User:Parker Aerospace communications department/sandbox

__NOINDEX__ Boeing 737 incidents In the 1990s, three accidents involving Boeing 737 aircraft occurred. The causes of the accidents were the subject of extensive, years-long investigations, with mixed and uncertain findings.

On March 3, 1991, United Airlines Flight 585, a 737-200, crashed in Colorado Springs, Colorado, killing 25 people. The National Transportation Safety Board (NTSB) concluded that the accident “could not identify conclusive evidence to explain the loss of United Airlines flight 585.”

In its initial probable cause statement, the board indicated that it considered the two most likely explanations for the sudden uncontrollable upset to be a malfunction of the airplane’s directional control system or an encounter with an unusually severe atmospheric disturbance.

The board stated in its probable-cause statement that, although anomalies had been identified in the accident airplane’s rudder control system, it was unable to identify a condition under which any such anomaly would have produced a rudder movement that the pilots could not have easily countered by the airplane's roll controls.

Instead, the board concluded that weather conditions in the Colorado Springs area at the time of the accident were conducive to the formation of a horizontal axis vortex (rotor) and that some witness observations supported the existence of a rotor at or near the time and place of the accident. [14] The Parker rudder power control unit (PCU) was not implicated as the cause of the accident. On September 8, 1994, USAir Flight 427, a 737-300, crashed near Pittsburgh, Pennsylvania, killing 132 people. The investigation lasted five years and faced many challenges, including a flight data recorder (FDR) that only recorded 13 basic flight and engine parameters, but not flight control input or surface positions. (Today’s typical FDR records hundreds of parameters, including multiple flight control inputs and surface positions.)

The rudder PCU, which hydraulically powers the movement of the rudder flight control surface and is manufactured by Parker Hannifin Corporation, was recovered from the wreckage. It was put through extensive and detailed acceptance testing to determine condition and operation at the time of the accident. The acceptance tests did not reveal any disqualifying anomalies.[15]

Further examination and testing of the PCU was performed, subjecting it to extreme conditions that were highly unlikely to occur on the aircraft. The PCU was subjected to a -40⁰ F cold soak without hydraulic pressure applied to the unit. It was then instantly pressurized with hot (170⁰F) fluid. At this point, a hesitation could be produced under these unrealistic conditions.

Under normal operation, the rudder PCU would be constantly pressurized by 70⁰F fluid which would keep the PCU warmer than ambient temperature. Any separate hydraulic component failures causing an increase in hydraulic fluid temperature would happen gradually, and the PCU would not be instantly subjected to such a large thermal shock.

Further examination of the test data indicated that the servovalve secondary slide momentarily jammed to the servovalve housing, and that the subsequent overtravel of the primary slide resulted in an increase in system return flow that could cause a rudder actuator reversal (travel in the direction opposite to that commanded).

Although reversal of the PCU actuator was not noted by any of the participants or observers during the tests, the periods of anomalous hydraulic system fluid flow observed in the data were consistent with the misporting of the hydraulic fluid from the effects of the jammed secondary slide and overtravel of the primary slide, resulting in a momentary output command opposite to the input command. [15]  However, there was no evidence that a rudder PCU reversal occurred on flight 427, as noted by all NTSB investigators in their reports.

Despite no evidence of a reversal, the NTSB concluded that the probable cause of the accident was “a loss of control of the airplane resulting from the movement of the rudder surface to its blowdown limit. The rudder surface most likely deflected in a direction opposite to that commanded by the pilots as a result of a jam of the main rudder power control unit servovalve secondary slide to the servovalve housing offset from its neutral position and overtravel of the primary slide.”[15]

The Federal Aviation Administration (FAA), Boeing, and Parker disagreed with the NTSB’s statement of probable cause. The FAA stated “While the investigation has produced evidence which support the scenarios where the rudder moved to a full-left position after an encounter with wake turbulence, the cause of the movement is still at issue.

