used to slow the aircraft. A modern aerodynamic thrust  reverser system consist of a translating cowl, blocker doors,  and cascade vanes that redirect the fan airflow to slow the  aircraft.  [Figure 3-49]  If the thrust levers are at idle position  and the aircraft has weight on the wheels, moving the thrust  levers aft activates the translating cowl to open closing the  blocker doors. 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. 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 clam shell.  [Figure 3-48]  This 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 light in weight, 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.  In the aerodynamic blockage type of thrust reverser, used  mainly with unducted turbofan engines, only fan air is  A thrust reverser must not have any adverse effect on engine  operation either 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 clam shell doors  or the blocker door 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 systems. The  systems are locked in the stowed position until commanded to  deploy by the flight deck. Since there are several moving parts,  maintenance and inspection requirements are very important.  While performing any type of maintenance, the reverser  system must be mechanically locked out from deploying while  personnel are in the area of the reverser system. The terms afterburning and thrust augmentation generally  pertain to military engine applications. The terms are used to  describe the same system. Normally, this is used to increase  the thrust of the engine up to double the original thrust. The  required additions to the exhaust nozzle for this system are  a flame stabilizer, fuel manifold, flame holder, igniter, and a  variable area exhaust nozzle.  [Figure 3-50]  After the engine  has reached full power under normal operation, the power  lever can be advanced to activate the afterburner. This allows  more fuel to flow into the exhaust nozzle where it is ignited  and burned. As energy and mass are added to the gas flow,  the exhaust nozzle must open wider to allow greater flow.  As the power lever is moved back out of the afterburner, the  exhaust nozzle closes down again. Some low-bypass turbofan  engines used in military aircraft use bypass (fan air) to flow  into the exhaust nozzle. Just as in a ducted fan, this air is  used in the afterburner. It contains more oxygen and assists  combustion in the afterburner. Since fuel is being burned in  the exhaust nozzle, the heat buildup around the nozzle is a  problem. A special type of liner is used around the nozzle to  allow cooler air to circulate around the nozzle. This operates  Figure 3-48.  Engine exhaust gases are blocked and diverted in a  reserve direction during thrust reversal. Afterburning/Thrust Augmentation  3-29 Forward thrust Reverse thrust