Where are the thrust reversers

There are several reasons reversers don’t make sense for the light jets. But all manufacturers share concern over several issues.

Perhaps the most important reasons not to install reversers on a light jet are weight and cost. The structure and operating mechanism of a reverser is heavy, and all light jets struggle with a restricted weight budget.

Think about it. To capture a large portion of the takeoff thrust of a jet engine, and turn it nearly 180 degrees is no simple task. And all the weight of the necessary mechanism, which is eating into invaluable cabin payload, goes far aft on the airplane, which already is typically at or near its aft CG limit when empty.

While the actual operation of a target type reverser—the kind with the buckets that swing out from the nacelle to block and reverse the engine exhaust—may not be complex, certification requirements are. That’s because a manufacturer must demonstrate that it is very unlikely a reverser will deploy in flight. And if that happens, it must be demonstrated the airplane is controllable until the unexpected reverse situation is resolved.

Some airplanes have a so-called “throttle snatcher” that is a mechanical link from the reverser mechanism to the throttle cables. If a reverser deploys in flight the movement of the reverser mechanism also grabs the throttle cable and physically snatches it to idle. It’s still a hairy situation for the crew, but idle reverse in flight is one heck of a lot better than takeoff power in reverse.

The universal implementation of full-authority digital engine controls (FADEC) in current light jet production has made the inadvertent reverser deployment more manageable because the computers that control engine power can cut power to idle without mechanical intervention. Still, demonstrating that a test pilot can maintain control during a reverse in flight is no simple task during certification.

Aggressive reversing on an icy runway may complicate control, especially in windy conditions.Another important reason light jets don’t have thrust reverse is that it really would do little to improve runway performance.

In general, the rules don’t give credit for stopping distance in a normal landing, or for distance required to stop in a high-speed takeoff abort. So, the weight and cost of reversers won’t give the airplane a flight manual advantage in required runway distance.

The other reason reversers wouldn’t do much for light jet runway performance is the low takeoff and approach speeds of the jets, and their relatively small mass. It’s the old mass and velocity thing that yields the energy that must be absorbed to stop. A light jet operating at comparably low runway speeds just doesn’t have the same amount of energy to get rid of when stopping as the larger jets do.

Approach and landing airspeeds in light jets are right around 100 knots, or even less. It takes at least a few seconds for thrust reversers to deploy and power to come up after the airplane is firmly on the runway. In those seconds the brakes will have slowed the landing jet considerably. And to avoid ingesting any debris thrown forward by the reverse thrust into the engines the pilot must have the power at idle reverse by a speed typically of 60 knots or so. That means the actual period of effective thrust reverse during the stopping phase will be close to nil.

Thrust reversers also create a challenge for engine nacelle designers. The aft end of the nacelle where the exhaust exits is called a nozzle, and its size and shape are critical to maximizing thrust and managing noise. The structure necessary for a reverser limits the options a designer has for the nozzle so there is probably at least some small performance loss compared to the optimized nacelle without reverser buckets. Another cost, no matter how you slice it.

The Lear Jet, the original light jet, did have thrust reversers. It also had a drag chute like early jet fighters. That was because it needed them.

The wing technology of the Lear was so rudimentary compared to today’s advanced airfoil design that takeoff and landing speeds were high. Reversers had time to contribute to stopping after the high-speed touchdown, and the chute was there as a last resort.

In addition to advanced wing design reducing runway speeds, some current light jets have effective ground spoiler systems that plant the airplane firmly after touchdown to add drag and improve braking effectiveness. Some also use a “lift dump” system that drives the wing flaps to extreme angles to add drag and kill lift for the same braking advantages.

Another contribution to runway performance has been FADEC, which can reduce residual thrust compared to the previous all-mechanical controls. Flight idle in a jet engine is restricted by the time it takes to advance from idle to full power. If the engine speeds up too slowly then the flight idle stop must be set higher. That means there’s more unwanted thrust after touchdown.

Cessna addressed this issue in the first few models of CJ with “thrust attenuators,” a paddle that moves into the exhaust stream to deflect it sideways thus reducing thrust. FADEC has the ability to lower flight idle, and in some cases quickly switch to ground idle after touchdown, so the attenuators went away.

It’s possible reverse could be helpful during slippery conditions. But aggressive reversing on an icy runway may complicate control, especially in windy conditions. Once you start sliding all bets are off, even with reverse.

The one situation where reverse can be very handy is during light gross weight taxiing. Deploying reverse, even on one engine, can avoid the need to ride the brakes and heat them up before departure while keeping taxi speed under control.

But, you say, turboprops have reverse thrust capability and most turboprops weigh less than the light jets and cost less. True. But reversing the angle of the propeller blades adds comparatively little cost and weight because the prop blades already must articulate from normal angles to feather. And there is no performance penalty because the prop efficiency is unaffected by its ability to transition to reverse. Not quite a free lunch, but a lot closer to free than reversing jet engine thrust.

While I understand the reasons thrust reverse doesn’t make sense on light jets, I do miss having those piggyback levers to pull upon touchdown. And the resulting noise and vibration when braking. In fact, I made it a point to mention to our regular passengers before the first few flights of my company’s new light jet not to worry when they didn’t hear the engine power come up, and feel the shaking as we stop, as they are accustomed to in other jets. I’m sure they noticed at first, but now nobody misses that extra racket.

J. Mac McClellan is a corporate pilot with more than 12,000 hours, and a retired aviation magazine editor living in Grand Haven, Michigan.