Definition
The maximum interval allowed between required inspections and tests of an aircraft's static pressure system, altimeter instrument, and automatic altitude reporting (Mode C) equipment when the aircraft is operated under Instrument Flight Rules (IFR) in controlled airspace, as specified in 14 CFR 91.411 and 91.413. The inspection must have been performed within the preceding 24 calendar months.
Plain English
The altimeter system, static system, and altitude reporting equipment must be tested and certified at least once every two years for an aircraft to legally fly IFR in controlled airspace.
Context Anchor
Seen in aircraft maintenance records and preflight planning when checking whether the altimeter system inspection is still current.
Derivation
A month originally referred to a cycle of the moon. In FAA inspection timing, the useful point is that months are calendar months, not just a rough count of days.
Why Pilots Care
Ensures altitude accuracy and compliance with legal requirements for safe IFR operations and traffic separation.
Intuition Check
Do not read 24 months as just “about two years.” In this inspection context, it is a calendar-month requirement that must be shown by the aircraft records.
Example Sentence 1
Before filing the IFR flight plan, the pilot checked the maintenance logbook and confirmed the altimeter system inspection had been completed within the last 24 months.
Example Sentence 2
Maintenance records showed the last static system check was completed 23 months ago, so the aircraft remained legal for IFR.