Definition
A turbine engine exhaust nozzle whose internal cross-sectional area is sized so that the gas flow reaches the speed of sound (Mach 1) at the nozzle's narrowest point during normal operation. Once flow is choked at that point, the mass flow through the nozzle is set by upstream conditions and cannot be increased by lowering the downstream pressure.
Plain English
A jet engine exhaust opening shaped so the hot gas leaving it hits the speed of sound at the tightest spot. Once that happens, the gas can't flow any faster through the nozzle no matter what's happening behind the aircraft.
Context Anchor
Seen in aircraft engine carburetor descriptions, especially when studying how fuel and air are mixed before entering a piston engine.
Derivation
‘Choke’ comes from the Old English ‘ceoke,’ meaning to block or restrict the throat. In fluid flow, a passage is ‘choked’ when it reaches a physical limit on how much can pass through it — like a throat that can't swallow any faster. That's exactly what happens here: the nozzle reaches its flow limit at the speed of sound.
Why Pilots Care
A worn or blocked choke nozzle can cause hard starting, rough idle, or overly rich mixtures that affect engine reliability on the ground.
Grounding Statement
At full power, the exhaust gas is moving so fast at the narrowest part of the nozzle that it physically cannot move any faster — it has hit the sound barrier inside the engine.
Intuition Check
Do not read choke nozzle as a part that simply shuts something off. In this term, choke refers to the narrowed part of the carburetor, and nozzle refers to the small fuel outlet located there.
Example Sentence 1
At takeoff power, the engine's exhaust reaches sonic velocity at the choke nozzle, fixing the mass flow leaving the turbine.
Example Sentence 2
A damaged choke nozzle allowed too much air into the mixture, resulting in repeated start failures.