Incident Overview
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) in Ontario, California. The flight was scheduled to land at Lihue Airport (LIH) in Lihue, Hawaii, but diverted to Daniel K. Inouye International Airport (HNL) in Honolulu, Hawaii. The diversion was prompted by an inoperative horizontal stabilizer and the need for a longer runway. The airplane landed at HNL without incident. The flight was conducted as a Title 14 Code of Federal Regulations Part 121 cargo operation with two flight crew members on board.
Flight History
During descent into LIH, the autopilot repeatedly disconnected. The crew executed the autopilot disconnect checklist, which did not resolve the issue. At about 10,000 feet, the crew began hand-flying the airplane and reported a noticeable down pressure on the yoke was required to keep the airplane descending. The crew observed that the horizontal stabilizer was inoperative, indicating 3.4 units on the stabilizer position indicator. Attempts to use the alternate electric trim switches were unsuccessful, as the stabilizer remained stuck.
The crew leveled the airplane at 3,000 feet and selected flaps 5 to reduce the control column force needed to maintain the desired pitch. They then performed the stabilizer trim checklist from the quick reference handbook (QRH), but the problem persisted. The unscheduled stabilizer trim checklist also yielded no improvement. The crew reported no “STAB TRIM” engine indicating and crew alerting system (EICAS) message or 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
The horizontal stabilizer is a movable airfoil at the rear of the airplane that provides longitudinal trim. Its function is to alter its angle of attack during flight to maintain maneuvering and speed stability. The stabilizer control system on the incident airplane is controlled by electrical inputs, which are received by two Stabilizer Trim Control Modules (STCMs). These modules direct hydraulic pressure from the left and center systems to the horizontal stabilizer trim actuator (HSTA). The HSTA, located forward and above the stabilizer center section, includes two drive hydraulic motors and two hydraulically released secondary brakes. These components work together to rotate the ballscrew, moving the horizontal stabilizer about its center section pivot points to change its angle of attack.
Investigation
The National Transportation Safety Board (NTSB) was notified of the incident on April 29, 2025. The investigation was initially graded as a class 5 investigation and later upgraded to class 3 on September 8, 2025. About one month before the incident flight, the airplane underwent scheduled maintenance during which an overhauled HSTA was installed. By the time of the incident, the overhauled HSTA had accumulated 11 cycles and 31 flight hours. After landing, the HSTA was removed and sent to the component manufacturer for examination under NTSB direction. Flight data from the incident is under review, and the investigation is ongoing.