Boeing 737 rudder issues
[1] Similar rudder issues led to a temporary loss of control on at least one other Boeing 737 flight before the cause of the problem was ultimately identified.
[2]: ix [3]: 49 This was only the fourth time in the NTSB's history that it had closed an investigation and published a final aircraft accident report where the probable cause was undetermined.
This lack of precision led to it being possible for Boeing to interpret the data differently from the way the NTSB did, leading the manufacturer to suspect and insist that the pilot had responded incorrectly to a wake turbulence incident.
Unlike the two prior incidents, the rudder issues on Flight 517 spontaneously resolved and the pilots were able to safely land the aircraft.
[3]: 51 When the plane abruptly rolled to the right, the captain applied left aileron and attempted to move the rudder, but the rudder pedal controls felt stiffer than normal and did not seem to respond to his input; when his flight control inputs did not immediately resolve the roll upset, he also advanced the throttle of the right engine in an effort to compensate.
[3]: 71, 81–85 Testing revealed that under certain circumstances, the PCU's dual servo valve could jam and deflect the rudder in the opposite direction of the pilots' input.
[3]: 81–85 Thermal shock testing revealed that the uncommanded rudder movement could be replicated by injecting a cold PCU with hot hydraulic fluid.
The NTSB concluded that the probable cause of the Flight 427 crash was rudder reversal due to the PCU servo malfunction.
The following Boeing 737 aircraft incidents were also suspected of being caused by a rudder PCU malfunction: On December 19, 1997, SilkAir Flight 185 crashed in Indonesia, killing 104 people.
As the crash involved a Boeing 737-300 rolling and diving toward the ground at a steep angle, investigators considered the possibility of rudder hardover due to PCU servo malfunction.
[12] The Indonesian National Transportation Safety Committee, the lead investigating agency, concluded in its December 14, 2000 final report that it had found "no evidence to explain the cause of the accident.
[13]: 24–25 [14] In 2004, following an independent investigation of the recovered PCU/dual-servo unit, a Los Angeles jury, which was not allowed to hear or consider the NTSB's conclusions about the accident, ruled that the 737's rudder was the cause of the crash, and ordered Parker Hannifin, the PCU/dual-servo component manufacturer, to pay US$44 million to the plaintiff families.
[5] Mayday separately dramatized the SilkAir crash investigation and lawsuit, including its connection to the 737 rudder controversy, in a 2012 episode titled "Pushed to the Limit" (broadcast in some countries as "Pilot Under Pressure").
