Wet V1 Speeds

[Woof etc]

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??

[Runaway Gun]

It takes much longer to stop on a wet runway, so the V1 is reduced.

[Woof etc]

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'

[Gary Lager]

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.

[chornedsnorkack]

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.
But just why is a takeoff from wet runway to 15 feet screen (which is allowed) more safe than takeoff from a dry runway to 15 feet screen (which is forbidden)?

[Piltdown Man]

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.

[Woof etc]

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.

If so, why the 35 ft requirement for a dry TO? Surely the possibility of an engine failure is equally remote - why should the wetness of the runway influence this?

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.

[PantLoad]

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

[Gary Lager]

That doc is good value and can be found here:

http://www.wingfiles.com/files/performance/gettingtogripswithaircraftperformance.pdf

Which bits are nonsense?

[AlR]

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 .

[Alex Whittingham]

'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.

[santiago15]

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

[Gooneyone]

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.:)

[gearpins]

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:)

[Gary Lager]

I'm a think a couple of comments here have confused me slightly:

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.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.

V1 is stop / go consideration speed.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!

[chornedsnorkack]

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.

Obvious from the numbers chosen as example. Is it at all obvious why Vr generally should be independent of runway 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
Or alternatively, I presume that if you rotate early, somewhere between the minimum unstick speed and the normal Vr, you are flying slower and having more drag than if you had rotated at the normal Vr - therefore worse climb gradient and you also end up arriving at a lower screen height. Correct?

[captainpaddy]

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.

[airmen]

By what number and /or where to look for, do you reduce V1?
In the exemple I have seen it is always 10 kts...

[chornedsnorkack]

By what number and /or where to look for, do you reduce V1?
In the exemple I have seen it is always 10 kts...
Is the reducing done by a fixed number, or by looking through the list of applicable restrictions/different line-in-the-sand test bad case scenarios?

[santiago15]

GaryLager,

a couple of comments here have confused me slightly

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

[chornedsnorkack]

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

So, the question b) is somewhat answered, but not quite.

If you make performance calculations for a runway that is dry, and find you cannot clear 35 feet screen, you are not allowed to attempt takeoff. Not unless you unload the plane to the point where it can clear 35 feet.

If the runway counts as wet, you are allowed to clear the screen by 15 feet. You do not have to unload the plane until it can, starting from the wet runway V1, clear the screen by 35 feet.

Why?

[gearpins]

distance measured till you reach 15ft does not absolve you of the requirement to attain V2 by 35 feet. ref page one my previous post

[santiago15]

You do not have to unload the plane until it can, starting from the wet runway V1, clear the screen by 35 feet. Why?


Because you accept a reduced safety margin: 15' not 35' screen ht.

Imagine this scenario:

Screen ht has to be 35' for wet and dry R/W. You taxi out to a dry R/W with conditions and ac weight such that, with a critical engine failure at V1, you will just make the screen ht. Now imagine that, during the taxi out it pours it down with rain. Now, are you going to taxi back and reduce the ac weight or adjust V1 such that you will now only going to make the screen ht by 15'?

Again, I hope I'm not causing confusion here. Different types may have different ways of dealing with wet V1s; my knowledge is limited to my type where we just reduce the dry V1 by 10 kts for a wet R/W.

[Denti]

Wonder how using wet performance as normal performance data for all take offs except contaminated and reduced braking action figures into that all.

That is the case in my company, operating under JAR OPS too, performance data is supplied by some swedish company (EAG?), previously it was BA performance, wet performance as standard case as well.

[chornedsnorkack]

Because you accept a reduced safety margin: 15' not 35' screen ht.
Imagine this scenario:
Screen ht has to be 35' for wet and dry R/W. You taxi out to a dry R/W with conditions and ac weight such that, with a critical engine failure at V1, you will just make the screen ht. Now imagine that, during the taxi out it pours it down with rain. Now, are you going to taxi back and reduce the ac weight or adjust V1 such that you will now only going to make the screen ht by 15'?

What would you do if during the taxi out, a blast of tailwind rose such that while the runway is still dry, you would not clear the screen height by 35', but only by 15'?

Would it not be more sensible that if rain is likely to come, the plane would be loaded so that it can still take off if the rain does come?

[santiago15]

What would you do if during the taxi out, a blast of tailwind rose such that while the runway is still dry, you would not clear the screen height by 35', but only by 15'?


Perf A allows for a reduced screen ht on a wet R/W. What you cannot do is start making-up allowances for dry R/Ws.

[PantLoad]

Old Smokey....Where are you??? You are desperately needed!!!!!

PantLoad

[PantLoad]

Old Smokey....Where are you??? You are desperately needed!!!!!

PantLoad

[Old Smokey]

The essential difference between Dry and Wet runway performance is that both cases meet entirely different certification criteria. The data from one condition cannot be transposed to the other.

