Performance on a damp runway vs wet/dry
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Performance on a damp runway vs wet/dry
When working out take off performance on a damp runway do you use dry or wet runway performance figures?
The FAA does not make any reference to damp runways, whereas JAR-OPS 1.475 states that a damp runway can be considered dry in terms of takeoff performance.
But how accurate is this?
On one hand using wet performance reduces margins by reducing screen height to 15ft and including the use of reversers in the calculation.
On the other hand using dry performance assumes a braking action/friction coefficient that you are unlikely to fully achieve particularly as you approach the rubber coated touchdown zone of the reciprocal runway.
What do you personally use and why? And what guidance, if any, does your company provide?
The FAA does not make any reference to damp runways, whereas JAR-OPS 1.475 states that a damp runway can be considered dry in terms of takeoff performance.
But how accurate is this?
On one hand using wet performance reduces margins by reducing screen height to 15ft and including the use of reversers in the calculation.
On the other hand using dry performance assumes a braking action/friction coefficient that you are unlikely to fully achieve particularly as you approach the rubber coated touchdown zone of the reciprocal runway.
What do you personally use and why? And what guidance, if any, does your company provide?
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I personally comply with company-SOPs because I don’t like to be fired. 
Our OM states: For performance considerations, a damp runway shall be considered wet.
Nevertheless, certain runways may be treated differently after specific analysis… (which has not been done yet).
Our OM states: For performance considerations, a damp runway shall be considered wet.
Nevertheless, certain runways may be treated differently after specific analysis… (which has not been done yet).
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Ditto for dolly737....my company's sop..states that a damp rwy is to be considered dry for performance calculations..if there is any visible patches of water(shiny areas),and regardless of the depth..the wet rwy data shall be used..which is a drag if you ask me since we'll have to have both wet and dry figures available and brief for both cases
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The SOP that I've written into our Operating Manuals is that if the runway is less than dry, up to the maximum permissible water depth for a "Wet" Takeoff, both the Dry runway, and the Wet runway Takeoff Weight limits be obtained, and the aircraft operated at the lesser of the two as the RTOW. (There are SOME circumstances where, because of the Reverse Thrust and reduced screen height concession, it's possible to derive a wet runway RTOW higher than for a dry runway).
The JARs do require such a consideration for wet runways (but a grey area remains for damp runways), but not all regulatory authorities impose such a requirement.
For the record, we also consider a dust or sand covering on the runway as a wet runway, and apply the same operating policy specified above.
A bit of JAR, a bit of Boeing, and a bit of our own thinking, that's the way that it was submitted it to the regulatory authority for compliance for the AOC, and it met with their full approval.
Regards,
Old Smokey
The JARs do require such a consideration for wet runways (but a grey area remains for damp runways), but not all regulatory authorities impose such a requirement.
For the record, we also consider a dust or sand covering on the runway as a wet runway, and apply the same operating policy specified above.
A bit of JAR, a bit of Boeing, and a bit of our own thinking, that's the way that it was submitted it to the regulatory authority for compliance for the AOC, and it met with their full approval.
Regards,
Old Smokey
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"a wet grooved rwy should be considered dry", yes, that is so, but not legally so. Grooving greatly increases safety on a wet runway, but legally, a wet runway is a wet runway, grooved or not.
Sand will improve the braking coefficient on a wet or contaminated runway, but most certainly not on a dry runway (Boeing's words, not mine).
Regards,
Old Smokey
Sand will improve the braking coefficient on a wet or contaminated runway, but most certainly not on a dry runway (Boeing's words, not mine).
Regards,
Old Smokey
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During our paper performance days we did that, used wet performance data for every take off thus reducing our workload by quite a bit. However if a contaminated or reduced braking action runway came into play it was quite a bit more work.
Nowadays with our EFB most of us do use wet performance data for a damp runway, however we are nearly never performance limited in the first place so we have quite a big margin in any case.
Nowadays with our EFB most of us do use wet performance data for a damp runway, however we are nearly never performance limited in the first place so we have quite a big margin in any case.
Intruder, I don’t follow your argument, unless you imply that the use of wet speeds always requires full power. Surely an operator could use wet speeds and still flex – runway length, obstacles, etc allowing?
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The problem with using Wet runway data on a Dry runway is that it's illegal.
The normal rules require that a 35 ft screen height be obtained within the TODA, and that one means of retardation (inevitably reverse thrust) be kept in reserve. That's the rules.
Then, along comes a DISPENSATION for Wet runways (Wet, not Dry or Damp) which allows the use of Reverse Thrust, and allows for a reduced screen height to 15 ft. The dispensation is allowed to be used, ONLY IF THE RUNWAY IS WET!
