What happened
On the early morning of the accident, Korean Air Flight 801, a Boeing 747-300, was conducting a scheduled flight from Seoul to Guam. After cruising at 41,000 feet, the flight crew began their descent toward Guam under the direction of the Guam Combined Center/Radar Approach Control (CERAP). During the descent, the crew encountered deteriorating weather conditions, necessitating a deviation to avoid severe thunderstorms.
As the aircraft approached runway 0CL, the controller notified the crew that the glideslope was unusable. Despite this, there was confusion within the cockpit regarding the status of the glideslope. While the crew performed landing checklists and managed the approach, the aircraft continued to descend through 1,800 feet. The Ground Proximity Warning System (GPWS) issued several alerts, including a "sink rate" warning as the aircraft descended at 1,400 feet per minute.
At approximately 01:42:19, the crew attempted to initiate a missed approach. However, as the aircraft descended through 670 feet, the nose pitched up and power was applied, but the aircraft struck trees and hilly terrain at Nimitz Hill. The impact caused a post-impact fire.
Findings
- The aircraft was descending below the minimum descent altitude (MDA) without being detected by air traffic control.
- A software patch intended to reduce false alarms had rendered the Minimum Safe Altitude Warning (MSAW) system ineffective.
- There was significant confusion among the flight crew regarding the operational status of the glideslope during the final approach.