Recent Boeing 737 rudder malfunctions are not being linked to rudder control unit failures suspected of causing fatal 737 crashes in 1991 and 1994, senior US Federal Aviation Administration officials say (Flight International, 3-9 March, P13).

Preliminary rulings refer to a MetroJet Boeing 737-200 incident on 23 February, involving a sudden unexplained roll during cruise, and a United Airlines 737, the rudder power control unit (PCU) of which was removed after the flightcrew and a maintenance crew reported difficulty in moving the aircraft's rudder pedals during ground checks.

Release of tentative conclusions on these latest 737 incidents comes on the eve of the US National Transportation Safety Board's meeting to publish the "probable cause" verdict on the crash on 8 September, 1994, of a US Airways 737-300 near Pittsburgh. That accident followed the unexplained loss of a United Airlines 737-200 near Colorado Springs three years earlier. In both cases, an uncommanded rudder hard-over is a suspected, but unproven, cause.

The MetroJet 737 flew with the modified Parker Hannifin rudder PCU that the FAA required to be retrofitted to early-model 737s two years ago. Tom McSweeny, the FAA's associate administrator for regulation and certification, says investigators were unable to fault the PCU and other control systems after a two-day inspection, adding: "We are not really sure at this point what happened. Our thinking is that it is just as likely not to be a problem with the PCU. This does not sound like a malfunctioning PCU event-we are looking very extensively for misaligned linkages and cables."

He says investigators determined that, in the latest United 737 incident, a mechanic failed to complete a functional check when installing a rudder PCU.

On the US Airways crash, McSweeny says the uncommanded rudder hard-over may have been prompted by a rudder reversal, rudder system malfunction or crew response to a wake vortex. He says the data do not support any verdict. McSweeny adds that changes ordered since 1997, including the PCU redesign, yaw damper modifications, addition of a rudder limiter and improved flightcrew training, should help in avoiding future accidents.

Source: Flight International