Thrust reversal, also called reverse thrust, is the temporary diversion of an aircraft engine’s thrust so that it acts against the forward travel of the aircraft, providing deceleration. Thrust reverser systems are featured on many jet aircraft to help slow down just after touch-down, reducing wear on the brakes and enabling shorter landing distances.
Some aircraft are able to safely use reverse thrust in flight, though the majority of these are propeller-driven. Many commercial aircraft, however, cannot. In-flight use of reverse thrust has several advantages. It allows for rapid deceleration, enabling quick changes of speed. It also prevents the speed build-up normally associated with steep dives, allowing for rapid loss of altitude, which can be especially useful in hostile environments such as combat zones, and when making steep approaches to land.
The C-17 Globemaster III cargo aircraft is one of the few modern aircraft that uses reverse thrust in flight. This airlifter is capable of in-flight deployment of reverse thrust on all four engines to facilitate steep tactical descents up to 15,000 ft/min (4,600 m/min) into combat environments (a descent rate of just over 170 mph, or 274 km/h).
But what happens if a C-17 pilot accidentally deploys the aircraft thrust reversers in flight?
‘I used to teach in C-17s. The thrust reversers are used in flight for steep fast descents, like 15–18,000 feet per minute, which is what the plane was built to do so they don’t have any complicated interlock to keep them from deploying in flight.
‘Aboard airliners the throttles you hold in flight aren’t the ones you use for reverse, you reach up and move different handles that have interlocks to keep them from deploying in the air. The C-17 just had a hump that you pull up and over.
‘So they’re easy to deploy. They still have electronic logic to keep the engines at reverse idle and not rev them up beyond that level.’
‘I was teaching a pilot air refueling. Not initial qualification, just recurring training.
‘We also practice this maneuver called a breakaway where we try to get away from the tanker as fast as possible. In a real emergency that involves hitting the boom disconnect switches (all of them) pulling the power to idle, extending the speed brakes and descending back and down from the tanker. It’s very aggressive and the slipstream separation throws the tanker around so on training we do all of that very slowly, steady, and methodically.
‘So as I recall we briefed it up and the tanker’s boomer called “breakaway” and the pilot flying got his sleeve caught around the #1 throttle somehow and pulled it into reverse with 2 through 4 in forward idle. It was a little surprising to him and as he tried to pull his arm away I think it just made him more confused and then he started to pull #2 into reverse but I’d already grabbed the throttles and his hand and put them back to forward and used some rudder to keep the plane level because it wanted to roll left with that engine directing thrust back and up. The loadmaster was downstairs and yells “What the hell are you doing up there” or something. Anyway it was funny and we debriefed it.’
‘Anyway, no, reverse thrust won’t make you stall IN flight as long as you get the nose down and use gravity to keep the speed. With all 4 engines in reverse we usually flew a deck angle of -20 degrees at .8 Mach to 320 knots down to as low as 2,000 AGL.’
Photo credit: Airman 1st Class Luke Milano / US Air Force and Alastair Bor via Wikipedia
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