Weight penalty on WET runway

[alatriste]

Our Gross Weight Chart provider has just changed its lay out and information.
When runway is wet it is reasonable to decrease the RLW ( runway limited weight) cause of stopping capability might be reduced even though acceleration remains the same.
BUT in the new charts TOW limited by OBSTACLES is also reduced.

¿ANYBODY COULD FIGURE OUT ANY REASON FOR THAT DECREMENT?
I´m talking about just a wet runway not a contaminated one.

Thanks for your help.

[Genghis the Engineer]

Without seeing the chart, possibly because on take-off the ground run is increased by the wet runway, and therefore the distance along the ground to clear a fixed obstacle height is also increased? Thus, one solution is to reduce the weight limit so as to give the same obstacle clearance distance?

G

[alatriste]

Thank you G, but why is the take-off run increased if the runway is just wet, when the acceleration is not reduced only the stopping capability?
With other GWCs I have dealt with, WET runway only penalized the runway limited weight, not the obstacle not the second segement.
Studying JAR/FAR 25.113-109 Sub B. I cannot find out reasons for that penalty.

Thanks for your time and interest.

[Hotel Mode]

If V1 dry is 130 Kts V1 wet 120kts and VR 140 with a engine failiure at V1 the wet case will have slower acceleration for an extra 10 kts over the dry case to make VR due engine out, using up more runway and spoiling obstacle clearance. Er i think....

[FlightDetent]

So far I believed that climb out performance is calculated from DERA irrespective of actual lift off point. Now the required screen height at this point is 35 but for wet runway it is only 17 (???). I wonder if this tiny diference - well it is actually half/twice as much - solves your dilemma?

FD.
(the un-real)

[oldebloke]

OK chaps,Under JAR using a reduced 'screen'height of 15',at the end of the 'clearway'one should still miss the obstacles!!!
(JAR25.113b)
The Gross horizontal distance from the start of the Takeoff roll to the point at which the aircraft reaches the 15',assuming a critical power failure at Vef,achieved in the same manner consistent with obtaining V2 by 35'(end of clearway)...ergo one must be about 10' over the end of the Hard suface runway..(amj 25.1591 para3.2.1b1
Now you can see why Co's revalue the obstacle clearance with Wet ops....
A320 suffers no payload penalty with a wet (3mm) runway provided one uses a reduced V1-3knots,and the 15'screen-and reverses operative(FCOM 2.04.10)
cheers:ok:

[Old Smokey]

alatriste,

It might serve you well to check if the regulatory authority allows the benefit of the 15ft screen height offered by the Wet runway performance data.

Wet runway performance data makes 2 mojor concessions not normally available for Dry runway operations, namely -

(1) Full credit for reverse thrust is allowed for accelerate-stop. and

(2) Screen height is reduced from 35 ft to 15 ft.

In some circumstances, in SOME aircraft, there are some occasions where reference to Wet runway data can lead to a higher RTOW than if Dry runway data is used. This was so for the last aircraft for which I created the full performance analysis / Airport analysis / Special Procedures for an operator's Air Operator's Certificate. It necessitated insertion of an instruction that if the runway was Wet, then the Wet runway RTOW should be compared to the Dry runway data, and the least of the two weights used as the limiting weight.

For the same aircraft, whilst the regulatory authority (Australian) allowed full credit for reverse thrust for the rejected takeoff, it firmly rejected any credit for a reduced screen height, necessitating "restructuring" of the Wet runway continued takeoff AFM performance data to allow for a 35 ft screen height, and obstacle clearance height. This had a very significant effect upon field length and obstacle limited data (difficult to separate the two). Consider the minimum (for 2 engined aircraft) 1.6% gradient subtended back to the TODA for an increase of 20 ft in screen height provided in the Wet runway data, on a level runway this reduces the available TODA by 1250 ft (381 M), a significant penalty.

Your operator may have been subject to the same limitation by the regulatory authority.

Regards,

Old Smokey

[mutt]

Old_smokey,

Do the Australian CAA have contaminated runway regulations or is the requirement based upon the POI?
I havent seen any Boeing software that allows the screen height to be adjusted to 35' for a wet runway.

Mutt

[Old Smokey]

Mutt,

The Australian regulations do allow for contaminated runway operations, provided that the applicable performance data is FAA approved. In our case (or at least for the operator for whom I worked the performance data), full AFM data was available, but not yet FAA approved, thus, the created software lies in wait for the FAA to approve the AFM data.

There are a considerable number of Australian operators / aircraft which DO make use of the 15 ft screen height, but, as a matter of policy in acceptance of our AOC, the 35 ft requirement for screen height / Net obstacle clearance was required for all operations. I'm not privy to their policy for new AOCs for other operators, but in our case there were 3 concessions to be made -

(1) Line-Up allowance calculated as per the JAR formula,
(2) No credit for 15 ft screen / obstacle clearance height, 35 ft minimum must be retained, and
(3) OEI missed approach must meet 2.5% Gross in lieu of the AFM provision for 2.1%.

For the aircraft that I was involved with, there was no manufacturer's software available for performance, only the AFM (thankfully to a very high degree of precision). Thus, I had to create the software for all operations, and this was done in full. To resolve the dilemma of having to meet an increased screen / obstacle clearance height from that provided in the Wet runway AFM data, it was not necessary to re-write any portions of the software programmes, but simply to increase the obstacle heights by 20 feet for any Wet Runway computer run. This included insertion of a 'phoney' 20 ft obstacle at the end of the TODA to address the screen height problem. I would think that such a technique might work with the Boeing software, i.e. no software change necessary, simply 'jacking up' the obstacles artificially by 20 ft, and insertion of the 'phoney' 20 ft obstacle at the end of the TODA.

P.S. Don't be in too much of a rush to lead the revolution in having all manufacturers provide software for their product's performance. I'm making some nice $$$ in addition to my 'day job' from those that don't.:ok: :ok: :ok: :ok:

Regards,

Old Smokey

[oldebloke]

Boeing produce performance figures for the customers operating authority(FAA/JAR)performance A or B ..Started i believe when the Dutch acquired the B747F,,?..Perf 'B'aplies the 'wet' limits,and the FAA(until they accept total 'Harmonization'of theJAR rules )regard the 'wet' info as nice to know,but not totally approved.The ATA have lobbied that the Harmonization doesn't apply to the present fleet(only 'new' types),as they might have had to apply 'penalties'to the wet Takeoff case.So every time it rains remember that all the European,and a lot of Asian 747's are using different data than the FAA Carriers departing from US airports....:ok: :ok: