Boeing 737 Rudder Reversal: The NTSB Investigation and Findings
The safety of commercial aviation relies on the absolute predictability of flight controls. However, a series of mysterious incidents involving the Boeing 737 led the National Transportation Safety Board (NTSB) into a complex investigation regarding a phenomenon known as rudder reversal—a critical failure where the aircraft's rudder moves in the opposite direction of the pilot's command.
The breakthrough in understanding this failure came through the study of Eastwind Flight 517. Unlike previous accidents, Flight 517 landed safely, providing investigators with a surviving aircraft and crew who could offer first-hand accounts of the malfunction.
[ไม่มีภาพประกอบ]The Experience of Eastwind Flight 517
During a post-accident interview, the captain of Eastwind Flight 517 reported that the flight had been free of turbulence. However, during the landing descent, he experienced a sudden "kick" or "bump" from the rudder, despite neither pilot touching the rudder pedals. This caused the plane to roll abruptly to the right.
In an attempt to recover the aircraft, the captain applied left aileron (the controls that tilt the wings) and tried to use the rudder. He noted that the rudder pedal controls felt stiffer than normal and failed to respond to his inputs. To compensate for the roll upset, he also advanced the throttle of the right engine.
Technical Analysis of the Power Control Unit
To identify the root cause, NTSB and Boeing engineers conducted rigorous testing on the Power Control Units (PCUs)—the hydraulic actuators that move the rudder. They tested units from Flight 517 and Flight 427, as well as brand-new PCUs and those from other aircraft. (The PCU from Flight 585 was too damaged for these specific tests).
The Role of Thermal Shock
Testing revealed that the PCU's dual servo valve could jam under specific conditions. Investigators discovered that thermal shock—which occurs when a cold PCU is suddenly injected with hot hydraulic fluid—could replicate the uncommanded rudder movement.
This thermal shock caused the servo's secondary slide to jam against the servo housing. Once the secondary slide was jammed, the primary slide could move to a position that caused the rudder to deflect in the opposite direction of the pilot's intended input.
[ไม่มีภาพประกอบ]Key Facts
- Primary Cause: Rudder reversal caused by the jamming of the PCU secondary slide and excessive travel of the primary slide.
- Trigger: Thermal shock (hot hydraulic fluid entering a cold PCU).
- Affected Flights: United Flight 585, USAir Flight 427, and Eastwind Flight 517.
- Regulatory Action: The FAA mandated the replacement of servo valves on all Boeing 737s by November 12, 2002.
- Pilot Training: New protocols were implemented to help pilots manage unexpected flight control movements.
Investigation Conclusions and Outcomes
After a four-year investigation, the NTSB issued its probable cause finding for USAir Flight 427 on March 24, 1999, attributing the crash to rudder reversal due to the PCU servo malfunction. Two years later, the NTSB amended the report for United Flight 585, concluding it shared the same probable cause.
| Incident/Action | Finding/Requirement | Date/Deadline |
|---|---|---|
| Flight 427 Probable Cause | Rudder reversal via PCU servo malfunction | March 24, 1999 |
| Flight 585 Amended Report | Same probable cause as Flight 427 | 2001 |
| FAA Servo Valve Mandate | Replacement of valves on all 737s | November 12, 2002 |
| FAA Training Mandate | New protocols for unexpected control movement | Post-investigation |
Frequently Asked Questions
What is a Power Control Unit (PCU)?
A PCU is a hydraulic component responsible for translating the pilot's pedal inputs into the physical movement of the aircraft's rudder.
What exactly is rudder reversal?
Rudder reversal is a dangerous malfunction where the rudder moves in the opposite direction of the pilot's command, potentially causing the aircraft to roll or yaw unexpectedly.
How did thermal shock cause the failure?
Thermal shock occurred when hot hydraulic fluid entered a cold PCU, causing the secondary slide of the servo valve to jam against its housing, which allowed the primary slide to move the rudder in the wrong direction.
Which flights were linked to this specific PCU failure?
The NTSB concluded that United Flight 585, USAir Flight 427, and Eastwind Flight 517 were all affected by this rudder reversal issue.
What did the FAA do to prevent this from happening again?
The FAA ordered the replacement of servo valves on all Boeing 737 aircraft by November 12, 2002, and introduced new training protocols for pilots to handle unexpected flight control movements.