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Mike O'Brien: Mike O'Brien is Director of Sales and Marketing, Dowty Aerospace, Los Angeles, USA
ACKNOWLEDGMENT: [copyright] Dowty Aerospace.
When an aircraft is making a landing approach in harsh weather and touches-down on a wet or slippery runway - passengers can take comfort in the fact that the aircraft is equipped with a system capable of stopping it, without full reliance on wheel brakes and tyre traction. This is referred to as the thrust reverser system and the above scenario is the primary reason for its existence, although it is commonly employed on routine landings.
The engine thrust reversers act as brakes by redirecting the engine exhaust air from an aft direction (forward thrust) to a forward direction (reverse thrust). This provides alternate emergency braking capability in event of an aborted or refused take-off. The thrust reverser therefore performs a critical function, as does the actuation system which powers it. Both must be reliable at all times, i.e. not only in the normal deploy/retract mode of operation but also to guarantee that the reverser will never deploy while the aircraft is in flight.
A little background information is in order to understand the various nuances of the reverser itself. The thrust reverser is a movable section of the engine nacelle which surrounds the jet engine core. It is typically categorised in one of two design types:
(1) the target door type; or
(2) the transcowl sleeve (cascade) type.
In the former, the reverser moves to block and divert both the engine core exhaust and fan by-pass airflow. The latter moves to block and divert only the by-pass airflow, which is the major thrust provider in high by-pass ratio engines. In both cases, the blocked airflow is re-routed by forcing it out and forward into the airstream through openings (cascades) in the side of the engine nacelle.
The actuation system to power these reversers must be tailored to the peculiarities of both these systems as they are quite different in design. Although pneumatic actuation is used in...