Wet V1 Speeds
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Wet V1 Speeds
Can anyone explain the concept of a wet V1 speed to me - the explanations I have read don't really make sense to me.
Why the reduced screen height (15 ft as opposed to 35 ft with dry TO)? Surely under wet conditions your take-off distance required should increase so whats with the reduced screen height requirement??
From what I understand a wet V1 is not a 'real' decision speed and the dry V1 remains the actual speed at which a decision is made whether to abort or continue a take off. Then why bother with the wet V1??
Why the reduced screen height (15 ft as opposed to 35 ft with dry TO)? Surely under wet conditions your take-off distance required should increase so whats with the reduced screen height requirement??
From what I understand a wet V1 is not a 'real' decision speed and the dry V1 remains the actual speed at which a decision is made whether to abort or continue a take off. Then why bother with the wet V1??
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Sure, but what about the continued take-off case then??? And why the reduced screen height requirement?
And why is a wet V1 not a 'real' V1 speed.
From 'Ace the Technical Pilot Interview':
' The wet V1 is not a V1 speed because it does not imply any abitlity to continue the take off following and engine failure, and unlike a dry V1 this speed may be less than Vmcg'
And why is a wet V1 not a 'real' V1 speed.
From 'Ace the Technical Pilot Interview':
' The wet V1 is not a V1 speed because it does not imply any abitlity to continue the take off following and engine failure, and unlike a dry V1 this speed may be less than Vmcg'
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There are quite a few mistakes in that book. Have a search for previous threads here on PPRuNe. If what you read contradicts what you previously understood, don't panic straight away!
However, it's my understanding that the reduced screen height is in some part due to the possibility that the aircraft may be committed to takeoff with an engine failure shortly after Wet V1, but still a few knots below the usual dry V1.
Remember that unless you are field length limited, then there will be a range of speeds from which a safe takeoff or a safe stop are both possible.
Much depends on how your company has elected to conduct its performance calculations (balanced vs unbalanced field etc) - best to ask them for the definitive answer.
My company uses optimised runway-specific data, with separate tables for WET conditions, so our 'V1' can account for both a safe RTO or a safe takeoff in the event of an Eng failure, albeit with a reduced screen ht.
However, it's my understanding that the reduced screen height is in some part due to the possibility that the aircraft may be committed to takeoff with an engine failure shortly after Wet V1, but still a few knots below the usual dry V1.
Remember that unless you are field length limited, then there will be a range of speeds from which a safe takeoff or a safe stop are both possible.
Much depends on how your company has elected to conduct its performance calculations (balanced vs unbalanced field etc) - best to ask them for the definitive answer.
My company uses optimised runway-specific data, with separate tables for WET conditions, so our 'V1' can account for both a safe RTO or a safe takeoff in the event of an Eng failure, albeit with a reduced screen ht.
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There are quite a few mistakes in that book. Have a search for previous threads here on PPRuNe. If what you read contradicts what you previously understood, don't panic straight away!
However, it's my understanding that the reduced screen height is in some part due to the possibility that the aircraft may be committed to takeoff with an engine failure shortly after Wet V1, but still a few knots below the usual dry V1.
Remember that unless you are field length limited, then there will be a range of speeds from which a safe takeoff or a safe stop are both possible.
Much depends on how your company has elected to conduct its performance calculations (balanced vs unbalanced field etc) - best to ask them for the definitive answer.
My company uses optimised runway-specific data, with separate tables for WET conditions, so our 'V1' can account for both a safe RTO or a safe takeoff in the event of an Eng failure, albeit with a reduced screen ht.
However, it's my understanding that the reduced screen height is in some part due to the possibility that the aircraft may be committed to takeoff with an engine failure shortly after Wet V1, but still a few knots below the usual dry V1.
Remember that unless you are field length limited, then there will be a range of speeds from which a safe takeoff or a safe stop are both possible.
Much depends on how your company has elected to conduct its performance calculations (balanced vs unbalanced field etc) - best to ask them for the definitive answer.
My company uses optimised runway-specific data, with separate tables for WET conditions, so our 'V1' can account for both a safe RTO or a safe takeoff in the event of an Eng failure, albeit with a reduced screen ht.
