How common is an airliner activating reverse while still in flight? Around 1977, I watched a DC-9 landing at Guacimeta airport, Lanzarote, Canary Islands, Spain, opening the flaps reversing thrust of both engines just over the airstrip end, but some 9 feet over the ground. Of course, it fell down that height as fast as a stone. Is this usual?
The system will allow deployment based on various criteria, and radio altitude height may be one of them, so even for a system not designed to operate "in flight", you still can deploy the TR's a few feet off the ground. Pretty ugly, rough touchdown when you do, though, and certainly not advised. How often that happens would be speculation, but probably very rare.