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 mis-alignment 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.