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The certification authorities assume that the chance of an engine failure at the critical point (V1) is a sufficiently remote enough possiblity to allow a reduced screen height and subsequently not too much of a risk for you to fly the N-1 profile. You may also see that reverse thrust is may be allowed on a wet runway as well. The reason for this is to boost your wet runway performance so that you can carry a very similar (or notionally greater, but reduced to Dry) load to that if it was dry. In practice it means that you don't have to chuck people off just before take-off because it's just started to rain.
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And is the wet V1 an actual decision speed? If so, how can it be below Vmcg?
Gary Lager - know what you mean about Ace the Interview - learnt this morning from the book that a headwind increases range!
Thanks for the replies so far.
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Wet V Speeds
Once upon a time, in a majical land far away, you were screwed when taking off on a wet runway. Fortunately, today, there is some help.
I suggest you consult the Airbus Series "Getting to Grips..." They have an excellent treatise on performance. It's concise and complete...as well as the French are able to communicate. This will eliminate the nonsense that some of the replies are providing.
Regards,
PantLoad
I suggest you consult the Airbus Series "Getting to Grips..." They have an excellent treatise on performance. It's concise and complete...as well as the French are able to communicate. This will eliminate the nonsense that some of the replies are providing.
Regards,
PantLoad
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That doc is good value and can be found here:
http://www.wingfiles.com/files/perfo...erformance.pdf
Which bits are nonsense?
http://www.wingfiles.com/files/perfo...erformance.pdf
Which bits are nonsense?
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Wet v1
Wet V1 is sopping condsideration on a wet / contaminated runway. V1 is stop / go consideration speed. With wet V1 you must consider continuing the T/O knowing you insufficent braking action to stop but still haven't accelerated to a go decision yet. Whereas V1 is a stop/go speed.
It can be somewhat confusing...tyr to thing of it as being a fix for a wet runway that will allow you calibrated stopping distance .
It can be somewhat confusing...tyr to thing of it as being a fix for a wet runway that will allow you calibrated stopping distance .
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'The wet V1 is not a V1 speed because it does not imply any abitlity to continue the take off following and engine failure, and unlike a dry V1 this speed may be less than Vmcg'
This is a quote from CAP 385, the now defunct UK CAA reference doc for Performance A and based on the L1011-1. The V1 wet the original author was referring to was a max abandon speed on a contaminated runway and derived from a graph (which had several uses) as 'V1 wet'.
A real V1 wet is a real decision speed, I fear the confusion here may have been caused by a phrase copied out of context.
This is a quote from CAP 385, the now defunct UK CAA reference doc for Performance A and based on the L1011-1. The V1 wet the original author was referring to was a max abandon speed on a contaminated runway and derived from a graph (which had several uses) as 'V1 wet'.
A real V1 wet is a real decision speed, I fear the confusion here may have been caused by a phrase copied out of context.
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Woof etc,
With a dry V1 you should be able to suffer a critical engine failure, right up to V1, and still be able to abort the T/O and remain on the R/W. Now, if the R/W is wet your braking action would be degraded. Thus, if you suffer an engine failure at V1, on a wet R/W, you may go off the end off the R/W before coming to a complete halt. Therefore, to guarantee that you can abort, and stop before the end of the R/W, you must reduce V1 - ie. bring your decision point nearer to the threshold at which you commence the T/O roll. This is a wet V1.
Now, for the sake of argument, let me put some random figures to this:
Dry R/W: V1=120kts and Vr=140kts
Wet R/W: V1=110kts and Vr=140kts
Note Vr is, obviously, independent of the R/W condition.
From the above figures you should be able to see that in the dry case, with a failure of the critical engine at V1, you would have to accelerate by 20kts, without an engine, before hitting Vr. However, with the wet case, you would have to accelerate by 30kts before hitting Vr. From that I hope you can see that the wet V1 case would require the ac having a longer take-off roll.
Now, the climb gradient from unstick to screen height is exactly the same in the wet and dry case; however, due to the wet case having a greater TORR, the wet case will see the ac arrive at a lower screen height than the dry case.
I've tried to avoid technical spiel with that explanation, hope I haven't caused further confusion.
S15
With a dry V1 you should be able to suffer a critical engine failure, right up to V1, and still be able to abort the T/O and remain on the R/W. Now, if the R/W is wet your braking action would be degraded. Thus, if you suffer an engine failure at V1, on a wet R/W, you may go off the end off the R/W before coming to a complete halt. Therefore, to guarantee that you can abort, and stop before the end of the R/W, you must reduce V1 - ie. bring your decision point nearer to the threshold at which you commence the T/O roll. This is a wet V1.
