Definition
An unintended opening in the engine's induction system that allows outside air to enter the intake downstream of the carburetor or fuel injection metering point, bypassing the normal air measurement and fuel-air mixing process. The extra unmetered air leans the fuel-air mixture beyond what the pilot has set with the mixture control, causing rough running, power loss, high cylinder head temperatures, and abnormal engine instrument indications.
Plain English
A crack, loose clamp, or bad seal somewhere in the engine's air intake plumbing that lets extra air sneak in after the point where fuel and air are supposed to be measured and mixed. The engine ends up with more air than the pilot intended, so the mixture runs too lean.
Context Anchor
Seen when checking abnormal engine indications, especially rough running, unusual idle behavior, or signs that the engine is not receiving the correct air-fuel balance.
Derivation
Induction comes from the Latin inducere, meaning to lead in. The induction system is the plumbing that leads air into the engine. A leak in that system lets air in where it shouldn't be.
Why Pilots Care
Left uncorrected it can cause detonation, engine damage, or sudden power loss during critical phases of flight.
Analogy
It is like drinking through a cracked straw: extra air gets in through the crack, so the flow is no longer what you expect.
Grounding Statement
Picture the engine trying to breathe through a controlled air path, while an unwanted gap lets extra air sneak in after that control point.
Intuition Check
Do not think of an induction air leak as a fuel leak. The problem is unwanted air entering the engine’s intake path, which can still cause serious engine-running problems.
Example Sentence 1
After noticing one cylinder running noticeably hotter than the others, the mechanic traced the problem to an induction air leak at a cracked intake tube.
Example Sentence 2
After landing, maintenance found the induction air leak by pressurizing the intake and listening for escaping air near a cracked hose.