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VHF-C / Acars Incompatible?

Sat Apr 08, 2006 6:30 am

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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".

Two questions...

1. Why?
2. What happens if VHF-C is used for ATC Comm when ACARS is operational?
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Goldenshield
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RE: VHF-C / Acars Incompatible?

Sat Apr 08, 2006 6:57 am

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.
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IFEMaster
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RE: VHF-C / Acars Incompatible?

Sat Apr 08, 2006 7:30 am

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.

Thanks for the info.

It just surprised me to see a plaque like that. I mean, to me it would be like putting a plaque in the cockpit that says "Do not rotate before V1" or "Do not land with landing gear up". I would also have thought that, given today's system capabilities, there would be a redundant measure in place to prevent VHF-C being used for ATC comm while ACARS is operational without having to remind the pilot.
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fr8mech
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RE: VHF-C / Acars Incompatible?

Sat Apr 08, 2006 9:42 am

The Center VHF (VHF-C) normally acts as the back-up for the left & right VHF. The warning is just telling the flight crew that ACARS data mode must be disabled, by enabling voice mode, before using VHF-C for voice comm. In the past you used to be able to hear the data, much like some modem squeals, but it's been a long time since I've heard that. The squeals are probably supressed.
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avioniker
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RE: VHF-C / Acars Incompatible?

Tue Apr 11, 2006 9:48 pm

The reason is because the Communications Management Unit will give control of the dedicated VHF (C in this case, #3 in others) to the ACARS after a set length of time with no transmission from the pilots.
That would change the frequency back to the ACARS freq, possibly introduce noise into the headsets that the pilots really don't want to hear, but most importantly, when the ACARS takes back over the pilots will miss any incomming calls from ATC or whoever, if they aren't monitoring the frequency on another radio.
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