National Transportation Safety Board
Office of Public Affairs
The National Transportation Safety Board has been investigating two recent events involving a United Airlines B-737-300 and a Metrojet B-737-200. Both airplanes were equipped with the new servo valve design. Information gathered by Safety Board investigators follows:
On February 19, 1999, the flight crew of the United Airlines plane reported a "stiff" or sluggish rudder response while performing a flight control check while taxiing. The crew returned to the gate and requested maintenance technicians to investigate the problem. After the main rudder power control unit (PCU) was replaced, the rudder pedal forces returned to normal. The NTSB examined the PCU and found that a valve spring guide was mispositioned. When the guide was properly positioned, the PCU passed the standard acceptance test procedure. Further testing of the servo valve is planned for next week.
On February 23, 1999, a Metrojet plane en route from Orlando, Florida, to Hartford, Connecticut, experienced an unintended heading change while on autopilot and cruising at approximately 260 knots at 33,000 feet over Salisbury, Maryland.
The pilots declared an emergency - noting a problem with the rudder - and diverted to Baltimore-Washington International Airport for a precautionary landing. NTSB removed the flight data and cockpit voice recorders, the main rudder PCU and several other parts for its investigation.
To date, the NTSB has not found anything in its inspection of the Metrojet airplane and the components that would explain the in-flight event. Following the event, the rudder PCU passed the required acceptance tests; it is undergoing further examination. Preliminary analysis of the FDR appears to indicate that a rudder deflection occurred, but further analysis of the data is being conducted. The CVR had recorded over the event, so its data was useless to the investigation.
Examinations of the servo valves from the PCUs of both aircraft showed no sign of the cracking that has been detected on some production models of the new PCUs.
NTSB Office of Public Affairs: (202) 314-6100
The National Transportation Safety Board (NTSB) is an independent federal agency charged with determining the probable cause
of transportation accidents, promoting transportation safety, and assisting victims of transportation accidents and their families.