Analysis of sounds on the cockpit voice recorder (CVR) of the crashed EgyptAir flight 990 has yielded no evidence of an explosion or mechanical failure, says the US National Transportation Safety Board (NTSB).

Investigators are unable to provide an explanation for what appears to have been a pilot-induced steep descent in the early stages of the accident. Egyptian officials suggest that an explosion caused the Boeing 767 to crash into the Atlantic (Flight International, 24-30 November).

The NTSB continues to lead the investigation, saying that more evidence is needed before deciding whether this is a criminal matter. If it were, the US Federal Bureau of Investigation would take over leadership, with the NTSB continuing its technical investigation.

NTSB chairman Jim Hall says routine air traffic control (ATC) communications are heard throughout the 32min recording. "At no time did a member of the flightcrew-advise ATC of either an emergency or a mechanical failure or concern," says Hall.

Hall says the final flight data recorder (FDR) data has been fed into a Boeing 767 simulator to re-run the critical part of the flight, allowing investigators to see the control deflections and flight display readouts that the crew are likely to have seen. The FDR does not record the flight instrument displays, so the assumption is made that they would reflect precisely what the aircraft did.

Because the simulator's construction does not allow a demonstration of the 7° split elevator condition shown on the EgyptAir FDR (port deflected up, starboard deflected down), this was tried in a Boeing 767 on the ground.

Hall says CVR transcripts in English and Arabic have been produced, but adds: "Further refinements are possible before they are considered complete." Responding to concerns over speculation over what CVR details have revealed, Hall says: "The public would be better served by waiting for the release of the transcript in conjunction with other factual reports."

Source: Flight International