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I've flown on a few commercial flights, and on several of them I recall a phase of the flight during descent when the engine seemed to go idle (or at least, the engine noise subsided substantially) and then all of the sudden, the engine got really loud and I could feel the plane slowing down substantially, as in, I felt the same as if I was in a car that was slowing down suddenly. This seemed to last between 5 and 15 seconds, or possibly slightly longer, and then the engines got more quiet and the noticeable deceleration stopped.

Living under an incoming flight path for a few years I have also noticed many times that a plane will be going overhead by the fact that suddenly the engine will get few loud for about the same amount of time.

This seems to be rather common, and I always assume this was reverse thrust. But today I am reading two separate questions: "Is it possible to use reverse thrust while airborne?" and "Why can't most jets safely use reverse thrust while airborne?", the answers to both of which seem to imply that using reverse thrust while airborne is very rare.

So what have I experienced / heard? I'm pretty sure it's not an airbrake, because far as I can tell a speed brake is a more passive device that doesn't involve the engine, and it would seem to defeat its purpose to spool up the engine while using it. I've never been in a seat where I could actually see the back of an engine to tell if a reverser was deployed, but I think at least in some cases the speed brakes were also deployed; from in front of the wing this made it impossible to see the back of the engine.

Update: Everyone is saying speed brakes... how long are these usually deployed for?

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    $\begingroup$ FWIW, on 747-100 and -200 aircraft it is theoretically impossible to use reverse thrust in flight if things are working as they should. There's air or ground logic that has to show the aircraft is on the ground before the reverse levers are unlocked. $\endgroup$ – Terry Jun 30 '15 at 20:20
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It is normal for engines to spool up during the approach. The initial part of the approach, from cruise level down to approximately 10 miles from the airport, is flown at flight idle power. This is the most efficient way to get down. In the final part of the approach, flaps and landing gear add so much drag that the engines need to be well above idle power.

At low speeds required for landing, the wing of the aircraft will not produce enough lift to sustain flight. To compensate for the low speed, flaps are extended. They do not only increase the lift, they also increase the drag. When the flaps are extended, suddenly the drag increases, resulting in a deceleration. Extending the landing gear has a similar effect.

Normally the flight crew will limit the time that they fly at low speed and high drag. The first couple of degrees of flaps deployment are relatively efficient;they do not add a lot of drag but allow the aircraft to fly slower. When the aircraft is about 7 to 10 nautical miles from the runway, the crew will transition the aircraft to landing configuration for the final approach. Often there is a horizontal segment before the aircraft starts a 3 degree glide path down to the runway. The crew will ensure that enough flaps (drag) is added before they start this final segment, otherwise they have difficulty reducing speed (they usually try to prevent use of speed brakes in that part of the flight). Often there is a sequence of flap and gear extension before the final descent segment starts. This causes the aircraft to decelerate from the efficient high speed configuration (flaps 2 or less on the B737) to the landing configuration. At the end of that configuration sequence the aircraft has a lot of drag and is decelerating rapidly towards the final approach speed. To stop the deceleration extra power is commanded which spools up the engines and stops the deceleration. To reduce the spool up time of the engines, the engine control system (FADEC) will give a short burst of power. This what you hear while still decelerating. When the engines are spooled up they start to produce thrust which is the end of your deceleration.

This youtube video shows how it happens. When at 28:20, the aircraft (Boeing 737) is flying horizontally, flaps 2, and is about to start decelerating and descending for landing. Keep a look at the thrust levers(right bottom corner of the screen) and the airspeed (on the left of the artificial horizon).

It begins at 28:28 when the captain ask for flaps 5, and reduces the thrust in order to reduce to final approach speed. 15 seconds later he asks for gear down and flaps 15. The speed is now reducing rapidly. Starting at 28:50 the thrust levers are moved forward, until they are reduced again at 29:08. This is the power boost you hear. In 40 seconds the aircraft is transformed from low drag / low thrust to landing configuration.

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    $\begingroup$ Excellent explanation, thanks. This sequence of steps is very helpful in understanding exactly what is going on when. $\endgroup$ – Michael Mar 3 '15 at 16:31
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The noise you are most likely hearing from inside the cabin is the speed brakes (also known as spoilers). In addition to drag, they create a lot of turbulence, which creates a lot of noise. After looking at some videos, it doesn't seem to come through on video as well. There does seem to be more noise at lower frequencies though. The FAA Airplane Flying Handbook notes:

There is usually a certain amount of noise and buffeting associated with the use of speed brakes...

