CVR Channels - Where is the audio sourced?

[PineLemon]

Been looking into the CVR system, and based on what I've found in its documentation, there are 4 channels that record audio, which are all individually tested on the panel.

1: Cockpit spare microphone
2: Copilot microphone
3: Pilot microphone
4: Cockpit area microphone

This leads me to my questions, firstly, where and what is the spare microphone? Secondly, where exactly would the pilot/copilot audio be sourced from? My first thought was the headset, but to accommodate for the oxy mask microphones, would the audio be sourced from somewhere that encompasses everything?

Sorry for the particularly niche question, I know there is a massive wealth of knowledge here though, so I thought it wouldn't hurt to ask.

[PineLemon]

I've just discovered that the 1st CVR channel in the CMM wiring diagram is labeled to go to "3rd crew member or PA". Does this hold true for the 744?

My assumption is that it would go to the first observer station with PA also being recorded on the same channel.

[Jeroen Hoppenbrouwers]

As far as I know, only the cockpit area channel is recording audio permanently. All other channels are only recording when the respective PTT button is pushed. These audio streams are picked off the cockpit mixing consoles -- the Audio Control Panels. This means that either the headset or the oxy mask or a separate loose mic will work, they all first go to the onside ACP.

Another channel on the CVR that is nowadays mandatory if you use FANS or LINK2000+ or another CPDLC system is to record all datalink messages between pilots and ATC. This is obviously not an analog channel really, there is no audio, but only a digital stream of messages. The reason why this is put on the CVR and not the FDR is that these messages don't fit easily in a continuous stream of data parameters. A CVR is much better suited for the occasional burst of data these messages generate.


Hoppie

[PineLemon]

As far as I know, only the cockpit area channel is recording audio permanently. All other channels are only recording when the respective PTT button is pushed. These audio streams are picked off the cockpit mixing consoles -- the Audio Control Panels. This means that either the headset or the oxy mask or a separate loose mic will work, they all first go to the onside ACP.

Another channel on the CVR that is nowadays mandatory if you use FANS or LINK2000+ or another CPDLC system is to record all datalink messages between pilots and ATC. This is obviously not an analog channel really, there is no audio, but only a digital stream of messages. The reason why this is put on the CVR and not the FDR is that these messages don't fit easily in a continuous stream of data parameters. A CVR is much better suited for the occasional burst of data these messages generate.


Hoppie

Thank you for that, definitely clears it all up for me.

[John H Watson]

As far as I know, only the cockpit area channel is recording audio permanently.

The headset boom mics are permanently live for CVR usage. I don't know if that was an FAA, etc, requirement. The jack panels had to be modified for this to happen.

Some aircraft even had an additional external headset jack for monitoring cockpit conversations in case of hijacking, before the new cockpit door security regulations came into effect. Pretty redundant now. The Classic 747's even had a secret headset jack in one of the main gear wells if I remember correctly to monitor cockpit conversations.

The spare microphone may be referring to a handheld microphone. Qantas fitted at least one handheld mic with a special pushbutton tone generator. I recall this was for HF (or VHF?) usage and the tones enabled the control of directional ground station antennas (for better transmission/reception) 

[John H Watson]

Here's a photo of the jack which can be used to monitor the pilots conversations (if you were really bored waiting for the pushback request)




... although I don't understand the "Freighter use only" label.

[Will]

One good source of information is from NTSB accident investigations, because they always go into detail about the sources of the data they use for analysis. This is from a recent 747-400 accident:

"The CVR records audio in a digital format, and is active whenever electrical power is available. The CVR memory will retain the audio from the most recent 2 hours of CVR operation, which may or may not be continuous. The recorded audio data is separated by the Honeywell download software into 2 sets of audio files: a) a 2-channel recording containing the last 2 hours of audio, and b) a 4-channel recording containing the last 30 minutes of audio. The 2-hour portion contains one channel of audio information from the cockpit area microphone (CAM) and the other channel contains a mixture of three audio sources: the Captain's audio panel, the First Officer's audio panel, and the spare channel which may be used to capture a third pilot station or observer station and/or the Public Address system. The 30-minute portion of the recording contains 4 channels of audio data, one channel for each of these 4 sources... On this recording, the spare channel captured the same audio as the First Officer's audio panel channel."

[Jeroen Hoppenbrouwers]

Back in 2012 I was aboard the late N194UA and that one also had a handheld mike with DTMF keys. United used it to beep their way into the automated maintenance/dispatch phone system using SATCOM phone calls, if I remember well.

Far left, dangling from the knob on the glareshield wing.


Hoppie



Ah, good times...