View Full Version : Takeoff Performance Data
30th Aug 2008, 16:19
When calculating takeoff field length for a wet runway the most limiting factor is accelerate stop distance.
Wet runway performance charts should consider the reduced braking efficiency for the aborted takeoff.
A book I am reading states that all takeoff performance data are based on dry runways ONLY and therefore a correction to V1 should be made, to assure stopping ON the runway, for an aborted takeoff.
What is your knowledge on that subject?
30th Aug 2008, 16:47
Our manuals and our performance computers all have charts/entries for wet/slippery runways and various reported braking actions. What book are you reading?!?
30th Aug 2008, 17:14
It is called "Fly The Wing"
30th Aug 2008, 17:39
Obviously they need to start somewhere making performance calculations and that is the dry runway in normal conditions. Wet or contaminated runways require corrections.
If you need to do corrections on "dry figures" yourself depends on the situation. In my company we have separate charts for wet conditions. No need to do corrections yourself. For sure these figures are derived from the dry data (with corrections) but someone else did the work for us. When we have a contaminated situations we need to do these things ourselves. Starting point is dry figures (maybe that is what the book means?)
30th Aug 2008, 18:12
V1 wet concept is not longer applied. According JAR 25.113 new definitions are set for ASDR, TODR,TORR when the runway is wet/contaminated. The use of the reverse thrust is considered and Clearway lenght is not taken in account for take off calculations.
"Fly the wing" is a very nice book BUT consider that it was first published around 1970 and 2º 1990. Certification rules are always changing, nowadays different criteria are used.
31st Aug 2008, 10:55
Very interesting, thank you for shedding light on that subject!
31st Aug 2008, 13:44
When calculating takeoff field length for a wet runway the most limiting factor is accelerate stop distance.
Are you sure?:confused:
I would agree with you that, at least for the aircraft that I do the Performance Engineering for, that is true in about 70% of cases. The other 30% or so are Accelerate-Go limited.
Consider this. Using wet runway data invariably leads to significantly lower V1 speeds, so far, so good, that V1 reduction is necessary to allow for the reduced stopping capability of the aircraft. As V1 is the Stop/Go speed, if then confronted with a "Go" situation, the aircraft must then accelerate from a lower V1 speed to Vr, and this will use SIGNIFICANTLY more runway than in a Dry Runway OEI Takeoff, 3 times the V1/Vr delta is common in work that I regularly do. From this, it is frequently found that Accelerate-Go is more limiting than Accelerate-Stop.(If it weren't for the reduced screen height to 15 ft, Accelerate-Go limits would then predominate over Accelerate-Stop limits).
1st Sep 2008, 08:10
As we all start by reducing V1 to be able to stop on the wet runway. Isn't the ability to continue the takeoff artificially degraded?
What I'm trying to say is: if we start with lowering our V1 to meet wet runway conditions, that is the limiting factor. The side effect is that accelerate go weight is reduced due to the lower V1.
I agree the end result is the same, but the logic is slightly different.
1st Sep 2008, 18:47
Sixfeet-,remember that 'flying the wing' is only predicated of US FAR 25.
The FAA have introduced 'wet 'ops via AC91.
One wouldn't enter the runway with less Acc/stop than required.
Wet ops (JAR)use a reduced V1 in conjunction with a reduced screen(15').
Ergo ,with longer to 'run' on reduced thrust(Eo)from Vi,one has less to leap to clear the screen.
Wet ops allow the use of reverse thrust credit in the acc/stop case.It has been stated in the 320 FCOM that reducing the V! by3knots,on a 3mm wet runway requires little reduction in Takeoff weight.
For the JAR/EASA standards go to: Britflight....Performance...Getting to grips with aircraft Performance.It coversall the JAR stanards.
As a fillet'Boeing'says that on a wet runway your brakesare worth 50%...
Plenty of authorities have stipulated the additional runway required for Landing,only the JAR have stipulated Takeoff requirements.
1st Sep 2008, 18:50
Sorry I should have said that only the B777 has demonstrated the 'wet' acc/stop..
1st Sep 2008, 20:14
Old Smokey, could you give us a concrete example of how you deal with the reduced V1 (wet) and its consequences on go-accelerate distance? My brain is pretty much going along the same lines as Green Cactus...
The stuff they give you in most JAR Perf books spend very little time talking about how real life T-O tables are put together (mostly because the aim of the books is help you deal with the dreaded CAA graphs...)
1st Sep 2008, 22:52
I rather deal (and operate) with an "Accelerate-GO" limitation, than an "Accelerate-STOP" limitation...
To me, accelerate-stop is a very "hard-defined" distance, where the "far edge" of your usable T/O distance consists of mud and metal or trees, which will have detrimental effect on your jet-powered flying machine. The steel of the light posts at the other end of your runway is much "harder" than the aluminum of the nose of your airplane.
Should you venture to "abort your takeoff" and end-up in the mud by 20 meters beyond the end of the runway, you will become the subject of a thread and 719 postings by the Pprune accident investigators. In a "accelerate-go" non-incident you survived, passing the "screen" with only 10 (or less) feet above your obstacles, will not be known to that SLF, a reporter for the Daily Mirror, who happens to have enjoyed two "before takeoff whiskies" in his seat nº 3C of your cabin...
As per flight safety, I always "knocked" a few "knots" from V-1 speed on wet runways. And yes, when I started airline flying in 1969, I eventually read "Fly the wing" by Jim Webb. And "Handling the big jets" by D.P. Davies became (and still is to this day) my bedside favorite book, next to my bible.
My decision to continue takeoff (not to abort) is briefed and indicated to the other pilot and flight engineer by my hands going OFF the thrust levers. I do not waste time in 1 hour-long briefings during taxi, about what V1 is or will be, and what we will be doing, should an engine fail. We all know what V1 means anyway, since the first time we sat in a cockpit.