“The FAA, upon review of the evidence, cannot conclude that a failure mode which resulted in an uncommanded rudder movement on Flight 427 has been identified. Any causal findings, to be legitimate, must have conclusive evidence to support findings of a hard over or reversal rudder. Such evidence has yet to be found. Consequently, a specific causal finding of this nature may not be appropriate.” [15]

The Boeing submission stated that “three of the hypothetical system-related scenarios (a dual slide jam, a secondary slide jam with primary slide overtravel, and an input linkage jam) and one hypothetical flight crew input scenario evaluated during the investigation all “potentially fit a kinematic analysis.”

However, with regard to the three hypothetical system-related scenarios, Boeing’s submission commented that “evidence does not support finding as probable cause.” Boeing’s submission further stated that “there is no evidence to support a conclusion that an uncommanded full rudder deflection occurred.

While there is no evidence of a crew commanded, sustained left-rudder input, such a possibility is plausible and must be seriously considered, especially given the lack of evidence of an airplane-induced rudder deflection.” [15] After the USAir 427 investigation report was completed, the NTSB revised the United 585 probable-cause statement, making the cause identical to USAir 427, despite no evidence that this had occurred.[14] There was strong disagreement with this revision. Boeing and Parker noted that there was no evidence in either case that linked Parker’s PCU with the accidents’ causes.

In 1997 a Boeing 737 used in SilkAir Flight 185 crashed in Indonesia. The Indonesian National Transportation Safety Committee could not determine the cause of the crash, due to the near total lack of physical evidence and complete destruction of the aircraft. [7] However, the US National Transportation Safety Board disagreed and determined that the crash was caused, possibly intentionally, by the pilot.[8][9]    The NTSB commented on  the NTSC report,  stating that “It is more likely that the nose-down flight control inputs were made by the captain than by the first officer.”

The NTSB noted that “the investigation of the captain’s background developed evidence that revealed he had experienced multiple work-related difficulties, particularly during the six-month period before the accident. Additionally, the investigation found that the captain was experiencing significant financial difficulties about the time of the accident, and there were indications that the captain’s behavior and lifestyle had changed before the accident.” [16]

There was no evidence in any form that the Parker PCU had any role whatsoever in the crash of Flight 185. However, in 2004, a Los Angeles jury ordered Parker Hannifin to pay US$43 million to the plaintiff families of Flight 185. Parker Hannifin subsequently appealed the verdict, which resulted in an out-of-court settlement for an undisclosed amount.

On November 12, 2002, the FAA ordered an upgrade of all Boeing 737 rudder control systems. Boeing argued that the Parker PCU components were not at fault, citing that the 737 and its equipment has one of the safest records in its class, but the FAA airworthiness directive (AD) went through regardless.[10]

According to the FAA AD: “The redesigned rudder control system will incorporate design features that will increase system redundancy, and will add an active fault monitoring system to detect and annunciate to the flightcrew single jams in the rudder control system. “If a single failure or jam occurs in the linkage aft of the torque tube, the new rudder design will allow the flightcrew to control the airplane, using normal piloting skills, without operational procedures that are unique to this airplane model.

“During our reviews of the new rudder control system, we have found that the new main rudder PCU design is equivalent to two independent PCUs. The main rudder PCU is an assembly with two PCUs arranged in tandem. The new main rudder PCU will have two independent servo valves in lieu of the existing common dual concentric servo valve. Two separate input linkages will control the position of these valves on the main rudder PCU. The pilot can override each of these input linkages and also override the linkage for the standby PCU. The function of the override capability is to enable the pilot to control the airplane in the event of a jam in any one of the three input linkages or associated servovalves in the rudder control system.”[10]

The airworthiness directive was completed in 2008, with all existing and new Boeing 737 aircraft equipped with the revised PCU design. [10]

Source documents added to original list:

[14] NTSB Aircraft Accident Report, United Flight 585, DCA91MA023, NTSB/AAR-01/01 PB2001-910401, Dated March 27, 2001. [15] NTSB Aircraft Accident Report, USAir Flight 427, DCA94MA076, NTSB/AAR-99/01 (PB99-910401), Dated Nov 4, 1999. [16] NTSC Aircraft Accident Report, Silkair Flight MI 185, Boeing B737-300, 9V-TRF, Musi River, Palembang, Indonesia, 19 December 1997, Jakarta 14 December 2000.