📘14 CFR §91.121 – Altimeter Settings

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🚩 Overview

Accurate altimeter settings are critical for maintaining safe vertical separation from terrain and other aircraft. Pilots must understand when and how to set altimeters correctly based on FAA regulations.

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✅ FAA Altimeter Setting Requirements

🛫 1. Below 18,000 Feet MSL

• Use the altimeter setting reported by a station along your route and within 100 nautical miles of your aircraft.

• If no altimeter setting is available, use the current reported altimeter of an appropriate available station.

• If neither is available, set your altimeter to the airport elevation before departure.

✈️ 2. At or Above 18,000 Feet MSL

• Set the altimeter to 29.92 inches of mercury (Hg).

• Operate in flight levels (FL180, FL190, etc.).

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🛩️ Example Scenario

You're departing an uncontrolled airport with no weather reporting facilities. Before takeoff, set your altimeter to the airport elevation listed in the Chart Supplement. After departure, as soon as practical, obtain and set your altimeter based on an appropriate nearby station within 100 NM.

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📌 CFI Pro Tips

• Check Early, Check Often: Teach your students to frequently update altimeter settings en route, especially when flying near terrain or in variable weather conditions.

• Standardize Procedures: Incorporate altimeter setting checks into your students' cruise and descent checklists, reinforcing safety habits.

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📚 Further Reading and Official References

• 14 CFR §91.121 (Full FAA Regulation): https://www.ecfr.gov/current/title-14/chapter-I/subchapter-F/part-91/subpart-B/section-91.121

• FAA Aeronautical Information Manual (AIM) – Altimeter Setting Procedures: https://www.faa.gov/air_traffic/publications/atpubs/aim_html/chap7_section_2.html

• Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25B): https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak