http://www.pprune.org/forums/showthread.php?p=2470374
and
http://www.pprune.org/forums/showthread.php?t=148251
http://www.pprune.org/forums/showthread.php?p=2470374
and
http://www.pprune.org/forums/showthread.php?t=148251
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Question - "'Which is better: failure 10 kts before V1 and continue T.O or failure 10 kts after V1 and abort T.O?"
Answer - NEITHER
Assuming that you were in a Balanced Field situation where Accelerate-Stop and Obstacle limits are equal, reject from V1+10 would inevitabely result in an overrun. For a dry field you will have the small bonus of reverse thrust being available, but this would probably be insufficient to dissipate the additional 10 knots of kinetic energy.
For the continued Takeoff from engine failure at V2-10 you MIGHT get away with it, as the margins for the continued takeoff are somewhat better, BUT, what if the original V1 was Vmcg limited and you're now attempting a continued takeoff with inadequate directional control? - You crash. And again, even if not Vmcg limited but field length limited, do you have the performance available to accelerate an extra 10 knots with an engine inoperative from Vef to V2, I doubt it - You crash.
And what of the comment somewhere here that we regularly reduce V1 for Wet runways? True, we do, and apply a usually hefty weight limit to go with it, but, on a dry runway your limiting weights were calculated for the higher V1 - You crash.
And what of the enquiry afterwards (assuming that you survive)? - You crash.
The numbers work, they're well proven, use them properly, know the means by which your aircraft was certified, and don't fiddle with them.
Regards,
Old Smokey
Answer - NEITHER
Assuming that you were in a Balanced Field situation where Accelerate-Stop and Obstacle limits are equal, reject from V1+10 would inevitabely result in an overrun. For a dry field you will have the small bonus of reverse thrust being available, but this would probably be insufficient to dissipate the additional 10 knots of kinetic energy.
For the continued Takeoff from engine failure at V2-10 you MIGHT get away with it, as the margins for the continued takeoff are somewhat better, BUT, what if the original V1 was Vmcg limited and you're now attempting a continued takeoff with inadequate directional control? - You crash. And again, even if not Vmcg limited but field length limited, do you have the performance available to accelerate an extra 10 knots with an engine inoperative from Vef to V2, I doubt it - You crash.
And what of the comment somewhere here that we regularly reduce V1 for Wet runways? True, we do, and apply a usually hefty weight limit to go with it, but, on a dry runway your limiting weights were calculated for the higher V1 - You crash.
And what of the enquiry afterwards (assuming that you survive)? - You crash.
The numbers work, they're well proven, use them properly, know the means by which your aircraft was certified, and don't fiddle with them.
Regards,
Old Smokey
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Old smokey: Did you mean V1-10 and not V1+10 knots for a RTO SPD? a little typo I guess.
BTW looking for myself at TODR vs WAT and OEI-Climb gradients vs WAT-assumed from the screen as well as scheduled V1's for particular limiting conditions (first actions initiated BY V1(TL's closed, spoilers up, I assume) and the continued climb one you definitely make it on either side. So checkout various combo's on the WAT curves for yourself they all schedule safe margins.
56% of overruns occur after V1; others times there have been W@Bal errors contributing, and at other times improper handling of aborts. It may be of little help because of the real life startle factor, but all you airlines guy's if you can i would ask if (if extra time remains on the sim) for a V1 cut after EFATO very soon before V1 under WAT limiting conditions; and a WAT limited OEI climb at V2 with EFATO occuring just before V1 so that you will have proven to yourself that the schedules work on either side.When flying small multi's ( about 40 hrs). I have to rely entirely on the performance data and many times one contends with a lot of "grey area" combinations where you can't STOP and can't GO and have to wait until it cools a bit outside
Feel grateful for a jet's performance IMHO V1 and V2 sound very comforting.
Oh... I've read Old Smokey's response more carefully, it's just seeing V1+10 cause an involuntary motor-eye reflex.
BTW looking for myself at TODR vs WAT and OEI-Climb gradients vs WAT-assumed from the screen as well as scheduled V1's for particular limiting conditions (first actions initiated BY V1(TL's closed, spoilers up, I assume) and the continued climb one you definitely make it on either side. So checkout various combo's on the WAT curves for yourself they all schedule safe margins.
56% of overruns occur after V1; others times there have been W@Bal errors contributing, and at other times improper handling of aborts. It may be of little help because of the real life startle factor, but all you airlines guy's if you can i would ask if (if extra time remains on the sim) for a V1 cut after EFATO very soon before V1 under WAT limiting conditions; and a WAT limited OEI climb at V2 with EFATO occuring just before V1 so that you will have proven to yourself that the schedules work on either side.When flying small multi's ( about 40 hrs). I have to rely entirely on the performance data and many times one contends with a lot of "grey area" combinations where you can't STOP and can't GO and have to wait until it cools a bit outside
Feel grateful for a jet's performance IMHO V1 and V2 sound very comforting.Oh... I've read Old Smokey's response more carefully, it's just seeing V1+10 cause an involuntary motor-eye reflex.
Last edited by rhovsquared; 28th Apr 2006 at 22:58.
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Assuming the limiting runway scenario,at V1+10 YOU crash in an overrun..(if as smokey says the dry brake and reverse thrust don't save you.)
V1-10 one doesn't meet the the 35'screen,but who cares ,
one doesn't meet it anyway on Wet ops(15').
GO man GO....
V1-10 one doesn't meet the the 35'screen,but who cares ,
one doesn't meet it anyway on Wet ops(15').
GO man GO....
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to quote one of my prior post
"normally V1 should not be tampered with unless the AFM allows a V1 range either V1 min. For field length limitations (such as when a clearway exist that can be overflown within the TODA and the aircraft meets appropriate screen heights for rwy conditions -ASDA v. ASDR- and obstacle clearance on the first and second segments of the net flight path. and everything is scheduled in the perf. tables or afm or the perf pages of the afm or the FMC. and nothing is overtly limiting ). or pick a V1 max for wat limited or obstacle limited departures where you want to stop as late as possible where field length is obviously not limiting and AOTA)) and as y'all said V1 can never be less than Vmcg cuz you'd drift of the side at high power".
with a small digression added
"normally V1 should not be tampered with unless the AFM allows a V1 range either V1 min. For field length limitations (such as when a clearway exist that can be overflown within the TODA and the aircraft meets appropriate screen heights for rwy conditions -ASDA v. ASDR- and obstacle clearance on the first and second segments of the net flight path. and everything is scheduled in the perf. tables or afm or the perf pages of the afm or the FMC. and nothing is overtly limiting ). or pick a V1 max for wat limited or obstacle limited departures where you want to stop as late as possible where field length is obviously not limiting and AOTA)) and as y'all said V1 can never be less than Vmcg cuz you'd drift of the side at high power".
with a small digression added
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play with this http://www.caa.co.uk/docs/64/DAP_ACD_TYPEA_EGHH.pdf and
thishttp://www.boeing.com/commercial/airports/acaps/767sec3.pdf and
your V1's and oei climb charts and really convince yourself
767 choosen randomly to exclude a bias
thishttp://www.boeing.com/commercial/airports/acaps/767sec3.pdf and
your V1's and oei climb charts and really convince yourself
767 choosen randomly to exclude a bias
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oldebloke,
Yup, up to a point I agree with you, you might possibly get away with a continued OEI Takeoff from V1-10 and tolerating a much reduced screen height, but what concerns me is that the screen jeight might be zero, that is, you haven't yet become airborne at all. The acceleration from Vef to Vr and V2 is a massive performance demand for any aircraft, particularly the 2 engined aircraft. Nevertheless, the GO decision in these 'odd-ball' circumstances is probably the lesser of 2 evils.
All of these discussions, of course, relate to engine failure, other events such as tyre/tire failure are clear cut GO cases.
If I can harp on about the lower V1's that we accept with a lower screen height for Wet runway cases, it must be remembered that this performance is based upon a lower Takeoff Weight limit, and to apply Wet runway data and procedures to a Takeoff at the higher weights permissible for a Dry runway is very risky business.
rhovsquared,
You make good points, but in FAR25 (and it's equivalents) aircraft rarely operate to the WAT limit, takeoff is usually limited by Accelerate-Stop and/or Obstacle clearance. Certainly, for an aircraft such as the B737 operating from excessively long (say, 3,000M/10,000 ft plus) runways, WAT will probably be the limiting factor, and 'messing with the speeds a bit' probably wouldn't do any harm, no guarantees. Similarly, if the aircraft is meeting the minimum OEI 2nd segment gradients, but no obstacles exist (e.g. over water), then again, you'll probably get away with it, again, no guarantees.