The dispensation entirely removes the ONLY spare performance capability remaining on a Dry runway for a RTO, and significantly reduces obstacle clearance for the continued Takeoff.
There's absolutely nothing wrong with using the more limiting of the Wet and Dry RTOW for any runway condition (except Contaminated), provided that the chosen Thrust and V-Speeds used with the actual weight are for the actual prevailing conditions, i.e. Dry (including Damp) or Wet.
Regards,
Old Smokey
The normal rules require that a 35 ft screen height be obtained within the TODA, and that one means of retardation (inevitably reverse thrust) be kept in reserve. That's the rules.
Then, along comes a DISPENSATION for Wet runways (Wet, not Dry or Damp) which allows the use of Reverse Thrust, and allows for a reduced screen height to 15 ft. The dispensation is allowed to be used, ONLY IF THE RUNWAY IS WET!
The dispensation entirely removes the ONLY spare performance capability remaining on a Dry runway for a RTO, and significantly reduces obstacle clearance for the continued Takeoff.
There's absolutely nothing wrong with using the more limiting of the Wet and Dry RTOW for any runway condition (except Contaminated), provided that the chosen Thrust and V-Speeds used with the actual weight are for the actual prevailing conditions, i.e. Dry (including Damp) or Wet.
Regards,
Old Smokey
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...also in some cases if you were to use Wet figures your Wet RTOW would be higher than your Dry RTOW because of taking use of reverse thrust and lower screen height on a Wet runway into consideration, so how exactly is that safer....? Which RTOW and figures do you use then? A mix and match?
Would be interesting to find out the legalities in the event of an incident when facing the investigation panel...
Dry figures for dry RWY and wet figures for a wet RWY...
Would be interesting to find out the legalities in the event of an incident when facing the investigation panel...
Dry figures for dry RWY and wet figures for a wet RWY...
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Intruder, I don’t follow your argument, unless you imply that the use of wet speeds always requires full power. Surely an operator could use wet speeds and still flex – runway length, obstacles, etc allowing?
The problem with using Wet runway data on a Dry runway is that it's illegal.
The normal rules require that a 35 ft screen height be obtained within the TODA, and that one means of retardation (inevitably reverse thrust) be kept in reserve. That's the rules.
Then, along comes a DISPENSATION for Wet runways (Wet, not Dry or Damp) which allows the use of Reverse Thrust, and allows for a reduced screen height to 15 ft. The dispensation is allowed to be used, ONLY IF THE RUNWAY IS WET!
The normal rules require that a 35 ft screen height be obtained within the TODA, and that one means of retardation (inevitably reverse thrust) be kept in reserve. That's the rules.
Then, along comes a DISPENSATION for Wet runways (Wet, not Dry or Damp) which allows the use of Reverse Thrust, and allows for a reduced screen height to 15 ft. The dispensation is allowed to be used, ONLY IF THE RUNWAY IS WET!
The wet runway case also assumes a longer stopping distance; also, IIRC, an additional 15% runway length buffer is required for the T/O roll, hence a lower V1 and usually a higher thrust requirement. Only if the screen height or the wet runway stopping distance is limiting, would the V1 or thrust requirement of the dry runway case be more restrictive.
I have not seen an instance, where I have worked out both cases for actual performance, where the dry runway requires more thrust or a lower V1. maybe you can come up with a real-world example...
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"I have not seen an instance, where I have worked out both cases for actual performance, where the dry runway requires more thrust or a lower V1. maybe you can come up with a real-world example... "
If you're looking for real world examples or comparisons, and, seeing as you are working out actual performance, try this for a convincing demonstration -
Calculate the Wet runway performance as normal (that is, with credit for reverse thrust for the RTO, and a 15 ft screen height for the GO case), and then do the same calculation for the Wet runway for Reverse Inop, and for a 35 ft screen height. NOW you have a direct comparison between the requirements for a normal takeoff and that with the allowed concessions. You might be surprised at the huge difference. I routinely produce such RTOWs as a series for each runway, and more often than not, Reverse Inop keeps the aircraft on the ground.
If the legality of using Wet runway data on Dry runways is doubted, then consider this -
(1) Does a Dry runway takeoff allow credit for Reverse Thrust during a RTO? I think not, the law does not allow it.
(2) Does a Dry runway takeoff allow a screen height of 15 ft for a continued takeoff? I think not, the law does not allow it.
If you use Wet runway data on a Dry runway, you're doing exactly what I've specified here, and the law does not allow it.
