Can commercial airliners theoretically taxi backwards using reverse thrust?
If this is possible, why isn't it common?
I can already imagine some safety reasons...
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asked Aug 11, 2014 at 8:03
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2$\begingroup$ Very common on turboprops as the prop angle can be changed to produce reverse thrust a lot more efficiently than a jet thrust reverser! $\endgroup$– Brian KnoblauchAug 11, 2014 at 19:14
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1$\begingroup$ I have seen it done with DC-9s many years ago. $\endgroup$– CimbianAug 12, 2014 at 18:04
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1$\begingroup$ @Cimbian Yes, this used to actually be very common with DC-9 class aircraft. I saw DC-9s to powerbacks all the time in the 90s. With fuel prices nowadays, though, it's no surprise that they almost universally opt for tugs instead now. $\endgroup$– reirabDec 9, 2014 at 15:14
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5$\begingroup$ I've powered back King Airs, Metroliners, and 727s without difficulty when tugs were not quickly available. An acquaintance powered back a 747-200 when a tug couldn't be maneuvered into a position that it could do it. His is the only incidence of a 747 being powered back that I'm personally aware of. $\endgroup$– TerryFeb 23, 2015 at 21:10
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3 Answers
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This is called Powerback, most aircraft can do it, but it is not done very often.
In a jet aircraft, the three main problems are:
- Reverse thrust tends to throw a lot of debris into the air because the exhaust is deflected to the sides and up and down too. This debris can damage the engine itself, other things on and around the aircraft or injure someone on the ramp. It is less of a problem for aircraft with tail-mounted engines (so DC-9s often used powerback), but that engine configuration is not used as much any more in transport aircraft, as it is less aerodynamically efficient.
- A related problem is that the compressor might suck in its own hot exhaust gases. This might lead to temperature-induced damages in the last (high-pressure) stages of the compressor and health issues for passengers, since the air conditioning system works with bleed air from the compressor.
- Reverse thrust is rather inefficient on jet engines, so it uses a lot of fuel.
- As Casey mentioned in the comment below, the pilot has to be careful using the brakes during powerback: the main wheels are very close behind the centre of gravity, so harder braking can lift the nose wheel, causing the pilot to lose control and even causing damage to the tail if it hits the ground (“tail-tipping”).
So a tug is both cheaper and safer.
In propeller aircraft, reverse is more efficient and does not throw up as much debris, so it is sometimes used. But a tug is still preferred because transport aircraft don't have any rear visibility, so the pilots can't see where they are taxiing. With tug the driver can see behind the aircraft, and a ground marshaller walks along side each wingtip with an intercom connected to the plane.
Lastly, since aircraft are able to turn almost on the spot using differential braking and thrust and 180-degree nosewheel steering, at airports with few facilities there is generally enough space around the aircraft to permit the pilot to reverse out easily.
answered Aug 11, 2014 at 8:26
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1$\begingroup$ I didn't think about the fuel consumption. Nice answer, thanks! $\endgroup$ Aug 11, 2014 at 11:21
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3$\begingroup$ @AndréStannek wheels are aft of CG, so braking will create a nose-up torque and sit the airplane on its tail. Standard procedure during powerbacks is "feet on the floor" to avoid inadvertent brake application. $\endgroup$– caseyAug 12, 2014 at 12:00
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1$\begingroup$ Isn't the prevention of compressor stall also another reason that powerbacks aren't used in most aircraft? This is also one of the reasons that reverse thrust has to be cancelled relatively early in the landing rollout, too, right (along with FOD prevention?) $\endgroup$– reirabDec 9, 2014 at 15:24
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1$\begingroup$ @reirab: I don't know. I always only heard the FOD argument, but I don't know enough to claim it is not true. $\endgroup$ Dec 9, 2014 at 20:40
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$\begingroup$ Powerbacks can also create projectile hazards on the ramp... $\endgroup$ Mar 21, 2015 at 1:36
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It can be done, in fact the DC-9 and MD-80 aircraft are approved for backing up using reverse thrust. It is called "powerback".
It is rarely used since it is quite fuel consuming, noisy and increases the risk of sucking up debris near the gate area causing damage to the engines.
Here's a video of an MD-80 backing up.
answered Aug 11, 2014 at 8:22
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1$\begingroup$ Those reversers deploying look like something out of Transformers! $\endgroup$ Aug 11, 2014 at 10:35
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$\begingroup$ @DavidRicherby would you believe that it's only 2 links and an actuator to get that movement? $\endgroup$ Aug 11, 2014 at 10:42
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$\begingroup$ Wow, seeing a plane back up be itself just seems wrong :-D Thanks! $\endgroup$ Aug 11, 2014 at 11:18
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2$\begingroup$ @SkipMiller It is a trade-off between fuel consumption (and perhaps maintenance costs) for powerback, or labor costs and equipment costs for pushback. The 30-40 seconds may not be much compared to the total fuel costs but it may still be more expensive than a lowly paid pushback truck driver for 5 minutes. It is a beancounters decision in the end. $\endgroup$– DeltaLima ♦Aug 12, 2014 at 15:48
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Can commercial airliners theoretically taxi backwards using reverse thrust? - After an airshow at RNAS Yeovilton, Somerset in the 1980s a British Airways Concorde found it could not taxi out for departure as it had been parked too close to an adjacent hangar (and didn't have the required turning circle)... there was no suitable towing gear on site, or within easy reach... so after a discussion with his engineers the pilot decided to reverse taxi a short distance using reverse thrust.... the centre of gravity was adjusted by transfering fuel to the forward fuel tanks, fingers were crossed, and... it worked! (I was there to see it, working in the control tower)
