Difference in Wet and Dry Screen Height
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Is the braking action really that much worse on a runway that is slightly discoloured (the definition of damp) than a dry runway
BA is about stopping; screen height, which is the discussion topic, is about continuing.
BA is about stopping; screen height, which is the discussion topic, is about continuing.
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Airmann - "The definition is that it's always wet unless it's completely dry"
I would be interested to know where your quote is from. A look at Perf section of EASA 2012 gives ..
EASA CAT.POL.A.105 (d) "For performance purposes, a damp runway, other than a grass runway, may be considered to be dry."
I would be interested to know where your quote is from. A look at Perf section of EASA 2012 gives ..
EASA CAT.POL.A.105 (d) "For performance purposes, a damp runway, other than a grass runway, may be considered to be dry."
Last edited by zoigberg; 10th Feb 2017 at 20:46. Reason: Spelling of Airmann
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but I mean I find it strange that they haven't also lowered the dry screen height to 15' and just left it at that.
Unfortunately, I don't have the written rationale behind the wet reduction so I can only surmise. My view is that 35 ft is the standard and, for wet, 15 ft should be viewed as a concession.
The USAF, perennially underpowered reduced the screen height, wet or dry, to ZERO
I wasn't aware of that approach ... making TORR super-serious. Given that the performance numbers aren't precise, although reasonably indicative, what sort of percentage of takeoffs at critical weights throw up some dirt and grass off the runway end ?
if missing the hotel by 15' is OK on a rainy day
.. but keep in mind that we are looking at gross and net flight paths. A serious real in-close obstacle is a worry ... one at 5 miles, not so much ...
On some runways it is possible to have a higher weight on a wet runway than dry, but some operators don't allow you to do that:
..along with the regulator .. refer back to the FAA video link earlier
35' is safer than 15', perhaps,
As GF observed earlier, it's just a number, a line in the sand, albeit based on rational historical data risk assessment. What we have up our sleeves is the gross to net margin .. It really doesn't warrant endeavouring to pull out your tape measure and fret over the last millimetre.
The new definition is that its always wet unless its completely dry
While I have no problem with damp being dry, the end result doesn't appear to have a problem either which way. Have I missed something along the way ?
If you took off at TOPL weight AND ..
We need to keep in mind that the OEI figure is but one of a number of cases considered .. the worst of which defines the TO weight
The reduced runway braking action is the only reason we use wet performance, its all about stopping.
Not quite, if you have standing water, increased rolling and impingement drag (ie go considerations) are very relevant
Unfortunately, I don't have the written rationale behind the wet reduction so I can only surmise. My view is that 35 ft is the standard and, for wet, 15 ft should be viewed as a concession.
The USAF, perennially underpowered reduced the screen height, wet or dry, to ZERO
I wasn't aware of that approach ... making TORR super-serious. Given that the performance numbers aren't precise, although reasonably indicative, what sort of percentage of takeoffs at critical weights throw up some dirt and grass off the runway end ?
if missing the hotel by 15' is OK on a rainy day
.. but keep in mind that we are looking at gross and net flight paths. A serious real in-close obstacle is a worry ... one at 5 miles, not so much ...
On some runways it is possible to have a higher weight on a wet runway than dry, but some operators don't allow you to do that:
..along with the regulator .. refer back to the FAA video link earlier
35' is safer than 15', perhaps,
As GF observed earlier, it's just a number, a line in the sand, albeit based on rational historical data risk assessment. What we have up our sleeves is the gross to net margin .. It really doesn't warrant endeavouring to pull out your tape measure and fret over the last millimetre.
The new definition is that its always wet unless its completely dry
While I have no problem with damp being dry, the end result doesn't appear to have a problem either which way. Have I missed something along the way ?
If you took off at TOPL weight AND ..
We need to keep in mind that the OEI figure is but one of a number of cases considered .. the worst of which defines the TO weight
The reduced runway braking action is the only reason we use wet performance, its all about stopping.
Not quite, if you have standing water, increased rolling and impingement drag (ie go considerations) are very relevant
JT,
Underpowered is another word for over gross, I guess. We used 2.3% as minimum gradient a significant reduction from the FAR 25 3.0% for quads. Being a quad, AEO take-off run was quite close to OEI BFL, so one saw a lot of runway. The 115% rule provided some protection. That said, climb gradient and obstacle clearance tended to be more restrictive than runway performance, so take-offs didn't usually dust off the overrun.
Underpowered is another word for over gross, I guess. We used 2.3% as minimum gradient a significant reduction from the FAR 25 3.0% for quads. Being a quad, AEO take-off run was quite close to OEI BFL, so one saw a lot of runway. The 115% rule provided some protection. That said, climb gradient and obstacle clearance tended to be more restrictive than runway performance, so take-offs didn't usually dust off the overrun.
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Airmann - "The definition is that it's always wet unless it's completely dry"
I would be interested to know where your quote is from. A look at Perf section of EASA 2012 gives ..
EASA CAT.POL.A.105 (d) "For performance purposes, a damp runway, other than a grass runway, may be considered to be dry."
I would be interested to know where your quote is from. A look at Perf section of EASA 2012 gives ..
EASA CAT.POL.A.105 (d) "For performance purposes, a damp runway, other than a grass runway, may be considered to be dry."
Not quite, if you have standing water, increased rolling and impingement drag (ie go considerations) are very relevant
Airmann, in EASA land there is no longer any such thing as a damp runway, there are only three states considered, dry, wet and contaminated. I don't know what has driven this change which came in a few years ago (?) if memory serves but it would be interesting to know. It may be the difficulty in reporting the state and certainly in the UK it has been many years since a runway was reported as damp.
It is now quite common to hear a runway being reported as wet when it is very obviously dry as far as actually stopping on it would be concerned which probably means that wet performance is being used far more than it was before and I am not convinced that safety has been improved.
Edited to add: Having thought about it I guess its obvious that the change has come about to try and reduce runway over runs which is how quite a lot of high speed rejects end up, perhaps there is data to show that these are happening on damp runways and had wet perf. been used it might have been avoided. All good stuff but its a difficult balance, just failing to stop at the end of the paved surface at most airfields is just embarrassing, just failing to clear an obstacle is going to kill you, 15ft is not much of a margin even with the gross/net buffer.
Last edited by Max Angle; 11th Feb 2017 at 12:03.
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Stop Stop or Go Go! ??
Damp rwy reported all the time in Europe.
With the 737 800 on long trips from some medium rwys, I hate using wet figures when it is realy dry and often grooved.
Why? Because I would like to do a rejected T/ O rather then go airborne with the problem.
So , I end up on the gras doing 4 knot, bu hu.
With the crew we fly with these days, I give it a 50 - 50 chance of survival if we have a engine fail or never mind severe damage and fire.
The notion of, " I would rather go airborne then risk a critical RTO to me is interesting.
Going airborne when rwy limited is no picknic. Rather taxi in for a coffee and quick debrief.
Then again I do live in a simple world.
With the 737 800 on long trips from some medium rwys, I hate using wet figures when it is realy dry and often grooved.
Why? Because I would like to do a rejected T/ O rather then go airborne with the problem.
So , I end up on the gras doing 4 knot, bu hu.
With the crew we fly with these days, I give it a 50 - 50 chance of survival if we have a engine fail or never mind severe damage and fire.
The notion of, " I would rather go airborne then risk a critical RTO to me is interesting.
Going airborne when rwy limited is no picknic. Rather taxi in for a coffee and quick debrief.
Then again I do live in a simple world.
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which probably means that wet performance is being used far more than it was before and I am not convinced that safety has been improved.
Aero 11 - Rejected Takeoff Studies
"About 2 percent occur at speeds in excess of 120 kt. The overruns and incidents that occur invariably stem from these high-speed events."
"Approximately 80 percent of the overrun events were potentially avoidable by following appropriate operational practices."
"Using the Boeing data, we initially approached the FAA with a proposal to call a reduced V1 the "decision speed" and treat it as a V1 speed. The flight crew would remove their hands from the thrust levers, and the takeoff would continue. The initial proposed speed was 10 kt less than published V1."
