Definition
In aviation maintenance, a permitted limit, deviation, or condition specified by the manufacturer, regulatory authority, or maintenance manual. An allowable defines what is acceptable for continued safe operation — for example, the maximum allowable wear on a part, the maximum allowable crack length, or an allowable difference between a measured value and the specified standard.
Plain English
How much wear, damage, or variation is permitted before a part or system must be repaired, replaced, or pulled from service. If something is within allowable limits, it can stay in service; if it exceeds them, it cannot.
Context Anchor
Seen in aircraft manuals, regulations, maintenance instructions, weight-and-balance information, and performance charts when a limit or approved condition is being stated.
Derivation
From the verb 'allow,' from Old French 'alouer' meaning 'to permit or approve.' In aviation it carries that exact sense: a value, condition, or amount that has been formally permitted by the relevant authority.
Why Pilots Care
Maintenance decisions, airworthiness, and legal return-to-service all hinge on whether something falls inside or outside its allowable limits. A pilot reviewing maintenance records or discussing a discrepancy with a mechanic needs to understand that 'allowable' is a defined limit, not a casual judgment.
Intuition Check
Do not read “allowable” as “probably okay” or “left to personal judgment.” In aviation, it means allowed only because a rule, manual, or approved procedure permits it.
Example Sentence 1
The mechanic measured the brake disc and found the wear was within the allowable limit, so the aircraft was returned to service.
Example Sentence 2
The part was replaced because the wear had gone beyond what was allowable.