What happened
On February 3, 2008, a Boeing 737-600 operating a scheduled flight from Stockholm to Milan-Linate experienced a series of hydraulic system failures while cruising at FL390. Approximately one hour and 50 minutes into the flight, the engine-driven pump for hydraulic system A (EDP ENG 1) failed. Two minutes later, the electric pump for hydraulic system B (EMDP ELEC 1) also ceased operation. Roughly ten minutes after the second failure, the second engine-driven pump for hydraulic system B (EDP ENG 2) failed, resulting in a total loss of pressure in system B.
Following the loss of the third pump, the flight crew decided to divert the aircraft to Nuremberg, Germany. The aircraft landed at Nuremberg Airport without further incident. There were no injuries to the 69 passengers or 5 crew members on board.
The investigation
The BFU investigation examined the aircraft's maintenance history and hydraulic architecture. Records indicated that on January 31, 2008, a flickering low-pressure indication had been noted on the EDP ENG 2 pump during an approach, which led to the cleaning of the low-pressure switch electrical contacts.
Investigators analyzed the flight data recorders and cockpit voice recorders to establish the exact timeline of the pressure drops. While the pressure in system A remained stable at approximately 3000 PSI, the pressure in system B dropped from 3000 PSI to 0 PSI following the final pump failure. Physical inspection of the aircraft revealed no external leaks or structural damage.
Findings
- The investigation identified that water had entered the pneumatic system.
- This contamination affected the pressurized air used to feed hydraulic fluid from the reservoirs to the pumps.
- The presence of water in the pneumatic system is a known issue, for which a manufacturer service bulletin had previously been issued.