N393HA

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 SaintOuen 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 274454, 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...