Occurrence
On 29 July 2012, a Boeing 737-800, registration 4X-EKB, operated by EL AL Israel Airlines as flight LY-333 from Tel Aviv to Brussels, was cruising at flight level 340 over Serbian airspace, approximately 170 nautical miles south of Budapest, when the crew reported a fuel system problem to Belgrade air traffic control. The flight crew announced their intention to divert to Budapest as an alternate airport. During the descent, the crew declared a Mayday emergency but stated that no technical assistance was required for landing. In response to an ATC query, the crew explained that the aircraft had 6.2 tons of fuel on board, but 3.8 tons in the right-hand tank were unusable. The aircraft landed safely on Runway 31R at Budapest Liszt Ferenc International Airport (LHBP).
Investigation
The Hungarian Transportation Safety Bureau (TSB) received notification from the control tower duty service, which did not initially mention the emergency declaration. As a result, TSB personnel did not attend the scene. Upon learning the full details, the investigator contacted the maintenance organization responsible for troubleshooting and requested documentation. The investigator also contacted the head of the Israeli investigating organization to obtain the captain’s report and requested a transcript of the voice communication from the Hungarian aviation authority.
Findings
The investigation established that the flight departed Tel Aviv with 14,700 kg of fuel distributed as follows: left main tank 3,900 kg, center tank 7,100 kg, and right main tank 3,900 kg. Fuel consumption from the center tank proceeded normally. When the center tank emptied, consumption from the left main tank commenced, but no fuel was consumed from the right main tank. The crew performed a fuel leak check as per the Quick Reference Handbook (QRH) and detected no leakage. They then attempted to correct the imbalance but encountered difficulty because the open position indicator for the cross feed valve did not illuminate when the valve was opened. Despite this, the crew suspected the valve was open, as the engines continued to operate. The captain decided to land in Budapest. The crew attempted the next QRH step, which required turning off both pumps on the low side (the right side). Fearing an engine flameout, they did not turn both pumps off simultaneously; instead, they turned one pump off, then back on, then the other. Without feedback on the cross feed valve position, the crew requested diversion.
With only fuel from the left main tank – somewhat more than 2,000 kg – and thunderstorm activity reported in Budapest, the captain declared an emergency but requested no technical assistance. The aircraft landed smoothly. After landing, the consumable fuel remaining was exactly 2,000 kg.
Maintenance Findings
During troubleshooting, maintenance personnel found that the direct current motor driving the cross feed valve had broken down. After replacing the electric motor and performing necessary maintenance, the aircraft was returned to service.
Conclusion
The TSB concluded that the crew acted in compliance with the operational manual requirements. The bureau found no circumstances warranting a safety recommendation and closed the technical investigation.