dry/ wet screen height

[captain coldfront]

Could any of you clever chaps tell me why there is a 35' screen height for dry runways and a 15' screen height for wet?

[The Flying Cokeman]

hello,
The screen height on a wet runway is reduced to 15 ft because you are using a wet v1 speed. This is due to a portion of the airborne distance being added to the ground run as a result of the increased ground run used between the wet v1 and vr if an engine fails at the worst point, after the wet v1. :D
tfcm

[kenparry]

It's a fudge factor allowed by the regulators to reduce the performance penalty of a wet runway compared with a dry one. Does it reduce safety? Yes, by that 20 feet.

[Old Smokey]

The original 50' screen height, later amended to 35' for heavy transport aircraft are ARBITRARY, but reasonable values. The 15' screen height is one of two major concessions allowed by the regulatory authorities for wet runway operations, the other major concession being the allowance of Reverse Thrust as a credit in the Accelerate-Stop calculation.

Basically, the two concessions are economic in origin, if aircraft had to conform to the normal requirements for wet runway operations, the payload penalties would be considerable. I had first hand experience with this in the certification of one aircraft, where the regulatory authority accepted the 'Reverse Thrust concession', but insisted upon the 35' screen height being retained. It resulted in about 25% reduction in payload on most runways.

Under some unusual circumstances, use of the two concessions will result in a Wet Runway RTOW exceeding that for the Dry Runway. It was necessary for me to include in the Ops Manual, that, for Wet Runways, the lesser of the Dry and WET Runway RTOW be the limiting weight.

What The Flying Cokeman says about wet runway performance is absolutely correct, but as a consequence of conforming to the wet runway procedures and legislation, not as a direct consequence of wet runway operations per se.

I think that kenparry is close to the mark in asking "Does it reduce safety?". Yes, most decidedly it does, it removes my two quite small reserves of safety in both the Accelerate-Stop and the Accelerate-Go cases. In the Accelerate-Stop case, my small stopping reserve, reverse thrust, is consumed in it's entirety leaving nothing in reserve. In the Accelerate-Go case, the screen height is reduced to a mere 15', almost nothing at all.

Even with the elimination of the reasonable margins of safety when using wet runway performance, I still see F/Os reaching for the Wet Runway RTOWs when the runway is, at most, a little damp. There seems to be a horrendously incorrect belief that these data are more conservative. In GENUINE wet runway conditions, of course, you'd be a damned fool to use anything but the Wet Runway data, because other factors such as reduced acceleration and reduced stopping capability are considered.

I hate those days when the ATIS says "Runway Wet", and there's just a few little puddles on the runway or it's a bit damp. Then I'm obliged to use the Wet Runway data, because at the investigation............. (You fill in the spaces between the dots)

Regards,

Old Smokey

[Mr Levitator]

Old Smokey, very interesting reading (revision for me....). I understand your point of the lower screen height (the accel go case) being less desirable if the runway is damp.
I often go down that route because of the Accel Stop case. If the runway is damp, the concessions allowed for a truly dry runway will be used (if dry perf is taken), because it is damp. Some may say that is what the concessions are for.
I would rather be sure of adequate Accel Stop performance and take the lower screen height, afterall, although lower its still safe. So my question is, although a lower screen height is used, what is wrong with using it??

Mr L

[BANANASBANANAS]

The figures are not exact for every aeroplane in any particular fleet of course. There are safety factors built in but the idea is for the performance to relate to an average aeroplane flown by an average pilot etc. Given a wet runway, a little slow to get TO thrust set, a fraction late with the rotate, a bit of rudder juggling after V1 engine failure and that 15 feet will feel very, very small if you are at anything like limiting weight.

I have tried it in the sim and have absolutely no desire to experience it in reality.

[Old Smokey]

Fair comments Mr Levitator, I feel that if this thread continues much longer that it will boil down to an argument of when does Damp become Wet.

For a runway that's only Damp, or has a few puddles, I'd suggest that only minor reduction in acceleration and stopping performance occurs, and you're better going with Dry Runway data, in that way you'll have Reverse Thrust as an additional and bonus retardation method, in other words, you have a bit in reserve. With Wet Runway data, you have expended ALL of your reserves, if you're at Accelerate-Stop limited MTOW, and the wind is 1 knot less favourable than as planned, then an overrun is on the cards. The only bonus in hand is - Can you initiate the RTO in a lesser time after Vef than the 1 or 2 seconds assumed?

Having said that, I cannot argue more strongly than to say that if the runway is TRULY wet, then you must use the Wet Runway data, as, even though all of your reserves of performance have been used, the data considers the significant degradation in acceleration to Vef, and the stopping performance thereafter.

There's nothing wrong with a 15' screen height if all goes as planned, but consider again that 1 or 2 knot less favourable wind than planned, if it knocks 10 to 15 feet off the standard 35 feet, then we all live another day, but knock 10 to 15 feet off 15 feet, and it's another story.