A historian once said that given the choice of living your life with Adolf Hitler or Josef Stalin, you would be living in great danger in either case, but more likely to survive your time with Adolf Hitler. I think that Herr Hitler would favour the GO case from V1-10, and that's about the only time that I'd agree with him.
Regards,
Old Smokey
Yup, up to a point I agree with you, you might possibly get away with a continued OEI Takeoff from V1-10 and tolerating a much reduced screen height, but what concerns me is that the screen jeight might be zero, that is, you haven't yet become airborne at all. The acceleration from Vef to Vr and V2 is a massive performance demand for any aircraft, particularly the 2 engined aircraft. Nevertheless, the GO decision in these 'odd-ball' circumstances is probably the lesser of 2 evils.
All of these discussions, of course, relate to engine failure, other events such as tyre/tire failure are clear cut GO cases.
If I can harp on about the lower V1's that we accept with a lower screen height for Wet runway cases, it must be remembered that this performance is based upon a lower Takeoff Weight limit, and to apply Wet runway data and procedures to a Takeoff at the higher weights permissible for a Dry runway is very risky business.
rhovsquared,
You make good points, but in FAR25 (and it's equivalents) aircraft rarely operate to the WAT limit, takeoff is usually limited by Accelerate-Stop and/or Obstacle clearance. Certainly, for an aircraft such as the B737 operating from excessively long (say, 3,000M/10,000 ft plus) runways, WAT will probably be the limiting factor, and 'messing with the speeds a bit' probably wouldn't do any harm, no guarantees. Similarly, if the aircraft is meeting the minimum OEI 2nd segment gradients, but no obstacles exist (e.g. over water), then again, you'll probably get away with it, again, no guarantees.
A historian once said that given the choice of living your life with Adolf Hitler or Josef Stalin, you would be living in great danger in either case, but more likely to survive your time with Adolf Hitler. I think that Herr Hitler would favour the GO case from V1-10, and that's about the only time that I'd agree with him.
Regards,
Old Smokey
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Smokey,remember Boeings reference to this in 84 AIRLINER figure 4 shows the schematic of 'variable decision speeds on the takeoff performance'
to quote the article"It can be seen that a takeoff continuation with an engine failure recognition speed 5 knots below V1 would place a 2 engine at 18' instead of 35' required by regulations.However,an abort decision 5kts above the scheduled V1 would cause the airplane to continue over the end of the runway at a speed between 50 and 72 knots depending on weight..
Boeing go/no go...
At the risk of being cheeky I note your out of Singapore,ergo wouldn't you have the WET info in your AFM...You don't sound convinced of the wet data's 15 screen??
regards
to quote the article"It can be seen that a takeoff continuation with an engine failure recognition speed 5 knots below V1 would place a 2 engine at 18' instead of 35' required by regulations.However,an abort decision 5kts above the scheduled V1 would cause the airplane to continue over the end of the runway at a speed between 50 and 72 knots depending on weight..
Boeing go/no go...
At the risk of being cheeky I note your out of Singapore,ergo wouldn't you have the WET info in your AFM...You don't sound convinced of the wet data's 15 screen??
regards
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'Which is better: failure 10 kts before V1 and continue T.O or failure 10 kts after V1 and abort T.O?'
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It isn't all that silly. The long and short of it is it's better to continue v1-10 than v1+10 and abort. Because to continue means a screen height of 15 feet or so on the MD-80 (no different then adjusting v1 for contamination) however aborting at v1+10 means going off the end of the runway at 80knts.
At American Airlines they make a big deal of this b/c of the number of high speed aborts. We even have charts to figure the effects of different abort or continue speeds and its effect on the crossing height over the end of the runway.
Just my 2 cents
At American Airlines they make a big deal of this b/c of the number of high speed aborts. We even have charts to figure the effects of different abort or continue speeds and its effect on the crossing height over the end of the runway.
Just my 2 cents
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It isn't all that silly. The long and short of it is it's better to continue v1-10 than v1+10 and abort. Because to continue means a screen height of 15 feet or so on the MD-80 (no different then adjusting v1 for contamination) however aborting at v1+10 means going off the end of the runway at 80knts.
At American Airlines they make a big deal of this b/c of the number of high speed aborts. We even have charts to figure the effects of different abort or continue speeds and its effect on the crossing height over the end of the runway.
Just my 2 cents
At American Airlines they make a big deal of this b/c of the number of high speed aborts. We even have charts to figure the effects of different abort or continue speeds and its effect on the crossing height over the end of the runway.
