Sequence of events
Shortly after selecting the landing gear up after departure on a flight from Brisbane to Barcaldine on 27 November 2003, the crew of the de Havilland Canada DHC-8-202 (Dash 8), registered VH-SDA, observed the number-two hydraulic pump caution light illuminate, followed by a zero hydraulic pressure indication. The number-two hydraulic system powered the aircraft's roll spoilers, ground spoilers, parking brakes, nose wheel steering and landing gear extension and retraction. The crew diverted back to Brisbane and extended the landing gear manually for the landing. The failure of the hydraulic system pressure was traced to the failure of a flexible hydraulic hose in the nose landing gear actuation system, which allowed the loss of system pressure and hydraulic fluid.
The failed component was forwarded to the Australian Transport Safety Bureau in Canberra for examination. The flexible hydraulic hose, part number DSC252B4-0124, failed at 12,369 cycles since new and had been installed since the aircraft's manufacture. When first introduced into operation, the hose life limit was set at 15,000 cycles. However, the aircraft operator indicated that a company replacement life limit of 13,000 flight cycles had been introduced in response to a history of failure of the same or similar hoses in the Dash 8 landing gear system.
Examination by the ATSB found the hose had failed by localised rupture at the point of swaged connection to an end fitting. Associated with the rupture was evidence of fatigue cracking and breakage of the external reinforcing braid wires, with cracking also found to a lesser degree on the opposite side of the connection. There was no evidence suggesting that a manufacturing or material defect had contributed to the hose failure. Assembly diagrams showed that the hose failed at the point of maximum flexure when the landing gear was extended or retracted.
Failure of the hose was attributed to the localised fatigue cracking and breakdown of the external braided hose reinforcing sheath and the subsequent rupture of the tubular core in the absence of the support afforded by the sheath. In-service flexures of the hose and pressure cycles and pulsations inherent in the operation of the aircraft's hydraulic system were considered to be likely contributory factors.
The hose failed 631 flight cycles before it was due for removal in accordance the aircraft operator's maintenance schedule. Given that the failure was a result of a fatigue cracking mechanism and there has been a history of similar occurrences, it would be expected that the identified cycles-to-failure for any such component would lie within a distribution represented by lower and upper limits ('outliers'). While most of the failures would occur between those limits, there remains a probability that some components may fail before the lower limit, or indeed after the upper limit. The establishment of a service life limit for a component based on the distribution takes into account the criticality and acceptability of component failure and hence, the necessary confidence level in defining a service limit for removal of the item before failure.
The evidence to hand suggests that this flexible hydraulic hose failure was a distribution 'outlier'. Given that the reliability of the assembly is being managed by life limiting the susceptible component, and the loss of a hydraulic system in this case was able to be managed using existing aircraft flight manual procedures, no Bureau recommendation has been made with respect to the maintenance philosophy applied to that system.
|Date:||27 November 2003||Investigation status:||Completed|
|Time:||1115 hours EST|
|Release date:||07 December 2004||Occurrence class:||Technical|
|Report status:||Final||Occurrence category:||Incident|
|Highest injury level:||None|
|Aircraft manufacturer||de Havilland Canada|
|Type of operation||Air Transport Low Capacity|
|Damage to aircraft||Nil|
|Departure point||Brisbane, QLD|