Gearhead News

August 3, 2021
Watch This C-17 Engage Its Thrust Reverses In Mid-Air To Make An Extremely Rapid Descent

The ever-impressive C-17 Globemaster III military transport aircraft has recently been the focus of The War Zone’s attention when it comes to getting off the ground, in the form of an eye-catching short-field tactical departure. Now, thanks to the U.K. Royal Air Force (RAF), we’ve got the chance to see another of the big airlifter’s unique attributes — the ability to deploy the thrust reversers on its engines in-flight to provide a quick descent from its cruising altitude perch.

The C-17’s thrust reversers are normally used to deflect the airflow from the four Pratt & Whitney F117-PW-100 turbofan engines upward and forward when landing. By diverting the thrust in the opposite direction, it can reduce the distance required to come to a stop to around 3,000 feet or less, and provide access to smaller, austere airstrips.

Crown Copyright
An RAF C-17 performing a ‘touch-and-go’ at its RAF Brize Norton base.

On the ground, with the engines running in reverse idle, the thrust reversers also keep jet wash away from the rear of the aircraft, which is especially useful when loading or unloading via the ramp.

The Tweet below from 99 Squadron showcases the manuever:

In fact, so effective are the thrust reversers that they can even be used to propel the aircraft backward on the ground, which can be ideal for reversing backward on shorter airstrips, for example. They can even push a fully loaded Globemaster up a two-degree slope if required.

Another function of the thrust reversers is less well known, but thanks to a recent video from No. 99 Squadron, the RAF’s sole C-17 operator, we have a rare chance to see a “reverse idle tactical descent” during a seven-hour training sortie around the United Kingdom on July 29.

In this particular procedure, the thrust reversers are engaged in flight to enable a maximum-rate descent. As the caption to No. 99 Squadron’s video explains, the C-17 seen here drops from a cruising altitude of 30,000 feet to 5,000 feet in just two minutes, although this is “not even close to max performance for this beast!”

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Flightpath of RAF C-17 ZZ174 on its July 29 training sortie. Reverse idle descents were conducted in the Norfolk and Cotswolds/Worcestershire areas, shown here in blue.

As the squadron goes on to explain, once the thrust reversers are engaged in flight, the pilots ensure that the C-17’s speed is maintained by adjusting the aircraft’s attitude. That can be seen in the video, with the dramatic relationship between the horizon and the angle of the wing.

“The low nose attitude needed to maintain speed is part of what causes the massive rate of descent,” No. 99 Squadron explains, “Speed brakes are also used, for an even higher decent rate. Higher speed also means higher rates [of descent].”

The unit goes on to state that the in-flight thrust reversal is primarily used for “Getting on the deck as quickly as possible” and it is “unlikely to be used in an emergency due to the highly dynamic nature of it, but you never know.” 

One can imagine how the maneuver could be useful for dropping from normal cruise to low altitude for penetrating into or near contested airspace or even landing at an airfield in hostile territory without flying low over dangerous areas that a normal descent could mandate. 

As for the physiological effect on the crew, this is reportedly limited to vibration felt through the aircraft: “From the cockpit, the amount the wings and nacelles shake with all four [engines] in reverse is also quite incredible.” Otherwise, the automatic cabin pressurization system ensures this maneuver is relatively kind on the ears and body, and the forces imparted are somewhere between 0.0 and 2.0G.

The War Zone spoke to former RAF C-17 pilot Andy Netherwood about the reverse idle tactical descent, which he described as a “great party trick and fun to fly,” although noting that even without the thrust reversers engaged, the C-17 has a seriously impressive rate of descent.

“There is no risk of stall and it’s fairly simple to fly,” Netherwood explained. “Idle, slow down, configure, idle reverse then chase the flight director as it drops off the bottom of the head-up display! The amount of ground in the windscreen is more than transport pilots are used to seeing, but otherwise, it’s straightforward.”

While the maneuver could potentially be a vital tool to ensure the survival of the C-17 in a combat zone, it also speaks to the versatility of the airlifter, a hallmark that ensures its status as one of the most in-demand military transports among the air arms fortunate enough to operate it.

Contact the author: thomas@thedrive.com

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