isca-isca -
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Civilian airplanes only have VHF 121.5 capability.
We always select 121.5 on our nš 2 VHF radio and monitor.
It really should be on nš 1 in case of partial electrical system loss (favors nš 1).
It goes on nš 1 if on oceanic flights.
Absolutely no UHF equipment... so forget 243.0 for them.
Airplanes with HF also can use 2182 kHz as alternate guard frequency.
But HF is of limited use, and that frequency is bad for daytime.
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Happy contrails