FAA Instrument Flight Instructor (FII)Instrument RegulationsEasy
A pilot is flying IFR at 8,000 feet MSL, below FL180. According to 14 CFR 91.121, which altimeter setting must the pilot use?
- AThe destination airport's altimeter setting, obtained before departure and never updated.
- BThe current reported altimeter setting of a station along the route of flight, or an appropriate substitute if none is available.
- CThe departure airport's altimeter setting, maintained unchanged for the entire flight.
- D29.92 inches Hg, the standard setting used throughout the flight.
Show answer & explanationAnswer & explanation
Correct answer: B. The current reported altimeter setting of a station along the route of flight, or an appropriate substitute if none is available.
14 CFR 91.121 requires pilots operating below FL180 to set their altimeter to the current reported setting of a station along the route, updating it periodically, or to use an appropriate altimeter setting if no station report is available.
Why the other options are wrong
- A. Using only the destination setting throughout the flight would produce inaccurate readings en route.
- C. The setting must be updated along the route, not held from departure only.
- D. 29.92 is used at or above FL180, not below it.
Altimeter Setting Procedures (91.121)
Below FL180, pilots must use the current altimeter setting of a station along the route, updated periodically; at/above FL180, 29.92 is used.
- Below 18,000 ft MSL: use current station setting along route
- At/above FL180: set 29.92 inches Hg (standard)
- Use appropriate substitute setting if no station report available
Memory trick: 'Below 18, dial it in; at 18, dial 29.92.'