Distress and emergency frequencies and transponder codes
This section covers distress and emergency frequencies and transponder codes: the dedicated VHF and military frequencies, the three special squawks, and when each is used.
Core rules
- The international aeronautical distress and emergency frequency is 121.5 MHz; 243.0 MHz is the military counterpart. These frequencies are guarded for emergency use and should not be used for routine conversation.
- Squawk 7700 for a general emergency, 7600 for radio failure, and 7500 for unlawful interference such as hijacking.
- If able, set the code before or while transmitting the emergency message; do not delay a critical transmission solely to change the transponder.
- Mode C gives pressure altitude to ATC. If altitude information is incorrect, report it; do not leave ATC with misleading data.
- Use 121.5 for distress or urgency when no other frequency is available, and return to the assigned frequency when able.
Worked example
A pilot discovers smoke in the cockpit and elects to divert immediately. The pilot transmits “MAYDAY MAYDAY MAYDAY, 5N-ABC, smoke in cockpit, 25 nm east of LOS, 6,000 ft, requesting immediate landing, three persons on board” and squawks 7700. Smoke in the cockpit is a distress condition, so MAYDAY and 7700 are appropriate; 7600 would wrongly indicate radio failure.
Traps
Squawking 7600 for an emergency; using 7500 for radio failure; using 121.5 for routine calls; and forgetting to report incorrect altitude information.