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In a Microsoft Flight Simulator 2004 Boeing 747 landing tutorial, it was said to cut the reverse thrust after decelerating to 40 km/h.

What happens if that was ignored all the way to a complete stop? Would the plane start moving backwards?

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    $\begingroup$ You'd risk ingesting unwanted objects into the engines and in turn damge them $\endgroup$ – securitydude5 Jan 28 '18 at 18:50
  • $\begingroup$ What are odds of that happening on an international airport (Lets say London Hearthrow) ? $\endgroup$ – Delta Oscar Uniform Jan 28 '18 at 18:53
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    $\begingroup$ Answer already available in Can reverse-thrust be used to push back from the gate? $\endgroup$ – mins Jan 28 '18 at 20:38
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    $\begingroup$ @mins Aircraft is on the runway and decelerating, this question is a different situation. $\endgroup$ – Koyovis Jan 28 '18 at 22:00
  • $\begingroup$ @securitydude5 Well, in a flightsim, the risk is pretty much limited... ;-) $\endgroup$ – kebs Jan 29 '18 at 23:43
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The question speaks of a very old 747 landing tutorial. And as such I take that to mean it would be of a 747-100/200 aircraft, and that's the reference point I'm answering from.

First, concerning the comments below the question that wonder whether you have to worry about unwanted objects on a runway at an international airport, there is no such thing as a completely clean runway, even after sweepers have just finished clearing the runway. There is always some grit—it's a matter of degree—and while ingesting it isn't going to cause immediate problems, over time it will abrade the fan blades and thus the engine performance. This may not apply to higher, body-mounted engines rather than engines mounted below the wing, at least we didn't worry about it when I was flying 727s. If our taxi speed with engines idle was getting a little fast, we would sometimes deploy the reversers of the center, S-duct engine in the tail for the taxi.

Next, why take any risk if there's little benefit to be gained. The greatest effect of reverse power is immediately after touchdown when the aircraft speed is highest. As the speed slows, reverser effect decreases rapidly.

(...) cut the reverse thrust after decelerating to 40 km/h.

40 km/h is only 22 knots. That's a taxi speed. I spent 10 years on 747-100/200 aircraft at two different carriers, and we never used powered reverse anywhere near taxi speed.

The 747-100/200 reverse levers are on the forward edge of the thrust levers. You can't raise them unless the thrust levers are in the idle position and the air-ground sensors show you on the ground. When you do raise them, they first come to a near vertical position that deploys the reversers but leaves engine power at idle. If you pull the reverse levers aft to get greater than idle reverse thrust, you're doing so against a spring loaded force such that if you relax the pressure, the engines will return to idle.

What happens if that was ignored all the way to a complete stop? Would the plane start moving backwards?

Staying in reverse down to 0 km/h is simply not done. However, let's say you did it. If you had the reversers deployed but were at idle power, I doubt the aircraft would move backwards. Reverse thrust is not as efficient as forward thrust. Depending on the engines and how heavy you are, 747-100/200s can start to move forward at idle forward thrust, but the usual procedure is to come to "breakaway" thrust to get the aircraft moving and then go back to idle.

If you came to 0 km/h in reverse with significant engine power, then, yes, you would start to move backward. But, of course, the chief pilot would hear about it, and probably fire you.

I know of one instance where a 747 was backed up using reverse thrust to clear a runway in an extreme situation in a third world country, so it can be done.

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    $\begingroup$ "I know of one instance where a 747 was backed up using reverse thrust to clear a runway" Seeing how the typical 747 lacks rear view mirrors, that must have been an interesting experience for the pilots. Makes me wonder if they did something like actually open the side window of the cockpit and look out that way. $\endgroup$ – a CVn Jan 29 '18 at 8:46
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    $\begingroup$ Here are some exceptions noted, especially older bigger planes with rear mounted (or higher mounted) engines. But even than it's loud and burns much fuel. $\endgroup$ – Peter Jan 29 '18 at 16:57
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    $\begingroup$ @MichaelKjörling Just as a matter of information, the side windows of the cockpit in 747-100/200 aircraft can't be opened. My guess is that's true of all 747 models. In the instance I cited, they did have a problem in that one of the main gear trucks, probably a wing truck, reportedly went off the runway edge while they were backing. $\endgroup$ – Terry Jan 29 '18 at 19:17
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The big concern is that thrust reversers blast fan or exhaust air outwards and forward of the engine nacelle. If there is loose debris on the ground, or on unpaved airfields, this debris is kicked up into the air around the engines. At higher speeds, the engine intakes will remain clear of it, but at lower speeds, there is a possibility of the debris being ingested into the engines, causing damage. This is also done when landing turboprops with the power levers in reverse to prevent debris from being kicked up into the propeller discs.

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