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Question 1: If I am flying in uncontrolled airspace and I have an emergency, should I report on 121.5MHz if no ATC is within line of sight communication (which means I cannot hear the ATC and the ATC cannot hear me)?

Question 2: Which frequency should I use to report the emergency in the following scenario? When the emergency happens, the aircraft is in uncontrolled airspace but near or approaching controlled airspace. There is an ATC station which is in charge of that airspace and the aircraft is within the line of sight of ATC (which means I may hear them), but my onboard radio coverage (the green circle indicated in the picture) cannot reach the ATC station (which means ATC cannot hear me). Should I report on the ATC frequency as maybe some aircraft are flying in the area and can relay the emergency to notify the ATC? Or should I just keep reporting on the common radio frequency like 121.5MHz?

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    $\begingroup$ If you are not actively talking to somebody, always switch to 121.5, the nearest ATC facility will be listening to that. If you are already talking to somebody, just keep talking to them. $\endgroup$
    – Ron Beyer
    Commented Dec 3, 2018 at 0:22
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    $\begingroup$ That should be an answer Ron. $\endgroup$
    – John K
    Commented Dec 3, 2018 at 1:13
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    $\begingroup$ Closely related $\endgroup$
    – Pondlife
    Commented Dec 3, 2018 at 1:18
  • $\begingroup$ Being in uncontrolled airspace doesn't mean that ATC won't be able to hear you. Monitoring of 121.5 is extensive, so there is a very good chance they'll be able to hear you regardless of where you are. If not, they likely won't hear you on any other frequency either. $\endgroup$
    – StephenS
    Commented Dec 4, 2018 at 17:42
  • $\begingroup$ I'd contact the nearest TRACON ("approach") or ARTCC ("center") $\endgroup$
    – Steve Kuo
    Commented Dec 5, 2018 at 17:55

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The safest approach is to use 121.5 until someone instructs you to try another frequency. ATC will monitor 121.5 at all locations it has antennas at. Additionally, military, some airlines, some medvac, and other operations will monitor 121.5 So your coverage will be greatest if you just use 121.5, unless you are already talking to ATC.

Addendum: Really in uncontrolled airspace? If you are really in uncontrolled airspace, in the CONUS, you are likely flying rather low. 121.5 will be your best best in that case as well. With enough time, you might work out a relay with higher traffic, perhaps on a center frequency. But 121.5 remains the go to frequency.

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  • $\begingroup$ If the ATC does not contact me, but I could hear people communication on the ATC frequency, that means I am within the line of sight communication of some aircrafts, should it be a better way to talk to the ATC frequency rather than 121.5 Mhz？ Maybe the aircrafts could help me to relay the emergency and also some ATC told me not everyone will monitor 121.5 Mhz $\endgroup$
    – VvV
    Commented Dec 3, 2018 at 3:49
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    $\begingroup$ Not all aircraft monitor 121.5, but many do. My understanding is that all radar communications locations, towers and FSS outlets do. Aircraft can relay, and if you can't raise ATC they will relay. In CONUS above 5000 AGL it is rare to not reach ATC. In the US all ATC monitor and can communicate on 121.5 unless they have an equipment failure. $\endgroup$
    – mongo
    Commented Dec 3, 2018 at 9:21

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