By John Hayes
No matter how fast you go, bringing everything to a safe stop is vital in an aircraft. That’s why many turbine aircraft have the capability of reversing thrust to provide extra stopping power. Reverse thrust not only saves wear and tear on brakes, it can significantly reduce landing distance under a variety of conditions. Understanding the operation, limitations, and potential hazards of reverse thrust is essential for all operators of thrust-reverser-equipped aircraft.
Propeller blade-angle changes
There are a couple of ways thrust can be attenuated or reversed, depending on the type of aircraft. In turboprop aircraft, the propeller blade angle can be varied all the way from an angle that is mostly in line with the aircraft direction of travel (called the feather position) to a pitch angle that reverses airflow (the beta position). Depending on what is needed, pitch is controlled through both the throttle and the propeller control lever.
In most turboprops, the propeller control lever works the same as it does in most piston airplanes, except that by pulling the lever all the way aft, past a safety stop, the propeller goes into the full feather position. The prop is normally feathered for shutdown, but this is most valuable for reducing propeller drag in the event of an engine failure. Feathering a prop in a twin, whether it be a piston or turbine, greatly reduces yaw when an engine quits, and adds significant glide range in a single.
The propeller pitch can be reversed using the power lever handle. Pulling the power lever to the idle stop, and then squeezing the lever’s trigger(s) or lifting them up and aft, past a gate, will put the prop into beta (or reverse) mode. Moving the throttle farther back increases the engine power, to increase the reverse thrust. It’s pretty intuitive. Since turboprop engines often do not run at power settings low enough to allow a normal taxi speed without using the brakes, beta mode helps to control taxi speeds. Beta operation produces the characteristic growl that accompanies most turboprops during taxi and after touchdown.
It is important to test reversers before flight to ensure proper operation. If the reversers were to malfunction and “unlock” after departure, the aircraft might become uncontrollable within seconds.Beta mode is only available for ground operations. Many single-engine turboprops have low propeller ground clearances, so it is vital to minimize beta thrust in contaminated areas to avoid engine and prop damage from dirt and debris. A few creative pilots have tried using beta thrust in flight to increase descent rates; however, some of those who have tried that trick wound up at the bottom of a smoking hole. The use of reverse thrust in flight is strictly prohibited in virtually every type of aircraft. That’s why most turboprop propeller controls have in-flight reverse-thrust lockout systems. Unless it’s approved for your aircraft, don’t even think about it.
Reversing jet thrust
In a jet aircraft, reverse thrust is produced by systems that deflect engine thrust forward. Most small jet engines have reversers that deploy split deflectors behind the engines to redirect the exhaust toward the front of the airplane. The reversers are deployed with small reverse-thrust levers that can be lifted past a gate when the throttles are in the idle position.
It is important to test reversers before flight (typically during taxi to the runway) to ensure proper operation. If the reversers were to malfunction and “unlock” after departure, the aircraft might become uncontrollable within seconds. A recent tragic Cessna Citation 550 accident that occurred in Venezuela after a reverser deployed shortly after departure emphasizes the importance of instinctively knowing the location of the emergency reverser-stow switches so that they can be immediately activated. This is a malfunction to practice in a simulator, and it requires fast action to survive this kind of emergency.
Staying on centerline
Wheel braking requires adequate friction between the wheel and the runway surface, whereas thrust reverse works well on contaminated runway surfaces when braking action is poor. Contaminated runways include those covered in standing water, slush, snow, or ice. In spite of the stopping power provided by reverse thrust, pilots should be aware that the likelihood of a runway excursion or overrun increases as runway friction decreases.
This is particularly true when the runway is slippery and there is any significant crosswind. In order to track the centerline on a slippery runway, an aircraft must touch down slightly crabbed into the wind, so that a sideways component of engine thrust counteracts any force from the crosswind. Applying reverse thrust in this situation reverses the force holding the aircraft on the centerline and can cause a rapid, unexpected runway excursion. The only way to prevent an excursion is to quickly reverse the crab angle as the thrust is reversed. This is a difficult maneuver that is best practiced in a simulator.
No matter how the aircraft is equipped, it is wise to divert to another airport when faced with any significant crosswind and the braking action is poor. Remember that wheel friction not only helps with stopping, it’s what keeps the aircraft on the runway in a crosswind—with or without reversers.
Controlling your speed during landing is what keeps things under control and when properly used, reverse thrust is a powerful tool that helps shorten runway requirements under a variety of conditions.
John Hayes is an optical engineer. He is an ATP, CFI, and type rated in Cessna C500 and C510 Citations.