AIM Text
- Up to but not including 18,000 feet MSL, state the separate digits of the thousands plus the hundreds if appropriate.
- At and above 18,000 feet MSL (FL 180), state the words “flight level” followed by the separate digits of the flight level.
Source: FAA Aeronautical Information Manual · current edition · paragraph 4-2-9.
Research Notes
AIM 4-2-9 covers ground station call signs — the identifiers ATC facilities use to identify themselves.
Standard format: Location name + service type. Examples:
- Tower: "Anchorage Tower" (city + Tower)
- Ground: "Anchorage Ground"
- Approach: "Anchorage Approach" (or sometimes "Anchorage Departure" for outbound)
- Center: "Seattle Center" (Center name = ARTCC name)
- Clearance: "Anchorage Clearance" or "Anchorage Clearance Delivery"
- FSS: "Kenai Radio" — FSSs use "Radio" as their service identifier
Why this matters: Calling the wrong facility wastes time and often gets ignored. "Anchorage Tower" can't give you an IFR clearance — that's Clearance Delivery. "Seattle Center" doesn't handle airport operations — that's Tower or Ground.
Multi-facility sites: Major airports have several facilities co-located. You'll switch frequencies between them as you progress through the flight phase. ATC will instruct you on the switch ("contact Departure on 119.4").
Reference: AIM 4-2-9.