FAR 91.121 — Altimeter Settings
FAR 91.121 explains altimeter setting rules: use local stations below 18,000 MSL and 29.92" Hg at or above FL180. Study guide for pilot students.
In Plain English
FAR 91.121 tells you how to set your altimeter so that everyone in the sky is using a common altitude reference. Getting this right keeps you legally at your assigned altitude and safely separated from terrain and other traffic.
- Below 18,000 feet MSL: Set your altimeter to the current reported altimeter setting from a station along your route within 100 nautical miles. If no station is available in that range, use the nearest appropriate station's setting. If your aircraft has no radio, set it to the departure airport elevation or an appropriate setting obtained before takeoff.
- At or above 18,000 feet MSL (the flight levels): Set the altimeter to 29.92" Hg. This is the standard setting that defines flight levels (e.g., FL180, FL250).
When local pressure is low, the lowest usable flight level rises — you must add an adjustment (up to 3,000 ft when the setting is 26.92–27.41) to any minimum altitude from §§ 91.119 or 91.177 to convert it to a legal flight level. This prevents flying a charted altitude that's actually lower than indicated when pressure drops.
Regulation Text
14 CFR § 91.121§ 91.121 Altimeter settings.
(a) Each person operating an aircraft shall maintain the cruising altitude or flight level of that aircraft, as the case may be, by reference to an altimeter that is set, when operating—
(1) Below 18,000 feet MSL, to—
(i) The current reported altimeter setting of a station along the route and within 100 nautical miles of the aircraft;
(ii) If there is no station within the area prescribed in paragraph (a)(1)(i) of this section, the current reported altimeter setting of an appropriate available station; or
(iii) In the case of an aircraft not equipped with a radio, the elevation of the departure airport or an appropriate altimeter setting available before departure; or
(2) At or above 18,000 feet MSL, to 29.92″ Hg.
(b) The lowest usable flight level is determined by the atmospheric pressure in the area of operation as shown in the following table:
(c) To convert minimum altitude prescribed under §§ 91.119 and 91.177 to the minimum flight level, the pilot shall take the flight level equivalent of the minimum altitude in feet and add the appropriate number of feet specified below, according to the current reported altimeter setting:
3,000
27.41 through 26.92
2,500
27.91 through 27.42
2,000
28.41 through 27.92
1,500
28.91 through 28.42
1,000
29.41 through 28.92
29.91 through 29.42
None
29.92 (or higher)
Adjustment factor
Current altimeter setting
27.41 through 26.92
27.91 through 27.42
28.41 through 27.92
28.91 through 28.42
29.41 through 28.92
29.91 through 29.42
29.92 (or higher)
Lowest usable flight level
Current altimeter setting
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Oral Exam Questions a DPE Might Ask
Q1How are you required to set your altimeter when cruising below 18,000 feet MSL?
Per FAR 91.121, I must set it to the current reported altimeter setting of a station along my route within 100 nautical miles, or the nearest appropriate station if none is available within that range.
Q2What altimeter setting is required at or above 18,000 feet MSL, and why?
FAR 91.121 requires 29.92" Hg at or above 18,000 feet MSL. This standard setting establishes the flight levels so all aircraft in the high-altitude structure share a common pressure reference.
Q3If the local altimeter setting is 28.50", how does that affect the lowest usable flight level?
Under FAR 91.121(c), with a setting between 28.42 and 28.91, you must add 1,500 feet to the minimum altitude when converting it to a flight level, raising the lowest usable flight level accordingly.
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Related Sections in Part 91