The Piper PA -31 (Navajo) aircraft with eight persons on board had departed on a charter flight from Mount Isa for Century Mines in north-west Queensland. The pilot reported that the landing gear did not retract when selected up after take-off. The lever was reselected without success. He then placed the landing gear lever into the down position, however the green down and locked light for the nose landing gear failed to illuminate. The pilot also tried, unsuccessfully, to extend the gear by using the emergency hand pump. He then conducted a flyby of the control tower where observers confirmed that the nose landing gear was not fully down. Emergency services were placed on stand-by and the aircraft completed a normal approach and touch-down. During the latter part of the landing roll the nose gear collapsed and the aircraft slid to a stop on the runway. All persons on board were able to exit without injury. The aircraft sustained impact and abrasion damage to both propellers, the nose landing gear doors, and the pitot head.
A subsequent examination by maintenance personnel revealed that a rigid hydraulic pressure line for the landing gear, which attached directly to the hydraulic power pack, had cracked and partially separated beneath the collar section of a line connector. As a result, all the fluid from the hydraulic reservoir, including that portion contained in the power pack emergency sump, had drained out. The loss of fluid meant that the pilot was unable to retract or extend the landing gear; either by the normal or emergency systems.
The operator reported that prior to the last flight, maintenance personnel had attended to a leak in the area of the failed hydraulic line. At that time several line connectors were checked for tightness but none was found to be loose. The operator also reported that the hydraulic reservoir was topped up and the aircraft returned to service.
The ATSB specialist engineering examination 200100023 stated:
"Identification:
The supplied component was identified as a hydraulic line from the nose landing gear assembly of a Piper PA-31 (VH-KAD) that sustained substantial damage on landing resulting from the failure of the nose landing gear to fully extend. The component did not carry any visible identification markings - inspection found that it was produced from a single length of seamless aluminium alloy tubing and employed conventional `B-Nut' connections at each end.
"Failure:
The hydraulic line presented cracking approximately 8.5mm from the flared connection at one end of the line. The cracking was located beneath the B-nut sleeve, which extended to 10mm from the end of the fitting. The cracking extended around 3/4 of the tube circumference and was associated with visible axial twisting of the tube, producing a permanent circumferential displacement of approximately 0.6mm. Branching of the cracking was not evident, nor was any evidence of pre-existing mechanical damage or defects.
"Fracture:
Separation of the crack surfaces allowed close visual and scanning electron microscope examination of the fracture morphology. Specific detail was difficult to resolve due to the extent of surface contact damage, however the suspected point of fracture initiation was identified and placed roughly mid-way between the crack ends. No indications of material defects or other anomalous features were noted.
While specific fracture detail was not evident, the general transverse nature of the cracking, the absence of branching and the lack of any plastic deformation associated with the cracking are all features typical of a fatigue cracking mechanism. The examination failed to find any evidence of contributory material or manufacturing defects.
The axial twisting distortion shown by the cracked region indicated the presence of pre-existing torsional loading on the hydraulic line. Pre-loading or residual loads add to operating loads and compound the level of stress experienced by components in service. In such cases, the potential for the initiation and propagation of fatigue cracking increases in response to the greater applied stress levels.
In the case at hand, torsional or bending pre-loads were most likely introduced during assembly, where one fitting was tightened sufficiently to prevent free movement of the line when the opposite end was brought into position."
Examination of the aircraft's maintenance documents did not reveal if the line was fitted during original aircraft manufacture or during a subsequent repair action. The aircraft had a total of 12,745 hours "time in service" at the time of the incident.
Many technical publications are available regarding the precautions to be taken during installation and maintenance of rigid pipes fitted to aircraft. One of those, the Civil Aircraft Inspection Procedures Manual, advises in section 2, AL/3-14, 3.2.2(a) "When connecting pipes with standard brazed, flared or flareless couplings the following points should be verified:- subpara (iii) That the pipe ends align correctly with their mating parts. Pipes should never be forced into position, since this may introduce considerable stress into the connection and result in subsequent leakage or fatigue damage".
The investigation was unable to determine when the tightening, that induced the axial twisting distortion, occurred.