Reverse thrust in air?

They won't. These aircraft need weight on the main gear before the reversers will deploy. I do know that the Il-62 could reverse before touchdown to reduce it's landing roll. I also know this aircraft is banned in most countries. Many turboprops can go into reverse in mid air, but it doesn't do as much as simply discing the prop at low pitch due to the large amount of cavitation. There was a very bad accident not too far from where I type this where a 737-200 landed on a runway with a snowplow still on it. The aircraft was on the ground and reversers were already deployed when they decided to go around. Because the landing gear and flaps were selected up immediately, they took pressure from the reversers so only one stowed. The aircraft spiraled and crashed next to the runway. http://en.wikipedia.org/wiki/Pacific_Western_Airlines_Flight_314

Even if you could do it, not too bright to try. I am wondering about MrAUDU's remarks about spoilers though. I use spoilers on my sailplane when I am airborne. I have seen spoilers in use on a 727 to lose altitude rapidly while maintaining the level flight attitude of the aircraft (not dropping the nose to spook the passengers). Maybe they don't do that anymore. I have not been on a commercial airliner for a long time. They treat people like cattle, so I don't fly anywhere I can drive. Spoilers are used to destroy lift, to keep an airframe from lifting off again after it has touched down. There was a bad accident related to this in Toronto back about 1970, I don't recall the exact year, but I recall the accident. I think it was a 707. If I remember the details correctly, the co-pilot pulled the spoilers while the aircraft was still airborne, about 50 ft off the runway. The wings lost lift and it rammed the gear up through the wings. The pilot tried to go around, and they didn't make it. Maybe you could remark on that in an edit, MrAUDU. I would be interested to hear your opinion.

The C-17 can use reverse in flight. It's called a tactical decent. Watch the levers at about 1:32. http://www.youtube.com/watch?v=lUUU-C-7o98&NR=1

You can't do that. Thrust reversers and spoilers only operate when the weight of the plane is on the gear.

Unless there is a mechanical linkage preventing it the reverse thrusters COULD always be deployed because electrical items can and do fail more often than mechanical ones. That is not to say that a mechanical linkage can't fail it is just less likely. What would happen? the plane would loose forward speed quite rapidly, loose lift and loose altitude VERY quickly. If only one deployed then bad things would happen quite rapidly.

While people will tell you about the weight on wheel switch, it's not true of all airplanes. DC-8s and Concorde used it routinely. 707s 727s and early 737s can definetly make it happen, though the results might be less than spectacular. Not sure about "electronic" airplanes.

Other dudes are right, safety weight switch will prevent such a thing to happen. But it's not difficult to figure out what would happen. Drive your car down the highway and jam it in reverse. You'll get your answer pretty damn quick.

if they did, which they are designed to disable in the air, the planes would loose significant airspeed, airplane may start vibrating, if not disintegrate (unless use minimal reverse) and probably start descending.

You can't, only airplane that could use the buckets airborn was the DC-8 and then only on 2 and 3 (the inboards).