A320 Environmental Envelope - Low OAT
 

FCOM 3.01.20 - Environmental Envelope: Pressure Alt Vs. OAT.

With the rather cold upper air masses of late the above table has had to be consulted to avoid being out of the top left side of the envelope while at cruise altitude ...

Just wondering what factors give rise to those minimum temperature limits at any given altitude ?

This has been debated on this forum before but I don't have the patience to search for it myself - have a go and you might find the answer.

I seem to remember, though, that it was more a structural consideration than anything else. It's also worth bearing in mind that the shape of the environmental envelope varies from ac to ac up there and it's always worth a check when the ISA gets down to -8C or so in the upper levels.

Over to those more clever than me!

Cheers all,
mcdhu

CS 25.1527 / FAR 25.1527 Ambient air temperature and operating altitude
The extremes of the ambient air temperature and operating altitude for which operation is allowed, as limited by flight, structural, powerplant, functional, or equipment characteristics, must be established.

The result is the environmental envelope.