History of Flight
On 21 March 2021, SriLankan Airlines Flight UL606, an Airbus A330-343 (registration 4R-ALR), departed from Bandaranaike International Airport, Katunayake, Sri Lanka, at 2350 UTC, destined for Kingsford Smith International Airport, Sydney, Australia. The flight was operated as a three-crew operation with a Captain, a Cruise Pilot, and a First Officer. There were nine passengers and seven crew members on board. Weather and visibility were good at departure, and the aircraft had no reported defects.
Flight Crew and Segments
The flight was divided into three segments. The first segment was flown by the Captain and the Cruise Pilot. After approximately three hours, the crew changed seats for rest. During the second segment, the Cruise Pilot occupied the left seat as Pilot Monitoring (PM), and the relieving First Officer occupied the right seat as Pilot Flying (PF). The Captain went to the cabin to rest. The third segment was operated by the Captain and the Cruise Pilot.
Incident
At Flight Level 390, at approximately 0310 UTC, the aircraft experienced autopilot disconnections, followed by an altitude loss of about 154 ft. The incident occurred over the Indian Ocean at coordinates -7.823044, 101.449898.
Aftermath and Investigation
The incident was reported to the Civil Aviation Authority of Sri Lanka (CAASL) via a Mandatory Occurrence Report on 02 April 2021, after being detected during routine Flight Data Analysis. The three flight crew members were grounded immediately. The Director General of Civil Aviation appointed an investigation team. Assistance was requested from the Bureau of Enquiry and Analysis for Civil Aviation Safety (BEA, France) and Airbus, as the Digital Flight Data Recorder data was unavailable due to late submission. The investigation utilized Digital Access Recorder data.
Injuries and Damage
No injuries to persons were reported, and no damage to the aircraft or other property occurred.
Findings
The investigation's findings included that the aircraft was airworthy, the flight crew held valid licenses and medical certificates, and the operating procedures were properly followed. Specific findings regarding aircraft systems and crew actions are detailed in the full report.
Probable Cause
The probable cause was not explicitly stated in the provided source.