As per the FCOM limitations chapter there is an altitude limitation of 20000 feet for operating the flaps/slats on the A320?
Why this specific altitude?
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1$\begingroup$ Without any deeper knowledge I would guess it has something to do with compressibility of air at speeds in higher altitudes $\endgroup$– busdriverJan 27 '21 at 22:29
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Why would you want to deploy high lift devices at any juncture except for departure and arrival?
Holding is the predominant scenario.
Above 20,000ft ICAO holding speeds are higher thus allowing clean configuration.
Above 20,000ft, at slower speeds, the likelihood of encountering icing on the wings when moisture exists is also a very real consideration (upto about 28,000ft) and ice accretion is not what you want with high lift devices hanging out.
Extending slats/flaps in typical cruise or when we are flying in the Mach regime of flight, say about 29,000ft and above, is something that would cause much grief to the average Airliner, a possible jet upset scenario.
If other reasons already cited such as aircraft certification/cost of certification are a factor - most operators would not need a change from the 20,000ft figure.
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$\begingroup$ Are you looking for official references for a specific part of the answer? You want ICAO holding tables and Official definitions of icing conditions? I can try editing and adding text to make the underlying logic more obvious. I cannot provide an official source saying "the slats and flaps are not to be or cannot be extended above 20,000ft because . . . " $\endgroup$ Jan 25 '21 at 0:16
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$\begingroup$ okay I was mainly trying to get information regarding as to why the manufacturer has kept this altitude limit. Or any other official reference which states why not to take flaps above 20,000. $\endgroup$– JaiJan 26 '21 at 13:22
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$\begingroup$ I’ve been in a situation where ATC tried to slow us down below feasible clean speed above 20000’ $\endgroup$ Jan 27 '21 at 22:26
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$\begingroup$ I'm sure, but you can always say, "unable due aircraft performance". ATC shouldn't 'penalize' you for this but there can be situations where it is unavoidable, most commonly resulting in pulling you out of the traffic sequence (= possible delay). Most airliner type transports would have the same issue as already discussed. Also, on Boeings of current vintage it is not prohibited to fly below The maneuvering (UP) speed down till minimum maneuver speed (top of yellow bar) tho probably Co. SOP may not allow. BTW @busriver - what was the circumstance, and was the same asked of other airplanes? $\endgroup$ Jan 29 '21 at 18:20