Definition
An unintended escape of exhaust gases from the engine's exhaust system before they reach the tailpipe outlet, caused by cracks, loose connections, or failed gaskets in components such as the exhaust manifold, risers, muffler, or shroud. In aircraft using exhaust-heated cabin heaters, an exhaust leak inside the muffler shroud can allow carbon monoxide to enter the cabin air supply.
Plain English
Hot, toxic gases from the engine are escaping somewhere they shouldn't, instead of being carried out the back of the airplane.
Context Anchor
Seen during engine inspections, engine troubleshooting, and discussions of low manifold pressure in turbocharged airplanes.
Derivation
Exhaust comes from a Latin word meaning to draw out or empty. Leak comes from an old word meaning to drip or let something escape. Together, the term points to gases being drawn out of the engine but escaping where they should not.
Why Pilots Care
An exhaust leak reduces engine power output, distorts manifold pressure readings, and can introduce carbon monoxide into the cabin or create a fire hazard.
Intuition Check
Do not think of an exhaust leak as only a bad smell or a louder engine sound. In this context, it can directly affect engine power and manifold pressure, and it can create a serious cabin safety hazard.
Example Sentence 1
The mechanic found a small exhaust leak at the manifold flange and replaced the gasket before returning the airplane to service.
Example Sentence 2
After takeoff the manifold pressure dropped steadily until the mechanic found and repaired a cracked exhaust manifold joint.