Synopsis A Boeing 747-400 lost all information on its six electronic display units in cruise from Singapore to Sydney. Loss of primary flight data is a critical incident; aircraft have standby (backup) instruments, which rely on completely separate system components to avoid common-cause failure. The crew flew the aircraft using the standby flight data instrumentation while they trouble-shot the problem. Cycling the circuit breakers for the EFIS/EICAS interface units restored the electronic display units to normal operation.
On 2 December, 2003, the U.S. National Transportation Safety Board issued Recommendations A-03-55 and A-03-56. The short document may be read at www.ntsb.gov/Recs/letters/2003/A03_55_56.pdf. The note recounts the incident as follows:
A similar event occurred on another SIA B747-400 on November 6, 2001 ...
This earlier event occured to a Sydney-to-Singapore flight, during an emergency descent because of an cabin pressure warning. Maintenance personnel recycled the EIU circuit breakers on the ground and restored the IDUs to normal operation.The NTSB document describes the function of the flight displays are follows:
The EIUs are apparently responsible for data display on all six IDUs and preliminary investigation has indicated that all six IDUs blanked because all three EIUs stopped transmitting data. The EIUs are identical devices; the architecture is triple-redundant. The cause of this loss of data has not been determined, and no countermeasures have yet been identified that could inhibit the loss of all six IDUs again.
Boeing recommended cycling the EIU circuit breakers in such an event, and the NTSB recommended that this procedure be included in the quick-reference handbook used by the flight crew to access procedures in an emergency.
The NTSB letter was also reported in Flight International, 9-15 December 2003, p13; and by Frances Fiorino on p31 of Aviation Week and Space Technology, December 15, 2003.
I recounted this incident, in much the same words as here, in an article B747-400 Electronic flight displays rendered inoperative in the Risks Digest, Volume 23 Number 12, 12 January, 2004.