Definition
In aviation maintenance, a permitted dimensional difference between two mating parts, expressed as the intentional clearance or interference designed into the fit. It is the prescribed minimum or maximum gap (or overlap) that allows parts to function correctly when assembled.
Plain English
The amount of space or overlap that's deliberately built in between two parts that fit together, so they work the way they're supposed to.
Context Anchor
Seen in aircraft maintenance when checking, machining, or replacing parts that must fit together precisely, such as bearings, bushings, shafts, and holes.
Derivation
From the Old French 'alouer,' meaning 'to assign or permit.' In engineering, it kept that sense of something deliberately permitted or set aside — here, a permitted gap or overlap between parts.
Why Pilots Care
A wrong allowance can make a part fit too loosely or too tightly, which can lead to wear, binding, or failure after the aircraft returns to service.
Intuition Check
Allowance does not mean permission in the general sense here. It means the specific size difference allowed between parts so they fit correctly.
Example Sentence 1
The technician checked that the shaft and bushing met the allowance specified in the maintenance manual before reassembly.
Example Sentence 2
A negative allowance on the bushing required a press to install it into the housing.