What happened
On March 8, 1999, at 05:43 local time, a Boeing 737-200, registration LY-BSD, operated by Lietuvos avialinijos, departed Vilnius Airport for Istanbul. Shortly after climbing to an altitude of 9,600 meters, the crew observed a low-pressure warning for the System B N1 pump. Following the Quick Reference Handbook (QRH) procedures, the crew continued the flight, but 40 minutes later, a low-pressure warning appeared for the System A N2 pump.
As the hydraulic fluid level in System A approached zero, and with the aircraft approximately 110–120 miles from Vilnius, the crew decided to return to the airport. During the diversion, further failures occurred, including a low-pressure warning for the System B N2 pump and a low quantity indication for System B. The crew followed all necessary checklists and successfully performed an ILS approach, landing at Vilnius at 06:53. There were no injuries among the 51 passengers and 6 crew members.
The investigation
The investigation focused on the physical condition of the hydraulic systems and the communication between air traffic services and emergency responders. Investigators identified hydraulic fluid leakage in the main landing gear bay and the aircraft fuselage.
Physical examination of the components revealed a crack in high-pressure tube P/N 6 and a small hole in tube P/N 3. Additionally, the pressure modulator package showed a deformed filter element and damage to the check valve housing. The investigation also reviewed the sequence of notifications provided by the Minsk Service Control (SVC) to Vilnius emergency services, noting delays and procedural errors in how information was disseminated to fire and rescue teams.
Findings
- The primary cause of the hydraulic failure in both Systems A and B was a crack in high-pressure tube P/N 65-44546-3655 due to fatigue.
- This crack led to significant fluid loss, causing the pressure drop.
- A hydraulic shock, resulting from the sudden pressure drop in the failed tube, caused damage to the pressure modulator package and its check valve.
- There were deficiencies in the communication chain; the initial notification from the Minsk SVC to Vilnius authorities lacked specific aircraft details and did not follow the established order of notification for certain rescue services.
Safety action
- Technical and flight departments should be briefed on the circumstances of this incident.
- The manufacturer, Boeing Company, should be notified of the incident.
- Maintenance personnel and crews should pay particular attention to the integrity and leak-free operation of hydraulic systems and components.
- Air traffic services should undergo training regarding radio communication and information delivery to ensure brevity and proper phraseology.
- Improvements were recommended for the installation of a direct audio link between Vilnius airport control and emergency rescue services to expedite notification.