Reverse Thrust: An Essential Deceleration Mechanism

Function:

Reverse thrust is thrust projected in the opposite direction to provide an opposing, decelerating force on the plane’s direction of travel. It is used to decelerate an aircraft after landing, in the event of a rejected take off or, in some limited cases, during push-back from the gate. This system can be either hydraulically or electrically operated. It’s main function is to decelerate an aircraft during landing to enable shorter landing distances and to reduce wear and heat built-up within the wheel brakes. It’s also especially important when the runway is contaminated with rain or ice, reverse thrust can be crucial as brakes are much less effective. In addition, reverse thrust also helps in maintaining directional-control of the aircraft under these circumstances. However, certain aircraft such as the British Aerospace 146 (Bae-146), is not fitted with reverse thrust as its low take-off and landing speeds do not merit it.

Variations:

The workings of clamshell & cascade-type reverse thrust.
credit: 12charlie.com

There are three variations of reverse thrust used on modern aircraft today:

  • Clamshell (or bucket)
  • Cascade (or bypass flow)
  • Reverse Pitch Angle (on turboprops)

Clamshell reverse thrust is usually seen on turbojet engines where the entire stream of exhaust air is deflected forward with the help of hydraulically-actuated buckets when deployed. Cascade reverses are installed on turbojet engines: the bypass-air of the engine is blocked off and redirected forward with the help of blocking doors mounted within the engine nacelle. The reverse thrust mechanism used on commercial turboprops works using a hydro-mechanical system to change the blade angle of the propeller to direct thrust forward instead of backward.

The pitch of the turboprop’s propeller can be angled accordingly to accommodate the reverse thrust function.
credit: flightliteracy.com

Deployment & Control

The deployment and intensity of the reverse thrust is controlled by the engine’s throttle levers. On Boeing aircraft, secondary levers located on top of the main throttle levers are pulled back to deploy the reverses. Some Airbus aircraft can deploy the reverses by moving the main throttle levers backward from the normal “IDLE” position into the “REV” section on the engine control panel. On both aircraft, the power can be set anywhere between “IDLE” and “MAX REV”. The power is determined according to runway conditions and length of the runway.

Reverse thrust on this A320 can be selected by pushing the throttle levers backward into the dashed REV section on the engine control panel.
credit: pilotstories.net

The “Power-back” Procedure

In some cases, reverse thrust can be used to push-back an aircraft without the help of a tug. This is called a “power-back”. However, due to the increased fuel consumption and risk of airborne debris stirred up by the engines, this operation is not normally used during everyday airport procedures. Only under special circumstances, such as when a tug is not available (usually at smaller airports) or during special military procedures, is reverse thrust used to push back an aircraft. It’s important to remember that pilots have no “back-up camera” unlike automobiles, therefore special care is required when utilising this procedure.

The blade angle on the engine of this turboprop is changed so that thrust is redirected forward to push the aircraft back from the gate.
credit: youtube.com/buxtonaviation

So there you have it! Everything you’ve ever wanted to know about how the reverse thrust mechanism works on modern aircraft. Don’t be alarmed if you hear the engines rev up during landing! Controlling the speed of the aircraft 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.

Sources:

Cover Photo Credit: https://500px.com/photo/59783866/Rainy-Arrival-by-Marc-Riedel/

https://www.aopa.org/news-and-media/all-news/2017/march/pilot/turbine-reverse-thrust

http://www.12charlie.com/Chapter_15/Chap15Page021.htm

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