Definition
A maintenance inspection that measures the small amount of free movement, or play, between two meshed gears or between mating threaded parts. The check verifies that the clearance falls within the manufacturer's specified tolerance — neither too tight (which causes binding and wear) nor too loose (which allows slack, vibration, and inaccurate movement).
Plain English
A check to see how much wiggle room there is between two parts that mesh together, like gear teeth. Too little means they bind. Too much means they're worn or loose. The check confirms the play is within the allowed range.
Context Anchor
Seen in aircraft maintenance procedures for engines, accessory drives, gear assemblies, and some control systems.
Derivation
Backlash' combines 'back' (reverse direction) and 'lash' (a sudden movement or strike). It originally described the loose return motion in mechanical linkages — the small reverse movement before the driven part responds when direction is reversed. That free play is exactly what is being measured.
Why Pilots Care
Excessive backlash produces delayed or imprecise control response, which can degrade handling qualities and mask developing mechanical problems.
Analogy
It is like turning a loose doorknob and feeling it move a little before the latch starts to move. That empty movement is the kind of play a backlash check is looking for.
Intuition Check
Backlash does not mean a negative public reaction here. In aircraft maintenance, it means mechanical looseness or free play between parts.
Example Sentence 1
The mechanic performed a backlash check on the propeller governor gears before signing off the inspection.
Example Sentence 2
During the annual inspection the technician recorded the results of the backlash check on the rudder control cables.