Los Angeles, CA, USA
N393HA
AIRBUS A330-243
The flight crew reported that, when the airplane climbed through flight level 360, the trimmable horizontal stabilizer (THS) trim wheel began to move “very quickly” forward and aft, creating small pitching oscillations. No message appeared on the airplane’s electronic centralized aircraft monitor, and no flight deck alerts annunciated. The crew decided to turn off the autopilot to isolate the malfunction. However, when the autopilot was off, the trim wheel continued to move erratically. The crew then reengaged the autopilot, and small pitch oscillations resumed. These oscillations, which were below the threshold to trigger a fault, were partially countered by flight control system inputs, but the trim wheel continued its uncommanded movement. The first officer (the pilot flying) then turned off the autopilot and manually flew the airplane for the rest of the flight while the captain (the pilot monitoring) managed the situation. The flight crew contacted air traffic control to declare an emergency and advise that fuel would need to be jettisoned before the airplane returned to the airport. Afterward, fuel was jettisoned until the airplane reached its maximum landing weight. The airplane landed uneventfully, and the emergency was terminated after landing. The airplane’s Post Flight Report information showed the “PITCH TRIM ACTR [actuator] OVERRIDE SWITCH” fault message. The maintenance crew at the airport replaced the pitch trim actuator (a subcomponent of the THS actuator), and an operational test of the THS actuator was successful. One possible scenario for the uncommanded pitch trim wheel movement involved a failure or malfunction of a subcomponent of the THS actuator (the No. 1 digital electronic module, which digitizes the analog signals from THS position sensors. Extensive testing of the THS actuator and its subcomponents, including the incident pitch trim actuator and the No. 1 digital electronic module, was conducted at the manufacturers’ facilities. However, the testing did not identify the specific reason for the uncommanded pitch trim wheel movements during the incident flight. As a result, the investigation was unable to determine the cause of the anomaly based on the available evidence. It is important to note that this investigation found that the anomaly appeared to be isolated to components that were either removed and repaired or replaced on the incident airplane. Further, the incident airplane had no pitch or pitch trim flight control faults related to the circumstances of this incident since the THS actuator was removed and replaced in November 2022.
HISTORY OF FLIGHTOn September 22, 2022, about 1200 Pacific daylight time, Hawaiian Airlines flight 3, an Airbus A330-243, N393HA, experienced a pitch trim anomaly while climbing out of Los Angeles International Airport (LAX), Los Angeles, California. The flight crew declared an emergency, and the airplane returned to LAX without further incident. None of the 12 crewmembers and 278 passengers aboard the airplane were injured, and the airplane sustained no damage. The regularly scheduled domestic passenger flight, from LAX to Daniel K. Inouye International Airport, Honolulu, Hawaii, was operating under Title 14 Code of Federal Regulations Part 121. The flight crew reported that, when the airplane climbed through flight level 360, the trimmable horizontal stabilizer (THS) control wheel (also referred to as the trim wheel) began to move “very quickly” forward and aft, causing small pitch oscillations. No message appeared on the airplane’s electronic centralized aircraft monitor, and no flight deck alerts annunciated. The crew decided to turn off the autopilot to isolate the malfunction. The crew stated that, when the autopilot was off, the trim wheel continued to move erratically but did not affect flight control and stability. The crew then reengaged the autopilot, and the small pitch oscillations resumed. The first officer (the pilot flying) then turned off the autopilot and manually flew the airplane for the rest of the flight. The flight crew contacted air traffic control (ATC) to advise that the airplane had a flight control anomaly and would be returning to LAX. The crew then contacted the flight attendants to tell them that the airplane would be turning around and have them ask an A330 captain traveling in the cabin to come to the flight deck. The crew also called maintenance control in Honolulu for assistance. Maintenance control advised the crew to try a primary computer reset, but the crew did not perform this action because a table in the quick reference handbook (QRH) showed that this reset should only be performed on the ground. The traveling captain and an A330 first officer who was in the cockpit jumpseat looked but could not find an applicable operational engineering bulletin or QRH procedure to address the anomaly. The flight crew contacted ATC again to declare an emergency and advise that fuel would need to be jettisoned. The crew notified the flight attendants about the emergency and made an announcement to the passengers that the airplane would be returning to LAX and that fuel would be jettisoned. ATC provided vectors and assistance for returning to the airport and accomplishing the fuel jettison. The captain of the flight (the pilot monitoring) managed the emergency while the first officer flew the airplane. The captain stated that the traveling captain and traveling first officer “provided an excellent resource” and that all four pilots were accomplishing “necessary tasks” and keeping each other “in the loop.” The flight crew stated that, per the QRH procedure, fuel was jettisoned until the airplane reached its maximum landing weight. The airplane was cleared to land on runway 24L, and the flight crew briefed and performed one task at a time so that the trim wheel and airplane controllability in the landing configuration could be assessed between actions. The airplane landed at LAX uneventfully, and the emergency was terminated after landing. Hawaiian Airlines reported that the airplane’s Post Flight Report information, accessed through the airplane’s multifunction control and display unit, showed a fault message that indicated “PITCH TRIM ACTR [actuator] OVERRIDE SWITCH.” The fault message is triggered when the trim wheel is manually