What happened
On 18 July 2019, an Ethiopian Airlines Boeing 787-8, registration ET-ASG, was conducting a scheduled passenger flight from Manila to Hong Kong International Airport. While performing an Instrument Landing System (ILS) approach to runway 25R, the aircraft's autoflight system experienced a malfunction while attempting to intercept the localizer course at waypoint RIVER.
The aircraft overshot the intended course and drifted north toward higher terrain. During this deviation, the plane descended below the Minimum Sector Altitude of 4,300 feet. The flight crew noticed the deviation and the pilot flying immediately disengaged the autopilot to regain manual control and climb back to a safe altitude. Air Traffic Control (ATC) alerted the crew to the terrain proximity and subsequently instructed a go-around. There were no injuries and no damage to the aircraft, which landed safely following a second approach.
The investigation
The investigation examined the aircraft's Autopilot Flight Director System (AFDS) and the flight control electronics. Investigators analyzed the flight data and the behavior of the flight control module during the localizer interception. The inquiry involved international cooperation between the AAIA, the Ethiopian Accident Investigation Bureau, the NTSB, and the Civil Aviation Department of Hong Kong.
Findings
- The primary cause of the flight path deviation was a software problem embedded within the flight control module (FCM) of the autoflight director system.
- The error occurred specifically during the process of capturing the localizer course.
- No Enhanced Ground Proximity Warning System (EGPWS) alerts were triggered during the descent below the minimum sector altitude.