Now, for the sake of argument, let me put some random figures to this:
Dry R/W: V1=120kts and Vr=140kts
Wet R/W: V1=110kts and Vr=140kts
Note Vr is, obviously, independent of the R/W condition.
From the above figures you should be able to see that in the dry case, with a failure of the critical engine at V1, you would have to accelerate by 20kts, without an engine, before hitting Vr. However, with the wet case, you would have to accelerate by 30kts before hitting Vr. From that I hope you can see that the wet V1 case would require the ac having a longer take-off roll.
Now, the climb gradient from unstick to screen height is exactly the same in the wet and dry case; however, due to the wet case having a greater TORR, the wet case will see the ac arrive at a lower screen height than the dry case.
I've tried to avoid technical spiel with that explanation, hope I haven't caused further confusion.
S15
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Interesting thread. On the Dash8(100 / 300) a Wet V1 is something actually used when runway is wet - NOT contaminated - and distance limited (Supplement 51 AFM). The problem of Vmcg is addressed by a minimum V1 (.9Vr). Once the Wet V1 is calculated and found to be below .9Vr, you must use the .9Vr speed and go back into the performance charts as your ASD increases.
There is no allowance for use of reverse when using the Wet V1. With an engine failure after Wet V1 but before Vr you simply try to keep it on centreline and keep going - and believe me if you tried to stop and applied max reverse on the live engine you are going off the side of the runway.
An interesting note here - we seldom use a Wet V1, but when we got Supplement approval, we started training for it in the sim. The hardest part was for the crew to keep going. Invariably someone closed the power levers with very interesting results and an even more interesting de-brief.
There is no allowance for use of reverse when using the Wet V1. With an engine failure after Wet V1 but before Vr you simply try to keep it on centreline and keep going - and believe me if you tried to stop and applied max reverse on the live engine you are going off the side of the runway.
An interesting note here - we seldom use a Wet V1, but when we got Supplement approval, we started training for it in the sim. The hardest part was for the crew to keep going. Invariably someone closed the power levers with very interesting results and an even more interesting de-brief.
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The takeoff distance on a
wet runway is the greater of the following values:
•
TODdry = Takeoff distance on a dry runway
•
TODN-1 wet = Distance covered from brake release to a point at which the
aircraft is at 15 feet above the takeoff surface, ensuring the V2 speed to
be achieved before the airplane is 35 feet above the takeoff surface,
assuming failure of the critical engine at VEF and recognized at V1.
the above is from airbus hope it helps
aircraft is at 15 feet above the takeoff surface, ensuring the V2 speed to
be achieved before the airplane is 35 feet above the takeoff surface,
assuming failure of the critical engine at VEF and recognized at V1.
the above is from airbus hope it helps
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I'm a think a couple of comments here have confused me slightly:
Not strictly - V1 is a decision speed, i.e. you need to have recognised the engine failure and decided to stop by V1 at the latest. I believe the regs expect a 1 sec (?) decision time. So if you had an engine failure right at V1, it's too late to have think about it - you must continue the takeoff.
In other words, we must have already made the decision to stop by the time V1 is reached in order to stop safely.
I'd take issue with the use of the word 'consideration' speed in favour of something a little more black and white, as I was taught: decision speed. When talking about a/c in Perf category A there are really few failures which require a great deal of 'consideration' with reference to V1.
If it's an engine failure before V1 - stop. If it's an engine failure at or after V1 (see above) - go. If it's anything other than an engine failure then the ability of the a/c to stop or continue the takeoff is unchanged and V1 is only relevant if it is obviously necessary to abort.
From speeds anywhere near V1 situations requiring aborts ought to be very obvious, such are the risks associated with high speed RTOs, so 'consideration' of the situation will probably be limited!
With a dry V1 you should be able to suffer a critical engine failure, right up to V1, and still be able to abort the T/O and remain on the R/W.
In other words, we must have already made the decision to stop by the time V1 is reached in order to stop safely.
V1 is stop / go consideration speed.
