B727 flap use above FL200

[extreme P]

Originally Posted by anson harris

There's an interesting paragraph or two in "Handling the Big Jets" which explains the highly arbitrary nature of the FL200 limitation. And also why the manoeuvre margin changes at that level.

​​​​​​Use of flaps at high altitude

It would appear that in recent years pilots have developed the habit of using

their flaps at high altitude, presumably for holding or descent purposes.

This rather indiscriminate use of flaps has caused some concern. While the

captain of an aeroplane can of course take any action he considers justifiable

under a special set of operational conditions, there are good reasons for

not using flaps in other than the normal circumstances.

Firstly, flap design is based on the assumption that the flaps will be used for

take-off and climb, and approach and landing, and that they will be retracted

for the rest of the flight. On this assumption are based the strength and

fatigue calculations of the flaps. If they are used at other times then the

original design assumptions are compromised. Secondly, there is a lower

level of proof strength with the flaps extended; this is quite often + lg

increment instead of the + 1 ½g increment in the flaps retracted case. Lastly,

on some of those types fitted with stick shakers and stick pushers, the

design of these systems is such that, with the flaps extended above certain

altitudes, the shake and the push will not function in the correct fashion and

the aeroplane will be denied some of its normal stall protection (in the clean

configuration stall protection is provided at very high altitude by the marked

pre-stall buffet due to Mach number effect, regardless of the point at which

the stick shaker operates).

In this last instance specific limitations on the use of flaps above certain

altitudes will be found in the Flight Manual. They are there for good

reasons, so observe them. Unless there are specific operational reasons the

use of flaps should be confined to the normal low altitude fun􀁊tion. While

you are about it, do try to stay within the speed limitations; flight recorder

results show far too many occasions on which the flap speeds have been

exceeded, in some cases by large margins.