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.