Common Themes

Organizational Lapses

Because of parts availability, the engine rework of a spare lease pool engine was not completed per the applicable engine service bulletin before the engine was sent to Air Transat by the engine shop. Air Transat did not have a process to ensure completion of the unfinished engine service bulletin before the engine was installed on the airplane. Mismatched fuel and hydraulic lines during replacement of the right engine created interference between the two lines resulting in fuel line cracking. The accident investigators noted that there are no regulatory requirements to review non-mandatory service bulletins for required interacting service bulletin modifications.

Human Error

The Air Transat Maintenance Control Center technician and the lead maintenance technician decided to proceed with the engine change without first obtaining and consulting a service bulletin to ensure proper completion of a modification required for installation of the engine on the aircraft. They both acknowledged concern about not being able to access the service bulletin from the company computer network, but decided on a course of action without the service bulletin. This course of action was not in accordance with the service bulletin and resulted in interference between fuel and hydraulic lines and a subsequent fuel line crack.

The flight crew did not follow the correct procedure for a fuel imbalance indication, which requires that they first determine if there is a fuel leak before opening the crossfeed valve to balance fuel. The flight crew opened the crossfeed valve from memory without consulting the checklist.

Flawed Assumptions

There was an implicit assumption that standard operating procedures fuel quantity monitoring during cruise would detect all but the most critical fuel leaks. It was also assumed that the fuel leak procedures for detected fuel loss, combined with flight deck indications and associated procedures for abnormal fuel system and engine failures, would provide appropriate corrective action to stop the fuel loss and allow a safe diversion and landing. The Air Transat accident showed that standard cruise fuel quantity monitoring is not sufficient for high rate fuel leaks. This accident also showed the criticality of quickly detecting such leaks and stopping the loss of fuel in order to complete a safe diversion and landing.

Unintended Effects

Forward fuel transfer from the stabilizer trim tank that balanced fuel in the main fuel tank feeding the leaking right engine masked the fuel leak for about 25 minutes and contributed to an unnoticed loss of 3,500 kg (7,718 lbs.) of fuel.  Flight crew confusion over flight deck indications, with an initial assumption that lower than expected fuel quantities were a result of a fuel quantity indicating error, and automatic fuel transfer of fuel from the stabilizer trim tank resulted in delayed awareness of a critical fuel loss.

Preexisting Failures

A fuel leak developed following an engine change that was accomplished using a leased engine that was not properly configured relative to the hydraulic pump a fuel leak developed.  The leak was the result of interference between hydraulic and fuel lines, and developed over the early portions of the accident flight, progressing to the point that fuel loss made it impossible to reach the planned destination.

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