No incidents or accidents in my long career, ending for retirement 2 months from now.
Except (maybe) almost one time...
Dubai - early/mid 1980s - I am captain on a DC8-63F cargo - US air carrier, operating ACMI for Air India. We came from Madras (Chennai) and destination was Milano Malpensa. The experienced F/O was dealing with weight and balance, and I took care of performance and speeds. Next sector was my flight..
The Dubai agent asked if we could carry "extra payload" to MXP. He asked "how much" - The F/O said "twelve thousand". The agent acknowledged. The F/O had meant POUNDS as a good American boy he was, and the nice Air India agent understood KILOS... In that part of the world, 12,000 certainly becomes 13 or 14,000 in practice... Airplane got loaded, we taxied for takeoff, at a computed maximum weight of about 355,000 lbs, we were performance limited despite the long runway. The flaps were to be set at 18 for takeoff.
Close to reach published V1, it became evident to me that we were not going to "make it". Maybe the only day in my life that I exercised superior airmanship, I moved the thrust levers forward to the stops (F/E hand came to reduce some of it) and I called (actually yelled) for "flaps 23" which the F/O immediately selected without asking a question.
Should we have "aborted" the T/O, we would have been a statistic of the NTSB. Sure, more flaps reduced the "2nd segment" gradient but it also avoided the airplane stall. As I could not pull more "nose-up" (stretched DC8s are limited to 8º rotation), the extra lift got us to fly above the obstacles by the end of the runway, not by the "35 feet" of FAR 25... Who knows, we cleared them by only 10 or 15 feet...
By chance I had an excellent crew with experience, and we all learned of our mistakes. How heavy were we, I would estimate we were at 370,000 lbs.
I believe, having a crew (in the name of modern concepts of SOPs) reducing the power to maximum EPR, N1 or EGT limits, and ignoring the "flaps 23 call" because it is "not in the procedures" would have made the three of us become some name statistics of aviation accidents. And the reason would have been "pilot error - crew failed to compute proper weight, and failed to observe standard procedures".
Thanks for that Bel, good food for thought. Anyone else got any similar stories?
3rd Sep 2008, 15:13
I was afraid that some-one would bring that up, which would then require a more comprehensive explanation of "it all depends".
It all depends upon the vintage of the aircraft and the certification standards required. As oldebloke has pointed out "Sorry I should have said that only the B777 has demonstrated the 'wet' acc/stop..", more modern aircraft have fully accounted for wet runway considerations at the time of certification, and typically, the Wet Runway charts are for Balanced Field, which, at least in theory, means that Accelerate-Stop and Accelerate-Go are equal.
Another mode of certification in recent aircraft is full accountability for Wet Runway Stop or Go, not Balanced Field, but one set of charts providing limits for the most limiting of Stop or Go. For the daily user, there's little to indicate which factor is limiting, for the "Performance Nerds" (like myself), careful examination of "break-points" on the charts will indicate when either Stop or Go is more limiting.
Yet another earlier mode is the provision of Accelerate-Stop Charts, and Accelerate-Go charts, calculation of both limits, and limiting Takeoff weight to the least of the two. In this case, it will be clearly evident to the user whether the Wet Runway takeoff is Stop or Go limited.
In even earlier generations of aircraft, when wet runway performance was not considered at all, later "wet runway band-aids" were applied to dry runway V1s, with asociated performance penalties. These band-aids were primarily directed at the Stop case, but the better manufacturers would oft provide corrections for the Go case if the (usually nominal) V1 reduction was applied. If no correction was necessary, you were Stop limited, if further weight reductions were necessary, you were Go limited.
As stated at the beginning, "It all depends" (upon the era when the aircraft was certified).
3rd Sep 2008, 21:57
Thanks Old Smokey, very interesting stuff as usual.
Happen to know which of the methods you described was used for the NG? Balanced field or "most limiting" approach?
4th Sep 2008, 17:16
Sorry, haven't looked at B737 for 18 years, and the NG certainly came along much more recently than that.
Hazarding a guess, considering the era and Boeing philosophy at that time, I SUSPECT that the Balanced Field approach was used.
4th Sep 2008, 22:51
Perhaps something in the following links will aid the discussion; at least there are some diagrams to refer to.
Range of V1. (http://brahimtahiri.bravehost.com/02_RangeV1.pdf)
Takeoff Performance. (www.captainpilot.com/performance/04_Takeoff_AFM-DPI.pdf)
737 example. (www.captainpilot.com/performance/09_ProblemSolnSet.pdf)
As Old Smokey already stated, there is no general answer to what is limiting on a wet runway. You have to look at each case individually.
Factors are certification, aircraft type (number of engines), speed (V1) concept, airline policy, runway.
Dry runway T/O is based on no reverser and 35ft screen height, where wet runway T/O allows to account for the reverser and reduces screen height to 15ft. Thus, you might end up in a situation, where the dry runway T/O is more limiting than the wet runway T/O (same other parameters). Authorities addressed this point and require on a wet runway a crosscheck of the wet and dry data and the use of the lower weight.
In other words: you might calculate a wet RWY T/O and find out, that you are limited by the dry RWY field length limit, which, for a four engine aircraft usually is the all-engine-go case (due to the 15% penalty).
Wet/contaminated runway data have not been required for FAA certification. The data could be found in the "Performance Engineers Manual PEM" as advisory or guidance information only, but never really made it to the Airplane Flight Manual. For younger FAA certified aircraft you will find the data in the AFM, but they are still advisory information.
I recall a Boeing recommendation for B747-200 to knock of 10kts from V1 on wet runway without a weight decrease necessary.
Hope that helps a bit.