There is the bonus factor that only half of the Headwind and 1.5 times the Tailwind is considered to acommodate unforseen wind variations, true, but for a zero wind Takeoff, there's no factor whatsoever.

It seems as though Bananasbananas and I were simultaneously posting, and the factors that he mentions are very valid. I've done it for real many times with real failure at Vef during certification flying, and the clearance at the screen height is very very scary.

Regards,

Old Smokey

[oldebloke]

When the BCAR(UK) circa1980,first implimented the 15' screen they were the loudest in saying it was a COMPROMISE of the safest considerations at that time,for operations off a WET runway.
Safest in the fact that if one needed to stop,after the failure at Vef,then one had a Slower speed to stop from(reduced V1),but the longer acceleration,after the reduced V1,required more distance to GO.ergo the 15' screen height.
AS they say what constitues WET ,well the Airbus guys have their reference in Chapter 2.04.10(approved by the JAR),at the moment nodefinition exists in the FAA world,although Boeing sells performance 'B' to non FAA carriers.
The Certification bodies always felt that the 15'screen was at the end of the runway and it wasn't until 'Harmonization' meetings(FAA/JAR) that found out that according to the JAR rules it could be at the end of the clearway(ergo following the rules that the flight path to the screen must be over runway for 50%,one was left with crossing the end of the runway at 9' ?????).
I believe that the latest ruling by all bodies is that the 15'must be at the runways end..If this is the case then there can be little difference to crossing the end at 15'enroute to a DRY days 35' at the Clearway screen..
Facts from Airbus,On WET ops: Reverse thrust credit(if RevThrst inop ,offload payload ).....WET runway is 3mm of rain or shiny..:ok:

[captain coldfront]

Thanks for the replies everyone.

A very interesting thread by people who obviously know their stuff.

[novicef]

Just read this posting.So does one positively know that the Dry & Wet screen heights for Dry & Wet Rwys are at the end of the Rwy.

Has this been agreed on by both FAA & JAA?

In Australia does CASA deem the screen height at the end of the Clearway or Rwy??

[boofhead]

The average performance is equivalent to Gross performance values, and the figures we use in operations are Net. Net is Gross reduced by a certain factor depending on the number of engines, and is not always applied, but generally it factors in the small variations in operating technique, errors in wind information etc. In particular, the 35 foot screen height is Net, thus the real altitude you would expect to see will be higher than that.
Using the figures as presented by the manufacturer will not put you on the limit. The average line pilot should always be able to fly to these numbers or, usually, better.

[john_tullamarine]

Net/gross gets an airing in the climb segments ... can you lead some supporting evidence to support your contention that there is a similar factored approach to the runway numbers ?

I suggest that the 35ft screen is hard data. What you might be having a problem with is the fact that there are several cases to be considered. In the normal sort of situation one routinely sees the AEO takeoff case in action (which is distance factored) and it is easy to fall into the trap of presuming that there is a nice big fat pad for all the cases. However, if you are OEI TOD1/TOR1 limited, especially if you are using clearway .. and you have a near V1 failure with the takeoff continued, then you might very well have an up close and personal experience with the end of the runway ....

[Alex Whittingham]

As JT says the words net and gross are not usually applied to the take-off distances.

Gross performance is the average performance of a group of aeroplanes being tested according to an approved method. If you work to gross performance standards you have a 50:50 probability of achieving them. Net is the gross performance reduced so that the probability of an aircraft not achieving the standard is remote, defined in turn as 1:100 000 to 1:10 000 000.

I would say the distance to the 35ft screen height is gross in the engine out case, there being no margins applied other than the improbability of a failure, i.e. after the engine failure net = gross and you have a 50:50 chance of making the screen height if you go, and that the distance to 35ft in the all engine case is a net distance because of the 1.15 safety factor. i.e. net = gross x 1.15 and you have only a remote probability of not making 35ft on a normal all engines take-off.

[oldebloke]

NoviceF,until recently the FAA had no info regarding the WET screen(the B777 was the only aircraft to do Demonstrated Wet perf')having said that I believe the latest writeups in their Regs refer to the 15'screen.Remember the FAA,due to lobbying of the ATA re'economic considerations refuse to require 'old' a/c to have the charts in the manual.ATA figured that the poss' required payload penalties would incurr to great a financial penalty.Whereas every non FAA a/c dept from US terminals didn't complain..(on a wet takeoff if the technique of Reverser credit ,a reduced V1,and the 15' screen is utilized-payload penalties can be avoided)..The recent JAR(easa)ruling is that the 15' is at the end of the runway..
If your A/c operates in Europe it should have the wet' info in the AFM(Boeing provide Perf'B'(JAR rules)to their offshore customers)..ergo once one has 3mm of water(shiny)on the surface one should review the WET data..
Can't answer for Ausssie,but as they are years ahead with the conservative perf'(alignement distance etc)I imagine they have adopted the WET rules..
Cheers:ok:
The 'cost' of developing the wet charts ,for olde' a/c,was argued between the FAA/Air Trans Assoc.......??:*