Ramon Lopez/WASHINGTON DC and David Learmount/LONDON
The US National Transportation Safety Board's investigation into the USAir Pittsburgh crash has spawned airworthiness directives (ADs) requiring changes in the Boeing 737 flight-control system. This comes despite the fact that the investigation, the most exhaustive in the board's history, has failed to determine a cause for the crash.
Two years after the 8 September 1994 fatal crash of a USAir Boeing 737-300 outside Pittsburgh, Pennsylvania, the Federal Aviation Administration (FAA) has published nine ADs following a critical design review (CDR) of 737s prompted by the Pittsburgh accident and the similarly unsolved 1991 United Airlines 737-200 crash at Colorado Springs.
The most important changes are to the rudder and its yaw-damper. One AD, which may cost $2 million across the US 737 fleet, requires installation within 18 months of corrosion-resistant yaw-damper solenoids, and regular inspection in the meantime.
Some 70 yaw-damper malfunctions have been reported "over the years", says the FAA. The ADs also deal with galling and corrosion in the standby rudder actuation system, particularly the standby pressure control unit (PCU) which is rarely selected.
The CDR determined that the Boeing 737 complies with FAA airworthiness requirements and it had failed to uncover any design flaws that could have caused either accident. NTSB investigators made 27 recommendations, however, which are partly addressed by the nine ADs.
Although some ADs affect only early-model 737s (-100/-200), others affect the entire series from -100 to -500, of which 2,830 are operating worldwide - 1,037 in the USA. The FAA estimates that it will cost as much as $10 million for US carriers to comply with the ADs.
FAA Administrator David Hinson says that the ADs "are designed to make a safe plane even safer". Once finalised, the ADs will require corrective actions within 90 days to 18 months. The cost of implementing the changes to the US 737 fleet would range from $10,000 for inspection/replacement of a 737-300 control cable bracket to as much as $5 million to replace certain outboard and inboard wheel-rims with higher-strength units to reduce the chance of failure causing metallic debris hitting critical flight control systems in the wheel-well. Many US air carriers have already done this, the FAA believes.
Source: Flight International