During a Wet runway takeoff, rolling friction is increased slightly, but not to the point where the continued takeoff suffers significantly using normal Dry runway V-speeds. During a Wet runway Rejected takeoff, braking efficiency is significantly degraded, and this component of the takeoff inevitably becomes the most limiting. As a CONCESSION USING THE LAST AVAILABLE RESERVES, V1s are reduced, and Reverse thrust taken into account. The rejected takeoff, then, uses all of the available margins with no reserve. Because of the lower V1, the OEI acceleration from Vef to Vr and through to V2 is much longer, and that's where the reduced screen height to 15 ft comes into play.
I notice one poster bringing up the 15% additional stopping distance required with no reverse thrust. Not sure which direction where the post was coming from, but if an aircraft does NOT have a reserve means of stopping (e.g. Reverse Thrust or a Braking Parachute), a 15% penalty applies to the ASDR during certification. Was that where you were coming from?
Regards, and Merry Christmas,
Old Smokey
If you're looking for real world examples or comparisons, and, seeing as you are working out actual performance, try this for a convincing demonstration -
Calculate the Wet runway performance as normal (that is, with credit for reverse thrust for the RTO, and a 15 ft screen height for the GO case), and then do the same calculation for the Wet runway for Reverse Inop, and for a 35 ft screen height. NOW you have a direct comparison between the requirements for a normal takeoff and that with the allowed concessions. You might be surprised at the huge difference. I routinely produce such RTOWs as a series for each runway, and more often than not, Reverse Inop keeps the aircraft on the ground.
If the legality of using Wet runway data on Dry runways is doubted, then consider this -
(1) Does a Dry runway takeoff allow credit for Reverse Thrust during a RTO? I think not, the law does not allow it.
(2) Does a Dry runway takeoff allow a screen height of 15 ft for a continued takeoff? I think not, the law does not allow it.
If you use Wet runway data on a Dry runway, you're doing exactly what I've specified here, and the law does not allow it.
During a Wet runway takeoff, rolling friction is increased slightly, but not to the point where the continued takeoff suffers significantly using normal Dry runway V-speeds. During a Wet runway Rejected takeoff, braking efficiency is significantly degraded, and this component of the takeoff inevitably becomes the most limiting. As a CONCESSION USING THE LAST AVAILABLE RESERVES, V1s are reduced, and Reverse thrust taken into account. The rejected takeoff, then, uses all of the available margins with no reserve. Because of the lower V1, the OEI acceleration from Vef to Vr and through to V2 is much longer, and that's where the reduced screen height to 15 ft comes into play.
I notice one poster bringing up the 15% additional stopping distance required with no reverse thrust. Not sure which direction where the post was coming from, but if an aircraft does NOT have a reserve means of stopping (e.g. Reverse Thrust or a Braking Parachute), a 15% penalty applies to the ASDR during certification. Was that where you were coming from?
Regards, and Merry Christmas,
Old Smokey
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What worries me in all this is the alacrity with which some will use wet figures - often sparked by the Atis saying that the R/w is 'wet' when, in fact, it is dry but it has rained with in the last 24hrs!!
Where would we stand in the subsequent court martial if we had used wet figures because the Atis said the R/w was wet although we knew the R/w to be dry?
Any comments?
Cheers,
mcdhu
Where would we stand in the subsequent court martial if we had used wet figures because the Atis said the R/w was wet although we knew the R/w to be dry?
Any comments?
Cheers,
mcdhu
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My views:
If the ATIS says it's wet, then by all means, I'll count it as wet. It's often impossible to judge from the apron, the surfaces differ too much. A contaminated apron may well be a wet runway (admittedly though, the runways usually allow for more water to run off than the aprons and will therefore often be in better condition than the apron). My point here is that it is difficult to judge from the apron or gate, weather or not the runway is in fact wet. So if the ATIS says something, I'll use that information. It should hold up in court.
As for using wet or dry figures, your SOP should provide guidance. My SOP tells me to use dry when dry, wet and dry when wet (and use the most conservative value) and when braking action reported, use both BA, wet and dry, and when contaminated, use contaminated, BA, wet and dry. All of it is paperwork and manual calculations so it's a bit of a chore on those snowy days...
Because of the various special cases that arise with the different modes of calculation, by allways using the most limiting value, you "should" be on the safe side performance wise. I haven't, though, given this any deeper thought if this is truly the case, but it sounds reasonable to me.
merry x-mas!
