Originally Posted by
tdracer
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.
So on the C17, im fairly sure a similar probability loss model is used. If 2 of 3 interlock prox sensors indicate TR deployment in flight (or if 2 prox sensors fail in flight and loose resilience), the controller automatically commands flight idle on that engine. So its not impossible (nor difficult) to meet FAR/CS25 requirements in a modern aircraft design if this design were adopted. But again, structure would need to be beefed up (weight cost, complexity etc), and there is no role requirement for it, hence why its not a design feature.
If I were in a similar scenario in a commercial equivalent (Lauda scenario) the T-Handle would be the best place to start I suspect. But that would require very rapid actions and realization of what is happening pretty quickly.