Definition
A maintenance test performed on a turbine engine after shutdown, in which the time taken for the rotating compressor and turbine assembly to slow to a stop is measured. An unusually short coast-down time indicates internal friction or rubbing, often pointing to bearing wear, seal damage, or other mechanical problems inside the engine.
Plain English
After shutting down a turbine engine, mechanics time how long it takes for the spinning parts inside to come to a stop. If they stop too quickly, something is rubbing or dragging inside the engine, and it needs to be looked at.
Context Anchor
Seen in aircraft maintenance, especially during engine run-up, shutdown, and troubleshooting procedures.
Derivation
From 'coast,' meaning to keep moving under existing momentum without added power, and 'down,' meaning slowing toward a stop. The check literally measures how long the engine 'coasts' down to rest.
Why Pilots Care
Reveals early signs of bearing wear or excessive friction that could cause overheating, vibration, or failure in flight.
Analogy
It is like switching off a household fan and watching the blades slow down. If the fan stops almost at once or scrapes as it slows, that tells you something may be binding.
Grounding Statement
After shutdown, the mechanic watches the rotating parts wind down and notes whether they slow normally and smoothly.
Intuition Check
Coast-down does not mean an aircraft descending while gliding or flying near a coastline. Here it means rotating machinery slowing after power is removed.
Example Sentence 1
After every flight, the mechanic performed a coast-down check to confirm the engine spooled down within the expected time.
Example Sentence 2
A coast-down time shorter than specified indicated increased internal friction that required further inspection.