baobab72
9th Aug 2012, 14:44
Hi folks
i would like to know what is the criteria for limiting the max altitude of the aircraft in case the spoilers are inop. I do remember it has to do with the time required to descend to a certain altitude during an emergency descent but i can not find the source nor the exact figures.
many thanks
baobab72
Mad (Flt) Scientist
9th Aug 2012, 16:46
Typically it can be driven by the need to comply with:
CS 25.841 Pressurised cabins
(a) Pressurised cabins and compartments to be occupied must be equipped to provide a cabin pressure altitude of not more than 2438 m (8000 ft) at the maximum operating altitude of the aeroplane under normal operating
conditions. If certification for operation over 7620 m (25 000 ft) is requested, the aeroplane must be able to maintain a cabin pressure altitude of not more than 4572 m (15 000 ft) in the event of any reasonably probable failure or malfunction in the pressurisation system.
The emergency descent capability is combined with the rate at which the cabin depressurizes for the various failure cases, and the net cabin altitude has to stay below 15kft. If the a/c only just passes the check with full spoilers, then it'll fail with partial spoilers, unless the altitude at the start is lowered.
There may be additional guidance (AC, AMJ, etc) giving more details. I haven't looked into that.