B737 Flap Extension above 20000'
 

The B737 has a limitation of not extending the flaps above 20000'. Does this also imply that one cannot go ABOVE 20000' with flaps extended? For example, say the flaps would not retract after Takeoff and the departure airport was below landing minima, could one divert to another airport at a flight level above FL200?
Does anyone know the reason for the 20000' restriction?
Thanks all.

Possibly a certification issue. The Jet I fly is also limited to F200 with flaps. When I asked the OEM it was explained as this was the height it was certified too. Possibly a FAA requirement. I guess that there is not much requirement for flaps above F200. I think the highest jet airfield is 14000,somewhere in South America. Interested to hear other views

Cheers

It's to do with Mcrit. A 727 fell out of the sky many years ago due to inadvertant flap deployment in cruise.

This is taken from a 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.

They specified that there is no airport close to this altitude,so , no reason to certify the flap usage at FL200......fair enaugh....

btw if you take off in conditions below the landing minima then u will require a departure alternate which in most cases wouldn't require a crz altitude of above 20000'

Actually the incident you're talking about came from a non authorized procedure of pulling the CB's on the LE slats, and selecting the first flap. it allowed a higher cruise altitude. The problem was the FE came back from the lav and saw some cb's out, and reset them. the attendant yaw/roll was sufficient that he couldn't reach the other side cb's. needless to say this came out in the inquiry. Kind of like the go fast switch in the learjet.

Flaps, like the gear and the whole aircraft have a Speed Limit (CAS) at lower altitudes, and a Mach Number limit at higher altitudes. Many manufacturers (example Douglas :() published Mach Number limits for Flap operation as well as CAS limits. Boeing's solution to not having to establish and prove the Flap Mach No. limit was to simply apply an altitude limit which is below the limiting CAS/Mach crossover.

A simple solution, but yes, it does prevent you from deliberately exceeding 20,000 feet in the event of Flaps stuck in the extended position and diversion to alternate - you would be operating outside the certification envelope.

You might notice that the gear does have a Mach as well as a CAS limit, for the very good reason that gear may be used in an emergency descent from high altitude in the event of speedbrake failure.

Regards,

Old Smokey

Hello,
let's say you have a leading edge flap transit after takeoff, with 1 ore more leading edge devices out, you are limited to 230 Kias. With only one out, you are limited to 300 kias / .65 . The checklist does not mention the altitude restriction as it does not for a pack trip off , for example. The books say that flap extension is limited to 20'000 ft. Anyway this is only my personal opinion :cool:

In the case of the 727 incident it was later discovered that actually using flaps in the cruise did not improve performance.

The flight crew maintain to this day that they did not pull the CB and that it was a completely uncommanded deployment of the slats.

Most interesting observation re use of landing gear for emergency descent. Any chance of quoting the published reference? I would have thought the probability of having a emergency descent event and by rotten luck or a quirk of fate also having an inoperative speed brake at the same instant, would be a million to one...
On the other hand as there is no altitude restriction on landing gear use, it makes sense to have a mach number restiction as well as IAS limit

Gently sliding off topic but an interesting fact is the A320 family does have an altitude limit of 25,000 feet on gear extension. If commencing an emergency descent from a typical cruising level and your speedbrakes are inop the initial portion of the drop would be unwelcomingly slow and gentle. Not a problem in a Boeing but as you've explained this is an extremely unlikely scenario.