"In late 1992, after we received the Boeing Takeoff Safety Training Aid in draft form, we decided to again seek approval of the "decision speed" concept. This time we chose a speed of 8 kt for a reduction, which added approximately 2 seconds of recognition time. In the worst case the screen height was degraded to approximately 15 to 20 ft."
"We believe that this reduced V1 procedure provides a valuable increase in the safety margin over that provided in the AFM in the event of an RTO."
In order to reduce the chance of an over run from a poorly handled RTO, a reduced V1 (use of wet figures) helps.
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Ok , so 50- 50 chance was a bit gloomy. But lets say a fairly large chance of hole in the ground. 5% 10% or 15 % pick Your nr.
And ; no, my present airline is one of the best.
It is an educated guess, considering people do try go-around without power on 777,,,,?
And drill big holes in the Swedish mountains, just because the ADI goes for a loop,,,? Want me to carry on?
There is so much stuff going on these day that we see on OFDM.
You would take the train to Ibiza if you saw it.
With regards to Evergreen Intl.
I that the Yank operation with 5 aircraft according wiky?
Leaders on Flight training and Operations?
We have 2000 departures a day and they have!
Never mind, Flying Club stuff.
Train to Standard, Fly to standard.
And ; no, my present airline is one of the best.
It is an educated guess, considering people do try go-around without power on 777,,,,?
And drill big holes in the Swedish mountains, just because the ADI goes for a loop,,,? Want me to carry on?
There is so much stuff going on these day that we see on OFDM.
You would take the train to Ibiza if you saw it.
With regards to Evergreen Intl.
I that the Yank operation with 5 aircraft according wiky?
Leaders on Flight training and Operations?
We have 2000 departures a day and they have!
Never mind, Flying Club stuff.
Train to Standard, Fly to standard.
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Airmann - sorry, sense of humour fail from me there!
Speaking for the airline I am currently working for, interestingly I have noticed a few captains opting to look at the wet data when damp. However, in the (4) European airlines I have worked for, I haven't come across pilots being Intstructed to do so.
More noticeable from an anecdotal point of view, is the ATIS still giving wet or damp when the surface is clearly bone dry...... but that's another story
Speaking for the airline I am currently working for, interestingly I have noticed a few captains opting to look at the wet data when damp. However, in the (4) European airlines I have worked for, I haven't come across pilots being Intstructed to do so.
More noticeable from an anecdotal point of view, is the ATIS still giving wet or damp when the surface is clearly bone dry...... but that's another story
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15ft is not much of a margin even with the gross/net buffer.
Again, gross/net progressively improves whatever you have at runway head as the departure progresses.
So , I end up on the grass doing 4 knot, bu hu.
However, the reality of the dynamics of stopping is that the rate of speed decrease is quite high. The overrun is far more likely to commence at, say, 80-100 kt, than near stopped. On the runway, this speed is washed off quickly without damage (apart from very hot brakes) but, if you hit some rough things at speed, the damage may be significant
Again, gross/net progressively improves whatever you have at runway head as the departure progresses.
So , I end up on the grass doing 4 knot, bu hu.
However, the reality of the dynamics of stopping is that the rate of speed decrease is quite high. The overrun is far more likely to commence at, say, 80-100 kt, than near stopped. On the runway, this speed is washed off quickly without damage (apart from very hot brakes) but, if you hit some rough things at speed, the damage may be significant
We had duscussion here, eons ago, with Old Smokey et al, (might have involved SSG, JT) on the subject of overruns. First, the risk analyst those classify an overrun as "catastrophic" at, IIRC, something like 70 knots. Second, very few extra knots at the initiation of the high speed reject adds lots of knots at overrun point. Something like 5 knots extra at the first initiation results in 40-60 knots at the runway end.
FAA AC 25-32
Wet Runway.
A runway is wet when it is neither dry nor contaminated. For purposes of condition reporting and airplane performance, a runway can be considered wet when more than 25 percent of the runway surface area (within the reported length and the width being used) is covered by any visible dampness or water that is 1⁄8 inch (3 mm) or less.
Note: A damp runway that meets this definition is considered wet, regardless of whether or not the surface appears reflective.
Wet Runway.
