Incident Overview
Canadian Airlines flight 431 was on approach to land at The Pas, Manitoba, when the aircraft's A-system hydraulic pressure dropped to zero. The crew executed a missed approach and, after consulting with company maintenance control, decided to return to land at Winnipeg. The crew declared an emergency and requested priority handling. Winnipeg airport emergency response services were placed on standby. The aircraft landed safely. The runway was closed until the aircraft could be towed to the terminal and spilled hydraulic fluid was washed from the surface. There were no injuries.
Investigation Findings
Company maintenance personnel determined that the A-system hydraulic fluid depleted when the end cap failed on a hydraulic transfer cylinder installed in the left main landing gear wheel well. The failed transfer cylinder (Boeing Assembly No. 69-54600-2, Serial No. 585) was removed from the aircraft. The failed components of the end cap and the internal shuttle mechanism were removed from the cylinder. Both end caps installed on this hydraulic transfer cylinder were made of an aluminium alloy material. The failed end cap displayed evidence of a multi-surfaced fracture that emanated from an internal groove and proceeded around the circumference of the cap material. The failed components were forwarded to the TSB Engineering Laboratory Branch for analysis.
Maintenance History
The hydraulic transfer cylinder end caps are the subject of Boeing Service Bulletin (S.B. 737-32-1099), issued 14 September 1979, which recommended replacement of aluminium end caps with higher strength stainless steel end caps. On 10 January 1985, Canadian Airlines International created an engineering order (E.O. 3200-01033) that referenced the service bulletin and mandated replacement of the aluminium hydraulic transfer cylinder end caps at the next cylinder shop visit. The engineering order did not specify action to be taken regarding aluminium end caps installed on cylinder assemblies that were in inventory ready for installation on company aircraft. Following installation, the hydraulic transfer cylinders are maintained as "on-condition" units and are not required to be removed or overhauled in accordance with any specified time-frame.
The data plate on the failed unit indicated that the transfer cylinder was a ...-2 model. Company records indicated that the hydraulic transfer cylinder was a ...-1 model. The data plate serial number (585) was consistently displayed in all records for the component. This hydraulic transfer cylinder was installed from inventory stock on the occurrence aircraft on 30 July 1985. The cap failed approximately nine years and one month after installation, after 44,149.22 hours in service.
Cause Determination
The material composition of the failed end cap was consistent with the standard aluminium alloy (AA 7075 heat treated to a T-73 condition) used in the original end cap construction. The mode of failure was determined to be low-cycle fatigue; the fracture propagated under normal service loads from fatigue-generated pre-cracks originating at the top edge of a ring groove in the internal diameter of the end cap. No stress concentrations other than those caused by the geometry of the groove were detected.
The Transportation Safety Board concluded that the aluminium alloy end cap failed because of low cycle fatigue under normal service loads.