/LnS
If the ATIS says it's wet, then by all means, I'll count it as wet. It's often impossible to judge from the apron, the surfaces differ too much. A contaminated apron may well be a wet runway (admittedly though, the runways usually allow for more water to run off than the aprons and will therefore often be in better condition than the apron). My point here is that it is difficult to judge from the apron or gate, weather or not the runway is in fact wet. So if the ATIS says something, I'll use that information. It should hold up in court.
As for using wet or dry figures, your SOP should provide guidance. My SOP tells me to use dry when dry, wet and dry when wet (and use the most conservative value) and when braking action reported, use both BA, wet and dry, and when contaminated, use contaminated, BA, wet and dry. All of it is paperwork and manual calculations so it's a bit of a chore on those snowy days...
Because of the various special cases that arise with the different modes of calculation, by allways using the most limiting value, you "should" be on the safe side performance wise. I haven't, though, given this any deeper thought if this is truly the case, but it sounds reasonable to me.
merry x-mas!
/LnS
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As someone who has the responsibility of calling the runway state, I must admit I am finding this thread both educating and fascinating.
A Wet or Dry runway is an easy call. However, calling a damp runway always ends up with some discussion and some disagreement.
A grooved runway causes problems also. the tops can be dry but the grooves can hold water.
The Atis reflects the runway surface on the last inspection. We do inspect as soon as we can to call a change but obviously there are times when the runway surface changes before we get out there. This is especially the case when the runway dries out quickly.
A pilot who is unsure can always ask for a surface inspection prior to take off.
We pass the runway state in thirds of the runway length. So you can have Wet, Wet, Dry or even Wet, Dry, Wet, or any combination. Am I right in assuming a pilot would use Wet settings if any combination contained a third that was wet?
My observations are UK centric obviously, but, I imagine the same applies internationally.
Any comments that assist us to assist you are welcome.
Happy Christmas.
A Wet or Dry runway is an easy call. However, calling a damp runway always ends up with some discussion and some disagreement.
A grooved runway causes problems also. the tops can be dry but the grooves can hold water.
The Atis reflects the runway surface on the last inspection. We do inspect as soon as we can to call a change but obviously there are times when the runway surface changes before we get out there. This is especially the case when the runway dries out quickly.
A pilot who is unsure can always ask for a surface inspection prior to take off.
We pass the runway state in thirds of the runway length. So you can have Wet, Wet, Dry or even Wet, Dry, Wet, or any combination. Am I right in assuming a pilot would use Wet settings if any combination contained a third that was wet?
My observations are UK centric obviously, but, I imagine the same applies internationally.
Any comments that assist us to assist you are welcome.
Happy Christmas.
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Calculate the Wet runway performance as normal (that is, with credit for reverse thrust for the RTO, and a 15 ft screen height for the GO case), and then do the same calculation for the Wet runway for Reverse Inop, and for a 35 ft screen height. NOW you have a direct comparison between the requirements for a normal takeoff and that with the allowed concessions. You might be surprised at the huge difference.
First, the 15% runway length buffer is mandated in Part 121 (121.195(d)) for landing only, not for takeoff as I had previously mentioned. However, a 15% buffer on the takeoff run IS part of the "all engines operating" criteria for takeoff in Part 25 (25.113(b)).
Second, Part 121 does NOT have a 15' screen height concession. That is ONLY part of the performance criteria for the 'V1 cut' case in FAR 25.113(b)(2). Note also that the takeoff run is the GREATER of the "all engines operating" case to 35' (including 15% buffer) or the 1-engine-out case to 15'. Therefore, there can be no "concession" to the 15' screen height if the all-engines-operating case is more restrictive.
Third, reverse thrust MAY be used on a wet runway IF its use is part of normal operating procedures. Whether or not that might make a difference in the calculations is a matter of individual circumstances.
After all that, operationally we all use our charts or performance computers per our airline procedures. I don't know if your airline allows you to pick and choose from among the different screen height and reverser scenarios, but mine does not. Again, I have NEVER seen a case in the 747 (manual charts in the -400, OPS computer in the Classic) where the wet runway performance was less restrictive than dry runway performance. If you can come up with specifics, I would be very interested in seeing them.
Last edited by Intruder; 25th Dec 2007 at 21:07.
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When I look out the window and I think "mmm might be wet/ damp" I will use wet figures. If any doubt in my mind, I will always use the wet figures. It seems to me the safe option, and all I am losing is 20 feet at screen height if I lose an engine at V1(wet).
I use the ATIS as a guide, but I, as the captain, make the call on the runway state.
I use the ATIS as a guide, but I, as the captain, make the call on the runway state.
