12.2 The 121.5 MHz Emergency Frequency, Transponder Codes and the AROC

121.5 MHz — The International Aviation Emergency Frequency

121.5 MHz is the international VHF aviation emergency frequency. It is guarded by air traffic services, search-and-rescue organisations, and most military and civilian aircraft. For a remote pilot, 121.5 MHz is the frequency you would tune to declare an emergency, or to monitor if you suspect another aircraft is in trouble and you are the closest listener.

Key points about 121.5 MHz:

• It is a voice frequency, used for distress and urgency calls only — not for routine chat.
• It is continuously available on a VHF aviation transceiver.
• Transmissions should begin with the appropriate emergency prefix, spoken three times.

Distress Call — Mayday

A Mayday call signals distress: an aircraft is in grave and imminent danger and requires immediate assistance — for example an engine failure, fire, or loss of control. The format is:

1. "Mayday, Mayday, Mayday."
2. Called station (for example, "Brisbane Centre" or "All stations") if known.
3. Callsign.
4. Nature of the emergency.
5. Intent and present position, heading and altitude.
6. Number of persons on board (for manned aircraft).
7. Any other useful information.

For a Remote Pilot Aircraft (RPA), a Mayday might be declared if the drone has a fly-away over a populated area, a battery fire, or a loss of control with a risk to persons on the ground. State the nature, position, and what you are doing — for example, attempting to land in a clear area. Even though an RPA has no persons on board, the call still matters because the aircraft itself is a hazard to persons and property on the ground, and other traffic in the area needs to know to keep clear.

Urgency Call — Pan-Pan

A Pan-Pan (from the French "panne", meaning a breakdown) call signals urgency: an urgent situation that is not immediately life-threatening but could become so, or that otherwise affects safety. Examples for an RPA include a lost link that is partially recovering, a GPS anomaly over a rural area, or a low-battery condition that is managed but not yet critical. The format is identical to a Mayday except the prefix is:

• "Pan-Pan, Pan-Pan, Pan-Pan."

The distinction matters: overusing Mayday for non-life-threatening situations degrades the response for those who genuinely need it. If in doubt, start with Pan-Pan; you can upgrade to Mayday if the situation worsens.

Transponder Emergency Codes

A transponder (when fitted to a larger RPA or a manned aircraft) squawks a four-digit code. Three international emergency codes must be memorised:

Mnemonic: 7-7-00 = big trouble, 7-6-00 = radio gone, 7-5-00 = foul play. The descending middle digit (7, 6, 5) is an easy memory aid — as the middle digit drops, the severity narrows from general emergency, to communications failure, to hijack.

For most small RPA operations a transponder is not fitted, but the codes are examinable and you must know them before operating any equipped aircraft.

VHF Voice Band and Channel Spacing

Aviation VHF voice communications occupy the band 118.000–136.975 MHz. Each channel is separated by:

• 25 kHz (the older standard), or
• 8.33 kHz (the newer, European-mandated standard that triples channel capacity in congested airspace).

Most Australian CTAF and tower frequencies are 25 kHz-spaced, but 8.33 kHz-capable radios are increasingly common and are required in much of European airspace. A remote pilot buying or operating a radio should confirm which spacing is legal for the area of operation.

The Aeronautical Radio Operator Certificate (AROC)

The Aeronautical Radio Operator Certificate (AROC) is a separate qualification from the Remote Pilot Licence (RePL). It is not granted automatically when you obtain a RePL, but it is commonly taken alongside it because many useful RPA operations — particularly near controlled aerodromes or on a CTAF where transmission is required — need you to legally transmit on an aeronautical VHF radio.

What the AROC authorises:

• Transmission on aeronautical VHF frequencies for the purposes of aviation communication (CTAF coordination, ATC liaison, emergency calls).
• Operation of an aeronautical radio station in accordance with CASA and ICAO phraseology standards.

Without an AROC (or an equivalent endorsement), a remote pilot may monitor aeronautical frequencies but must not transmit. Monitoring is useful for situational awareness; transmitting is a licensed act.

AROC Exam Format

The AROC is delivered by CASA-approved training providers and invigilated by a CASA-approved invigilator; it is not a CASA centrally-run exam. Australian providers advertise the assessment as:

• About 30 multiple-choice questions
• Closed book, in a sitting of roughly 1 hour
• Marked on the spot by the invigilator, with the application forwarded to CASA
• Questions cover phonetics, phraseology, CTAF and Class G procedures, the emergency frequency, distress and urgency calls, and transponder codes — exactly the material in this chapter.

CASA does not publish a pass mark for the AROC, so treat any specific percentage you see quoted as a provider figure rather than a regulatory one — confirm it with your own provider. An English language proficiency (ELP or AELP) assessment is also required as part of the qualification package.

AROC training and assessment are recorded on CASA Form 64-ROC. Many RePL training organisations bundle the AROC into their course, so you sit both assessments together. If your operation will ever require you to key the microphone on an aviation frequency, plan to hold the AROC.