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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 '19 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 '19 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 '19 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 '19 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 '19 at 12:26
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Juan is correct, inflight reversing was a bit noisy; however, it was a MEL requirement to have the inboard reversers operational to operate above FL350. The procedure was as follows:

For a steep or emergency decent all throttles are retarded to idle and the inboard thrust brake levers are put into "Reverse Detent Thrust" and when the "Eng Thrust Brake" indicating lights come on the throttles are moved to the reverse power stop. The outboard engines will be at flight idle as reverse in engines 1 & 4 is not possible until the landing gear is extended. Additionally, the use of reverse thrust was to be discontinued below 190 knots.

Sorry Juan, no speed brakes on the DC-8, but yes on the DC-9. The spoilers on the DC-8 were used to augment flight control (three outboard on each side) but only when the gear was down. This made handling the aircraft in gusty weather much easier giving a quick roll response. On landing the spoilers(two inboard each side and three outboard) would deploy when armed by a signal from rear wheel spin-up or, as a back-up, by nose gear compression. The crew could also deploy the spoilers manually by pulling the spoiler lever back and up.

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  • $\begingroup$ Does this also apply to the variants with more modern CFM56 engines? $\endgroup$
    – trognanders
    Jul 10 '19 at 22:30
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    $\begingroup$ "Additionally, the use of reverse thrust was to be discontinued below 190K." - I'm guessing you mean 19k or FL190? I don't think DC-8s usually go up to 190k. $\endgroup$
    – Terran Swett
    Jul 10 '19 at 22:48
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    $\begingroup$ No, you guessed incorrectly! This is a DC-8 NOT a DC-3. The VNE (KIAS) at 10,000 ft is 352 Knots. MMO above FL29.3 is .88M. We normally tried to discontinue reversing on the ground below 80K; outboard engines first followed by the inboard engines to prevent compressor stall. $\endgroup$
    – Loren
    Jul 12 '19 at 14:19
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    $\begingroup$ @Loren, the problem is that 190K is a bit unclear and Terran clearly understood it as altitude. Do you mean speed 190 knots? $\endgroup$
    – Jan Hudec
    Jun 28 at 19:36
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    $\begingroup$ Knots is usually abbreviated as kt, not k. :) I will correct the mention of the speed brakes. $\endgroup$
    – Juan Jimenez
    Jul 6 at 5:09
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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 ground spoilers, but they were not meant to be used in flight. The one time they were used in flight, they caused one of the first fatal accidents on the type. The lack of spoilers/speedbrakes that could be used in flight 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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