The CVR Transcript Controversy

Chief Inspector of Air Accidents Ron Chippindale concluded his 1980 accident report into the Erebus tragedy by stating the accident’s probable cause was, “the decision of the Captain to continue the flight at low level toward an area of poor surface and horizon definition when the crew was not certain of their position…” (Paragraph 3.37) He further asserted that the flight engineers had expressed “apprehension” and “dissatisfaction” at the flight’s “continued descent towards a cloud-covered area.” (Paragraphs 2.25, 3.24). The basis for these claims was the content of the cockpit voice recorder (“CVR”) transcript appended to the report.

 

Investigating the Erebus cockpit voice recorder: a critique

The following is a critique of the published official cockpit voice recorder transcript from ZK-NZP, the DC10 aircraft involved in the accident at Mt Erebus, Antarctica, on the 28th of November, 1979, in which all 257 persons on board lost their lives.The critique provides an interesting study in what can happen when established and agreed upon investigative procedures and evaluation techniques are not subsequently followed.

In the following article, Captain Gary Parata (Chairman of NZALPA’s Accident, Incident and Safety Group and a flight recorder specialist with the IFALPA Accident Analysis and Prevention Committee) considers flaws in the procedure used for the production of the published transcript, and why – as a result of those procedural flaws – the Chief Inspector’s theory and conclusions were misleading.
  

Background Information

A cockpit voice recorder or CVR is a device that, as its name implies, records ambient voice (and other sounds) present on the flight deck of aircraft.

A Modern CVR Lab in Canberra.

It also records all passenger address announcements and radio communications between the aircraft and ground stations. Unlike modern solid-state units, the CVR fitted to ZK-NZP consisted of an earlier generation magnetic tape, contained within an impact-resistant module. It incorporated an ‘endless-loop’ mechanism, with conversations recorded more than about 30 minutes earlier being continuously recorded over by more recent data.
 
The CVR unit was found in moderately good condition, with the impact-resistant module not seriously damaged. The unit was sent to Sundstrand, the manufacturers of the unit in Seattle, Washington, in the USA, after the accident, where it was carefully disassembled and a working copy of the tape produced.
 
Much of the intra-cockpit communications were recorded only via a low-fidelity (but high sensitivity) cockpit area microphone mounted in the overhead panel between the two pilots. While both pilots’ voices were, for the most part, loud and clear, conversations taking place behind them were muffled, indistinct, and sometimes simply not intelligible. The sensitivity of the system was so high that voices from the forward galley area aft of the flight deck were also recorded. It would be accurate to say that this CVR system was not designed to accommodate a scenario where several persons positioned remotely from the cockpit area microphone might be speaking and be recorded. Despite these issues, it was still possible to obtain a good quality transcript from most CVR, provided certain protocols were employed for doing so 1.