When flying on a plane with two or more radios, should I set the second radio to 121.5 at all times? (Assume I'm not using it to listen on other frequencies)

I've heard that sometimes when an intruder penetrates restricted airspace, the military will broadcast on this frequency in an attempt to communicate with that aircraft. Is this a good way to avoid me getting shot?

What about monitoring in case someone around me has an emergency?

Is there a difference in general aviation vs commercial airlines?

EDIT: I'm asking about whether this is a good idea, not about regulations.

  • $\begingroup$ Are you asking about a specific country, or is it a general question? $\endgroup$
    – Pondlife
    Commented Sep 20, 2018 at 3:49
  • $\begingroup$ There is no requirement for this, if that is what you are asking? Are you asking about regulations? FAA recommendations? $\endgroup$
    – Ron Beyer
    Commented Sep 20, 2018 at 3:50
  • $\begingroup$ @Pondlife I'm asking about recommended procedures, as in is it a good idea. $\endgroup$
    – kevin
    Commented Sep 20, 2018 at 6:28
  • $\begingroup$ Are you expecting to penetrate restricted military airspace??? O_o $\endgroup$
    – FreeMan
    Commented Sep 20, 2018 at 12:15
  • $\begingroup$ Nah, you can wait til after the F-16 goes knock-knock, at that point, you picking up the radio and talking is the best possible outcome for them. $\endgroup$ Commented Sep 21, 2018 at 5:13

1 Answer 1


Airlines routinely set comm 2 to 121.5 when they don't need it for talking to company or getting ATIS. But for a light aircraft pilot in busy airspace, the second comm is a really useful tool for reducing workload by being able to preset frequencies, and dedicating the second comm to 121.5 all the time takes that feature away, for the very small chance that you're hear someone's ELT go off or broadcast an emergency that will be heard by ATC anyway. Very few GA pilots monitor guard frequency regularly.

On the other hand, if you are flying over a remote area, it's not a bad idea at all and you could save someone that crashed with a non-406 ELT.

  • $\begingroup$ There's also a new low-level frequency now in the UK $\endgroup$
    – Cloud
    Commented Oct 4, 2023 at 10:52

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