Text in force at 23-63: from 23-51, effective 12 Mar 1996.
[If an air filter is used to protect the engine against foreign material particles in the induction air supply--]
(a) Each air filter must be capable of withstanding the effects of temperature extremes, rain, fuel, oil, and solvents to which it is expected to be exposed in service and maintenance; and
(b) Each air filter shall have a design feature to prevent material separated from the filter media from interfering with proper fuel metering operation.
61 FR 5137 · Source