Flap retraction

[tommoutrie]

hello againn apruneuk

the issue is that it links to putting together cohesive arguments about NAP to the authorities. At the moment most people don't really understand performance and the way its taught confuses people.

For instance. The reason the Hawker manual says 2nd segment goes to 1500 feet or to 3.5 mins is thats the point at which you need to accelerate and take the flaps up. Its actually the flap going up that changes the segment - you go to third segment (level acceleration) if you need it to accelerate and because your engines are limited to 5 mins max thrust you level at 3.5 mins to accelerate enough to take the flap up and do a final segment climb to 1500 feet where you would level accelerate again and climb at Venr or Vfto whatever you choose to call it.

So if you raise the flap at V2+10 at any height you have entered final segment climb, if you keep accelerating to Vfto you are in the enroute climb. There are advantages to climbing at Vfto - there's no level segment needed at 1500 for the final segment climb because you're already at the required speed and its easier to fly with a failure. But this is also true if you are at V2+10 clean.

The argument regarding needing a further margin over V2+10 is also not valid - you actually need less lift in the climb than in the level segment and everyone seems happy to raise the flap at V2+10 in the level segment. The margins are already built in as explained in a previous post.

I don't understand the logic of flying a less efficient departure, climbing against unnecessary drag. It simply not true that its mandated - thats urban myth. I don't really care how people choose to fly the planes but we shouldnt make up rules that simply don't exist. This myth of a 400 foot flap retraction height and even the 1500 foot one just causes confusion in pilots and they don't really understand the problem. I've been accused of making up my own rules and all sorts on this forum simply for saying "why don't we do what the manual says" and its revealed all sorts of holes in peoples knowledge which is also why I object to how its taught. In many companies, a call of 400 feet is the trigger for flap retraction and its totally without logic.

anyway, as you say, got my knickers in a twist..

but they are very valid points eh..

[deefer dog]

Surely the question to answer is:

Will the aircraft, with OEI and flown at V2 or V2+10, make a better angle of climb, considering the conditions, with flaps in the take off position?

If a clean climb will produce a better climb angle (not rate), then tommie is surely correct, but can he prove that it will work in all conditions?

[Kak Klaxon]

Tom,

You are wrong, wrong wrong wrong wrong wrong.

[apruneuk]

Tom

From the Hawker manual, V2: take-off safety speed. V2 is equal to VR plus the increment in speed attained prior to reaching 35 feet with an engine failure.

VFTO: The final take-off climb speed. "When a positive rate of climb has been established, retract the landing gear. Raise the flaps at approximately 160 kts but not below final take-off climb speed. With both engines operating at take-off thrust, the airplane should be allowed to accelerate to an airspeed of 160 KIAS, this airspeed being maintained until obstacle clearance height is reached."

VERC: en route climb speed.

So for a two engine noise abatement departure we aim for 160kts whilst adjusting power so as not to exceed the maximum mandated pitch of 20 degrees. Flaps are then raised, the aircraft accelerated to VERC and climb power is set.

So for the Hawker, V2 plus 10, whilst unlikely to make the aircraft fall out of the sky, is not the most efficient speed to raise flaps whilst maintaining the close-in obstacle clearance profile (second segment). Or, put another way, retracting flaps at V2 plus 10 in the climb will degrade climb angle as opposed to waiting until VFTO. From the manual.

AP

[deefer dog]

why don't we sort the wheat from the chaff and forget about noise abatement! it was not brought up in the original argument....tommie's argument is based on his belief that the airplane will climb at a better angle with flaps up, and thus he believes that one will clear obstacles by a better margin with OEI if the airplane is cleaned early while both engines are operating, and before one of them quits.

there are a lot of self proclaimed experts here who seem to differ, but as far as I can see nobody has yet disproved the central premise that he is putting forward.....will it make a better angle with flaps in the take off position with OEI?

my gut feeling is that it will not....irrespective of the conditions, but i'll check my AFM tomorrow.

[Sillypeoples]

Tom is trying to convince you his cruise climb speed (V2+20)is an obstacle clearance speed..

In a nutshell he's trying to argue that Vy instead of Vx is what you use to get over obstacles..

Brilliant.

I vote to stick him back in a C172 and have him fly at hills all day long using Vx and Vy to get over them until he figures out the difference.

[tommoutrie]

deefer, that bit is interesting for my aircraft.

Comparisons of a departure at MAUW from sea level at 20 degrees give 5.8% for the final segment and 5.7% for 2nd segment.

40 degrees, 41,000lbs, sea level, second segment is 5.0 % and exactly the same for final segment.

I'll do more tomorrow (I'm visiting inlaws..)

