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.