7.2 Aeronautical Frequency Assignments

Why frequency knowledge is an ROC-A skill

You can memorize perfect PAN PAN wording and still fail operationally if you transmit on the wrong part of the band. RIC-21 Appendix E — Frequency Assignments gives the official Canadian study-guide table of band → service / primary use. Separately, §§6.3 and 7.3 fix 121.5 MHz as the aeronautical emergency frequency when the working air-ground channel cannot establish contact. This section welds those facts into operator judgment: right band, right purpose, emergency fallback only after (or when) the frequency in use fails.

Quick Answer: Learn Appendix E’s primary-use blocks (radionavigation, ATC, general aviation, operational control). Treat 121.5 MHz as the emergency voice channel after (or when) the frequency in use will not work. VHF is essentially line-of-sight—altitude and terrain matter. Never invent a “random open channel” as your first plan if a published air-ground frequency is already in use.

Appendix E — official band table (RIC-21)

RIC-21 states that the following table indicates, for each frequency band listed, the service and primary use of the band:

How to read the table for exams

1) Radionavigation block (roughly 108–118 MHz)
Two consecutive rows cover aeronautical radionavigation. This is the historic home of VHF navigation aids (for example localizer/VOR-class assignments in the broader navaid structure). For ROC-A voice-operator purposes, the critical point is primary use = radionavigation, not “free chat.” Do not treat navigation bands as a casual substitute for tower or en route voice channels.

2) Air Traffic Control Services (three separate blocks)
ATC service frequencies appear in three slices:

• 117.9750 – 121.9625 MHz
• 123.5875 – 128.8125 MHz
• 132.0125 – 136.0000 MHz

Towers, approach/terminal, centre, and related ATC voice functions live in these primary-use ATC bands. When charts, ATIS, or a controller tell you a frequency, it will normally fall into an appropriate service assignment—not into the radionavigation block.

3) General Aviation Communications (121.9825 – 123.5875 MHz)
RIC-21 Appendix A defines General Aviation Communications (GAC) as communications relating to all civil aviation operations other than for scheduled air service and non-scheduled air transport operations for remuneration or hire. The Appendix E band is the primary-use slice for that class of communications. Many local aerodrome, training, and general-aviation working channels students meet in practice sit in or near this conceptual space—always confirm the actual assigned frequency for the aerodrome or service you need.

4) Aeronautical Operational Control Communications (128.8125 – 132.0125 MHz)
Appendix A defines Aeronautical Operational Control Communications (AOCC) as communications related to the regularity of flight—company ops, dispatch-style operational control, and similar. Primary use of that band is not the same as ATC separation service. Mixing company operational chatter onto a live ATC frequency is a classic interference / priority failure.

Small gaps in the numeric edges

Notice the table is not one continuous “118–136” smear with identical use. There are deliberate band edges and a small numeric step between 121.9625 and 121.9825 before the general-aviation communications row. For exam purposes, memorize the published bands and labels; do not invent intermediate micro-allocations that RIC-21 does not list.

121.5 MHz — aeronautical emergency frequency

RIC-21 §§6.3 (distress) and 7.3 (urgency) both name 121.5 MHz as the aeronautical emergency frequency. The shared rule:

1. First try the air-ground frequency in use
2. If you cannot establish communications there, repeat the distress/urgency traffic on 121.5 MHz
3. Or use any other frequency available to reach a ground or aircraft station

Operator implications

• 121.5 is not your default “hello” channel for fuel orders or radio checks when a published working frequency exists
• 121.5 is the international civil VHF emergency channel for distress and urgency when the live working channel fails or cannot establish contact
• Many stations monitor 121.5 when practical; ELT homing tones historically also appear there (equipment section covers ELT purpose carefully)
• Military UHF emergency 243.0 MHz is not the civil VHF number RIC-21 teaches for aeronautical emergency voice in this guide

Choosing correct air-ground frequency before emergency fallback

Jumping straight to 121.5 while already talking to a controller on 119.1 can delay the people who already have your identity, clearance, and radar tag. Conversely, if you are lost with no contact and no known working frequency, 121.5 is exactly the tool §§6.3/7.3 give you.

VHF line-of-sight character (operator-level)

Aeronautical voice communications in these bands are VHF. At ROC-A depth, you need the operational concept—not an invented formula:

• VHF energy travels essentially in a straight line (line-of-sight)
• Altitude increases the radio horizon; higher aircraft can often talk farther than low aircraft or ground stations
• Terrain, buildings, and the curvature of the Earth can block or shadow signals
• This is not the same mechanism as HF long-range ionospheric skip used over oceans and remote areas

RIC-21 does not require you to compute a precise nautical-mile range equation for every altitude. What examiners and safe operators care about:

• If you cannot raise a station, consider altitude, terrain, and frequency selection before assuming the radio is dead
• Do not expect ground-to-ground VHF across mountain ranges as if it were a cell network
• Emergency fallback frequencies only help if someone within radio range is listening—relay by other aircraft may still be necessary (distress/urgency relay concepts from Chapters 6–7)

VHF vs HF (context only)

Putting Appendix E and emergencies together — worked scenarios

Scenario A — Normal tower contact
You are inbound to a controlled aerodrome. ATIS and charts give a tower frequency in an ATC Services primary-use band. You select that frequency, listen, call the tower. You do not open with 121.5.

Scenario B — Urgency while already with tower
You are talking to Winnipeg Tower on the frequency in use when a passenger becomes seriously ill but the aircraft is not in grave immediate danger. Transmit PAN PAN on the frequency in use (RIC-21 §7.3 first step). Only if you cannot establish or maintain needed contact would you move to 121.5 or another available frequency.

Scenario C — No contact, true emergency
You need distress or urgency help and the working channel is dead silent or unreadable. Repeat on 121.5 MHz or any available frequency per §§6.3/7.3.

Scenario D — Company ops vs ATC
You need regularity-of-flight coordination with company. That is operational control territory conceptually—not a reason to park company traffic on a busy ATC frequency. Use the proper assigned operational frequency when one exists.

Exam traps

Trap 1 — Confusing radionavigation bands with ATC voice. 108–117.975 primary use is radionavigation in Appendix E.

Trap 2 — Treating 121.5 as the first channel for all emergencies even when already in solid contact. Frequency in use comes first.

Trap 3 — Claiming VHF bounces off the ionosphere like HF. VHF is essentially line-of-sight.

Trap 4 — Mixing general aviation communications primary use with ATC primary use as if they were identical bands.

Trap 5 — Inventing exact range formulas not present in RIC-21. Stick to the altitude/terrain/line-of-sight concept.

Memorization method

Draw one horizontal line from 108 to 136 MHz. Block-shade: nav / nav / ATC / GAC / ATC / AOCC / ATC. Write 121.5 emergency as a star beside the lower ATC/GA area conceptually, remembering it from §§6.3/7.3 rather than as a separate Appendix E row. Practice saying which primary use applies when given a band edge. Then drill: “Working frequency first, then 121.5 or any available.”