Max Operating Altitude for Flaps [Archive]

BelArgUSA

17th May 2009, 03:12

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
:8
Happy contrails

411A

17th May 2009, 04:00

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...:ugh::ugh:

petr_pilot

17th May 2009, 04:52

Thanks for the nice explanation.

Petr

Fratemate

17th May 2009, 06:43

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.

Mad (Flt) Scientist

17th May 2009, 07:22

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.

hetfield

17th May 2009, 07:47

Rainboe

17th May 2009, 07:55

............................

hetfield

17th May 2009, 08:07

http://img20.imageshack.us/img20/6286/sf15.jpg

mutt

17th May 2009, 08:44

Read the answer from Mad (Flt) Scientist, the same applies for Boeing and Airbus.

Mutt

Rainboe

17th May 2009, 08:49

...............

hetfield

17th May 2009, 08:54

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.

BOAC

17th May 2009, 08:59

To avoid extended gumbashing (let alone flaps), may I refer you all to posts #4 and #8 here (http://www.pprune.org/tech-log/278705-b737-flap-extension-above-20000-a.html)? I would assume the same applies to AB.

blousky

17th May 2009, 09:32

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.

VinRouge

17th May 2009, 09:54

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.

Mad (Flt) Scientist

17th May 2009, 15:05

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.

Mad (Flt) Scientist

17th May 2009, 15:08

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).

Spooky 2

17th May 2009, 15:34

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:eek:

I believe that the flaps will not extend on the B777 even if the handle is moved out of the up position.

fantom

17th May 2009, 16:14

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

HiFli

17th May 2009, 17:45

I am lead to believe that the Hoot Gibson incident in a B727 in the seventies was resposible for the restriction.

hetfield

17th May 2009, 18:19

Stubs400

17th May 2009, 18:26

Consider this.
Most airplanes restrict available flap the higher elevation the airport.
At sea level, you've got gobs of flap to play with for takeoff or landing.
Go to some of them 10k+ airfields, and you're permitted very little, maybe slats only.
Why's that?
I figure it's because the denser air can press itself around the outer camber more easily and can stay laminar to a lower speed.
Get up into the thinner angel breath, and it's not so forgiving - you really have to coax those sparse molecules to stick with ya.
Thoughts?

Stubs400

hetfield

17th May 2009, 18:31

@stubs400

99% of T/O in our route system are done with slats only (A300)

The point is: Am I limited to stay at or below 20k feet if slats are extended?

Concerning A300, I say "NO".

Regards

Jumbo Driver

17th May 2009, 21:05

Why would you ever want to even think about extending flaps above 20,000'?

JD
:)

Stubs400

18th May 2009, 00:39

Why would you want to set autopilot on a CRJ200 at 40K on a repositioning flight?
Chuck Yeager syndrome?

FLEXPWR

18th May 2009, 03:52

Hi there,

Just been reading this thread and although explanations of very pertinent nature have been mentionned, I would like to humbly contribute, taking the risk of looking like a fool since my theory on principles of flight is many years old now.

The reason there is a MMO limit, say 0.82, is that beyond it, you will find some local areas on the wing that will reach or exceed Mach 1.
So I can easily see this local Mach value being largely exceeded if with the same speed, someone extends slats or flaps, increasing the camber of the wing, and having the airflow moving over the wing with higher velocity. The shockwave would be sufficient to disrupt, damage, or shear a component that wasn't designed to withstand such stress.

Feel free to correct if my statement is lacking accuracy, or has the wrong wording as english is not my native language.

Flex

BEagle

18th May 2009, 07:27

In one aeroplane type, the maximum landing gear extension speed was 300KIAS.

The aeroplane wasn't fitted with speed brakes and one day a pilot decided that lowering the landing gear at 300 KIAS would increase drag and allow a more rapid rate of descent.

They were doing slightly more than 300, so he raised the nose slightly, saw the speed reduce to less than 300, then lowered the landing gear.......

At Mach 3.2! This was in an SR-71. Fortunately it all held together and they landed safely after rather a noisy descent. From then on, the -1 was amended to include an IMN limit as well as an IAS limit.

Intentional gear down at M3.2 is still the record, although an X-15 had an uncommanded nosegear extension at over Mach 4!

john_tullamarine

18th May 2009, 08:13

... I don't suppose anyone checked the tyre temperatures during that exercise ?

Puts exceedances in a totally different light for me.

Mad (Flt) Scientist

18th May 2009, 20:24

The reason there is a MMO limit, say 0.82, is that beyond it, you will find some local areas on the wing that will reach or exceed Mach 1.

Sorry, Mmo is not determined by the speed at which the local flow is transonic. That would be Mcrit, the speed at which the transonic drag rise starts (if we ignore low speed high lift effects). Mcrit is usually BELOW Mmo.

Mmo is set either by considerations of airframe loading (where it's called VC in the regulations, but there's a link between VC and Vmo, and thus indirectly to Mmo) or by considerations of aircraft handling (various criteria must be met at Mmo) or by considerations of various "upset" cases, which start at Vmo/Mmo and must not exceed VD/MD (or VDF/MDF as appropriate).

