Definition
A temperature-sensing probe in which the thermocouple junction is enclosed in a slender, sheathed tube that is inserted into a well or fitting on the engine or component being measured. The probe is held in place by a spring-loaded twist-and-lock collar, similar to a bayonet-style mount, allowing quick installation and removal without tools. It is commonly used to measure cylinder head temperature (CHT) and exhaust gas temperature (EGT) on piston aircraft engines.
Plain English
A pencil-shaped temperature sensor that slides into a hole on the engine and locks in place with a quick twist. It tells the engine instruments how hot a particular spot is.
Context Anchor
Seen in aircraft engine instrument systems, maintenance descriptions, and troubleshooting of cylinder head temperature indications.
Derivation
‘Bayonet’ comes from the French town of Bayonne, where bayonet blades were originally made. Engineers borrowed the name for any fitting that locks with a short push-and-twist motion, like a bayonet clipping onto a rifle. ‘Thermocouple’ comes from Greek thermos (heat) and ‘couple,’ meaning a pair — because the device works by joining two different metals that produce a small voltage when heated.
Why Pilots Care
Accurate engine temperature readings help prevent overheating, detect problems early, and keep the engine operating safely within limits.
Analogy
It fastens much like a push-and-turn light bulb: push it into place, twist it, and the fitting locks so it cannot simply pull straight out.
Intuition Check
Bayonet does not mean a blade here. It means a push-and-turn locking style used to hold the temperature probe in place.
Example Sentence 1
During the annual inspection, the mechanic replaced the bayonet thermocouple probe on the number three cylinder after it began showing erratic CHT readings.
Example Sentence 2
During the annual inspection the mechanic cleaned the contacts on the bayonet thermocouple probe to restore steady temperature readings.