Originally Posted by
VinRouge
Does part 25/cs25 prohibit in flight reverse? Or is it not more of, it’s not a requirement of civilian jets to drop 20k/min and therefore an unnecessary feature?
No, in-flight reverse is not specifically prohibited. However, everything needs to comply with 25.1309 - which in short means that the probability of a catastrophic event has to be less than 1 in a billion (per flight hour) and that no single failure can have a catastrophic outcome (i.e. you can't use probability arguments for single failures, structural elements excluded).
Now, look at what happened on Lauda - the reverser deployed on one side (uncommanded) - the resultant upset caused complete loss of control in a few seconds and aircraft breakup shortly there after. In order to certify in-flight reverse use, you'd have to definitively show that Lauda couldn't happen if you got unsymmetrical reverse deployment.
I very seriously doubt you could get there (and I sure as hell wouldn't want to be on a flight test to try it).
Beside, yes, it's an unnecessary feature. Passenger O2 requirements assume an idle descent from the max certified altitude - no need to get down even quicker than that. And in the extraordinary event that you do need to come down faster than idle with speed brakes, you're best bet is to drop the landing gear - it may cause damage to the gear doors, but it's readily controllable.