B737 Flap Altitude Limit

[WallabyNott]

Hello,

Can anyone explain the B737 (NG in my case) Max flap extension altitude of 20,000 feet.

I've had someone tell me that it's simply that Boeing have not tested/certified them above this altitude and thus limit them to this altitude. I'm not sure that this is true.

Any other ideas?

[Draculas Teabag]

El correcto.

Like pretty much anything with Boeing it's a question of $$$ (e.g. MTOWs, MLWs) and if you pay more money they will certify it higher (or heavier).

[Chryse]

The limit was imposed primarily because of compressibility effect. At higher altitudes air takes on the added and more pronounced effect of compressibility. Operating High lift devices at these altitudes may place the aircraft in a regime where it has not been designed/tested.

Notso Fantastic:

Heres an example for you..

We operate to airfields with elevations in excess of 11,000ft, where runway slope is +3% on landing and -3% on takeoff (unidirectional runway) using Flap 15 as a Normal landing flap. As you can see, elevation, slope limits and Normal landing flaps are not within Boeings prescribed limitations. Boeing provided us with a NOC (No Objection Cert.) to carry out such operations, so off you go.. :ok:
These fields are located in narrow valleys in an area where the grid MORA is 24000ft. Nav aids only work in close proximity to the airfield. So descent into the valleys are completely visual and where, when and how to commence decent varies depending on cloud coverage. In addition, the valleys appearance varies depending on recent or prevailing environmental conditions. eg snow covered etc. Needless to say, there have been times when the wrong valley was entered. Therefore, prior to entering the valleys, speed is reduced to decrease the turning radius. This often entails selecting flaps in excess of 20000 ft albeit not much higher. I've never experienced any abnormality as far as flight controls are concerned while selecting flaps at this altitude.

Cheers

[wellthis]

Indeed the limitation is on the LEDs/slats and not the TE flaps. Simple case being the modified B727s with winglets with higher ceiling (I believe 41000') and farther range due to fuel savings. Part of the modification aside from chopping a few feet off the wings and slapping the winglets, is also drooping the TE flaps1-2 degrees to generate extra lift. Clearly this doesn't pose a threat at 41000'.

Some B727 crews used to do this as a little trick to fly higher by pulling the CBs for LED/slats and lowering Flaps 2 at cruise! (clearly non SOP) You might recall the accident where the unaware SO/FE, upon his return from the Lav, pusshed the CBs back in, extending the LEDs/slats putting the airplane on its back into a dive.....and the rest is history.

So, the limitation is indeed for the high lift devices.

[mutt]

This is taken from the Boeing Airliner magazine:

Several operators have asked Boeing why the Airplane Flight Manual has a limitation restricting the use of flaps above 20,000 feet. The reason for the limitation is simple; Boeing does not demonstrate or test (and therefore does not certify) airplanes for operations with flaps extended above 20,000 feet.

There is no Boeing procedure that requires the use of flaps above 20,000 feet. Since flaps are intended to be used during the takeoff and approach/land phases of flight, and since Boeing is not aware of any airports where operation would require the use of flaps above 20,000 feet, there is no need to certify the airplane in this configuration


Mutt. :)

[visualappr]

What about the following : you're taking off at on outstation. During the cleanup you get a LED in transit light with one device stuck in transit. Are you going to come back to the airport where technics may not be able to help you (think of some of the places you are going to). How about in Low Visibility ? All of a sudden you find yourself altitude restricted (in my QRH it doesn't mention that you're allowed to fly higher than 20.000 ft in this case) and you may have to divert because of lack of fuel for this minor problem.
Any thoughts?

[MasterBates]

There are all sorts of far out problems that we can not be prepared for (that would be one hefty QRH), but I have never heard of the 20000´ limit to create any hazard. Surely we could one day have to do something that is prohibited, or at least not recommended to safe our bacon, and if I would have to top a 21000 foot mountain with flaps 1 just to get to an usable airstrip, I would send Boeing the test results. :ok:
MB

[wellthis]

I believe that LED in transit (and other flaps malfunction for that matter) are covered in the QRH in detail.

If it is only one of the lights (overhead or centre console) and visually confirmed that the LEDs are up, then continue as normal.

If there's only one panel stuck in transit, then may continue to destination at planned cruising altitude with only speed restriction (I believe M0.65).

If it is more than one panel, then speed 210 and return for landing, either to the origin or TO alternate whichever the case may be.

And really can't expect more than this since you have flaps extended, therefore speed 210 and return for landing. Of course one can fly around with flaps extended at 210 kts to one's heart's content, but it is not deemed fuel efficient and good airmanship. Also, can not expect to continue to destination with any malfunction, example being loss of an engine.