Reliability of Flight Data Recorders
 

Here is a question for anyone familiar with Flight Data Recorders and their characteristics.

How common is it for a FDR to miss a complete cycle of readings, i.e. to miss all the sensor readings for one recording cycle? I notice that this happened with the China 747 CI611 aircraft about 30 sec before the FDR stopped for good and the a/c broke up. No other readings were missing from t/o to the end of the recording. The trace below was extracted from the FDR data published by the ASC in Taiwan.

http://home.infi.net/~blueblue/_uima...07-24-2002.gif

I suppose a missing cycle could also be due to an error in the read-back system, or damage to the tape. But just considering the FDR alone, how common is this sort of thing. Would a missing cycle be considered symptomatic of a power system problem?

Also, no transponder signals were received for about 2 mins earlier in the flight. The transponder was not intermittently on, it snapped off for two minutes, and then snapped on again, and stayed on for the rest of the flight. This could also be e receiver fault. What could such intermittent operation be due to, and is it common?

Its not particularly unusual for a mechanical tape DFDR to skip a whole cycle. This can be due to the tape sticking to the spool and moving away from the recording head, especially if the recorder is close to its 8,000 service. Also, immediately prior to an accident when an aircraft is experiencing high 'G' loads, the tape can be moved off the recording head surface by the 'G' forces and fail to record anything. In normal service mechanical recorders suffer an error rate of up to 1 0r 2% of the sample, anything higher than this being a cause for removal. FAA require a sample readout to be made every eighteen months and the UKCAA once a year.

Solid state recorders or SSFDRs are much more reliable and rarely have a sampling error much above 0.01%. I don't know which type was fitted in the accident aircraft but would suspect it had the older mechanical model of DFDR.

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Through difficulties to the cinema

You are correct, the ASC FDR report gives details and a photo of the tape being cleaned with Q-tips.
http://home.infi.net/~blueblue/_uima...07-29-2002.gif
http://home.infi.net/~blueblue/_uima...07-29-2002.gif
So one missed cycle for the flight would seem to be considered an excellent error rate of less than 0.06%, and of no significance w.r.t. the crash enquiry. Thanks for your information.