Definition
A type of igniter plug used in turbine engines in which the spark is forced to arc across a small, recessed gap inside the igniter tip rather than across an exposed surface. The recessed geometry shields the electrodes from the direct combustion gas path, reducing erosion while still producing the high-energy spark needed to light the fuel-air mixture during engine start.
Plain English
A turbine engine spark plug whose sparking gap is tucked inside the tip, so the spark fires in a protected pocket instead of out in the open. This makes it last longer in the harsh environment of a jet engine combustor.
Context Anchor
Seen in turbine engine ignition system descriptions, maintenance manuals, and start or relight discussions.
Derivation
Constrained means held within limits or confined. Gap refers to the space the spark jumps across. So a constrained-gap igniter is one where the spark gap is deliberately confined inside the tip, rather than open to the combustion chamber like older surface-discharge designs.
Why Pilots Care
Ensures dependable engine starts, reducing the risk of failed ignitions during critical phases of flight.
Analogy
It is like making a match strike in a small protected slot instead of out in the open; the protected space helps the spark occur in the intended place.
Intuition Check
Do not read “constrained” as meaning weak or limited power. Here it means the spark path is physically controlled and kept in a specific place.
Example Sentence 1
The maintenance manual specified a constrained-gap igniter for the turbine engine, noting its longer service life in continuous-duty applications.
Example Sentence 2
During preflight checks the technician verified that the constrained-gap igniters produced a strong spark before approving the aircraft for flight.