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For most jets, the use of reverse thrust to slow down in flight, where even possible, is extremely dangerous, and strictly prohibited.

Not so on the DC-8, which is certified for the safe in-flight use of reverse thrust on the two inboard engines throughout its entire flight envelope, and which, in fact, uses in-flight reversing as a standard part of a normal descent - this despite the severe disruption in-flight reversing causes to the airflow over the wing.

What is it about the DC-8 that makes it capable of safely using reverse thrust in flight, unlike other jets?

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    $\begingroup$ The IL-62 can do that, too; I think NASA modified a 727 for the Shuttle training to do the same. Trident allegedly could do it, as well, and on the military side, I believe the C-17 can use reverse in flight, too. There might be more! $\endgroup$ – Cpt Reynolds Jan 9 at 21:48
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    $\begingroup$ @CptReynolds -- not sure about T-tail/rear-engine designs, but my understanding is the C-17 is able to deploy its reversers in flight, something used to allow for steep tactical penetration descents. Maybe someone with a C-17 flight manual handy could illuminate us sometime? $\endgroup$ – UnrecognizedFallingObject Jan 10 at 4:03
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    $\begingroup$ All that is needed to allow this is a strengthened wing structure that will safely absorb the forces created by using thrust reversers in flight. Most aircraft don't need that because they have effective speed brakes. $\endgroup$ – Juan Jimenez Jan 10 at 9:08
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    $\begingroup$ Concorde also used reverse thrust on 2 engines when performing an emergency descent due to pressurization failure to get to save altitudes fast enough. $\endgroup$ – Bianfable Jan 10 at 10:22
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    $\begingroup$ No kidding, losing cabin pressure at 50-60k feet would definitely be a hair-raising event. I wonder if it ever happened on a commercial flight. $\endgroup$ – Juan Jimenez Jan 10 at 12:26
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It is still considered somewhat dangerous to deploy the thrust reversers on a DC-8, but because the pilot can only do so on the inboard engines, any extreme vibration or buffeting is transmitted closer to the wing root, which is safer than doing so on the outboard engines. Farther outboard the force of the buffeting might damage the wing's mechanisms or structure. Still, it was rarely used because even when the pilots warned the passengers, it still scared the daylights out of them.

The DC-8 also has speed brakes, but they were considered very ineffective. This made it very difficult to do steep approaches without excessively increasing airspeed, hence the ability to deploy the inboard thrust reversers in flight to compensate.

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