Aviation safety investigations & reports

Avionics system event involving a The Boeing Company 747-438, VH-OJO and Douglas Aircraft Company DC8-73, N796FT, Oakland Oceanic Airspace, on 26 January 1998

Investigation number:
Status: Completed
Investigation completed


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FACTUAL INFORMATION The Boeing 747 (B747) was maintaining flight level 350 (FL350) when the crew received a traffic alert and collision avoidance system (TCAS) resolution advisory, warning of conflicting traffic crossing their track at an altitude 400 ft below. They were able to contact the other aircraft, which was a Douglas DC8 maintaining FL330 in accordance with air traffic control (ATC) instructions. Both crews were able to maintain their respective levels and no evasive action was required. When the DC8 departed Auckland, ATC verified that the secondary surveillance radar altitude readout had correlated with the reported altitude from the crew. However, on arrival at Honolulu, ATC noted a 1,500 ft difference in the altitude reported by the crew to that showing on the radar display. The aircraft transponder was providing an erroneous altitude, but the crew had no flightdeck display to enable them to check what information the transponder was emitting. The equipment failure had occurred between departing radar coverage at Auckland and the point at which the B747 crew received the TCAS alert. Initial maintenance action by the company indicated a faulty encoder, but an ongoing investigation in the USA revealed that the central air data computer had been providing faulty inputs to both transponders. SAFETY ACTION As a result of this occurrence, the Bureau of Air Safety Investigation is investigating a perceived deficiency involving the lack of information available to flight crews regarding the serviceability of altitude encoders. In addition, the Bureau is investigating the lack of information displayed to crews regarding altitude information being transmitted by transponders. Any recommendations issued as a result of these investigations will be published in the Bureau's Quarterly Safety Deficiency report.
Download Final Report
[Download  PDF: 15KB]
General details
Date: 26 January 1998   Investigation status: Completed  
Time: 14:13 UTC    
State: International    
Release date: 20 August 1998   Occurrence category: Incident  
Report status: Final    

Aircraft 1 details

Aircraft 1 details
Aircraft manufacturer Douglas Aircraft Company  
Aircraft model DC8-73  
Aircraft registration N796FT  
Sector Other  
Damage to aircraft Nil  
Departure point Auckland New Zealand  
Destination Honolulu USA  

Aircraft 2 details

Aircraft 2 details
Aircraft manufacturer The Boeing Company  
Aircraft model 747-438  
Aircraft registration VH-OJO  
Sector Jet  
Damage to aircraft Nil  
Departure point Los Angeles USA  
Destination Sydney NSW