AIM Text
- FSSs and Supplemental Weather Service Locations (SWSL) are allocated frequencies for different functions; for example, in Alaska, certain FSSs provide Local Airport Advisory on 123.6 MHz or other frequencies which can be found in the Chart Supplement. If you are in doubt as to what frequency to use, 122.2 MHz is assigned to the majority of FSSs as a common en route simplex frequency.
- Certain VOR voice channels are being utilized for recorded broadcasts; for example, ATIS. These services and appropriate frequencies are listed in the Chart Supplement. On VFR flights, pilots are urged to monitor these frequencies. When in contact with a control facility, notify the controller if you plan to leave the frequency to monitor these broadcasts.
Source: FAA Aeronautical Information Manual · current edition · paragraph 4-2-14.
Research Notes
AIM 4-2-14 covers speeds in ATC communications — how airspeed and Mach numbers are expressed.
Airspeed in knots: Stated as digit-by-digit: "one-five-zero knots" for 150 KIAS. Always include the "knots" suffix to distinguish from headings or other digit groups.
Mach numbers: Used above FL280 (general practice). "Maintain Mach point eight-zero" or "Climb and accelerate to Mach point seven-five." The decimal is pronounced "point."
Common speed instructions:
- "Maintain present speed"
- "Reduce speed to one-eight-zero knots"
- "Maintain best forward speed"
- "Hold at present altitude"
- "Standard speed" — for IFR aircraft, comply with the 200/250 limits of § 91.117 unless ATC specifies otherwise
The 250 KIAS limit: Below 10,000 MSL, the speed limit is 250 KIAS per § 91.117(a). ATC may grant deviations but the pilot must explicitly request and confirm.
The 200 KIAS limit: Within 4 NM of Class B/C/D primary airport, below 2,500 AGL, the limit is 200 KIAS per § 91.117(b)(2).
Reference: § 91.117 (aircraft speed); AIM 4-2-14.