Originally Posted by
Bergerie1
...or not approved by the manufacturer and the CAA...
A bit of speculation and guesswork on my part but I think that you would need to show during testing that use of thrust reverse in flight is safe so that you can then incorporate this into the procedures and the type's performance data. This was obviously arranged in the case of the Trident and the DC-8. Perhaps they needed it for performance purposes. A quick glance at the procedures and limitations for the VC10 has not shown any text that specifically prohibits use in flight, but judging from the bit I copied and the rest of the manual, it would appear to be intended for use on ground only.