A runway is wet when it is neither dry nor contaminated. For purposes of condition reporting and airplane performance, a runway can be considered wet when more than 25 percent of the runway surface area (within the reported length and the width being used) is covered by any visible dampness or water that is 1⁄8 inch (3 mm) or less.
Note: A damp runway that meets this definition is considered wet, regardless of whether or not the surface appears reflective.
I suspect that, in the UK at any rate, this wheeze may have been originated in the mid-1960s by the navigation (performance) department of my own airline, BUA (the one that was later taken over by Caledonian to become British Caledonian).
When you are planing a scheduled operation to Entebbe or Nairobi non-stop from the then 8000-foot runway at Gatwick, using shiny new VC10s, you don't want nearly half the flights to have to tech-stop because the runway is not dry. In the 1970s, our performance instructor (George Seaton) indicated that the airline had come to an arrangement with the ARB (or whomever) that, provided the braking action was good, the screen height could be reduced from 35 ft to 15 ft in the wet-runway case, to accommodate a 10-knot reduction of the V1.
As an aside, George and his colleagues were ahead of the game on take-off performance on contaminated runways. This was arguably a two-edged sword as far as its effect on our jet operations were concerned. I well remember sitting for several hours one wintry day at Manchester on a BAC 1-11 around 1980 trying to explain to my passengers why other airlines' BAC 1-11s were departing to various destinations, whereas we couldn't depart for Gatwick.
When you are planing a scheduled operation to Entebbe or Nairobi non-stop from the then 8000-foot runway at Gatwick, using shiny new VC10s, you don't want nearly half the flights to have to tech-stop because the runway is not dry. In the 1970s, our performance instructor (George Seaton) indicated that the airline had come to an arrangement with the ARB (or whomever) that, provided the braking action was good, the screen height could be reduced from 35 ft to 15 ft in the wet-runway case, to accommodate a 10-knot reduction of the V1.
As an aside, George and his colleagues were ahead of the game on take-off performance on contaminated runways. This was arguably a two-edged sword as far as its effect on our jet operations were concerned. I well remember sitting for several hours one wintry day at Manchester on a BAC 1-11 around 1980 trying to explain to my passengers why other airlines' BAC 1-11s were departing to various destinations, whereas we couldn't depart for Gatwick.
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Chris: drifting I admit, but to follow on your story of the 2 edged sword and differing rules. In early 80's I was in Corfu B732 with a UK airline, departing for EDI from the northly; i.e. towards the mountains. We had RTOW tables from our in house performance guru. No mater how sharp the pencil we could not get the fuel on and had to plan a tech/fuel stop in BHX. Meanwhile, another operator, with the very same ac/ & engines, who was operating charters on behalf of my host airline, loaded up and blasted off for GLA in one foul swoop. How on earth?
I year later I was in a night stop hotel and found a crew from the other airline and asked the question. They did not have RTOW tales and used QRH figures, i.e. balanced field. I asked about the obstacles and mountains. they said this was assessed beforehand and they decided that if anything untoward happened before 1000' they would turn right avoiding the granite visually. Good game.
A few years later I flew for them and they had progressed from B732 - B733. That also was a catalyst for stringent SOP's & RTOW tables. They complained bitterly that 'the fun had gone out of it.' An easy job had become difficult. However, to be fair, they did wise-up and realise that the gung-ho days were over and they needed to join the party.
But this screen height difference I don't get. There needs to be a buffer in missing the terrain, that is a screen height, and a 'net' climb gradient. The net value has an inbuilt buffer in it. It's safe or it isn't. 15' that is. Why, when conditions are better, do you need more margin? If 15' is safe on Monday, why is it not on Tuesday? What does FAA use? I admit I do not know more than I was taught in basic Perf A, and Chris's story might be a key to it. Why then has it not been reviewed and revised? Is the FAA different? If yes, then EU pax on a code share flight with an FAA carrier will be subjected to differing safety standards in their ignorance. That was always the case about FTL's etc before EASA levelled the field. But in global terms is the field level with all the big boys? What rules do the Far East & M.Easy guys use?
