Definition
The initial period of operation of a newly overhauled or newly manufactured aircraft engine during which the engine is run under a controlled schedule of power settings to seat the piston rings against the cylinder walls and to confirm that all systems are functioning correctly before the engine is released for normal service.
Plain English
A break-in period for a fresh engine. After an engine is built or rebuilt, it is run on a test stand or in the aircraft using a planned series of power settings so the new internal parts can wear smoothly into place and any problems can be found before the engine flies.
Context Anchor
Seen in aircraft maintenance, engine overhaul instructions, cylinder replacement procedures, and maintenance log entries.
Derivation
From the everyday phrase 'run in,' meaning to operate a new piece of machinery gently at first so its parts settle together. The aviation use is the same idea applied formally to engines.
Why Pilots Care
A proper run-in prevents early engine failure, excessive oil consumption, and loss of compression that could occur if the engine is placed directly into flight service.
Intuition Check
A run-in does not mean accidentally hitting or colliding with something. In aircraft maintenance, it means deliberately operating an engine under controlled conditions after work has been done.
Example Sentence 1
After the top overhaul, the mechanic completed the engine run-in on the test stand before signing it off for return to service.
Example Sentence 2
The logbook entry noted that oil consumption stabilized by the end of the run-in, confirming the rings had seated correctly.