In establishing 'NORMAL' aircraft operations, i.e. operations from DRY runways, lines were drawn in the regulatory sand to allow for achievement of a screen height of 35 feet for a continued OEI Takeoff from V1 (for failure having ocurred at Vef), or the Accelerate-Stop manoeuvre conducted within the ASDA with one means of retardation held in reserve (usually Reverse Thrust). For the RTO, V1 could be fairly high (compared to the wet situation) because of better braking coefficient on the DRY runway, and the fact that a 'spare' means of retardation was held in reserve. With the higher V1, the distance to accelerate from Vef to V1, to Vr, and to V2, is less, thus a much lesser demand for the aircraft in a much reduced acceleration capability.

That ends the certification summary for the dry runway.

When it was finally realised some decades ago that under wet runway conditions the aircraft could not realistically be expected to safely meet the dry runway performance criteria it became necessary to account for increased rolling friction to Vef/V1, and the much reduced deceleration capability due to decreased brake effectiveness. If this was factored into the existing requirements, it was simply impossible to carry any reasonable loads by applying the existing rules to wet runways. Thus, a whole new set of certification rules were created for the wet runway certification, a new line drawn in the (wet) sand.

To allow for the much reduced braking effectiveness, V1 was reduced as far as possible, and the requirement for keeping one means of retardation in reserve removed (typically reverse thrust). Thus, credit is allowed for the use of reverse thrust, removing ALL safety margins for the wet runway RTO. The lower V1s, with similar or the same V2, required a much bigger 'ask' of the reduced OEI acceleration from Vef through to V2, requiring much increased continued Takeoff distance. Now, the screen height was 'cut to the bone' down to a mere 15 feet such that the OEI Takeoff distance (which terminates at screen height in all cases) allowed for improved weights.

Thus, the entirely different set of certification rules for wet runways removed all margin of safety for the RTO, and drastically reduced the screen height at the end of the continued takeoff.

Two sets of rules for two sets of circumstances, Dry is Dry, and Wet is Wet, and never the twain shall meet.

In certification work that I've done, it is possible in some circumstances to arrive at a higher RTOW for the Wet runway as compared to the Dry runway, due to the lesser margin of safety. In these cases it has been necessary for me to add a stern caveat to the AFM/FCOM that in such cases, the least of the two limiting weights must be observed.

If you use wet runway data or V speeds on a dry runway, you're a bl**dy fool, and operating outside the law. You'll be hung, drawn, and quartered at the enquiry:eek:

On a daily basis, I still see F/Os pulling out the Wet Runway data when the runway could be, at most, described as a bit damp. My question to them is "Why do you want to remove all of our safety margins?":uhoh:

Regards,

Old Smokey

[DC-Mainliner]

Great read Old Smokey, thank you for explaining the subtle details. Do you have any information on what year they went back and re-certified wet runway performance? Some aircraft I've flown in the corporate aviation world still DO NOT have a wet runway correction, whereas all Boeing and McDonnel Douglas / Douglas aircraft I've flown previously had these numbers as well as clutter V1 numbers.

You bring up an excellent point about wet and dry performance decision making in the cockpit:


On a daily basis, I still see F/Os pulling out the Wet Runway data when the runway could be, at most, described as a bit damp. My question to them is "Why do you want to remove all of our safety margins?":uhoh:
Regards,
Old Smokey

As you clearly point out, while it may appear to be more conservative at first to choose "wet" runway numbers, there are tradeoffs to be made. The damp runway at the departure end could hold a few tricks. For example, in order to favor one scenario over the other, the compromise is to make a judgment call on what the biggest hazard of the day might be - an over run on a damp runway that may have more water/oil/rubber on it than it appears, or a very tight 15 foot screen height with obsticles - assuming your technique is perfect on a V1-Go decision. The captain has to be paid for making some grey matter decisions, right? :eek: ;)

[Hanuman]

In my aircraft type, all data in GWC are calculated by improved climb or inceased V2 method whenever it's possible. For the most 2 engines aircraft, the factor that normally limits MTOW is the second segment climb requirement not the available runway length. So we can trade off the remaining runway length to increase V2 for better climb performance in second segment resulting a higher MTOW. Even V1, Vr and V2 are increased but we still get 35' screen height with this increased V2 at the end of runway. At the same time we're also able to stop on the runway in case RTO at this increased V1.
Some pilots would rather use the speeds from 10kts tailwaind table instead of the zero wind table especially when both tables giving the same flex temp. It might be because they feel more comfortable with the lesser speeds from the 10kts tailwind table. Is there any pitfalls in this practice apart from degrading the climb performance in second segment?

[Denti]

In certification work that I've done, it is possible in some circumstances to arrive at a higher RTOW for the Wet runway as compared to the Dry runway, due to the lesser margin of safety. In these cases it has been necessary for me to add a stern caveat to the AFM/FCOM that in such cases, the least of the two limiting weights must be observed.


