Roger, Wilco
and Mayday.
Radio has its own short words, called procedure words or “prowords”. Here is what they mean, quoted from the US FAA’s Pilot/Controller Glossary, the UK CAA’s CAP 413, the allied military manual ACP 125(G) and the IMO’s marine phrases. Wording differs slightly between them, so each one is linked to its source.
Everyday prowords
In the FAA’s glossary, ROGER means “I have received all of your last transmission”, and it should not be used to answer a yes-or-no question. FAA Pilot/Controller Glossary ↗
ROGER means “I have received your last transmission”. UK CAA, CAP 413 ↗
In ACP 125(G), ROGER means “I have received your last transmission satisfactorily”. ACP 125(G) ↗
WILCO: “I have received your message, understand it, and will comply with it.” FAA Pilot/Controller Glossary ↗
WILCO is short for “will comply”. UK CAA, CAP 413 ↗
Because ROGER’s meaning is included in WILCO, the two are never used together. ACP 125(G) ↗
In CAP 413, AFFIRM means “Yes”. UK CAA, CAP 413 ↗
AFFIRMATIVE simply means “Yes”. FAA Pilot/Controller Glossary ↗
NEGATIVE means “No”, “permission not granted” or “that is not correct”. FAA Pilot/Controller Glossary ↗
OVER: “My transmission is ended; I expect a response.” FAA Pilot/Controller Glossary ↗
OUT: “The conversation is ended and no response is expected.” FAA Pilot/Controller Glossary ↗
OVER means “my transmission is ended, and I expect a response”; OUT means the exchange is ended and no response is expected. UK CAA, CAP 413 ↗
SAY AGAIN asks for a repeat of the last transmission. FAA Pilot/Controller Glossary ↗
I SAY AGAIN means “I repeat for clarity or emphasis”. UK CAA, CAP 413 ↗
STAND BY means the controller or pilot must pause for a few seconds. FAA Pilot/Controller Glossary ↗
STANDBY is not an approval or a denial. UK CAA, CAP 413 ↗
SPEAK SLOWER asks the other person to reduce their speech rate. FAA Pilot/Controller Glossary ↗
WORDS TWICE, as a request, means “Communication is difficult. Please say every phrase twice.” FAA Pilot/Controller Glossary ↗
ACKNOWLEDGE: “Let me know that you have received and understood this message.” FAA Pilot/Controller Glossary ↗
READ BACK: “Repeat my message back to me.” FAA Pilot/Controller Glossary ↗
BREAK marks the separation between messages. UK CAA, CAP 413 ↗
CAP 413 says ROGER and WILCO are not enough to acknowledge an instruction to hold. UK CAA, CAP 413 ↗
SQUAWK is the word air traffic control uses to tell a pilot to set the aircraft’s transponder. FAA Pilot/Controller Glossary ↗
MAYDAY and PAN-PAN
MAYDAY is the international radiotelephony distress signal; said three times, it means grave and imminent danger and that immediate help is needed. FAA Pilot/Controller Glossary ↗
A distress message should begin with MAYDAY, preferably said three times; PAN-PAN is used the same way for urgency. FAA, Aeronautical Information Manual 6-3 ↗
A distress call starts MAYDAY, MAYDAY, MAYDAY; an urgency call starts PAN PAN, PAN PAN, PAN PAN. UK CAA, CAP 413 ↗
The word MAYDAY commands radio silence on the frequency in use. FAA, Aeronautical Information Manual 6-3 ↗
On hearing a distress call, all stations must keep radio silence on that frequency. UK CAA, CAP 413 ↗
Calls from aircraft in distress have priority over all other transmissions. UK CAA, CAP 413 ↗
PAN-PAN is the international urgency signal; said three times, it signals uncertainty or alert, followed by the nature of the urgency. FAA Pilot/Controller Glossary ↗
PAN-PAN warns other stations not to interfere with urgency calls. FAA, Aeronautical Information Manual 6-3 ↗
121.5 MHz is the International Aeronautical Emergency Frequency. UK CAA, CAP 413 ↗
In the US, 121.5 MHz and 243.0 MHz can be used for distress or urgency calls; 243.0 MHz is listened to by military aircraft. FAA, Aeronautical Information Manual 6-3 ↗
If a pilot in distress can’t reach air traffic control straight away, the FAA says to set the transponder to code 7700. FAA, Aeronautical Information Manual 6-3 ↗
UK civil controllers say “squawk Mayday”; military controllers say “squawk emergency” or “squawk 7700”. UK CAA, CAP 413 ↗
See also more facts, the history and all sources.