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I'm unsure of the answer to this question, you will need a knowledgeable 747 driver or dispatcher to answer this (not one of the ACARS wonders who just follows what the box says).
The 737 example is invalid, that is a 737 and it behaves like a 737, not a 747. You will find that aircraft with more than two engines do not suffer as drastically from greater flap extension, as two engine aircraft do. The climb limits are based on one engine out at the most critical point, on a 737 this is a tremendous loss of thrust. On Super 727s (2x JT8D-17 and 1x JT8D-217) your climb limits will usually not be a factor regardless of flap setting (even at 30* the climb limit still exceeds the maximum structural weight). Now, I have not flown 747s, but to get the correct answer someone who actually knows the performance characteristics of the aircraft would need to chime in. I would not be surprised if even Flaps 20 would not be limiting under normal/average conditions. |
.. in which case you should be able to quote from the user manual what the basis for the two calculations might be .. and that might give us a chance to offer pertinent comment ? The 737 example is invalid, that is a 737 and it behaves like a 737, not a 747. You will find that aircraft with more than two engines do not suffer as drastically from greater flap extension, as two engine aircraft do. |
The discussions so far are all about flap settings, take off performance and associated engine failure flight paths. After all, that was the point behind the opening question. But take off accident history should not be ignored either. A Boeing study several years ago, stated that high speed rejected take offs (whether warranted or not) have resulted in more accidents than CFIT due to a reduced climb gradient. Thus tyre failures at high speed are where danger exists rather than the statistical probability of ploughing through tree tops. Put another way, tyre failures are more likely to happen at high speed than low speed.
if you can use a lower rotate speed (less chance of tyre failure) which usually means a greater flap setting, then on balance it may be less risky than thundering down the runway at high speed with a lower flap setting simply to ensure a higher climb gradient than the performance or obstacle limited gradient. |
Concur.
I'm far more worried about the accel stop on a limiting or near-limiting runway than accel go. Plus, the gross to net margin gives an increasing amount of fat the further out the critical obstacle is located ... |
How the heck ASDR is slightly increasing? The TODR with flaps 15 is less than with flaps 5 (assuming equal V1,s) because V2 is less. That doesn't exclude somewhat higher drag during acceleration. |
Take off flap setting. Wet Runway
Originally Posted by PantLoad
(Post 3710512)
As usual, Old Smokey provides and excellent explanation of the issue at hand. Thank you.
This thread is related to another that is currently running about the use of Flaps 2 on the 737. It is important to note that airplane manufacturers have takeoff performance figures for several different flap settings. For example, Flaps 2 on the 737 is approved for some models. It is the option of specific operators regarding what data are to be provided to the line pilots. I remember years ago, my company provided Flaps 2 performance data for Flaps 2 (on our 737-200) for certain airports, in addition to Flaps 1 and Flaps 5 data. Generally (and the key word is 'generally'), Flaps 2 provided a nice balance between takeoff run and second segment climb performance (Flaps 1 vis'-a-vis' Flaps 5). My old company (I've been retired for several years, now.) published in the Pilots Operating Handbook (POH) the statement that "Flaps 5 [was] the preferred flap setting for takeoff". Unfortunately, many of my fellow pilots took this to mean that Flaps 5 was the ONLY flap setting to be used, unless conditions (weight, runway length, density altitude, etc.) mandated a lesser takeoff flap setting. In one specific situation, we were taking off from a sea level airport on a hot July day (OAT maybe 35c)...relatively heavy weight...but we had well over 10,000 feet of runway. We were legal and safe for either a Flaps 1 or a Flaps 5 takeoff. I can't remember the specific numbers, but here are the approximate figures: Actual takeoff weight: 127,500 lbs Max weights for Flaps 1 and Flaps 5: (In this particular case, we had no Flaps 2 data.) Runway Limit - Climb Limit ----------------------------------------------------------------------- Flaps 1 - 128,500 - 128,700 Flaps 5 - 129,200 - 127,800 As you can see, Flaps 5 provided a much better safety factor in the event of an RTO, but an engine failure at V1 (continuing the takeoff on one engine) would have been 'tight' as we were only 300 lbs below the max for second-segment climb. Conversely, Flaps 1, while generating higher speeds...and making an RTO at V1 minus 1 'interesting' (thus soiling underpants)...provided a better overall safety factor (in the event of either an RTO of continued climbout OEI). We had published speeds along with the above data (We received all this over ACARS around pushback time.), and the V1 speed for Flaps 1 was about four knots higher than the V1 speed for Flaps 5. Tire and brake energy speeds were respected in either case...as either flap setting was legal. I discussed this issue of takeoff flap selection with the first officer, and he insisted that we HAD to use Flaps 5...to be in compliance with the POH doctrine. My point of view was that we had a better safety cushion if we used Flaps 1. I believe the F/O's confusion existed because some of the captains with whom he had previously flown didn't, themselves, understand basic performance...and the fact that he didn't properly interpret the POH statement. We took off with Flaps 1...being legal and safe and in compliance with our company's SOP...but not without heated objection from the F/O. The moral of the story is that we should consider all factors when choosing takeoff flap settings....and look to the manufacturer's published performance figures (as supplied by your company) as a guide in this decision process. Another factor to consider, by the way, is runway surface condition. If it's rough...to where you really need four-wheel drive for your takeoff run..you might lean towards a higher flap setting...all other factors being approximately equal. Fly safe, PantLoad |
Boy oh boy, this is a blast from the past !
First, follow OEM guidance in the AFM. Second, one needs to be very circumspect regarding the depth of contamination and reasonably likely drag sequelae. This can affect both TOR and ASD performance in less than predictable fashion. Overall, this engineer/pilot would maintain his routine concern with ASD problems ... going off the end or the side, generally, is of more immediate concern than flight path obstacles |
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