Definition
A controlled period of operating a newly overhauled or rebuilt engine under prescribed power settings and durations to seat the piston rings against the cylinder walls, stabilize internal clearances, and verify that the engine performs within specification before release to service.
Plain English
Run in is the break-in period for a fresh or overhauled engine. The engine is run on a test stand or in the aircraft at specific power settings for set lengths of time so its internal parts wear together properly and any problems show up before the engine is put into normal service.
Context Anchor
Seen in aircraft engine maintenance, especially after an engine has been rebuilt, repaired, or had major parts replaced.
Derivation
From the everyday phrase 'to run something in,' meaning to break in a new piece of machinery by using it gently at first. The aviation use keeps that exact sense but adds a defined procedure with specific power settings and time intervals.
Why Pilots Care
A proper run-in prevents early engine damage, reduces oil consumption, and ensures reliable performance after maintenance.
Analogy
Similar to driving a new car gently for the first few hundred miles so the engine parts fit together smoothly.
Intuition Check
Run in does not mean accidentally hitting something or simply starting the engine once. In maintenance, it means operating the engine for a controlled period after work has been done.
Example Sentence 1
After the major overhaul, the shop performed a four-hour run in on the test stand before releasing the engine.
Example Sentence 2
During the run in, power was increased gradually while monitoring cylinder head temperatures and oil pressure.