What happened
While on approach to land at The Pas, Manitoba, the crew of Canadian Airlines flight 4/31 experienced a sudden drop in A-system hydraulic pressure to zero. Following the loss of pressure, the crew performed a missed approach. After consulting with maintenance control, the crew decided to divert to Winnipeg. An emergency was declared, and the aircraft was granted priority handling, with Winnipeg airport emergency services placed on standby. The Boeing 737 landed safely, though the runway was temporarily closed to allow for the removal of the aircraft and the cleaning of spilled hydraulic fluid. There were no injuries during the event.
The investigation
Investigators determined that the hydraulic fluid depletion was caused by the failure of an end cap on a hydraulic transfer cylinder located in the left main landing gear wheel well. Analysis of the failed component revealed that the end cap was constructed of an aluminium alloy. The fracture pattern indicated that the failure originated from an internal groove and spread around the circumference of the cap.
Records showed that the hydraulic transfer cylinder in question had been installed on the aircraft on 30 July 1985. The component was maintained as an "on-condition" unit, meaning it was not subject to scheduled removals or overhauls. At the time of the incident, the cylinder had been in service for approximately 44,149 hours.
Findings
- The failure of the aluminium alloy end cap was caused by low-cycle fatigue occurring under normal operating loads.
- A Boeing Service Bulletin issued in 1979 had recommended replacing these aluminium end caps with more durable stainless steel versions.
- While Canadian Airlines implemented an engineering order to replace the caps, the directive only applied to components sent to the shop for service; it did not mandate the replacement of existing in-stock assemblies.
- Because the specific cylinder had not been sent for a shop visit, the original aluminium end caps remained installed on the aircraft.