CVR Failures and Deficiencies

[TheShadow]

Trying to put together an informative article for my company's safety news-sheet on the subject of CVR failures (and/or failures to record, failure to stop recording).

In other words I am pursuing instances where investigators, after a serious incident or accident, were denied any useful CVR data due to:

a. System malfunction

b. CB having been pulled (Silkair style)

c. CVR running on and over-writing (either in the air or after landing)

If anybody can point me towards a source of such a compilation or offer up individual cases/instances (preferably with details or a link), or comment generally, it'd be much appreciated.

You can also send direct to james.smith#iinet.net.au (replacing hash with ampersand @ - of course)

[Centaurus]

I have struck an unserviceability with the Cockpit Area Microphone (CAM) on several occasions with both the B737-200 and the B737 Classics. The CVR test was normal (press the test button and note status light and tone). But when a headset is plugged into the headset jack and you say a few words out aloud, or jiggle the flap lever to hear the noise through the CAM, either you didn't hear a thing, or it was too muffled to pick up words. Either way it would have been useless as far as listening to crew conversation or specific noises in the cockpit.

I would write always up the defect in the maintenance log. A specific check of the CAM serviceability is not a part of the Boeing before start scan nor is it called for in the amplified procedures and for that reason I would not be at all surprised if unserviceable CAM's are rarely picked up. I made a point of checking the CAM serviceability on every flight - it's dead easy to check it out.

[Blacksheep]

CVR testing is included in the Approved Maintenance Programme, the interval at which it is performed is, or ought to be, governed by the operator's reliability monitoring programme. During the ten years or so that I worked in Base Maintenance, I found no more than three unserviceable units when performing these tests. During my subsequent twelve years in Technical Services we never found cause to either increase or reduce the testing interval.

Be aware that the CAM frequency response isn't ideal for voice and this microphone is not intended to pick up speech; it is primarily intended to record cockpit background sounds such as trim wheel operation, landing gear being lowered etc. A common problem for accident investigators is that, for obvious reasons of comfort, pilots often prefer to use the overhead speakers and hand microphones for communication. In this case, no continuous direct speech is recorded from the "Hot" mic on the headset, only when the transmit button is pressed.

On one airmiss incident that I was involved with, the crew had pulled the CVR circuit breaker to stop the recording of ATC transmissions being erased by overwriting - CVRs generally record only the last thirty minutes. During the subsequent transit stop somebody reset the circuit breaker and the evidence was lost - though we had thirty minutes of interesting conversation covering the let-down into home Base.

[matkat]

In My maintenance career I remember several CVRs having to be replaced by the failure of the test function,do not remember any DFDRs failing.

[ICT_SLB]

JAA has required 120 minute CVRs for several years. The new FAA requirements will also now be for 120 minutes, solid-state (no more tape) and an independent power supply to give an additional 10 minutes recording capability after loss of normal power. CVR & FDR must be on different busses & physically seperate. The ARINC standard for future CVRs will also require recording of datalink transmissions (ACARS).

BTW from my experience of cert testing CVRs, the area mic is often near unreadable in one pass (as in the self-test) but actually is good enough to pass the authority's review.

[Blacksheep]

Datalink on the CVR? Shades of that familiar old Modem warble eh? ;)

We modified all our CVRs and DFDRs to solid state a while back. Much better results, with an already excellent MTBUR disappearing off the clock. Tape recording quality has never been very good and most mechanical recorder faults were due to either the tape breaking or failure of the tape transport mechanism, in both CVRs and FDRs.

It's certainly a practical proposition to keep a CVR running after a total power failure - there'd be plenty to listen to - but I've long been slightly amused by the regular suggestions that the DFDR should have its own UPS. I'm not sure what useful data it would be recording, with all the ARINC 429 databusses shut down, along with more than 90% of the aircraft systems...