Everyone here loves the "Channel 9" option on the UA and now CO fleet. For those who do not know, Channel 9 is a direct broadcast of the active ATC channel from the cockpit. My question is how does this system work and how does the pilot turn Channel 9 off (nothing to hear) or on? From what I would imagine, the Channel 9 output is controlled the same way the pilot would broadcast VHF/HF audio to either the cockpit speakers, boom mic headset (when using the oxygen mask) or to their headset. Is one of the knobs on the audio panel used to control if Channel 9 is working? Can one of the UA pilots or mechanics explain the technical aspect of the system?
Here is an audio panel from a 737. When you want to allow audio from the respective radio to be heard you can push the knob and it'll pop up. You turn it left or right to adjust the volume. The SPKR is selected when you wish to remove your headset and still hear the audio over the cockpit speakers. The MIC SEL panel selects which radio you are transmitting on (you may listen to multiple radios but you can only transmit on one at a time). Selecting PA is what will allow the pilot to make announcements to the cabin. FLT is for speaking to FAs on the cabin interphone and SVC is for the "cockpit to ground" communication via the service panel (used during pushback).
[Edited 2013-01-22 19:04:26]