Although you can't account for the credit, the conditions for 2nd segment are with the APR (power reserve) triggered which is an extra 2% N1. The engines are producing significantly more thrust in this condition and you are limited to 5 mins. For the final segment tables the engine conditions are max continuous so significantly less thrust. Achieving the same percentage climbs with less power would indicate a more efficient climb but that is a hard one to argue so I can only claim the same gradients. I promise to post it if I find conditions where final is worse than 2nd segment.

Dont forget that the APR would be triggered in the event of a failure anyway so you would absolutely definitely out perform the quoted final segment numbers (engines delivering more power), there would only be a requirement to reduce power to max cont after 5 mins with no level (third) segment so dist obstacle clearance performance with the flaps retracted would ALWAYS out perform flapped to 1500 feet. You would still need to fly a level segment to accelerate to enroute but it would be shorter than from the flapped condition (you're already going faster and there's no segment where you are raising the flap).

Still failing to see the downside..
I re-read the Bombardier AFM for a normal take off today for the 601. It states flaps up at 1.25Vs which is typically V2+4 when light to V2+7 when heavy. Flaps up at V2+20 (quoted in the Bombardier training notes and on the checklist) already has a margin built in.

every days a school day ...

[tommoutrie]

sillypeoples, my cruise climb is 250 kts, my enroute climb is 190, my final segment is V2+20, second segment is V2. Your grasp of whats going on here is astonishingly weak but I have a copy of Kippers Kite that you might enjoy now that my kids finished with it.

[tommoutrie]

deefer, you have summed up the issue very well. Can you explain it to sillypeoples?

one thing - this isnt my suggestion. Its whats in the AFM. I'm suggesting that some training organisations bang their own SOPs over the top and they don't enhance the performance of the aircraft - in fact they degrade it. Using the raw data it should become obvious if this suggestion is correct or not and the numbers I've just got from the graphs suggest it does. But we should not forget that using the manufacturers method give more benefits - no (or a shorter) third segment, lower fuel burn (not by much but a bit), less noise (because you will climb better as you accelerate), lower workload in the event of a failure (because you are going faster and are already clean), and I honestly can't see any downsides!

Actually, there is one (just to scupper my own argument). If you take off and climb into a strong tailwind the slightly increased speed (V2+20 as opposed to V2) would mean the gradient is very slightly worse but I cant be bothered to do the maths as the difference will be absolutely minimal and nobody else is bothering to wind the take off path at all so I dont see why I should bother!

[Kak Klaxon]

You don't see any downsides because you are not seeing the bigger picture in what a Perf A aircraft needs to be able to achieve. I guess you never had to suffer a Lawrie Benn 5 day Perf A course back in the day.
Look again at the NTOFP and tell me how bringing the flaps up on speed at an unknown altitude provides you with guaranteed obstacle clearance .

[apruneuk]

Tom

Just another thought for you; as I have already mentioned, for the Hawker, the manufacturer's recommended flap retract speed is VFTO, not V2 plus 10, for a normal take-off. One very good reason for this could be that V2 plus 10 is usually reached before the gear has started to retract. By the time the gear is fully stowed, VFTO (approx V2plus 30) has been achieved and the aircraft can be cleaned up.

The question now is, what speed would you pitch for if an engine were to fail clean below 1500'? The charts give profiles for a climb to 1500' at V2, however it is unlikely that you have calculated V2 flap less if you have just taken off flapped. Of course, you could approximate or continue at VFTO, however neither method would give you a provable second segment gradient.

Hence, it is far easier for operators to mandate a height of 1500' for flap retraction to allow for an engine failure at any stage prior to that point. All that would then be required would be a pitch to your calculated V2 to maintain a gradient guaranteed to keep you safe to 1500'. Or you could just take off flap less if your type allows it.

AP

[tommoutrie]

kak thats an interesting comment.

I did my exams at london guildhall up on the top floor and remember lauries lectures well

Remembering his lectures has convinced me even more that the problem lies in the teaching. The only case that was calculated was OEI at V1. Thats all you needed to pass the exam. All based on a Tristar from what I can remember. The legacy is that pilots believe they have to degrade the performance of the aircraft on normal take offs so that in the event of a failure they can re-capture the worst case scenario. But it simply isnt true. Because people misunderstand the segments and whats actually going on, they think they have to resort to a flapped climb in the event of a failure to comply and its just not true. Its a very significant failure in the training and testing regime.