I year later I was in a night stop hotel and found a crew from the other airline and asked the question. They did not have RTOW tales and used QRH figures, i.e. balanced field. I asked about the obstacles and mountains. they said this was assessed beforehand and they decided that if anything untoward happened before 1000' they would turn right avoiding the granite visually. Good game.
A few years later I flew for them and they had progressed from B732 - B733. That also was a catalyst for stringent SOP's & RTOW tables. They complained bitterly that 'the fun had gone out of it.' An easy job had become difficult. However, to be fair, they did wise-up and realise that the gung-ho days were over and they needed to join the party.
But this screen height difference I don't get. There needs to be a buffer in missing the terrain, that is a screen height, and a 'net' climb gradient. The net value has an inbuilt buffer in it. It's safe or it isn't. 15' that is. Why, when conditions are better, do you need more margin? If 15' is safe on Monday, why is it not on Tuesday? What does FAA use? I admit I do not know more than I was taught in basic Perf A, and Chris's story might be a key to it. Why then has it not been reviewed and revised? Is the FAA different? If yes, then EU pax on a code share flight with an FAA carrier will be subjected to differing safety standards in their ignorance. That was always the case about FTL's etc before EASA levelled the field. But in global terms is the field level with all the big boys? What rules do the Far East & M.Easy guys use?
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John T
If exiting the Rwy end at 80 to 100 kts after an RTO close to V1 is the result, then something must have gone wrong , bigtime.
Would be interesting to see how this crew would have fared if they went airborne??
An RTO shure has to be executed properly , there is a margin but as opposed to many other procedures, not much.
One of the last manual and critical manouvers left in aviation! Have to say I like the Auto brake, try Max every now and then on landing, that gives an idea about the deceleration.
If exiting the Rwy end at 80 to 100 kts after an RTO close to V1 is the result, then something must have gone wrong , bigtime.
Would be interesting to see how this crew would have fared if they went airborne??
An RTO shure has to be executed properly , there is a margin but as opposed to many other procedures, not much.
One of the last manual and critical manouvers left in aviation! Have to say I like the Auto brake, try Max every now and then on landing, that gives an idea about the deceleration.
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If exiting the Rwy end at 80 to 100 kts after an RTO close to V1 is the result
I fear you may have missed my point .. perhaps my words were not well chosen ?
The reject is a seriously critical manoeuvre. Of great concern is that, for the stopping bit, the shape of the speed vs time (or distance) curve during the stop is quite steep. Ergo, if there is anything much wrong in the execution of the manoeuvre, and the aircraft ends up a bit further down the runway than intended (for a critical reject limiting runway), it will end up off the runway. Due to the high deceleration at that point, the speed may well be up in the 80-100 kt range with a significant probability of serious hull damage traversing the overrun.
So, yes, something has gone wrong .. a little distance has been squandered. It doesn't take anything bigtime, just squander a little bit of distance ..
For older certification aircraft, particularly, there is scant margin ...
I fear you may have missed my point .. perhaps my words were not well chosen ?
The reject is a seriously critical manoeuvre. Of great concern is that, for the stopping bit, the shape of the speed vs time (or distance) curve during the stop is quite steep. Ergo, if there is anything much wrong in the execution of the manoeuvre, and the aircraft ends up a bit further down the runway than intended (for a critical reject limiting runway), it will end up off the runway. Due to the high deceleration at that point, the speed may well be up in the 80-100 kt range with a significant probability of serious hull damage traversing the overrun.
So, yes, something has gone wrong .. a little distance has been squandered. It doesn't take anything bigtime, just squander a little bit of distance ..
For older certification aircraft, particularly, there is scant margin ...
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We had duscussion here, eons ago, with Old Smokey et al, (might have involved SSG, JT) on the subject of overruns. First, the risk analyst those classify an overrun as "catastrophic" at, IIRC, something like 70 knots. Second, very few extra knots at the initiation of the high speed reject adds lots of knots at overrun point. Something like 5 knots extra at the first initiation results in 40-60 knots at the runway end.
Very easy to do, just requires a bit of time.
Double integration with a short Δ, just assume constant realistic values for deceleration and you should have a very clear idea of whether this sentence can be deemed true or false based on simple science.