What happened
On October 3, 1996, a McDonnell Douglas MD-11, registration OH-LGC, was operating a scheduled flight from Helsinki to Beijing. Approximately two hours after departure, the crew received a second-level warning indicating a fuel quantity discrepancy or excessive consumption. While engine parameters remained normal and the flight continued to Beijing, the aircraft consumed 4,870 kg more fuel than expected, leaving 18,400 kg in the tanks. The leak was traced to a damaged gasket at the junction between the number one engine fuel pump and the filter housing. Following the arrival in Beijing, the flange and gasket were replaced, and the engine was tested at idle, showing no leaks. The aircraft then departed Beijing for the return leg to Helsinki.
During the takeoff from Beijing on October 4, 199 and the subsequent climb, the crew received the same fuel quantity warning. The fuel consumption rate increased significantly, with 2,000 kg of excess fuel used during the climb. The N1 rotation for the number one engine was 6% below normal, and fuel flow was 200 kg below normal. The crew responded by dumping fuel, shutting down the number one engine, and returning to Beijing. The return flight lasted 1 hour and 9 minutes. Following the incident, the fuel pump was replaced, and after a successful high-power test, the aircraft was cleared for its return flight.
The investigation
OTKES examined the fuel pump, a Sundstrand Aerospace unit (part number 5006834, serial number 1023), and the maintenance history of the aircraft. The investigation focused on the installation of helical inserts used to repair threads in the pump flange. Detailed analysis by the manufacturer revealed that the flange holes contained extra threads with an incorrect pitch (16 threads per inch instead of the required 14). These mismatched threads prevented a secure seal.
Investigators also reviewed GE Aircraft Engine Service Bulletin 73-175 Revision 1, which had mandated the replacement of these inserts due to issues with galling and corrosion. The investigation looked into the maintenance procedures used by the operator to implement this bulletin and found that while the primary cause was the hardware mismatch, there were discrepancies in the provided tooling and instructions regarding thread cleaning.
Findings
- The primary cause of the fuel leak was the presence of incorrectly pitched threads within the pump flange, which caused the helical inserts to displace under load.
- The mismatch between the 14-pitch original threads and the 16-pitch extra threads weakened the structural integrity of the connection.
- High-pressure fuel spraying into the hot engine compartment created a significant fire hazard.
- Maintenance personnel in Beijing did not observe the protruding threads during the gasket replacement, which prevented the flange from seating properly.
- While the investigation noted errors in the service bulletin and maintenance instructions regarding cleaning tools, it could not establish a specific maintenance error by the operator's hangar during the initial modification work.