But your comments about shocks inducing loads on components that they were not designed to take is valid.

Mad (Flt) Scientist

18th May 2009, 20:38

@Mad (Flt) Scientist

I'm awfully sorry to disagree. Like I already mentioned, my FCOM (A300) says

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

So it clearly distinguishes between slats extended and slats operation.

What do you think is the chart for (it is also of my FCOM and suggested if you fly for whatever reason with slats extended) ? Is there a doted or red line at 20k feet?

Kind regards

I think we are talking at slightly cross purposes.

The word "operating" related to operation of the aircraft. The word "operation" relates to activation of the slats. So indeed in your case, as worded, flight with "slats extended" is indeed permitted above 20k. (provided they remain fixed)

But the original poster's question was
Could somebody pls explain why 20000 feet( Airbus) is the max operating alt for Flaps Extension.
And in that case (as in your FCOM text) the word 'operating' is an aircraft term, because the word "slat" (or flap, or anything else) isn't attached to it.

Also, logically, the slat(flap) operation envelope must be more restrictive than the slat(flap) extended envelope. So if the flap extended case is restricted below 20k, so must also be the flap operation, at least.

In passing, I'd say the FCOM statement is made clumsy by combining two different concepts. It should be:
"Max. altitude for slat operation is 20 000 ft.
Max. operating altitude for flaps extended is 20 000 ft."
to distinguish the two more clearly.

hetfield

19th May 2009, 07:56

@Mad

Thanks for your clarification concerning operating/operation.

Kind regards

hawk37

19th May 2009, 11:41

Hetfield, can you help me with reading the graph you posted, is this example right?

For zero wind, slats 15, at a speed of 250 kias or .55 IMN whichever comes first, at 130 tons weight, "optimum" altitude for maximum range is FL 280, minimum drag at FL 260, if flight at FL230 at 250/.55 then 4% loss in specific range .

But if 20 kts tailwind, then no loss in specific range for flight at FL 230.

But...what does the "maximum altitude" line represent?

hetfield

19th May 2009, 21:33

@hawk

To be honest, I don't know.

Fortunately I never had to use that graph.;)

kijangnim

21st May 2009, 13:18

Greetings,

SLAPS, FLAPS, speeds are based on (E)TAS (equivalent True Airspeed)
at low altitude it is equal to IAS, however above 20000 ft, if you select your SLAT/FLAP using your IAS, you would exceed the limiting Speeds because the corresponding ETA would be much greater than the IAS. :}

slim pickings

21st May 2009, 13:22

Mad (FLT) Scientist is indeed correct. I raised the same question with my instructor at Alteon during initial. His response was that it was not deemed necessary to test the aircraft in any flap configuration above FL200. As such FL200 is the max alt with flap. Incidently I do recall during initial on the GIV that it was not considered safe to configure with flap above FL200 as it caused a disrupted airflow over the engines. FL200 being the limit on it as well. Given that, would it be fair to conclude that Gulfstream did test the aircraft in various configurations above FL200?

SP

Monom

22nd May 2009, 21:26

Why would you ever want to extend flaps above FL200? I remember some years ago (in the sim) having progressive HYD failure in cruise, ending in total HYD loss. As it is essential to get stable at the lowest possible speed (probably F5 Vref40) asp, an emergency decent was initiated to get below 200 in order to extend flaps. So that's why!

Jumbo Driver

22nd May 2009, 21:34

That sounds pretty obscure, monom - what aircraft type was that ?

JD
:)

Monom

22nd May 2009, 22:08

@JD
It was B757. Without any HYD, straight and level flight can be achieved at whatever speed total HYD loss occurs. Obviously the slower the better. It is quite hard to line up on finals at 260 IAS, so if it appears as if it might happen (and nearly did to me on the "Gripper"), get yourself into approach config asp. All this came out post the DC10 HYD loss.

flying_shortly

23rd May 2009, 10:58

Am doing PPL at mo and am studying IAS vs TAS at mo... Just trying to get it right in my head. Is mach similar to TAS in that as you go higher, depending on temp. and pressure alt., it will vary significantly to IAS? Like TAS being important for nav. high up for PPL Mach becomes important for both nav and op. limits(e.g. flaps) high up in commercial aircraft?

Thanks a mil...

kijangnim

23rd May 2009, 11:15

flying_shortly

Greetings,
AGAIN......

SLAPS, FLAPS, speeds are based on (E)TAS (equivalent True Airspeed)
at low altitude it is equal to IAS, however above 20000 ft, if you select your SLAT/FLAP using your IAS, you would exceed the limiting Speeds because the corresponding (E)TAS would be much greater than the IAS. http://static.pprune.org/images/smilies/badteeth.gif

john_tullamarine

24th May 2009, 20:50

I have to admit to some confusion in my mind, here .. what is equivalent true airspeed ? I have no problem with TAS or EAS but ... ?

Zippy Monster

24th May 2009, 21:03