When approximately 120 NM north of Williamtown, the crew of the
BAe 146 aircraft received a fire warning for the number 3 engine.
After confirming the indication and completing the appropriate
check list items, the crew shut the engine down and the fire
warning stopped.
The crew informed air traffic control of the engine shut down
and of their intention to continue to Williamtown. The emergency
services at Williamtown were placed on standby; the aircraft was
landed without further incident.
Preliminary inspection of the number 3 engine by the operator
indicated the presence of extreme temperature around the bleed-band
area and a cracked fuel line between the flow divider and fuel
manifold assembly. The engine had accumulated 922 hours since
overhaul by its manufacturer in the USA in June 2000 and 16,470
hours since new.
The cracked fuel line was removed to be examined by the
Australian Transport Safety Bureau. It carried the following
identification: 91547 - ASSY - 2 - 193 - 940 - 02 and CDA 99193.
The records indicated that the fuel line was fitted during the last
engine overhaul.
The fuel line was a fabricated assembly with stainless steel
unions that were gas-tungsten arc welded to each end of the
stainless steel tubing. The fuel line had failed immediately
inboard of the weld between the tube and union on the flow divider
end of the unit. The examination of the fracture surfaces showed
that the cracking emanated from a point on the inside bore of the
tube.
The cracking was consistent with a fatigue mechanism propagating
under high frequency, low magnitude vibratory loads. The fatigue
crack had propagated circumferentially and covered approximately
eighty percent of the cross-section. The remaining section failed
in overload. The investigation found no evidence of any physical
defect or prior cracking. The material of the tube and the unions
complied with the manufacturer's material specifications.
The fuel line incorporated an expansion loop to reduce the
assembly and operating stress. The issue of these stresses
contributing to fuel leakage at the unions has been previously
identified and was addressed by Textron Lycoming service bulletin
ALF 502R 73-2 Revision 1 of March 1992. The service bulletin also
stressed a need for the fuel line to be positioned so as to
preclude stresses before tightening the unions and securing
clamps.
While these issues may have contributed to the cracking, the
investigation was unable to conclusively determine the reason for
the fatigue cracking of the tube.