Engine failures are rarely as benign as we experience in the sim and the shock of a powerplant letting go, shedding bits, and vibrating like mad is not realistically replicated in the box. I genuinely feel that we should be armed with the most appropriate and easiest aircraft to fly in this event and clean and fast is the easiest configuration. I haven't got a hawker manual to read but I'm assuming that what apruneuk has posted is correct and perhaps its written that way because the enroute climb gradient is good enough to meet the criteria. The 125 series is derived from the dominie which was a IMC and IFR trainer for the RAF for a long time so wouldn't be surprising if the aircraft has some basic design features which make it easy to handle with failures. For the Hawker, the default configuration for take off should be flapless anyway because of the superior 2nd segment and easier handling with a failure. The flaps shorten the roll and give an earlier lift off and mean lower brake energy in the event of a critical stop but that seems to have got lost in the mists of time too.

I think this matters because there's a mixture of ideas about what to do with a failure higher up the climb. I've flown 1200 sectors as a line trainer and simply discussing this with pilots seems to cause confusion - its never been explicit in a part B or an AFM for a pilot to read. Even on here its clear that pilots confident enough of what should happen have conflicting ideas. I'm still not clear on what some of your plans are in the event that you retract the flap at 400 feet "above V2+x" and then the engine quits. Would you pitch up to reduce the speed? Would you hold the pitch and let the speed bleed off more gently? Would you reduce the pitch and bleed the speed off really gently? I've never heard a pilot brief it and I don't tend to bother either. There's a complex drag argument to be had for doing each of those three things whether you are flapped or clean when you have excess speed and as the PM I'd like to have a really clear idea of what the HP's plan is before it happens because if the mental model is different to mine when an engine fails the discussion is going to be compressed..

[BizJetJock]

one thing - this isnt my suggestion. Its whats in the AFM.

Err.... no, it's not. The AFM for a 601 just says flaps up at not less than 1.25 Vs. Therefore either method meets the letter of the AFM. Other types, as has been pointed out, do have at or not below 400' mentioned.

why don't we sort the wheat from the chaff and forget about noise abatement!

Because we're not allowed to. The original question was why do people climb to 1500' before flap retraction - answer, because that's the noise abatement departure as required by the operating rules. I've quoted this previously, but it's EU-OPS1.235

[Kak Klaxon]

Tom,

Again, please tell me how you are going to guarantee obstacle clearance by raising the flaps at an unknown altitude. This is the whole crux of the argument.

[tommoutrie]

and you're right biz but a critical part about the noise abatement departure is ignored. Very few pilots adhere to the speed schedule for NADP claiming that the body angle is too high (and they are right) but climbing with excess power against the drag causes more noise and degrades the climb. Everyone keeps saying that the increased drag is minimal and its not for long etc but ANY increased drag is increased noise and increased footprint. Also, I have never seen charts for climb gradient at speeds above the minimum flap retraction speed so you don't know by how much you have increased the noise and footprint.

This is a good reason to start a proper discussion about noise abatement procedures and come to an agreement with the authorities with regard to noise because at the moment I think very few departures are compliant with the rules.

[mutt]

Tom you might enjoy reading this....

http://web.mit.edu/amarch/www/files/March_thesis_SM.pdf

Mutt

[BizJetJock]

I don't think I ever said the rules were right...

[deefer dog]

mutt,

anyone who might possibly enjoy reading that needs their head tested.

[BizJetJock]

Not sure about enjoy, but it was interesting. Basically said configuration (i.e. flaps up/down) makes stuff all difference to noise. The two biggest factors are:
Thrust setting - so all a/c that are capable of it should use reduced thrust for takeoff
Speed - We should be going for max body angle rather than letting the a/c accelerate.

Since bizjets not on reduced thrust will invariably accelerate to a fairly high speed even at 15 degrees of pitch, it bring it all back to the question of what procedure you should use for an EF at 500'. I am firmly in the camp of leaving the flaps at the takeoff setting, since you are then in the configuration you have done the OEI planning for, and guaranteed to be above and able to stay above the calculated flight path.
While there is some logic to Tom's suggestion for some a/c types, where even at max body angle you have reached Vfto or equivalent (note, it must be Vfto and not some lower speed that would subsequently require acceleration in the event of an engine failure), and the clean climb gradient is always better than the 2nd segment (again, not true for all aircraft), logic only gives you a "probably ok" which is not good enough.

And that also ignores the fact that the more different contingent procedures you bring in (EF here I do this, EF there I do that, EF over there I do procedure 3), the more likely people are to get it wrong. Even with 2 procedures, normal and OEI, a significant number of people get it wrong. So having the same procedure, the only difference between normal and OEI being the target speed, hugely increases the chances of success.

And finally, there is the factor that IMHO the drag and reduced pitch angle of flaps on the initial climb are a benefit from a handling perspective, especially with a low initial level off. By keeping the RoC low - only about 3000fpm! - it greatly reduces the chances of a level bust.

[Kak Klaxon]

Tom, answer my question, how do you ensure you can get over an obstacle if you raise the flaps at an unknown height.
Your a line trainer, you are teaching people to do this, what is the answer.