manipulated for an extended period of time. The airline maintenance crew was unable to replicate the fault. The maintenance crew replaced the pitch trim actuator, consistent with Airbus’ A330 troubleshooting manual, and performed a successful operational test of the THS actuator according to the procedures in the aircraft maintenance manual. AIRCRAFT INFORMATIONPitch Control System Two elevators along the trailing edge of the THS provide pitch and pitch trim control. The THS moves hydromechanically via a ball screw actuator powered by two hydraulic motors and held by brakes. The THS actuator operates in two modes: electrical and manual. The electrical mode enables the auto trim function in auto or manual flight. With auto trim, the commands computed by the flight control system are distributed between the elevators and the THS. The mechanical mode provides a standby system that can be used in flight or on the ground to trim the THS. Mechanical control is achieved by the two flight deck control wheels (trim wheels), which are located on both sides of the center pedestal. Cables from the control wheels are connected to the mechanical input shaft of the THS actuator. Mechanical control of the THS actuator can override electrical control if a pilot applies sufficient torque on the trim wheel. When the THS actuator is operating in electrical mode, the electrical commands are converted to hydraulic actuation by the pitch trim actuator, which is a subcomponent of the THS actuator (see the figure). The pitch trim actuator has three DC motors, each of which has its own controller and digital electronic module. Each module adjusts its control command to compensate for the difference between the position feedback from the command (COM) sensor (as described below) and the target THS value from each of the airplane’s flight control primary computers. Figure. THS system. (Illustration courtesy of Airbus.) Note: The NTSB added to Airbus’ illustration by circling (in red) the THS and outlining (in green) the THS actuator. FCPC, flight control primary computer; PTA; pitch trim actuator; DEM, digital electronic module; RVDT, rotary variable differential transformer. The THS actuator is fitted with two separate units that each contain three identical rotary variable differential transformers (RVDT). The two units, known as RVDT “packs,” are used for electrical position detection. The COM RVDT pack is used to sense the output position of the pitch trim actuator and transmit the electrical signal to its associated digital electronic module (No. 1, 2, or 3). The monitor (MON) RVDT pack is used to monitor the position of the THS actuator ball screw jack and transmit the electrical signal to its associated flight control primary computer (No. 1, 2, or 3). As previously stated, after the incident flight, the airplane’s pitch trim actuator was removed and replaced. Additional flight control faults were annunciated on the airplane during flights on October 3 (pitch trim override), October 22 (flight control primary computer No. 3/pitch trim actuator electric motor), and November 3 and 5, 2022 (flight control primary computer No. 1 pitch fault/pitch trim actuator). In response, multiple THS components were replaced, including a flight control primary computer (No. 3), the MON and COM RVDTs, and another pitch trim actuator. Another flight control fault (STAB drift) was annunciated on a November 7, 2022, flight from LAX to Kahului Airport, Kahului, Hawaii. Flight data recorder data showed a drift in the measured horizontal stabilizer position sensed by the MON RVDT while the THS position remained stable. As a result, the THS actuator was removed and replaced in November 2022. During each of these events, no THS oscillations were reported. Further, as of the date of this report, no pitch or pitch trim flight control faults related to the circumstances of the incident flight have been reported on the airplane. System Testing The incident THS actuator and its subcomponents were examined and tested as part of this investigation; information about the examinations and testing appears below. THS Actuator The THS actuator was examined at the Collins Aerospace facility in Saint Ouen L’Aumône, France. No significant damage was noted on the THS actuator. The THS actuator was tested on a Collins test bench according to the relevant procedures in the component maintenance manual (CMM) for the actuator. (All CMM references in this report refer to CMM section 27 44 54, which contained the acceptance test procedure for the THS actuator.) The THS actuator passed the testing with no faults found. The THS actuator was able to be mechanically operated to its full nose-down and full nose-up positions. The actuator was commanded to its neutral position (0°) with no oscillations noted. The actuator was then commanded to its -2.5° position, which corresponded to one of the THS actuator positions recorded during the incident flight, and no oscillations were noted. An operational check of the override mechanism was successful. The accuracy of the MON and COM position transducers, with the THS actuator at the 0° and -13.5° (bottom stop) positions, was within CMM limits. (The COM and MON RVDT packs used during this test were not the units installed at the time of the incident.) The pitch trim actuator was removed from the THS actuator and placed in a climatic chamber at -40°C for 2 hours to approximate the environmental conditions during the incident flight. The pitch trim actuator was then remounted on the THS actuator, and the CMM acceptance test procedures were run again. No anomalies were observed. The THS actuator was retested about 2 months later with the incident COM RVDT pack reinstalled on the actuator. (The MON RVDT installed at the time of the incident was repaired and returned to service before the decision to conduct further testing was made.) The CMM acceptance test procedure was then performed. The THS actuator passed the procedure with only two minor discrepancies, both of which were not related to the removal of the COM RVDT and MON RVDT packs from the THS actuator after the incident. One of the discrepancies was related to the bonding resistance between the THS actuator and the RVDT attaching screws; the other discrepancy was related to the position of a MON RVDT transducer. Pitch Trim Actuator The