Page 85 - Beginner's Guide to Aviation

Have you ever thought about how a pilot would alert the ATC during emergencies?
When an aircraft is in grave and/or imminent danger, and requires immediate assistance, it is considered as a distressed condition. The pilot will signal “Mayday, mayday, mayday.”
This RTF signal has priority over other transmissions.
Some examples of mayday situations are hijacking, bomb threat and engine failure.
Mayday, mayday, mayday
On the other hand, if the safety of an aircraft/person on board is threatened, but does not require immediate assistance, it is considered an urgent condition. In such scenarios, the pilot will signal “PAN PAN, PAN PAN, PAN PAN.”
If the aircraft is used for medical urgency, the RTF signal will be slightly different: “PAN PAN, PAN PAN, PAN PAN MEDICAL.”
PAN PAN, PAN PAN, PAN PAN
Aircraft is lost
After receiving the PAN message, the ATC will acknowledge the message, provide assistance to aircraft and give them priority.
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