Boeing Co 747-238B, 21977, 33 km N Brisbane, Qld, 26 December 1999

While climbing through approximately 10,000 ft after take off,
the crew of the Boeing 747-238, VH-ECB, heard an audible bang and
felt a jolt through the airframe. The number 1 engine parameters
were observed to fluctuate momentarily and the indicated Engine
Pressure Ratio dropped by about .15 to 1.35. All other engine
parameters remained normal and the aircraft remained free of
vibration and control asymmetry. A visual engine inspection by the
crew revealed nothing abnormal. After consulting with ground
engineers in Brisbane, the crew decided to continue to Cairns. An
inspection at Cairns revealed that most of the engine's cold stream
nozzle was missing, a number of the outboard leading edge flap
panels were damaged and the outboard flap canoe was holed.

The investigation determined that cracking and delamination of
the acoustic lining skin at the top right side of the nozzle had
been reported during the transit inspection at Cairns on 22
December 1999. An Engineering Authority had been issued to allow
the aircraft to proceed after minor repairs to the acoustic liner
were effected. The thrust reverser had also been locked out as a
precaution and the aircraft then operated nine sectors before the
nozzle failed.

The operator advised that delamination of the acoustic liner is
a known defect. However, investigation determined that the cracking
had originated in the nozzle structure adjacent to the acoustic
liner. There is no requirement for inspection of this area during
service. The area cannot be inspected in-situ without the removal
of panels and the use of a visual aid.

A fleetwide check by the operator found a further six nozzles
cracked in the same area. The nozzles were replaced before further
flight. A detailed inspection of the removed nozzles determined
that the cracking had originated at the top rivet hole of the
standoff which supports the outer skin of the nozzle. The cracks
varied in length from about 2 cm to 21 cm. The incident thrust
reverser assembly, of which the nozzle formed part, had accumulated
19,621 hours since the last heavy maintenance inspection and 56,716
hours since new. The only inspection requirement is a visual
inspection of the thrust reverser assembly at approximately 15,000
hours or shop visit when this area of the nozzle is visible.
However, the nozzle assembly can be swapped between engines
depending on the operators requirements. As the units are not
serialised it is difficult to determine each nozzles time in
service.

The manufacturer's recommended inspection requirements failed to
detect cracking of the cold stream nozzle structure which resulted
in failure of the nozzle.