Created Monday, Jun 13th 2022 20:24Z, last updated Monday, Jun 13th 2022 20:24Z
A Hong Kong Airlines Airbus A330-200, registration B-LHA performing flight
HX-707 from Hong Kong (China) to Denpasar (Indonesia) with 280 people on board, was climbing out of Hong Kong's runway 25L when the crew stopped the climb at 6000 feet reporting a hydraulic leak. The aircraft continued the climb to FL090 while positioning for the return to runway 25L. Upon touchdown on runway 25L loud bangs were heard associated with a large cloud of white smoke, the aircraft rolled out without further incident and became disabled on the runway with two tyres blown. The passengers disembarked onto the runway and were bussed to the terminal.
The runway was closed for about 105 minutes until the aircraft was towed off the runway.
The airline reported the aircraft experienced a technical problem prompting the return.
A replacement A330-200 registration B-LNI reached Denpasar with a delay of about 5 hours.
On Oct 16th 2019 Hong Kong's AAIA reported ECAM messages about green and blue hydraulic system pressures being low appeared prompting the crew to declare Mayday and return to Hong Kong. During landing on Hong Kong's runway 25L tyre #5 burst disabling the aircraft on the runway for about 100 minutes until the wheel assembly was replaced. The occurrence was rated a serious incident and is being investigated by Hong Kong's AAIA.
On Jun 15th 2020 the AAIA released their preliminary report and wrote:
On 29 September 2019 at about 1223 hrs, a Hong Kong Airlines Limited Airbus A330-243 aircraft, flight HX707 took off from Runway 25L at the Hong Kong International Airport (VHHH). Upon reaching 1500 ft on climb out at about 1225 hrs, the Electronic Centralized Aircraft Monitoring (ECAM) warnings and the associated messages indicated that the pressures in the Green and Blue hydraulic systems were low.
The flight crew carried out the emergency procedures accordingly, subsequently declared MAYDAY and decided to return to VHHH for a full emergency landing.
The aircraft landed and stopped safely on Runway 25L with the No. 5 tyre burst. The runway was subsequently closed for about an hour and forty-five minutes.
The inspection of the left landing gear wheel well revealed that a manual valve had detached from the Green hydraulic system Ground-Service Manifold and a Blue hydraulic system return pipe was punctured.
In June 2022 the AAIA released their final report concluding the probable causes of the serious incident were:
Causes
The loss of the Green hydraulic system was caused by the complete detachment of one of the three manual valves in the Ground Service Manifold, due to the failure of the four attachment screws.
The loss of the Blue hydraulic system was caused by the puncture made by the detached Green hydraulic manual valve.
Contributing Factor
The attachment screws of the middle manual valve were broken due to fatigue failure.
The AAIA analysed:
Aircraft Condition
The detachment of the manual valve from the Green system GSM and the puncture of the hydraulic pipe of the Blue system resulted in significant loss of hydraulic fluid from both the Green system and the Blue system, and their subsequent failure.
As the anti-skid system of the aircraft was powered by the Green and Blue hydraulic systems, the anti-skid function of the aircraft was also lost, resulting in the burst of the No.5 aircraft tyre during the landing roll where brakes were applied.
Failure Mode Analysis of Screw
Static Failure
Each manual valve is subjected to 3,000 psi (20.68 MPa). As the bore diameter of the valve is about 27 mm, the force exerting on the bore end is 11,842.9 N. That is, each screw would need to take a quarter of the force, i.e. 2,961N.
For the 0.190 inch diameter screw with 32 teeth per inch, the minor diameter is 0.1562 inch (3.967 mm). Stress is the ratio of force over the area (S =R/A, where S is the stress, R is the internal resisting force and A is the cross-sectional area). Using 3.967 mm as the minor diameter, the stress in the screw is 240 MPa.
Since screws of P/N NAS1101-3H8 are specified with a minimum strength of 160,000 psi (1,103 MPa), there is no basic problem with the strength that can be provided by the screws. Thus, static failure is not expected.
Hydrogen Embrittlement Failure
NAS1101-3H8 screw is cadmium plated with chromate treatment. These are electroplating processes that involve acid solution. In particular, hydrogen is evolved at the screw surface. Normally, a suitable process after electroplating is required to minimise the hydrogen remaining on the surface of the screws. An example of a suitable process is baking.
The remaining screws on the GSM were confirmed to have chromium and cadmium present on the surface, indicating that they had been subjected to cadmium plating and chromate treatment.
During the examination of the fracture surface, there appear to be locations with suspected hydrogen damage. It is unknown that there could be a problem with the manufacturing process, especially for hydrogen elimination. Although the exact root cause of screw failure cannot be fully established, the possibility of hydrogen embrittlement cannot be eliminated.
Normally, hydrogen embrittled components take a short number of operating cycles to fail. Since the screws had taken a few years to fail, even if there were hydrogen damage, the extent must have been minor.
Fatigue Failure
Crack initiation was at the fillet position between the screw head and the shank. In each screw, the crack propagated to a size of about 1 mm before a fast fracture occurred.
Since each broken screw started to have cracking below the maximum working stress of 240 MPa, it is clear that the screws failed through the mechanism of fatigue.
In addition, three of the eight remaining unbroken screws were found to have cracked. They were undergoing similar fatigue fracture processes as that in the four broken screws.
The failure mode identified by the examinations matches with the EATON Aerospace investigations on the broken screws of previous “pop-out” events of the manual valve.
...
Cause Analysis of the Punctured Blue System Hydraulic
Pipe The damaged surface of the opening shows that the burst was not from the inside out, indicating the failure was not due to the pressure cycle induced fatigue.
The surface of the pipe was punctured through by a foreign object.
The location of the pipe was at the opposite side of the Green system GSM with a manual valve detached.
When the manual valve detached under 3 000 psi pressure inside the manifold, the valve catapulted and punctured a hydraulic pipe of the Blue system inside the same wheel well area.
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