The Runway Magazine Jan/Feb 2017 | Page 12

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The clamshell door system is a pneumatically operated system . When the pilot activates the reverse thrust system , the clamshell doors rotate to uncover the ducts and close the inner fan airflow . Therefore , when the direction of flow is changed , the thrust created by the fans of the engine will be less . However , this will not affect normal engine operation . The thrust created by the primary airflow will be same . Since 80 % of the full thrust is generated by the fan airflow , we should only affect that airflow .

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The bucket target system is a hydraulically operated system that uses bucket type doors to reverse the hot gas stream ( exhaust airflow ). The thrust reverse doors are operated by a conventional hydraulic powered pushrod system . The actuator incorporates a mechanical lock in the extended position . In the forward thrust mode the bucket doors form the convergent-divergent final nozzle for the engine .

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The cold stream reverse system is controlled by an air motor . The output is converted into mechanical movement by a series of flexible drives , gearboxes and screw jacks . During normal operation , the reverse thrust cascade vanes are covered by the blocker doors . On selection of reverse thrust , the actuation system folds the blocker doors to blank off the cold stream final nozzle , thus diverting the airflow through the cascade vanes .
In passenger aircraft , reverse thrust is only used when on the ground . There are typically interlocks in the system that prevent the thrust reversers from deploying if the aircraft does not sense that it is on the ground . Once the aircraft touches down , the pilot will deploy the reverse thrust . But in military aircraft , thrust reversers can be used in the air . As it will enable them to perform very steep descents .

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