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I use the ATIS as a guide, but I, as the captain, make the call on the runway state.
I am not disputing your responsibility for the final call, I am just trying to understand the situation from the captains seat to improve my own knowledge and hopefully be of more help in these circumstances.
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A little bit pregnant...
I always get a bit tickled with the differences between JAA and FAA. One which comes to mind is that of protected glide slope signals. In the U.S., either you're screwing up the signal, or you're not. Under JAA, you have protected areas for CAT I, CAT II, CAT III.
Same with the "calculations" for takeoff performance. Under FAA, the runway is either dry, or it's not. There is no 'damp' in the eyes of FAA. (Again, a little bit pregnant.)
In terms of actual airplane takeoff perfomance, there is no way the plane will perform better on a wet runway vs. a dry runway. That's the reality. But, as we say, figures lie, and liars figure. It's all in the calculations.
Depending on what system you use for doing your RTOW charts, it IS possible to have 'better performance' on a 'wet' chart vs. a 'dry' chart. And, there's the provision of using reverse thrust on a wet runway for stopping...you don't have this on a dry runway. And, the screen height is 35 feet for dry...15 feet for wet....and the list goes on.
In the event you have a performance calculation system that, sometimes, will show a higher allowable RTOW under wet conditions, you MUST (as per the authority of granting permission to utilize such a performance system) take the more conservative of the two. That is to say, if you can go to 100T dry, but 110T wet...and it's wet...you are limited to 100T.
Some systems (most of what I've seen used in the U.S.) already take into account for this, thus relieving the crews of having to compare the two figures (wet and dry) and taking the more conservative of the two.
My company uses a computerized system ('rented' from Sabre) that is outstanding. It does everything for you. Little chance for error. The FAA loves such a system.
But, FAA approves a lot of crazy stuff. For example, one carrier in the U.S. (It will remain nameless.), has the first officer with a clip board and a calculator glued to that clipboard...and he does it all by hand. Not much better than doing a takeoff performance calculation for your Cessna 172. (I've seen private pilots who fly single-engine airplanes have very sophisticated weight-n-balance and takeoff/landing performance programs in these little programable calculators....)
Of course, over 30 years ago, when I flew co-pilot on a 727...the captain, an ex-WW II guy who flew the Hump)...used to tell me, "You don't need all that fancy paperwork...you push the thrust levers up...and when she feels like she's ready to fly, you ease back on the yoke and fly the damn thing. It ain't rocket science!"
Ah, as crazy as it sounds, I used to love to fly with those guys....
PantLoad
Same with the "calculations" for takeoff performance. Under FAA, the runway is either dry, or it's not. There is no 'damp' in the eyes of FAA. (Again, a little bit pregnant.)
In terms of actual airplane takeoff perfomance, there is no way the plane will perform better on a wet runway vs. a dry runway. That's the reality. But, as we say, figures lie, and liars figure. It's all in the calculations.
Depending on what system you use for doing your RTOW charts, it IS possible to have 'better performance' on a 'wet' chart vs. a 'dry' chart. And, there's the provision of using reverse thrust on a wet runway for stopping...you don't have this on a dry runway. And, the screen height is 35 feet for dry...15 feet for wet....and the list goes on.
In the event you have a performance calculation system that, sometimes, will show a higher allowable RTOW under wet conditions, you MUST (as per the authority of granting permission to utilize such a performance system) take the more conservative of the two. That is to say, if you can go to 100T dry, but 110T wet...and it's wet...you are limited to 100T.
Some systems (most of what I've seen used in the U.S.) already take into account for this, thus relieving the crews of having to compare the two figures (wet and dry) and taking the more conservative of the two.
My company uses a computerized system ('rented' from Sabre) that is outstanding. It does everything for you. Little chance for error. The FAA loves such a system.
But, FAA approves a lot of crazy stuff. For example, one carrier in the U.S. (It will remain nameless.), has the first officer with a clip board and a calculator glued to that clipboard...and he does it all by hand. Not much better than doing a takeoff performance calculation for your Cessna 172. (I've seen private pilots who fly single-engine airplanes have very sophisticated weight-n-balance and takeoff/landing performance programs in these little programable calculators....)
Of course, over 30 years ago, when I flew co-pilot on a 727...the captain, an ex-WW II guy who flew the Hump)...used to tell me, "You don't need all that fancy paperwork...you push the thrust levers up...and when she feels like she's ready to fly, you ease back on the yoke and fly the damn thing. It ain't rocket science!"
Ah, as crazy as it sounds, I used to love to fly with those guys....
PantLoad