CVR and FDR

[jaded88]

I'm just curious, FDR last 25hours straight so after every flts to you guys reset the thing? Or it will just restart? What about CVR? It last 30 mins and after that it will auto restart? Care to share tech crew?

[Old Fella]

Jaded88. The FDR is unable to be 'reset' by cockpit crew and has the info downloaded by maintenance following an incident in which the info may be useful, or at appropriate intervals depending on recording duration. The CVR however is able to be erased and in some airlines it is included in the Shutdown checklist to be erased. At least that is how it used to be in my time.

[Cyclone733]

In my company the FDR and CVR are not reset by the flight deck. At the end of recording period the units simply start recording over the first data recorded so there is a rolling log of the preceding period. The engineers can erase the CVR units using a switch on the outside of the aircraft if required

Additionally there is a QAR which automatically uploads data to the company's Flight Safety department where it is checked against a set of predetermined limitations. This list is pretty comprehensive and covers such things as 'gates' on the approach eg fully configured by a certain point on the approach. This is used to assess trends within the fleet.

[Nigd3]

Jaded88
FDRs have to record for a minimum of 25hours before they start to overwrite the first recorded data. You will sometimes get longer recordings depending on the amount of data being sent to it.
Most modern CVRs are now 2 hours of audio rather than 30mins and again it just loops and overwrites the oldest data continuously.

[dixi188]

Old CVR's used a tape loop of 30 mins. So it overwrites the previous info.
However accident investigators have managed to retrieve info from more than 30 mins. by clever audio processing since the erasure is not always complete.
Newer recorders record 2 hrs as some incidents had a critical event more than 30 mins. prior to tape stopping .

[Blacksheep]

CVRs and FDRs write continuously, as long as an engine is running. (Usually the oil pressure switches turn the system on and off). Starting from a clean recording medium (newer recorders are mostly solid state - SSFDR and SSCVR) they write until the storage medium is full and then continue, over-writing the older data as they go. Maintenance never erase recorded data, there is no need to do so.

For most commercial operations, flight recorder data is also stored in a Quick Access Recorder (QAR) whence it is recovered post flight on a disk or PCMCIA card and the data fed into a Flight Operations Quality Assurance (FOQA) system for analysis. Engineering also acquire flight recorder data for trouble-shooting, reliability & performance analysis or to identify defects in the flight recorder system itself.

The latest SSFDRs can record 50 hours data; SSCVRs 3 hours.

CVR data can only be erased on the ground. Circuit baulks prevent operation of the bulk erase function until the air/ground sensing is in ground mode and the parking brake is set.

Note that with SSCVRs, erased data can still be recovered by accident investigators in the same way that forensic detectives can read erased data on a computer hard disk.

[john_tullamarine]

as long as an engine is running

or, more generally, when power is on the ship ? - ref CVR (http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=df01c9fcb82109fc9518696efacb84b5&rgn=div8&view=text&node=14:1.0.1.3.11.6.197.57&idno=14) and FDR (http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=df01c9fcb82109fc9518696efacb84b5&rgn=div8&view=text&node=14:1.0.1.3.11.6.197.58&idno=14), noting the usual caveat that present rules may not apply specifically to a given installation on an older Type.

can record 50 hours data

and, for some, longer. The Fairchild (L3) F1000 (http://www.seaerospace.com/loral/f1000.htm) usually gives us around 100 hours with a bit of variation, presumably due to what the actual history is and how this affects compression algorithms... we generally see 80-110 hours

[Blacksheep]

As I understand it, the reason why the recorders aren't usually powered during time on the ground, is to preserve the recorded data. Power for the recorders still comes from the essential busses as required by the design regulations.

Data recorded all the time the aircraft is powered up would overwrite flight data with useless ground data. A typical airliner is powered up pretty much constantly between the power-off stages of major checks, but will be airborne only about 60% of the time - 14 flight hours / day would be considered a typical utilisation for a civil airliner. (The highest utilisation I can recall from my own civil operations experience is 16 hours/day.). So a permanently powered FDR would have about 40% of its useful data overwritten by irrelevant ground data. As I pointed out above, we "back room" engineers and Flight Ops QA use DFDR data for much more than accident investigation.

[jaded88]

Oh, now i get it. The newer ones (SSFDR and SSCVR) is so much useful. Being able to last so long. I'm not too surprised thou that investigaters are able to retrieve data's that have been overwrite. It's just like what Blacksheep said, similar to retrieving data's from hard disk. Anyway thanks for the replies. At least now I understand more clearly how FDR's and CVR's works. I'm only a cabin crew aspiring to be a pilot. Trying to learn as much as possible.

[HAWK21M]

Out here....The erase mechanism on CVRs by setting parking brake has been required to be deactivated by card removal on all Aircraft regiistered out here.
regds
MEL

[212man]

Data recorded all the time the aircraft is powered up would overwrite flight data with useless ground data.

That's EXCATLY what does happen, frequently, following serious incidents to aeroplanes that then remained powered on stand - subsequently causing a hinderance for the accident investigators who then find no trace of the incident data itself.

I had a similar experience myself a year ago when, following a forced landing in a remote area in a helicopter (in jungle) I subsequently powered up the APU to use the satphone to talk to base, several times. Subsequently, when we listend to teh CVR to see exactly what the crew response to the problems in flight was, we found the 2 hour recording started 26 seconds before the first indications, had 5 minutes in the air and the rest was 1:50 of telephone chat! At the time, pulling the CVR cb was not top of our priority list!

[HAWK21M]

Out here....Post Incident....CVR CB has to be pulled.
regds
MEL