When outside the aircraft, you may be hearing the engines spool up. During approach and landing, the pilots are extending flaps and landing gear. This and the maneuvering can cause the autothrottle to spool up the engines to compensate for added drag, or the pilots may be manually adjusting the throttle, to maintain or reach a certain speed. Speed brakes are generally used when the airplane is higher and faster on approach, and when heard from the ground, the plane is more likely to be lower and slower, using throttle to adjust speed.

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  • $\begingroup$ interesting... so the noise is loud enough to be heard from the ground? $\endgroup$ – Michael Mar 3 '15 at 3:14
  • $\begingroup$ Not sure about speed brakes... I'll edit that in. $\endgroup$ – fooot Mar 3 '15 at 3:17
  • $\begingroup$ ok thanks, so I may be hearing two separate things. The weird thing about hearing it from the ground is that it seems to only last for a few seconds... I would expect if it was engines spooling up they wouldn't spool back down so fast. But maybe, like you said, it's just doing some compensating. $\endgroup$ – Michael Mar 3 '15 at 3:52
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My guess would be that you're feeling accelerations more upwards rather than to the rear. It is common when flying a plane to need to adjust descent rate for various parts of the approach, either because different phases of the approach actually need different descent rates or to correct your glide path. When you need to decrease your rate of descent (or even level out) for a part of the approach, you do that by temporarily adding engine power. This will also usually be accompanied by pitching up a bit. I'm guessing that the forces you're feeling are the descent rate slowing down rather than the actual forward speed slowing down (though you might be doing some of that, too, if they pitch up enough.) I've experienced this commonly on commercial flights during approaches. We also make use of the engine to slow a descent/correct a glide path when flying light aircraft. Once the desired descent rate has been achieved, engine power will be reduced again, so this results in the few-to-several-second-long bursts of engine power that you're describing.

As fooot mentioned, when the gear and flaps are dropped, engine power is also added to compensate for the increased drag in order to maintain airspeed. However, this is not generally just a brief burst of power as in the above case of needing to arrest a descent rate.

As others have mentioned, there is some noise when spoilers (speed brakes) are being used to quickly slow down the aircraft in flight, but, at least in my experience, this sounds nothing even remotely like the engines spooling up. I would certainly not expect that you'd hear this from the ground.

It is, however, rather certain that you are not feeling thrust reversers. As far as I know, these are never intentionally deployed in flight on current commercial airliners (as UFO mentions, the DC-8 was actually certified for that, but I doubt you hear many DC-8s flying over these days.) There have been a few cases where malfunctions have caused them to deploy in flight, but I don't remember hearing about any cases of them being intentionally deployed in flight on current-model airliners.

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    $\begingroup$ Decent rate slowing feels different... that's more of a pushing down in the seat. $\endgroup$ – Michael Mar 3 '15 at 3:54
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    $\begingroup$ @Reirab -- DC-8 pilots use in-flight reversing both for normal and abnormal/emergency procedures, as the DC-8 lacks flight spoilers/speedbrakes. $\endgroup$ – UnrecognizedFallingObject Mar 3 '15 at 3:55
  • $\begingroup$ @Michael You'll feel some of both when this happens because of the changing pitch. $\endgroup$ – reirab Mar 3 '15 at 3:58
  • $\begingroup$ @UnrecognizedFallingObject Interesting! I've never flown on a DC-8, so I wasn't aware of that. I'm fairly certain that no current airliners do that, though. $\endgroup$ – reirab Mar 3 '15 at 4:04
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    $\begingroup$ @UnrecognizedFallingObject - as did Concorde $\endgroup$ – NobodySpecial Mar 4 '15 at 4:56
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Quite not possible because of the nature of reverse thrust (may not be as directional and controllable as required.) Can't say of future developments, but as of now, spoilers are the maximum possible "Airborne Braking" that can be deployed.

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  • $\begingroup$ Well, I'd say pitching up is an even more effective form of 'airborne braking.' :) Using spoilers along with it helps to prevent undesired altitude gain, though. $\endgroup$ – reirab Mar 3 '15 at 16:08

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