The Boeing 767-300ER aircraft had departed Sydney for Honolulu on a scheduled passenger service. While on climb through flight level 105, the left engine fire warning light illuminated. The crew carried out the fire drill, shutting down the engine and discharging the engines' fire bottle number 1. Fire bottle number 2 was discharged shortly after due to the reactivation of the left engine fire warning. The fire warning lights continued to fluctuate on and off.
Air traffic control was advised of the emergency and issued a clearance for the aircraft to return to Sydney. After landing at Sydney, rescue and fire fighting services followed the aircraft to the parking area but were not required.
An examination by the operator revealed that a high-pressure duct (Wye-Air Cooling Part No 1456M55G03) had fractured transversely through the shorter of two stub-sections. High-pressure, high-temperature air that had escaped from the cracked duct, impinged on wiring to the engine fire detection loop. The insulation of the wires was damaged, and the wires disrupted. The duct that ruptured was part of the engine's stage 11 cooling system. Air is ducted from stage 11 of the high-pressure compressor to cool the stage 2 high pressure turbine nozzles.
Examination of the duct by the Australian Transport Safety Bureau determined that the duct rupture was a result of fatigue cracking consistent with high-cycle, vibratory loads. The crack initiated at the base of a reinforcing strap brazed to the duct neck. There was evidence of a misalignment of approximately 2 degrees in the connection of the fractured stub section to the adjoining section. There was no evidence of material or manufacturing defects.
The ruptured duct was replaceable with either of two later part numbers introduced by the engine manufacturer's Service Bulletins (SB) 72-757 and 72-761. A further option was introduced by SB 75-0156, dated February 2001, that replaced the duct with individual pipes. None of the service bulletins had been actioned by the aircraft operator, nor were they required to be actioned.
The engine was fitted with a "Kidde" fire warning loop that relies on changes of resistance of the internal thermistor material. When subjected to heat, the resistance of the material decreases to a pre-set point which then activates the cockpit warning system. Once the source of heat is removed, the resistance of the material returns to the original value and the fire warning ceases.
The Boeing 767-300ER engine fire extinguishing system has two fire bottles located in the fuselage which are both available to either engine. The engine fire drill required that if an engine fire warning light remained illuminated after activation of the number 1 fire bottle, the crew wait for 30 seconds before they discharge the second fire bottle. As the light remained on, the second fire bottle was discharged. That action meant that the entire fire extinguishing system for both engines was exhausted.
The operator advised that the engine fire detection loop was inspected after the incident and found to be fully serviceable. Temporary repairs were effected to the damaged wiring and the wires were subsequently replaced. Photographs of the damaged and disrupted wires, and the corresponding wire loom on another aircraft, are available on the ATSB website, www.atsb.gov.au, or from the Bureau on request. The continued fluctuation of the fire warning was due to damage and disruption of the wires to the engine fire detection loop, rather than a signal from the loop itself.
Individual wires to the engine fire warning loop were in a loom and the loom was positioned between the duct, pipes and the compressor outer case. Compliance with SB 75-0156, that removed the duct altogether, would still not remove the pipes that carry the high-pressure, high- temperature air through the area.