Thrust reversers help reduce an aircraft’s landing distance by redirecting engine thrust to oppose the aircraft’s forward motion after touchdown. By supplementing wheel brakes and other deceleration systems, thrust reversers improve stopping performance and reduce brake wear. Understanding the different thrust reverser designs and their operation is important for safe aircraft operation and effective maintenance.
As aircraft have increased in gross weights with higher landing airspeeds, the problem of stopping an aircraft after landing has greatly increased. In many instances, the aircraft brakes can no longer be relied upon solely to slow the aircraft within a reasonable distance immediately after touchdown. Most thrust reverser systems can be divided into two categories: mechanical-blockage and aerodynamic-blockage.
Mechanical-Blockage Thrust Reversers
Mechanical-blockage is accomplished by placing a removable obstruction in the exhaust gas stream, usually somewhat to the rear of the nozzle. The engine exhaust gases are mechanically blocked and diverted at a suitable angle in the reverse direction by an inverted cone, half-sphere, or clamshell. [Figure 1]
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| Figure 1. Engine exhaust gases are blocked and diverted in a reserve direction during thrust reversal |
The clamshell is placed in position to reverse the flow of exhaust gases. This type is generally used with ducted turbofan engines, where the fan and core flow mix in a common nozzle before exiting the engine. The clamshell-type or mechanical-blockage reverser operates to form a barrier in the path of escaping exhaust gases, which nullifies and reverses the forward thrust of the engine.
The reverser system must be able to withstand high temperatures, be mechanically strong, relatively lightweight, reliable, and “fail-safe.” When not in use, it must be streamlined into the configuration of the engine nacelle. When the reverser is not in use, the clamshell doors retract and nest neatly around the engine exhaust duct, usually forming the rear section of the engine nacelle.
Aerodynamic-Blockage Thrust Reversers
In the aerodynamic-blockage type of thrust reverser, used mainly with unducted turbofan engines, only fan air is used to slow the aircraft. A modern aerodynamic thrust reverser system consists of a translating cowl, blocker doors, and cascade vanes that redirect the fan airflow to slow the aircraft. [Figure 2]
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| Figure 2. Components of a thrust reverser system |
If the thrust levers are at the idle position and the aircraft has weight on the wheels, moving the thrust levers aft causes the translating cowl to open and the blocker doors to close. This action stops the fan airflow from going aft and redirects it through the cascade vanes, which direct the airflow forward to slow the aircraft.
Since the fan can produce approximately 80 percent of the engine’s thrust, the fan is the best source for reverse thrust. By returning the thrust levers (power levers) to the idle position, the blocker doors open and the translating cowl closes.
System Actuation and Maintenance Safeguards
A thrust reverser must not have any adverse effect on engine operation whether deployed or stowed. Generally, there is an indication in the flight deck with regard to the status of the reverser system.
The thrust reverser system consists of several components that move either the clamshell doors or the blocker doors and translating cowl. Actuating power is generally pneumatic or hydraulic and uses gearboxes, flexdrives, screwjacks, control valves, and air or hydraulic motors to deploy or stow the thrust reverser system.
The system remains locked in the stowed position until deployment is commanded from the flight deck. Since there are several moving parts, maintenance and inspection requirements are very important.
Quick Review: Thrust Reverser Systems
What is the mechanical difference between mechanical-blockage and aerodynamic-blockage thrust reversers?
- Mechanical-Blockage Reversers: Place a physical obstruction (like an inverted cone or clamshell doors) directly behind the exhaust nozzle. This blocks and diverts the combined core and fan exhaust gas streams.
- Aerodynamic-Blockage Reversers: Utilize a translating cowl, internal blocker doors, and cascade vanes. This design blocks only the cold fan airflow—leaving the core exhaust unaffected—and redirects it forward to decelerate the aircraft.