N347AZ

On April 25, 2025, about 1814 Hawaii standard time, an Amazon Prime Air Boeing 767-319 operated by Air Transport International departed from Ontario International Airport (ONT), Ontario, California and intended to land at Lihue Airport (LIH), Lihue, Hawaii but diverted to Daniel K. Inouye International Airport (HNL), Honolulu, Hawaii due to an inoperative horizontal stabilizer and the need for a longer runway. The airplane landed at HNL without issue. There were two flight crew members on board, and the flight was conducted as a Title 14 Code of Federal Regulations Part 121 cargo flight.

History of Flight

On descent into LIH, the autopilot repeatedly disconnected. The crew ran the autopilot disconnect checklist, which did not resolve the issue. The crew started hand-flying the airplane at about 10,000 feet and reported that “noticeable down pressure on [the] yoke [was required] to keep [the] airplane descending.” The crew noticed the horizontal stabilizer was inoperative at 3.4 units on the stabilizer position indicator. They tried to use the alternate electric trim switches, but the stabilizer remained stuck.

The crew leveled off the airplane at 3,000 feet. They selected flaps 5 to reduce the control column force required to maintain the desired pitch. Next, they performed the stabilizer trim checklist per the quick reference handbook (QRH), but the issue persisted. The crew also performed the unscheduled stabilizer trim checklist, with no improvement. The crew reported that they did not receive a “STAB TRIM” engine indicating and crew alerting system (EICAS) message or the associated cockpit warning light at any point during the flight. The crew declared an emergency and diverted to HNL, where they landed safely with flaps 25 selected.

Horizontal Stabilizer Trim Control System Description

The horizontal stabilizer is a moveable airfoil at the back of the airplane which trims the airplane longitudinally. Its overall function is to alter its angle of attack during flight, as necessary, to keep the airplane in a condition of maneuvering and speed stability. The stabilizer control system on the incident airplane is controlled by electrical inputs. These inputs are received by two Stabilizer Trim Control Modules (STCMs). The STCMs direct left and center hydraulic system pressure to the horizontal stabilizer trim actuator (HSTA).

The HSTA is located forward and above the stabilizer center section and is attached to bulkhead structure at the top and to stabilizer structure at the bottom (figure 1). The HSTA includes two drive hydraulic motors and two hydraulically released secondary brakes. These work together to rotate the ballscrew, thereby moving the horizontal stabilizer about its center section pivot points and changing its angle of attack.

Figure 1. HSTA location and components labeled (Source: Boeing, modified. Image copyright © Boeing. Reproduced with permission).

Investigation

The NTSB was notified of this incident on April 29, 2025, and it was originally graded as a class 5 investigation. After initial investigation, it was upgraded to a class 3 investigation on September 8, 2025.

The airplane had undergone scheduled maintenance about one month before the incident flight during which an overhauled HSTA was installed. At the time of the incident flight, the overhauled HSTA had been installed on the aircraft for 11 cycles and 31 flight hours. The HSTA was removed after landing and sent to the component manufacturer for examination under the direction of the NTSB.

The flight data from the incident is being reviewed, and the investigation is ongoing.