Under JAR-OPS no such caveat is necessary as JAR-OPS 1 Subpart G 1.490 (b) (5) clearly states:

On a wet or contaminated runway, the
take-off mass must not exceed that permitted for a
take-off on a dry runway under the same
conditions.

As you say we may be foolish in using wet performance as our standard performance data, however saying it is outside the regulation is simply wrong at least over here. Using wet performance data as standard is possible under operation according to JAR OPS 1 Subparts F and G (but then only 35 ft is mentioned in those Subparts and not 15 ft) and of course our performance manual (as all our manuals) is approved by the relevant regulator, in this case the LBA.

Anyway, would love to see a direct link to relevant JARs concerning performance calculation for normal day to day operation.

However, wet performance data in our operation data doesn't include the use of reversers and is pretty much the same as dry (35ft screen height for example) except that it takes into account the increased rolling friction but does not allow for any braking action below Good (0.40 friction coefficient). For anything less than good braking action or with a contamination of more than 3mm water or slush (or 10 mm dry snow) we have to use degraded braking action or contaminated runway performance, whichever is more limiting of course.

[Woof etc]

Great info - thanks everyone.

It seems that the answer to my question is that with a wet V1 you are accepting lower safety margins in the case of a continued take off following engine failure, in exchange for improved margins in the accelerate stop case.

But is the wet V1 a 'real' V1 speed? And is it true that V1 wet is allowed to be less than Vmcg? How could this be allowed?

[PantLoad]

Yes, V1 is "real". No, V1 is not less than Vmcg.

PantLoad

[chornedsnorkack]

Does Vmcg remain unchanged on a wet runway, or does it not? (When the aircraft accelerates on asymmetric thrust after V1 and before Vr, does the friction of nosewheel contribute to holding the aircraft on runway?)

Also, what about asymmetric reverse? On dry runway, the stopping distance after V1 reject is computed with no reverse thrust - and one major reason to reject takeoff just before V1 is an engine out, because the plane cannot reach screen height or clear obstacles with one engine out before V1. But on a wet runway, what does it mean to reject takeoff exactly at V1, so that the stop distance is enough with full reverse, but one engine actually produces no reverse and the other engine/s generate asymmetric reverse?

[Woof etc]

'Vmcg,the minimum control speed on the ground, is the calibrated airspeed during the take off run, at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the aircraft with the use of the primary aerodynamic controls alone (without the use of nosewheel steering) to enable to enable the take off to be safely continued using normal piloting skills.'

[mutt]

In order to make this a meaningful discussion, one must remember that the certification requirements of the FAA/JAA/CASA are extremely different, for instance, DC-Mainliner asked Do you have any information on what year they went back and re-certified wet runway performance?. Guessing that he is based in Washington DC and operating under FAR's, the answer is that they HAVENT re-certified wet runway performance. You will find reference to wet runway performance in FAR25-109 (from memory), but it isn’t retroactive. Boeing/Douglas have supplied data for older aircraft, but this data is based upon FAA AC91-6B, it isnt certified data.

In JAA land, accountability for wet AND contaminated runway performance was established in the 1960's, I believe that it had to do the Hadley Page crash containing the Manchester United football team, but i stand to be corrected..

Denti, initially i will say that using WET runway Performance data on a DRY runway isn't legal, we would certainly have a hard time in a court of law justifying its use! :) Captain, why were you happy to accept a 15 ft screen height rather than the 35ft required by regulation?

However in your case, you have stated that you aren't using thrust reversers and you are using 35ft screen height, so the only difference that you have between wet/dry data is the MU factor! This is VERY different to WET runway performance data supplied by most manufacturers. BTW, are you flying a Brazilian jungle jet?

Mutt

[Woof etc]

Pantload / Gary Lager

Thanks for the link to the Airbus Performance document. Downloaded it today - what a gem and full of useful info.

[Denti]

No Mutt, no scary stuff like that at all, just good old boeings (737-5/3 and soon 7/8), all operation certified according JAR OPS (including using wet data as standard data). The thing you say about different regulations holds true, sadly even within JAR as each country has its own version of the JAR and they are, after all, different.

We do have in some cases dry performance data or can aquire it from our OPC, however the day to day standard is wet runway performance. That doesn't mean that wet covers reduced braking action or any kind of contamination. It just covers taking off from dry or wet (more limiting) runways with a water depth less than 3mm and a braking action of good or better (>= .40 friction coefficient). And of course this is completely legal ;) after all our SOPs have to be approved by the relevant regulatory body which in this case is the LBA.

[ray cosmic]

I suppose Denti is mostly flying into places where there is no performance limit. A lot of runways in Europe are anyway over 3000m, so maybe they wanted to save the hassle of carrying different table.
Probably they could save their engines a bit by optimizing for the prevailing conditions, but who cares to save an engine a littlebit of fuel...
Funny kind of opeation though.