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There is a little plaque above the radio that says "Do not use VHF-C for ATC Comm with ACARS operational".
2. What happens if VHF-C is used for ATC Comm when ACARS is operational?
|Quoting Goldenshield (Reply 1):|
1) Becuase the ACARS is using the 3rd VHF transceiver to do it's stuff, that's why.
2) The ACARS loses it's ability to talk to a ground station, which means that no one can contact the plane, and the plane can't contact the central computer.