Aviation Investigation Report A98H0003
2.2.2 Cockpit Voice Recorder
- 126.96.36.199 - Cockpit Voice Recording Duration
- 188.8.131.52 - Recorder Electrical Power Source
- 184.108.40.206 - Clarity of CVR Recording
The 30-minute cockpit voice recorder (CVR) recording on SR 111 was insufficient to provide the amount of data needed to fully analyze all of the factors that may have played a part in the occurrence or that could have led to the identification of further safety deficiencies. Longer audio recordings can provide additional background information to assist in assessing the relevance and importance of events, isolating the information that has a direct bearing on the occurrence, and resolving investigation issues more quickly. In the absence of audio recordings, such evaluations using other means can consume considerable time and resources. A minimum two-hour CVR recording capability would have enabled a quicker and possibly more in-depth assessment of events that occurred earlier in the flight. For example, the investigation would have benefited considerably if CVR information had been available to help analyze earlier events such as the time period of the 13-minute, very-high frequency (VHF) communications gap.
The JAA has implemented Joint Aviation Requirements (JAR)-OPS 1.700, which require that airline transport category aircraft certified after 1 April 1998 be equipped with a two-hour CVR recording capacity. However, the MD-11 was certified in 1991; therefore, only a 30-minute recording capacity for SR 111 was required and applied under JAR-OPS 1.710. The United States of America and Canada still only require a 30-minute CVR duration for transport category aircraft.
Had the CVR been equipped with an independent power source to allow for continued operation after its aircraft electrical power source was lost, the resulting additional recorded information could have facilitated a more thorough understanding of the circumstances faced by the crew in the final minutes prior to the crash, and permitted an evaluation of the associated potential safety deficiencies.
In aircraft where the CVR and the flight data recorder (FDR) are both powered from the same aircraft generator bus, both recorders would stop recording at the same time if that generator bus went off-line. Although the current requirement to power the recorders from the most reliable bus available seems prudent in principle, it leaves both recorders vulnerable to a single-point electrical failure. Powering the recorders from separate buses, with some separation of the wiring and separation of respective circuit breakers (CB), would provide an opportunity for one recorder to remain powered in the event of the loss of a bus, and therefore, improve the likelihood that additional useful information would be recorded.
Some portions of the recording from the cockpit area microphone channel of the CVR that were potentially important to the investigation were difficult to decipher. Experience with numerous other CVR recordings confirms that it is significantly easier to decipher the words when flight crews use their boom microphones. The improved clarity results in significant time saving during investigations, increased transcription accuracy, and higher likelihood of identifying and validating safety deficiencies. Currently, there is no regulatory requirement for the use of boom microphones in all phases of flight, nor is such usage standard practice among airline companies.
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