10.5 Exhaust Systems, Thrust Reversers, and Temperature Patterns

Reciprocating Exhaust, Carbon Monoxide, and Turbo Energy

Exhaust systems remove combustion gases after useful energy is extracted. In reciprocating engines the system includes exhaust stacks/pipes, mufflers, heat exchangers (cabin-heat shrouds), turbocharger turbines, and tailpipes, and it must both contain hot gas and protect the cabin from carbon monoxide (CO). In turbine engines, exhaust ducts, the exhaust cone/tailpipe, mixers, and thrust reversers shape thrust, temperature distribution, noise, and structure. The ACS expects mechanics to understand normal function and the consequences of cracks, leakage, restriction, and faulty reverser operation.

Reciprocating exhaust leaks are never cosmetic. Hot gas cuts metal, burns hoses and wiring, and can ignite nearby fluid. Where the exhaust supplies cabin heat through a shroud/heat exchanger, a crack can route carbon monoxide into the cabin air - which is why muffler and heat-exchanger inspections are mandatory items and why a soot trail, burned area, unusual smell, or CO complaint is treated as leakage until inspection proves otherwise.

Turbocharged engines add a second concern: the turbine is driven by exhaust energy, so a leak upstream of the turbine reduces the energy available to spin it, causing low manifold pressure or slow boost response and overheating nearby components. A leak downstream of the turbine is less likely to affect boost but still creates heat and structural hazards. Location controls the symptom.

Turbine Exhaust, the Convergent Nozzle, and Reverser Types

Most subsonic turbojet and turbofan engines use a fixed convergent exhaust nozzle: the duct converges to a smaller area at the exit so the gas accelerates, converting pressure/heat energy into the high-velocity jet that produces thrust. Nozzle area is calibrated - a distorted or damaged nozzle changes back pressure and EGT and can shift the engine's operating line.

A damaged tailpipe, mixer, or reverser blocker door disturbs flow and lowers efficiency, so rising EGT for the same power may signal engine deterioration with exhaust-path damage contributing. Cracks grow from thermal cycling and vibration; discoloration, distortion, missing hardware, and rub marks are practical clues.

Thrust reversers redirect engine thrust forward to decelerate the aircraft after touchdown. The three common types are:

• Cascade (cold-stream) reverser - used on high-bypass turbofans. A translating cowl moves aft, blocker doors fold into the fan duct to block rearward bypass flow, and cascade vanes turn that cold fan air forward and outward. It reverses only the cold (bypass) stream, leaving the hot core flowing aft.
• Clamshell / pivoting-door reverser - pneumatically or hydraulically actuated doors pivot to close the normal exit and deflect gas forward.
• Bucket / target reverser - two clamshell "bucket" doors swing into the hot exhaust stream at the tailpipe and turn the full jet forward; used on lower-bypass engines.

The dangerous reverser faults are unintended deployment, failure to deploy, failure to stow, unlocked indications, and asymmetry. Troubleshooting verifies lock integrity, actuator travel, proximity/position switches, hydraulic or pneumatic supply, rigging, and agreement between commanded position, actual position, and cockpit indication.

Exhaust/reverser checklist:

1. Look for soot, burned structure, cracks, missing clamps, distortion, and heat-shield damage.
2. For reciprocating heat exchangers, treat any crack or heater complaint as a CO risk.
3. For turbocharged engines, locate leaks before or after the turbine to predict boost effects.
4. For turbine engines, compare EGT trend, thrust response, vibration, and visual damage.
5. For reversers, verify commanded vs. actual position, lock status, symmetry, and indication agreement.

The key exam word is often location: a leak before a turbo turbine changes boost; a leak in a heater shroud changes cabin safety; an unlocked reverser changes dispatch and ground safety.

Reciprocating exhaust inspection methods are tested directly. Slip-joints and ball-joints are checked for security and freedom; clamps, gaskets, and hangers are checked for cracks and looseness; and mufflers/heat exchangers receive special attention because internal baffles can shed and block flow while shroud cracks leak CO. A common field check is a pressure or differential test of the muffler/heat-exchanger combined with a carbon-monoxide detector in the cabin during ground run.

Exhaust gas leaks announce themselves with a gray or white powdery deposit and a fluttering or burned area at the leak. For turbine systems, EGT (or TGT/ITT) is the primary health trend, and the exhaust gas temperature gauge uses multiple thermocouples in parallel sensing the gas after the turbine; an open or shorted thermocouple, not engine deterioration, can cause an erroneous reading, so the indication system is verified before the engine is condemned.

Stainless and Inconel exhaust parts are inspected for cracks at welds and clamp areas where thermal cycling concentrates stress.