Wet V1 Speeds
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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) 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
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?
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You do not have to unload the plane until it can, starting from the wet runway V1, clear the screen by 35 feet. Why?
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.
Last edited by santiago15; 20th Oct 2006 at 08:42.
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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.
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.
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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'?
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'?
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?
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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'?
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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
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?"
Regards,
Old Smokey
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
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?"
Regards,
Old Smokey
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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:
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? 
You bring up an excellent point about wet and dry performance decision making in the cockpit:
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Improved climb
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?
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?
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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.
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.
take-off mass must not exceed that permitted for a
take-off on a dry runway under the same
conditions.
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.
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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?
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?
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Vmcg
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?
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?
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'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.'
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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
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
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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.
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.
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.
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.
