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A pilot has done a preflight inspection on their Cessna and noticed the altimeter is over-reading. They can't recall the error tolerance for flight, and the answer is buried in a manual. AskWingman pulls it out:

Question

How do I know if the only altimeter in my Cessna 172 is ok for an IFR flight?

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Checking your altimeter for IFR flight — single system

The requirements for checking a single pressure altitude system before IFR flight are set out in Chapter 10 › 10.04, Page 59 of the Part 91 (General Operating and Flight Rules) Manual of Standards 2020.

The basic test

Check the altimeter against the known site elevation at your departure aerodrome using an accurate QNH. The pilot in command is responsible for the check.

One system required, one fitted

Under section 10.04(4):

Error 60 ft or less → serviceable; conduct IFR flight normally.
Error 60–75 ft → "permitted to conduct a flight to the first point of landing where the accuracy of the system can be rechecked." A limited permission only — recheck on arrival.
Error still over 60 ft on recheck → "the pilot in command must consider the system to be inoperative for further IFR flight."
Error over 75 ft at any point → under section 10.04(1), the system is inoperative for the flight.

Summary

Error at preflight check	Action permitted
≤ 60 ft	Serviceable — IFR flight normal
61–75 ft	Fly to first landing point for recheck only
> 75 ft	Inoperative — IFR not permitted
61–75 ft on recheck	Inoperative — no further IFR

Caveats

These tolerances apply where one system is required and one is fitted — the rules differ for two systems (section 10.04(2)–(3)).
VFR allows a broader ±100 ft (±110 ft above 3,300 ft test sites) under section 10.05(1) — this does not apply to IFR.
The equipment must also meet IFR fitting requirements under Table 26.08(4), Page 121.

Source

Part 91 MOS 2020 — Chapter 10 › 10.04, Page 59; Chapter 26 › 26.08, Page 121.

Sources Part 91 MOS 2020 · p. 59 Part 91 MOS 2020 · pp. 59–60 Part 91 MOS 2020 · p. 121

⚠ AI-generated answer. May contain errors, omissions, or outdated information. A reference aid, not professional advice — verify against the original document before acting.

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