Hello again all
Make yourself a coffee and read this properly - have a biscuit on me..
The reason you will clear obstacles is that you are always above the gradient required to clear them. (before you stop reading and just bang out another post, just hear me out).
I will ignore all the benefits which you can't account for because either you don't have the data or we do but currently don't account for it.
I am not suggesting for a moment that anything at all changes in terms of planning. We have to plan for a failure at any stage, obviously, so lets all be absolutely clear that in the event of a failure from V1 up to flap retraction, nothing changes. So lets just think a little more about that case? What is the reason we don't retract the flaps at a lower height in the event of a failure at V1? The reason is we don't have the performance to climb at the calculated gradient AND accelerate at the same time. So we plan to accelerate at a known height and accelerate there. The only point of this acceleration is to raise the flaps. What is the point of raising the flaps? If we can achieve a better gradient with the flap at the take off setting, what would be the point in raising the flap? Why would we not continue climbing at V2 with the flaps set at take off? If the aircraft does not produce much drag, why is it so important to remain in that rather narrow band (V2 to approx V2+10 or maybe a bit more) with the flap at the take off setting in order to achieve the stated gradient?
The reason we accelerate raise the flap at 1500 feet in the event of an engine failure at V1 is to improve the climb rate and for many aircraft 1500 feet is the certified platform at which we do this. It is desirable to raise the flap not only because it improves the gradient and also the rate of climb but the aircraft is also easier to fly.
So here's the thing. If you don't agree that the aircraft climbs better and is easier to fly, what is your justification for raising the flap at 1500 feet? Do you do it only because thats what the manufacturer says or because thats what the bloke that taught you perf A says you should do? If so, noddy pilot, need to think more. Consider how much easier the aircraft is to trim, how much less rudder you need because you've moved the centre of lift, how much more effective the fin is, and how (because its easier to fly) the auto-pilot will now manipulate the controls effectively enough for you to deal with the screaming captain who's just wet himself and can start working the problem.
Does everyone agree that raising the flap at 1500 feet in the event of a failure at V1 has benefits? Or does anyone think the aircraft performs better with the flap down? If anyone thinks the aircraft will continue to climb better with the flap down then you need to lobby the manufacturer and your company to change your OEI V1 procedures to leave the flap down.
Right..
Now lets think about what the 1500 feet is for. It is absolutely not a flap retraction height. It is a level platform which you can use and plan for which will allow you to accelerate and ONLY IF YOU ACHIEVE THE SPEED raise the flap. I'm just going to reiterate that point for sillypeople. You don't raise the flap because you achieve a height. You only retract the flap because you achieve a speed.
Does anyone disagree with that?
So hopefully we have all got to three agreed points so far.
1 - the aircraft climbs and performs better if you retract the flap otherwise you are certifiably insane for raising the flap at 1500 feet.
2 - the 1500 foot platform is for accelerating not for retracting the flap.
3 - we only retract the flaps because we have achieved the correct speed.
The gradient clean is normally called the final segment gradient and should be in all AFM's because you use it to complete a climb to 1500 feet in the event that you run out of time with the remaining engine at max thrust and have to reduce to max continuous. You also can use it to determine reduced acceleration altitude procedures which will enable you to clear distant obstacles which you may otherwise hit. This is all basic perf A and there was always a question in the PerfA exam which was to do with the maximum take off weight you could go with and it was a sneaky trick question because you could take slightly more if you reduced the AA. Nobody ever bothered to work out the answer to this question - it was the slightly higher of the two close weights...
Lets get back to the original question - am I nuts to suggest that we can do what the men that build the plane say and raise the flaps at V2+x. Once we have accelerated to V2+x we have achieved part of the purpose of the level (third) segment and we have attained the speed at which we can retract the flaps. Tremendous! Now, as agreed above, we can trend the aircraft from the slightly poorer gradient that we have with the take off flap to the better gradient achieved clean. You cannot disagree with that statement because if you do you have to leave the flap down at 1500 feet and come up with a good reason to raise them at all.
What happens if the engine fails just as we flick the switch, move the lever, that makes the flaps travel? It doesn't matter - we are already at the speed we need to make the thing climb at the better gradient so all the time the flaps are travelling the gradient is actually getting slightly better. There is a feeling that we are sinking because we have to change the angle of attack but in reality, as long as the speed remains at V2+20, the gradient will remain as good or better than we had with the flaps at take off. Again, if you don't agree with this, don't raise the flaps at 1500 feet as you will obviously not climb so well.
Ok so now we have all agreed, without using any of the extra factors like shorter take off roll than a failure at V1 or take off climb increment because we need the gradient from the end of the runway not our take off point, or the improved climb to wherever the failure actually occured at V2+20, that we have achieved at the very least the same gradient that we would have had in the event of a failure at V1.
By the way, I haven't forgotten what the rest of the job of the 1500 foot acceleration altitude is for - to accelerate to Venr. At the enroute climb speed you will definitely get a better rate of climb than at V2+20 but now your forward speed is greater so the gradient may be less. If you need the gradient for distant obstacle clearance you can continue the climb at V2+20 clean (extended final segment) at max continuous rather than max thrust. This is why final segment climbs are quoted at max continuous (OEI). This is also why we will always exceed the final segment climbs when we use them at low level (ie when we look at the gradient following a failure at V2+20 we will exceed the quoted gradient because we can still have the engine at max thrust rather than max continuous). This is another bonus that we can't quantify but is good to have in your back pocket.
So now we can be sure that at the very worst we will achieve the same climb gradient that we would have achieved if the engine failed at V1. Not a guess - absolutely certain because its in the manuals as I posted. If you don't think its in your manual I'm happy to have a look at the manual but bear in mind that the manufacturer would not certify the aircraft to have the flaps retracted at V2+x unless it were true. There would be big bold letters saying "you have to achieve this height, you have to reduce speed in the event of a failure to V2" blah blah blah and we've already seen from lots of other posts that no manual says that (except perhaps Coughs in his airliner and I completely agree that if it says it in the manual then that's what you do).
(dammit, lost my thread, had to read Hairy Maclary to my daughter)
oh yes. The AFM - the one published which was written by the people that certified the aircraft - doesn't say that. I think its a good plan to do what it actually says. All the benefits are on our side should the worst happen if we do what it says.
Tom
ps.. not facilitation learning because the audience is far too hostile and unwilling to consider an alternative idea. This is called primacy, and its the reason this subject needs to be taught better at an early stage.
pps.. call me odd but that paper Mutt posted is very interesting indeed. The footprint reduction calculation far exceeds what I expected. Its especially interesting that modelling techinques have advanced so much since I left uni as it wasn't possible to model turbulent flow that accurately when I was an undergraduate. This may mean we can develop a more appropriate noise technique for business jets that we can actually fly!
Last edited by tommoutrie; 1st Dec 2012 at 18:05.