Max ALT for gear extension

[zonnair]

Hello,

On my a/c there is the following limitation:
Maximum altitude at which the landing gear may be extended ....... 21000 feet

Does anyone has an (aerodynamic) explanation why this limitation only concerns gear extension and not gear retraction?

Has it to do with compressibility, hence airspeed (EAS) limited?

Thanks a lot.

[Cough]

You could read that statement in another way.

'The maximum altitude at which the gear may be down : 21000ft'

[BOAC]

Type? Often a 'limitation' produced by the highest 'tested' or 'demonstrated' event during certification, as with the 737 flap altitude limitation of 20,000'

[blind pew]

Flap altitude limitation is normally due to local airflow becoming supersonic.

Classic example was a Trident one in the clacton hold at around fl250 when the second officer extended the droop - the aircraft lost somewhere around 5000 ft before they recovered! (captain in the loo)

Grey matter says the limit was 22000ft?

[Alycidon]

I would think that you'd have a bit of trouble maintaining altitude due to drag at typical cruise alt with the gear down!

Also, if the max retraction speed was 230kts, let's say, then you might also find yourself very near the low speed buffet before you could get the gear back up, although the drag penalty would get you back there much quicker than you'd think!

That's assuming the gear is operating normally. If it is locked down due to unserviceability, then you'd look at the gear down cruise tables in the ops manual to see how high you can climb.

[catiamonkey]

A330? There's a maximum altitude chart on FCOM 2.04.25 P3, which gives, for a flight with gear extended, the maximum altitude "In order to ensure that there is at any moment of the flight a minimum range of 20 kts between VLS and the maximum speed (limited by thrust or VMO/MMO), the aircraft must remain below the following profile."

21,000 ft corresponds to 210t. So that's likely chosen as a safe assumption, if you took off at MTOW and started climbing with the gear down.

[BOAC]

Flap altitude limitation is normally due to local airflow becoming supersonic.- Hmm. I think you'd be pushed to get a shockwave at 210kts at 20,000ft!

[Mad (Flt) Scientist]

You might be surprised. Accelerating flow around regions of high curvature of aerofoils can become locally supersonic at VERY low freestream speeds. So while I'm not saying locally supersonic flow is the reason for a flap altitude limitation, I wouldn't rule it out based on the freestream data alone.

[Slasher]

Set flap 1+F at F200 in the 320 sim once when we had some
spare time left over.

For the hell of it set 390 in the window and allowed speed to
follow Vx increase as altitude increased while not allowing it
to exceed 1+F limit. Passing 330 we got some buffeting and
deduced it was Mach effect. We continued climbing when Vx
encroached on Vfe 1+F, still buffeting but more pronounced.

Didn't fall out of the sky and might've made 390 if the sim
time hadn't run out.

But its just a sim after all and probably doesn't have the necessary
algorithms to fully reproduce these effects correctly.

[BOAC]

I would put money on the certification issue with flap - gear is another matter vis a vis an emergency descent. When I asked here ages ago about the 737 flap limit the 'considered opinion' was certification boundaries, not Mach. As was pointed out, who in h***l would want to do it anyway, so why test it?:).

[Mad (Flt) Scientist]

Environmental effects may also come into play. Gears are probably designed and qualified to operate with certain temperature limits. A gear in a bay at FL410 may get cold, but it doesn't have to do anything. But a deployed gear may have to be retracted, and if the hydraulics aren't intended to operate at the prevailing temps it may not work as advertised.

Certainly on some products the flaps and gear are qualified to an operational envelope which assumes low altitude use and so limits the low end of the temp scale.

Just a thought ...

[zonnair]

This limit is for extension (VLO extension)only. It's allowed to fly above this altitude with gear down as per fcom 2.04.25 up to FL350. Retraction however is not limited to 21000'.

Flaps are limited not to be extendED 20000', so no difference between extension or retraction.

It's indeed an A330.

Thanks again!

[blind pew]

BOAC - the equation was fl 250, speed just below 250 knots indicated and leading edge droop - Trident 1 - fowler flaps - which was a bodge that was done away with on the 2 and 3.