On 1 January 2003, the crew of a Boeing 767, registered VH-OGB, operating a scheduled flight from Darwin to Singapore, reported that approximately one hour and ten minutes into the flight, and while maintaining FL380, they received an Engine Indicating and Crew Alerting System (EICAS) message indicating an autopilot and autothrottle disconnect. They reported that their attempts to re-engage the systems were unsuccessful so they reverted to manual control. Additionally, some navigation and fuel calculation functions of the Flight Management Computers (FMC) were not available. The crew elected to continue the flight and descended the aircraft to FL310 to improve manual control. They rotated cockpit duties to avoid fatigue and made an uneventful landing at Singapore.
The operator reported that an examination of the aircraft systems by engineering staff found that both elements of the dual Total Air Temperature (TAT) probe unit had failed. The TAT probe was mounted on the left side of the lower forward fuselage and consisted of dual sensors to provide system redundancy in the event of failure of one sensor. Each sensor of the TAT probe provided air temperature data to its respective Central Air Data Computer (CADC). When temperature input was lost to both CADCs, the FMC disconnected the autopilots and autothrottles, and was unable to provide some of the navigation and fuel calculation information.
The failed TAT probe was removed and sent to the component manufacturer for a detailed examination. The manufacturer of the TAT probe reported that a visual examination of the failed probe revealed damage from electrical discharge that suggested it may have been subjected to a lightning strike. The operator's records for the aircraft showed that a lightning strike on the lower left fuselage had occurred on 29 August 1999, but an inspection carried out in accordance with the aircraft manufacturer's maintenance manual, that included the TAT probe, had not revealed any evidence of lightning damage to the TAT probe.
Examination of the failed TAT probe by the manufacturer found that a dimple in the inner element tube had `popped' outwards and all four ends of the heater element leads had contacted the inner tube of the unit creating an electrical short. The manufacturer reported that water freezing in the element tube may expand and push the dimple outward. Although the visible evidence of electrical discharge had led them to conclude that failure was most likely due to a lightning strike, the reason could not be positively identified. The three years and four months that elapsed from the time of the initiating event until failure also could not be explained.
On 14 August 1998, the operator reported that a dual element failure to the TAT probe occurred to another B767 of their fleet (ATSB Occurrence number 199702646). The operator expressed concern to the aircraft manufacturer that a single, dual-element TAT probe may represent a single-point failure as both elements could become inoperative following a single event. The aircraft manufacturer advised the operator that 'The TAT probe failure described … is extremely rare on the 767'. The manufacturer considered that the dual element met certification requirements and that following failure of both TAT systems, sufficient information was available to safely continue flight under manual control.
During the investigation the operator reported another dual element failure of the TAT probe to a B767 aircraft on 28 February 2004 (ATSB Occurrence number 200400759). The operator advised that following the third dual element failure of a TAT probe and their subsequent investigations, they were implementing a functionality check of all TAT probes into the scheduled maintenance program of their B767 aircraft, and incorporating the requirements of the manufacturer's Service Letter 767-SL-34-111 dated 31 March 1998. That Service Letter suggested that operators install, at a convenient maintenance opportunity, a replacement TAT probe that offered improved performance under severe icing conditions.
The operator advised the ATSB that accounts and analysis of the Crew Resource Management (CRM) issues of both recent TAT probe failures had been drafted for publication and will be published for the education of other crews. Additionally the operator advised that they had introduced the scenario into the CRM training discussions.
|Date:||01 January 2003||Investigation status:||Completed|
|Release date:||24 September 2004||Occurrence category:||Incident|
|Report status:||Final||Highest injury level:||None|
|Aircraft manufacturer||The Boeing Company|
|Type of operation||Air Transport High Capacity|
|Damage to aircraft||Nil|
|Departure point||Darwin, NT|