A Metrojet Boeing 737-200 suffered a sudden unexplained roll in the cruise at 33,000ft (10,000m). The 737, operated by the US Airways' low-fare subsidiary, made a safe precautionary landing and the US National Transportation Safety Board is investigating.

The in-flight incident follows changes to the 737 rudder system, prompted by two unexplained fatal crashes. One involved a US Airways 737-300 near Pittsburgh, Pennsylvania, on 8 September, 1994. In the other, a United Airlines 737-200 crashed near Colorado Springs on 3 March, 1991. In both cases, an uncommanded rudder hard-over is suspected to be the cause.

In the 23 February incident, the Metrojet 737-200 was over Maryland, en route from Orlando to Hartford, when an uncommanded control wheel deflection occurred while on autopilot at 260kt (480km/h) indicated airspeed.

One pilot told Safety Board investigators that the control wheel rotated left. The pilots also noticed that the rudder pedals were offset. They disengaged the autopilot and attempted to centre the rudder pedals manually, but there was no response. They then disengaged the yaw damper, activated the standby rudder system, and the rudder pedals then centred. They made an emergency landing at Baltimore-Washington.

Investigators are analysing the flight data and cockpit voice recorders and interviewing the crew. They are also examining the yaw damper coupler and rudder power control unit, two hydraulic fluid filters, and samples of hydraulic fluid.

Source: Flight International