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Max Operating Altitude for Flaps

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Max Operating Altitude for Flaps

Old 17th May 2009, 03:00
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Max Operating Altitude for Flaps

Hi,

I am not a commercial pilot. Could somebody pls explain why 20000 feet( Airbus) is the max operating alt for Flaps Extension.

Thanks a lot
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Old 17th May 2009, 03:12
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Not an Airbus pilot, but I am certain the explanation is same as for Boeings.
20,000 feet is an arbitrary limit.
Below 20,000 feet - flaps speed limits are V speeds. There are such limits.
Above 20,000, should be a Mach number limit, but none are given.
So they just limit flaps operation to 20,000 maximum.
Douglas, to compare, had no altitude limits (DC-8) -
They had flaps VFE 230 KIAS or Mach .46...
xxx

Happy contrails
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Old 17th May 2009, 04:00
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Flaps.

Long ago, we had a new Captain on the 'ole L1011 at SV who decided it was a good idea, having just received an ATC instruction to cross ElDaba at a later time, decide that flaps/slats at FL330 was a good idea.
Opps.
They extended alright....at M.75
One slat section was torn off straightaway, and one more departed on the diversion to Cairo.

He was sacked.

No altitude restriction was listed in the AFM, however...it did say max Mach number, .50.
Guess he failed to read that part...
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Old 17th May 2009, 04:52
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Thanks for the nice explanation.

Petr
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Old 17th May 2009, 06:43
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I have to admit I'd never really given this too much thought. Mr Boeing says don't chuck the flaps out >20000', so I don't. However, BelArg's post got me thinking (which is not pretty at the best of times).

We know we don't put the flaps out at too high a speed because they will get damaged by the number of Bernoulli's hitting them (I like to keep things simple in my mind ). The higher the speed, the more Bernoulli's. Now, we also know that the only measure we have of the number of Bernoulli's in the air is INDICATED airspeed. So, given that, why would a manafacturer either put an altitude limit or a mach number limit on the flaps? Surely you can just stick with an IAS limit because that will measure the Bernoulli's in the air no matter what your altitude.

It obviously doens't really affect the price of fish. I'm not going to use the flaps that high and I have no intention of chucking them out too fast but I was just wondering why they just don't stick to an IAS limit and have one fewer line in the limitations chapter.
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Old 17th May 2009, 07:22
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In some cases you may find that the alt restriction is as much due to not having tested there as any hard system or aerodynamic limitation.

No-one tests more than they need to for cert, so if *we* decided no-one will need flaps above alt X, that's all *we* will test.

Usually you think in terms of "what's the max alt we'll use for TO or landing" then "whats a reasonable margin above that for ops" and presto - flaps altitude limit.
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Old 17th May 2009, 07:47
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Another point is the terminus "operating"....

Let's say you have an aircraft which may take off with slats only. After take off you find slats are not willing to retract. For my understanding it's legal to climb above 20k feet observing Vfe (250kts) and continue to destination (short sector), fuel permitting, cause you don't "operate" = move the flaps/slats.
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Old 17th May 2009, 07:55
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............................

Last edited by Rainboe; 17th May 2009 at 16:10.
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Old 17th May 2009, 08:07
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Old 17th May 2009, 08:44
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Read the answer from Mad (Flt) Scientist, the same applies for Boeing and Airbus.

Mutt
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Old 17th May 2009, 08:49
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...............

Last edited by Rainboe; 17th May 2009 at 16:08.
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Old 17th May 2009, 08:54
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That's what my book says:

Max. operating altitude for slat operation or flaps extended is 20 000 ft.


And it also says if flights are conducted with Slats extended refer to the chart above to selct altitude.
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Old 17th May 2009, 08:59
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To avoid extended gumbashing (let alone flaps), may I refer you all to posts #4 and #8 here? I would assume the same applies to AB.
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Old 17th May 2009, 09:32
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The 737 FCTM says it quite nicely:
The limit of 20000ft for flaps extension is there only because there isn't any airport in the world where an extention of flaps above 20000 would be necessary.
Hence we didn't tested them above that altitude.
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Old 17th May 2009, 09:54
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Surely you can just stick with an IAS limit because that will measure the Bernoulli's in the air no matter what your altitude.
Nope wrong there I am afraid. Putting slats or flaps out at high mach will turn the airflow from managable transonic to unmanagable transonic or even supersonic... thats why there is a mach limit. Nothing to do with number of bernoullis at high alt, all to do with the bernoullis bunching up, causing shock wave effects MASSIVELY varying the centre of pressure on the wing and thus invalidating any design criteria the slats/flaps were originally designed to.

High lift wings can have transonic (and also >M 1.0) flow at aircraft subsonic speeds as low as M0.5-0.6, as all those high lift devices do wonders to pressure differentials. In other words, the wing goes transonic WELL before the aircraft does.

As an aside, one of the big design problems for designers isnt only wing transonics, its the flow between nacelles. You put them too close together and you get a lovely convergen divergent duct to accelerate flow with... and nasty weak shocks that do wonders for engine mount fatigue.
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Old 17th May 2009, 15:05
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Originally Posted by hetfield View Post
Another point is the terminus "operating"....

Let's say you have an aircraft which may take off with slats only. After take off you find slats are not willing to retract. For my understanding it's legal to climb above 20k feet observing Vfe (250kts) and continue to destination (short sector), fuel permitting, cause you don't "operate" = move the flaps/slats.
Nope.

The "operating" refers to operating of the aircraft - as in Vmo for example.

Getting the flaps out at 19,999ft and then happily flying around at 30,000ft+ is (a) a false interpretation of the manual; and (b) asking for trouble.
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Old 17th May 2009, 15:08
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Originally Posted by VinRouge View Post
High lift wings can have transonic (and also >M 1.0) flow at aircraft subsonic speeds as low as M0.5-0.6, as all those high lift devices do wonders to pressure differentials. In other words, the wing goes transonic WELL before the aircraft does.
Agree, but it can be even lower than the speeds quoted. Think of the gap between slat and fixed portion of the wing. it's a nice accelerating channel. Thing what's going to happen at even moderate Mach numbers to the flow in that channel.

Hell, even non-slatted aircraft can generate transonic flow locally at VERY low free stream Mach numbers, under the right conditions (CL etc).
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Old 17th May 2009, 15:34
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There were a number of inadvertant slat extensions on the MD11 before they redesigned the flap/slat handle. Made for some interesting gyrations when this happened at .83

I believe that the flaps will not extend on the B777 even if the handle is moved out of the up position.
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Old 17th May 2009, 16:14
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Now, here's a real anorak's Airbus 320 fact:

Who knows when VFEnext appears on the PFD speed tape during descent?

Those paying attention to the above will realise it is FL 200 (the limit for flap extension).

Not a lot of people know that.

f
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Old 17th May 2009, 17:45
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I am lead to believe that the Hoot Gibson incident in a B727 in the seventies was resposible for the restriction.
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