In Boeing's view, under the standards developed by the NTSB, there is insufficient evidence to reach a conclusion as to the probable cause of the rudder deflection. The airplane entered a stall and remained stalled for approximately 14 seconds and 4,300 feet of altitude loss. Transport pilot training widely used in the 1994 time frame did not prepare flight crews for recovery from the highly unusual roll rates and roll and pitch attitudes encountered by the crew of Flight 427. 737 yaw damper reliability enhancements are needed to reduce potential airplane contributions to upsets. Potential 737 rudder PCU input rod fastener failure mode.
Research is needed on better ways to detect and avoid wake vortices.
Existing 737 flight control anomaly procedures could be improved. The flight data recorder infor mation from this accident was inadequate to prove definitive events. 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.