incident pitch trim actuator was also examined and tested at the Collins Aerospace facility. An operational check of the actuators’ three microswitches found that each microswitch measured beyond the CMM limits for movement. An internal friction torque test measurement test found that the internal friction torque was above the CMM limit. According to Airbus, this out-of-tolerance measurement was unrelated to the airplane behavior observed during the incident flight. An automated test was performed twice to determine the voltage at which the pitch trim actuator microswitches would trigger. For both tests, the results were within CMM limits. A separate test found that the ground continuity of the pitch trim actuator was within CMM limits. Digital Electronic Module The No. 1 digital electronic module was tested because recorded flight data showed oscillations between consistent values, which is characteristic of a digital issue. The module converts analog data from the RVDTs to digital data and, as such, could be a possible source of the recorded oscillations. The oscillations were below the threshold to trigger a fault and result in a system changeover to another digital electronic module. The No. 1 digital electronic module—the unit that was active during the incident flight—was examined and tested at the Triumph Aerospace facility in Flintshire, England. Isolation and bonding tests showed that the digital electronic module was within CMM limits. The module was also found to be within CMM limits during automatic test equipment tests. The module failed the leakage test because the recorded pressure was below the minimum limit specified in the CMM. According to Airbus, this out of tolerance measurement was unrelated to the airplane behavior observed during the incident flight. The module was then placed in a climatic chamber at a temperature of -55°C, and the module passed the test because it resulted in no anomaly or oscillation. Tests involving two pins of a resolver-to-digital converter (RDC) microchip on the control board of an engineering digital electronic module were performed to replicate the behavior observed during the incident flight, specifically, the uncommanded and oscillatory movement of the trim wheel. The behavior could not be repeated. The two internal circuit boards of the digital electronic module—the control board and the power board—were disassembled. Examination of the boards under microscopic power found that the boards were in good condition with no moisture or corrosion, and no anomalies were observed on the interconnectors or the boards’ surface components. Examination of the RDC digital controller under higher magnification found that the connector pins were in good condition, with no faults on any pin solder joints. X-ray examination of the control board found no faults on the interconnection circuit, the surface components, or connection bonding. A computed tomography scan of the control board was conducted so that all RDC pins could be individually examined in three dimensions. No damage was observed on the RDC pins, and no anomalies were noted on the tracks where the RDC was attached to the control board. No faults were observed on the power board. The extensive testing on the No. 1 digital electronic module did not yield a cause for the oscillations, but the unit was taken out of service as a precaution. COM and MON RVDT Packs The COM and MON RVDT packs were examined at the Collins Aerospace facility. The COM pack was the unit from the incident flight; the MON pack was not. (As previously stated, the MON pack from the incident flight was repaired and returned to service before it could be tested as part of this investigation.) A visual examination of the boards found no evidence of damage or corrosion. The sealant on both boards was in good condition. The boards’ electrical connectors showed no anomalies. The RVDT COM position sensor accuracy at THS actuator positions of 0° and 14.9° (upper stop) was checked and found to be within CMM limits. A ground continuity check of each RVDT pack resulted in values that exceeded CMM limits for both units. According to Collins, these out-of-tolerance measurements were unrelated to the airplane behavior observed during the incident flight, and the values were typical for in service units. Insulation resistance for both units was within CMM limits. A check of the DC resistance for both units found that the COM sensors were within tolerances. Both RVDT packs were disassembled. No evidence of humidity, corrosion, or physical damage was observed. Both units were placed into a climatic chamber and soaked at -55°C for 85 minutes. Afterward, the RVDT COM and RVDT MON packs were again checked for DC resistance. All measured values for both units were consistent with the previously measured values. AIRPORT INFORMATIONPitch Control System Two elevators along the trailing edge of the THS provide pitch and pitch trim control. The THS moves hydromechanically via a ball screw actuator powered by two hydraulic motors and held by brakes. The THS actuator operates in two modes: electrical and manual. The electrical mode enables the auto trim function in auto or manual flight. With auto trim, the commands computed by the flight control system are distributed between the elevators and the THS. The mechanical mode provides a standby system that can be used in flight or on the ground to trim the THS. Mechanical control is achieved by the two flight deck control wheels (trim wheels), which are located on both sides of the center pedestal. Cables from the control wheels are connected to the mechanical input shaft of the THS actuator. Mechanical control of the THS actuator can override electrical control if a pilot applies sufficient torque on the trim wheel. When the THS actuator is operating in electrical mode, the electrical commands are converted to hydraulic actuation by the pitch trim actuator, which is a subcomponent of the THS actuator (see the figure). The pitch trim actuator has three DC motors, each of which has its own controller and digital electronic module. Each modu
Uncommanded movement of the trimmable horizontal stabilizer control wheel for reasons that could not be determined based on the available evidence.
Source: NTSB Aviation Accident Database
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