Altimeters on Approach

Reader David Novelli asked an innocent but probing question. His primary airport has an approach that says if the local altimeter setting is unavailable you should use one from a nearby field and adjust the minimums. Weve all seen that, although few of us have actually had to take advantage of it.

IFR Staff

Saturday, November 11, 2017 at 12:00 AM ET

Verified

Key Takeaways:

For any instrument approach in the U.S., a certified and authorized altimeter setting is legally mandatory; without one, the approach is unusable, even if nearby airport settings are also unavailable.
In a hypothetical, dire Part 91 emergency (not for hire) where landing is critical despite no available altimeter setting, a pilot *might* consider "guesstimating" a setting using local pilot reports or interpolating from nearby airports.
This emergency guesstimation is an intellectual exercise, only to be considered if weather is comfortably above minimums, requires significant safety margins added to approach minimums, and does not override the default rule to divert when an altimeter setting is unavailable.

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Reader David Novelli asked an innocent but probing question. His primary airport has an approach that says if the local altimeter setting is unavailable you should use one from a nearby field and adjust the minimums. We’ve all seen that, although few of us have actually had to take advantage of it.

Philosophically digging deeper, Novelli’s question was what to do when both the primary and the secondary airport aren’t reporting an altimeter. This yielded some interesting discussions of both the legal and the practical that are just the stuff we’re known for.