Just my 2 cents
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Thanks to all for your contributions.
Flightsafety, you make a very valid point and I'm sure we'd all preface our answer by saying that the qustion should be re-phrased 'which is the least reckless course of action, continue V1-10 etc...' then by showing that you undestand the issues raised you can demonstrate some good understanding of Performance A.
Flightsafety, you make a very valid point and I'm sure we'd all preface our answer by saying that the qustion should be re-phrased 'which is the least reckless course of action, continue V1-10 etc...' then by showing that you undestand the issues raised you can demonstrate some good understanding of Performance A.
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I would counter with the question; what is the limiting factor in the take-off calc? If it was runway or brake limited then of the two bad options go is better. If it was any climb or obst limit then maybe stop. if it was structural then who knows? next question?
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Round midnight. I agree.
However consider why anyone would continue with a failure at V1-10? The only way to know if this has a good probability of success with the right conditions, is to plan it ahead of time, but who does this? Without a plan for continuing at V1-10, you can't work all the factors in the few seconds between V1-10 to V1, so there's no way to guarantee a good decision. Nobody is trained to make this particular decision, so if a failure happens at V1-10, there's no training to fall back on except to abort, which is the correct high probability of success decision.
However consider why anyone would continue with a failure at V1-10? The only way to know if this has a good probability of success with the right conditions, is to plan it ahead of time, but who does this? Without a plan for continuing at V1-10, you can't work all the factors in the few seconds between V1-10 to V1, so there's no way to guarantee a good decision. Nobody is trained to make this particular decision, so if a failure happens at V1-10, there's no training to fall back on except to abort, which is the correct high probability of success decision.
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Going back to the original question, not enough information is provided.
To get the best answer one would need to know whether the takeoff is Accelerate/Stop limited, runway limited or obstacle limited.
Having said that, most airlines are using balanced field performance data and maximum available reduced thrust.
If you were in a 737 on a 12000ft runway with no obstacles if the takeoff path either decision would be okay as you would have enough runway to accelerate to Vr and go or alternatively stop well before the end without seriously hot brakes.
If the same aircraft (on a shorter runway) was using balanced field performance and was accelerate /stop limited it would definitely go off the end in the abort at V1+10. You might reduce the amount of damage if reduce thrust is available as it is not taken into account in stopping performance calculations and would be a bonus.
In the go case your screen height will be reduced from 35ft to 15ft but the aircraft will perform and outclimb any obstacles in the takeoff path. If you were using reduced thrust you have the potential to achieve even greater obstacle clearance by restoring TO/GA thrust.
The better decision in most cases is the go one and that's what your recruiter probably wants to here.
Because a lot of guys ask this question about continuing at speeds significantly below V1 I have run many of these scenarios in the simulator over the years and never seen a failed takeoff attempt from V1 -10. I have seen plenty of overruns from aborts above V1.
I'm also sure that plenty of people have tried single engine takeoffs in the sim for fun.
If it was me being asked the question I would have asked for more info but I am well past the stage of interviewing for new jobs....... I hope.
To get the best answer one would need to know whether the takeoff is Accelerate/Stop limited, runway limited or obstacle limited.
Having said that, most airlines are using balanced field performance data and maximum available reduced thrust.
If you were in a 737 on a 12000ft runway with no obstacles if the takeoff path either decision would be okay as you would have enough runway to accelerate to Vr and go or alternatively stop well before the end without seriously hot brakes.
If the same aircraft (on a shorter runway) was using balanced field performance and was accelerate /stop limited it would definitely go off the end in the abort at V1+10. You might reduce the amount of damage if reduce thrust is available as it is not taken into account in stopping performance calculations and would be a bonus.
In the go case your screen height will be reduced from 35ft to 15ft but the aircraft will perform and outclimb any obstacles in the takeoff path. If you were using reduced thrust you have the potential to achieve even greater obstacle clearance by restoring TO/GA thrust.
The better decision in most cases is the go one and that's what your recruiter probably wants to here.
Because a lot of guys ask this question about continuing at speeds significantly below V1 I have run many of these scenarios in the simulator over the years and never seen a failed takeoff attempt from V1 -10. I have seen plenty of overruns from aborts above V1.
I'm also sure that plenty of people have tried single engine takeoffs in the sim for fun.
If it was me being asked the question I would have asked for more info but I am well past the stage of interviewing for new jobs....... I hope.