If it's an engine failure before V1 - stop. If it's an engine failure at or after V1 (see above) - go. If it's anything other than an engine failure then the ability of the a/c to stop or continue the takeoff is unchanged and V1 is only relevant if it is obviously necessary to abort.
From speeds anywhere near V1 situations requiring aborts ought to be very obvious, such are the risks associated with high speed RTOs, so 'consideration' of the situation will probably be limited!
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Woof etc,
With a dry V1 you should be able to suffer a critical engine failure, right up to V1, and still be able to abort the T/O and remain on the R/W. Now, if the R/W is wet your braking action would be degraded. Thus, if you suffer an engine failure at V1, on a wet R/W, you may go off the end off the R/W before coming to a complete halt. Therefore, to guarantee that you can abort, and stop before the end of the R/W, you must reduce V1 - ie. bring your decision point nearer to the threshold at which you commence the T/O roll. This is a wet V1.
Now, for the sake of argument, let me put some random figures to this:
Dry R/W: V1=120kts and Vr=140kts
Wet R/W: V1=110kts and Vr=140kts
Note Vr is, obviously, independent of the R/W condition.
With a dry V1 you should be able to suffer a critical engine failure, right up to V1, and still be able to abort the T/O and remain on the R/W. Now, if the R/W is wet your braking action would be degraded. Thus, if you suffer an engine failure at V1, on a wet R/W, you may go off the end off the R/W before coming to a complete halt. Therefore, to guarantee that you can abort, and stop before the end of the R/W, you must reduce V1 - ie. bring your decision point nearer to the threshold at which you commence the T/O roll. This is a wet V1.
Now, for the sake of argument, let me put some random figures to this:
Dry R/W: V1=120kts and Vr=140kts
Wet R/W: V1=110kts and Vr=140kts
Note Vr is, obviously, independent of the R/W condition.
From the above figures you should be able to see that in the dry case, with a failure of the critical engine at V1, you would have to accelerate by 20kts, without an engine, before hitting Vr. However, with the wet case, you would have to accelerate by 30kts before hitting Vr. From that I hope you can see that the wet V1 case would require the ac having a longer take-off roll.
Now, the climb gradient from unstick to screen height is exactly the same in the wet and dry case; however, due to the wet case having a greater TORR, the wet case will see the ac arrive at a lower screen height than the dry case.
I've tried to avoid technical spiel with that explanation, hope I haven't caused further confusion.
S15
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Santiago15's post is quite interesting and accurate, but I think it may be worth mentioning a few other factors into the mix:
VR is not strictly independent of the runway condition. There is in fact a range of speeds within which the climb gradient requirements can still be met. This means that there may still be scope for a small VR and V2 reduction which would recover some of the tarmac lost while accelerating on one engine from the lower V1.
Also, if the V1 speed must be changed significantly because the runway is now wet, it implies that the runway was weight limiting in the first place. Therefore, a reduced RTOW would allow a recovery of effective distance available. It is a little confusing to compare the dry and wet cases for cases at equal weights. In reality the wet runway would require both a reduced V1 and likely a reduced RTOW also.
VR is not strictly independent of the runway condition. There is in fact a range of speeds within which the climb gradient requirements can still be met. This means that there may still be scope for a small VR and V2 reduction which would recover some of the tarmac lost while accelerating on one engine from the lower V1.
Also, if the V1 speed must be changed significantly because the runway is now wet, it implies that the runway was weight limiting in the first place. Therefore, a reduced RTOW would allow a recovery of effective distance available. It is a little confusing to compare the dry and wet cases for cases at equal weights. In reality the wet runway would require both a reduced V1 and likely a reduced RTOW also.
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GaryLager,
I don't want to touch on V1 as a stop/go/decision speed, it's another debate that's been covered in great depth a number of times on this forum.
I believe Woof etc was originally asking 2 questions:
a) Why would you have a different V1 for a wet runway?
b) Why would you have a reduced screen ht (15' as opposed to 35')
I believe I answered both of those questions:
a) Because of degraded braking.
b) Beacause of a greater TORR.
S15
a couple of comments here have confused me slightly
I believe Woof etc was originally asking 2 questions:
a) Why would you have a different V1 for a wet runway?
b) Why would you have a reduced screen ht (15' as opposed to 35')
I believe I answered both of those questions:
a) Because of degraded braking.
b) Beacause of a greater TORR.
S15