On September 8, 1994, USAir Flight 427, a Boeing 737-300 on a scheduled flight from Chicago to Pittsburgh, crashed while maneuvering to land at Pittsburgh International Airport. The airplane was destroyed by impact forces and all 132 persons on board were fatally injured. Three years of investigation has failed to yield conclusive proof of why the aircraft crashed. What is known is that the aircraft encountered wake turbulence from a preceding aircraft while at 6,000 feet and 190 knots. The effects of the wake should have been easily recoverable. However, a few seconds after encountering the wake vortex, the 737's rudder deflected full-left and remained in that position for 23 seconds until the aircraft impacted the ground in a near-vertical position.
In 1997, the National Transportation Safety Board asked major participants in the investigation to submit their findings and recommendations. The Air Line Pilots Association (ALPA), The Boeing Company, and US Airways all submitted lengthy reports, the full text of which are available here. We were fascinated by the diversity of findings among these three parties, and present summaries of all three here (with links to the full reports).
Based on the evidence developed during the course of this accident investigation, ALPA believes that the airplane experienced an uncommanded full rudder deflection. This deflection was a result of a main rudder power control unit (PCU) secondary valve jam which resulted in a primary valve overstroke. This secondary valve jam and primary valve overstroke caused USAir 427 to roll uncontrollably and dive into the ground. Once the full rudder hardover occurred, the flight crew was unable to counter the resulting roll with aileron because the B737 does not have sufficient lateral control authority to balance a full rudder input in certain areas of the flight envelope.
The B737 rudder control system design is unique among jet transport designs in that it utilizes a single panel rudder and a single rudder PCU. Since the B737 received its original FAA Type Certificate in 1967, the aircraft has had a history of uncommanded yaw incidents. The B737 rudder control system does not meet the current FAR requirements, FAR 25.671, with regard to malfunction probability and effects. During the course of the investigations of UAL 585, USAir 427, and Eastwinds 517 a number of failure modes have been identified with the B737 main rudder PCU which can lead to uncommanded full rudder hardovers and rudder reversals. The B737 main rudder PCU's design redundancy is ineffective if any of these failure modes occur and, as a result, the aircraft is not in compliance with the FARs. Some secondary valve jams leave no witness marks. USAir 427 experienced a secondary valve jam and reversal in the main rudder PCU that resulted in an uncommanded full rudder deflection. The B737 has limited lateral control authority which, at certain airspeeds and aircraft configurations, is unable to counter the roll due to sideslip caused by a full rudder hardover. In the case of USAir 427, the lateral control authority available was not sufficient to maintain a wings level attitude once the flight experienced the full rudder hardover.
Full text of ALPA's report to NTSB (HTML).
The NTSB has recognized that a theoretical explanation for an accident can only be elevated to the "probable cause" of the accident when there is "conclusive" and "decisive" evidence to support that explanation. Several elements leading to this accident are clear:
Perhaps the most significant findings from the investigation are:
Full text of Boeing's report to NTSB (PDF).
The data demonstrates, and all parties seem to agree, that USAir Flight 427's rudder moved to a full-left position shortly after the aircraft encountered wake vortices generated by a preceding aircraft. It is also clear that the wake vortex encounter did not directly cause the accident. The investigation revealed that the Boeing 737 rudder control system has certain anomalies which may have resulted in a rudder reversal or uncommanded full rudder deflection on the accident aircraft. As a result, the Board has issued recommendations to correct problems that might exist in the Boeing 737 rudder control system. In addition, US Airways has implemented procedures to deal with potential rudder control problems in the unlikely event they should occur.
The Captain and First Officer were trained, certificated and qualified for the flight in accordance with applicable regulations. Nothing in the flight crew's background suggests they would have had problems with disorientation or control of the accident aircraft. The flight crew's performance was not affected by illness, fatigue, or personal or professional problems. The aircraft was properly maintained in accordance with applicable regulations. Inspections of the rudder control system required by AD 94-01-07 had been correctly accomplished in a timely manner. It was daylight and the weather was clear with a distinct horizon at the time of the accident.
The accident aircraft's speed and configuration at the beginning of the accident event complied with the manufacturer's and operator's maneuvering speed schedules. At the beginning of the accident event, the aircraft was at or below the "crossover speed," which is the speed below which lateral flight control authority is insufficient to counter the roll induced by a full rudder deflection. The manufacturer did not advise the operator, prior to this accident, that there were speeds below which B-737 lateral flight control authority is insufficient to counter the roll induced by a full rudder deflection. The manufacturer's published maneuvering speeds for some weights and configurations of the Boeing 737 were too slow and did not provide sufficient airspeed margins to allow recovery from an uncornmanded, fully deflected rudder or rudder reversal. The accident aircraft's rudder moved uncommanded or reversed to the full-left position. At the onset of the full rudder movement, the accident aircraft's speed was at or below the "crossover" speed.
The manufacturer did not provide the operator, prior to this accident, with an emergency procedure for recovery of a Boeing 737 from an uncornmanded, full rudder deflection or rudder reversal. Based on information known to them at the time, the flight crew reacted correctly to the uncomrnanded, full rudder deflection or rudder reversal and resultant left roll by selecting opposite aileron and attempting to maintain altitude. After the onset of the full rudder movement, decreasing airspeed, increasing bank angle, and increasing aerodynamic loads kept the aircraft's speed below the "crossover" speed. With an uncommanded, fully-deflected rudder or rudder reversal and the aircraft below the "crossover" speed, recovery through techniques known at the time was not possible.
The probable cause of this accident was an uncommanded, full rudder deflection or
rudder reversal that placed the aircraft in a flight regime from which recovery was not
possible using known recovery procedures. A contributing cause of this accident was
the manufacturer's failure to advise operators that there was a speed below which the
aircraft's lateral control authority was insufficient to counteract a full
Full text of US Airways' report to NTSB (HTML).
Full text of US Airways' report to NTSB (PDF).