Chapter 24: The Exhaust System and Thrust Reversal

24.1 The Exhaust System

• The exhaust system directs the hot gases from the turbine rearward to form the high-velocity
• jet that produces thrust.
•  Components:
• Exhaust Cone: A cone behind the turbine that smooths the gas flow and prevents turbulence.
• Jet Pipe: The main duct carrying the gases to the nozzle.
• Propelling Nozzle: A convergent nozzle at the end of the jet pipe that accelerates the exhaust
• gases to their final exit velocity.
•  Choked Nozzle: In a pure turbo-jet, the exhaust gas velocity reaches the local speed of sound
• (Mach 1.0) at the nozzle exit at most power settings above idle. At this point, the nozzle is
• said to be choked, and the only way to further increase the exit velocity (and thus thrust) is to
• increase the temperature of the gas (e.g., by using afterburner/reheat).

24.2 Thrust Reversal

• Thrust reversers are used after landing to redirect the engine's thrust forward, providing a
• powerful braking force that reduces landing distance and saves wear on the wheel brakes.
•  Types:
• Clamshell or Bucket Doors: Used on pure jet or low by-pass engines, these doors swing into
• place to block the hot gas stream and deflect it forward through cascade vanes.
• Cold Stream (Blocker) Reverser: Used on high by-pass turbofan engines. Blocker doors
• are deployed inside the fan duct to block the cold by-pass air and redirect it forward through
• cascade vanes on the side of the nacelle. The hot core exhaust is not affected.
•  Safety: Thrust reversers have numerous safety interlocks. They can typically only be
• deployed when the throttles are at idle and when the aircraft's weight-on-wheels switches
• indicate it is on the ground.