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In reading this question (Would more throttle when using reverse thrust reduce stopping distance?), it prompted me to remember that on many of my flights (typically on 737s or CRJ type aircraft), it seems that thrust reversers are used for a few seconds after landing, but then are deactivated while the aircraft is still rolling at high speed. The pilot then utilizes the regular brakes to slow down to taxi speeds.

I am wondering why the pilot does not continue to use thrust reversers down to approximately taxi speeds, to reduce the amount of brake usage (and heating of the brakes) needed. My only guess would be to possibly avoid the reversers kicking up FOD back into the engine inlets at lower ground speeds, but not sure if that's the real reason or not.

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A minimum max reverse power speed is often an airplane operating limitation. It's mostly related to FOD (mostly sand grains and small gravel) and on some designs there may be compressor stall issues due to flow disruptions.

On the CRJ 700 max reverse is limited to 75kt although you can use up to 60% N1 down to zero speed. On a 900 you have to be at idle below 60kt.

In practice, you start to come off max reverse during the landing roll transition from high energy to low energy (on the RJs, there is a speed call at 90 kts for this). You come down to idle not too quickly and linger at idle for a second before stowing them; if you just slam the levers down to stow position, the reversers stow while the fan is still producing a lot of energy and you actually get a little kick of acceleration, which feels like you let off the brakes for a moment.

You can use idle reverse, which gives a little bit of braking, while taxiing if the alternative is riding the brakes and overheating them, say like a taxi along a downhill slope or along a slick icy surface where braking is marginal, and you don't want to taxi single engine, and in a pinch you may use reversers to try you save your butt if you start sliding off an icy taxiway with no braking control.

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  • $\begingroup$ If FOD is a concern at low speeds, how are aircraft able to do "power back" to reverse at airports without tugs? $\endgroup$
    – Skyler
    Commented May 7, 2019 at 20:15
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    $\begingroup$ You'll see that with turboprops more than fans, but yes you can do it if allowed in the aircraft's limitations and if the operator allows it. The CRJ's limitations do not allow backing up with reversers at all. If you are allowed to, the FOD risk is very high though especially with wing mounted turbofans. Ramp areas are frequently littered with zipper tabs from luggage, easily sucked up. The perfect object for causing thousands of dollars of damage to a compressor. A competent captain of something like an EMB190, even if authorized, will use reverse to back up only as an absolute last resort $\endgroup$
    – John K
    Commented May 7, 2019 at 20:24
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    $\begingroup$ Most of them aren't able to, or at least aren't allowed to. Note that aircraft that used to do it in the past, like the 727, MD-80 or DC-9, all have fuselage-mounted engines that sit much higher than today's underwing-mounted high-bypass turbofans. $\endgroup$
    – Jörg W Mittag
    Commented May 7, 2019 at 20:41
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    $\begingroup$ @JörgWMittag EVen on tail mounted engines it can have consequences. We once spent a couple of years trying to figure out why one particular airline was wearing out the spline joints between sections of a rotor far faster than anybody else with the same engine and aircraft type. It turned out they were using full power reverse thrust to power back from the ramp and then full power forward instead of the brakes when they reached the taxiway! Memo to pilots: Don't drive a Fokker F100 plane the same way as would drive an F100 race car ... $\endgroup$
    – alephzero
    Commented May 7, 2019 at 21:18
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    $\begingroup$ @John K, my company operates quite a large fleet of CRJs, and while our SOP prohibits it, the aircraft manuals from Bombardier not only explicitly approve backing up with the reversers, there is an AFM supplement detailing how to do it, in all the variants we operate (200,700,900) $\endgroup$
    – nexus_2006
    Commented May 7, 2019 at 22:27
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To avoid kicking up rocks and debris for potential damage and ingestion, along with hot gas ingestion. Also not needed. Kinetic energy is proportional to velocity squared, so once you've knocked off the high velocities, most of the energy is gone. Brakes are fine from there.

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I believe its to prevent sucking in hot air from the reverser back into the the inlet. For example for the JA-37 that is the reason why you are not allowed to thrust up very much at low speeds with reverser on.

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Thrust reversers are very effective at high speed and have poor retardation at lower speeds. Using the brakes in the landing roll gives them about a tenth or less of the wear that they get taxing for take-off, and greatly reduces the wear they experience taxying to the terminal. Paradoxically, using the brakes in landing reduces brake wear compared to not using it at all in the landing rollout. So the reversers increase your safety margin, which is always a good thing, whilst using the brakes as well reduces brake wear. There is almost no increase in brake wear if brakes (with or without spoilers/lift dumpers) are used in the landing roll without reversers being deployed. It's debatable whether or not reversers are worth the trouble, but they sure can help when things go wrong. But as discussed, they also impose their own risks and costs. Fortunately, they are MELable, and dependant on the airline, locking pins may be carried for pilot use in locking them out if need be for the MEL.

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