Legal Interpretations
Knauff-Knauff&Grove 2012
Summary
The Question
Must glider pilots, including those on student training and local flights, set their altimeters to MSL rather than AGL under 14 CFR 91.121?
Background
Knauff & Grove, Inc. reported confusion among local glider pilots about whether altimeters must be set to mean sea level when not using a cruising altitude, such as during student glider training and local flying.
The FAA’s Interpretation
Yes. Under 14 CFR 91.121(a)(1), cruising altitude below 18,000 feet MSL is maintained by reference to an altimeter set to the current reported setting of a station along the route within 100 nautical miles (or, without a radio, departure airport elevation or a setting available before departure). Transient periods at a particular altitude do not relieve glider pilots of this, so altimeters must be set to MSL, not AGL, including on student training flights.
- Requester
- Knauff
- Organization
- Knauff&Grove
- Issued
- January 20, 2012
- Subject
- General Operating & Flight Rules
Regulations Cited
- 14 CFR 91.12
- 14 CFR 91.121
Page 1
U.S. Department
of Transportation
Federal Aviation
Administration
Mr. Thomas Knauff
Knauff & Grove, Inc
Office of the Chief Counsel
3523 South Eagle Valley Road
Julian, PA 16844
Dear Mr. Knauff,
800 Independence Ave., S.W.
Washington, D.C. 20591
This letter is in response to your July 19, 2011 request for a legal interpretation regarding altimeter settings required in 14 CFR § 91.121. In your letter, you state that there is confusion among local area glider pilots as to whether altimeters must be set to mean sea level (MSL) when not using a cruising altitude, such as when conducting student glider flight training. Additionally, you indicate that glider operators are unsure if the regulation requires altimeters to be set to mean sea level (MSL) or above ground level (AGL) during student training and local flying.
The cruising altitude of an aircraft below 18,000 feet MSL shall be maintained by reference to an altimeter that is set to the current reported altimeter setting of a station located along the route of flight and within 100 nautical miles of the aircraft. If there is no such station, the current reported altimeter setting of an appropriate available station shall be used. In an aircraft having no radio, the altimeter shall be set to the elevation of the departure airport or an appropriate altimeter setting available before departure. 14 CFR § 91.121 (a)(l).
The cruising altitude is the MSL altitude or flight level maintained during en route level flight. For aircraft such as gliders, transient periods of time at a particular altitude do not relieve pilots from the requirements to operate the aircraft in reference to an altimeter that is set according to the requirements of§ 91.121(a). Thus, local area glider pilots must set their altimeters to MSL, not AGL, during glider operations, including student glider training flights.
Further FAA guidance on altimeter settings for glider operations can be found in the Glider Flying Handbook, FAA-H-8083-13, Chapter 4 (available online at http://www.faa.gov/library/manuals/aircraft/glider handbook/media/faa-h-8083-13 .pdf). We appreciate your patience and trust that the above responds to your concerns. This response was prepared by Nancy Sanchez, an attorney in the Regulations Division of the
Page 2
Office of the Chief Counsel, and was coordinated with the General Aviation and Commercial Division of Flight Standards. Please contact us at (202) 267-3073 if we can be of further assistance.
Sincerely,
P-f-lfdRebecca B. MacPherson
Assistant Chief Counsel for Regulations, AGC-200
Regulations Referenced
Snapshot retrieved August 6, 2026.