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I play a flight-sim called X Plane. The other day while I was approaching an airport a little too fast and knew I couldn't slow down in time. I decided to use the reverse thrust while I was still in the air and this helped dramatically and I was able to land safely.
But now I'm curious - Would using reverse thrust in a real-life Boeing 747 have the same effect, would it cause the plane to crash, or go out of control?
Commercial jets are not designed to use reverse thrust in flight. With engines mounted under the wing, the turbulence can affect the lift over that section of wing. Tail mounted engines could interfere with the tail. This, in addition to the huge increase in drag, is what causes loss of control, as in the incidents that RedGrittyBrick mentions. Speed brakes are designed to provide the needed drag for emergency descents or otherwise slowing down faster. If the pilots find themselves too high/fast for an approach, and deploying spoilers/gear/flaps won't fix it, then they should go around for another approach.
The loss of control is more of a risk when a thrust reverser deploys only on one engine. Other risks are still there, since those thrust reversers are designed to deploy in landing conditions, not flight conditions. Notable exceptions listed on Wikipedia include the Hawker Siddeley Trident, though it also mentions that the capability was not often used.
Military aircraft, such as the C-17, can be different. They tend to make extremely steep descents more often (called a tactical descent/approach), so thrust reversers can be used in flight.
I did some testing with the stock 747-400 in X-Plane. Deploying the thrust reverser only changes the force applied by the engine, but doesn't seem to affect the air flow. So the loss of lift is not reflected by the model.
It may depend on the aircraft and situation but an unintended deployment on one engine lead to the in-flight break up of a 767
At approximately 15 minutes into the flight and at approximately 25,000 feet altitude, one flight crew member commented that the reverser had deployed. This comment was immediately followed by evidence of a rapid airplane attitude change and subsequent in-flight break up, leading to airplane wreckage falling into remote jungle terrain approximately 94 nautical miles from Bangkok.
The Pilatus PC-6 (a turboprop) can be put the prop into reverse pitch and descend nearly vertically. Here's a demo https://www.youtube.com/watch?v=PU3RecxHRtk
As Skip Miller Mentioned the DC-8 thrust reverse was designed for in-flight operation.
Skip Miller:
It was years ago, but I was on a DC-8 that had been re-engined with turbofans. On this flight, they used reverse thrust in flight to lose altitude quickly. The Captain came on the intercom first to warn the pax of an unusual noise and vibration before he did it, and then he used the reverse thrust. Yes, it was noticible and a bit noisey in the cabin. I am glad he gave the pax warning. Concerning the pax may be a reason why this is generally not done, even if the plane is capable of this maneuver.
DC-8 Manual ,American International Airways, page 29
Descent rates of less than 4000 ft/min are sufficient for all normal Operations. A descent rate of 1 000 ft/min is about maximum that Will allow a precise comfortable Level off. This should be closely monitored when within 2 000 Feet of the ground. Reverse thrust, if necessary, may be used in normal descent at speeds above 1 90K IAS. When this is done it is limited to inboard engines only.
Lauda Air Flight 004 (NG0049) overview:
