Definition
The highest exhaust gas temperature reached as the fuel/air mixture is leaned. As the mixture is leaned from rich toward lean, EGT rises, reaches a maximum value (peak EGT), and then begins to fall as the mixture becomes too lean to burn efficiently. Peak EGT corresponds to the mixture setting where combustion is most complete, and is used as a reference point for setting cruise mixture either at peak, rich of peak, or lean of peak according to the manufacturer's instructions.
Plain English
The hottest the engine's exhaust gets as you slowly lean the fuel mixture. Once you find that hottest point, you use it as a reference to set the mixture for cruise.
Context Anchor
Seen when leaning the mixture in cruise and when using cruise performance charts that state engine settings based on peak EGT.
Derivation
Peak' comes from the idea of the highest point on a curve or mountain. As the mixture is leaned, exhaust temperature climbs, reaches a high point, then falls — the high point is the 'peak.'
Why Pilots Care
It marks the reference point for leaning decisions that affect fuel efficiency, power, and engine longevity.
Grounding Statement
In steady cruise, picture slowly reducing fuel: the exhaust temperature climbs, reaches its top value, then starts down.
Intuition Check
Peak does not mean best or safest by itself. It only means the highest EGT reading reached during leaning; the correct mixture setting depends on the aircraft handbook and engine guidance.
Example Sentence 1
After reaching cruise altitude, the pilot slowly leaned the mixture, noted peak EGT, and then enriched until the gauge showed 50 degrees rich of peak.
Example Sentence 2
They then enriched the mixture 50 degrees past peak EGT for cooler cruise operation.