What happened
On December 2, 2020, SriLankan Airlines flight UL190, an Airbus A321-251 bearing registration 4R-AND, was en route from Dhaka, Bangladesh, to Colombo, Sri Lanka. During the cruise phase at 36,000 feet, the aircraft experienced significant malfunctions involving its Air Data Reference (ADR) systems. The flight crew observed a discrepancy in the calibrated airspeed (CAS) readings, which triggered a reversion to Flight Control Alternate Law.
As the flight progressed, the aircraft's automation, including the autopilot, flight director, and auto-thrust, became unavailable. Due to the complexity of the multiple system failures and heavy rain reported at the destination, the pilot in command elected to divert the aircraft to Chennai International Airport in India. The aircraft landed safely in Chennai without injuries to the 77 passengers or 8 crew members.
The investigation
The investigation examined the aircraft's maintenance history and the flight crew's response to the technical failures. It was established that the aircraft had been operating under two Minimum Equipment List (MEL) items following a previous flight. Specifically, the aircraft had experienced an erroneous airspeed reading and a failure of the First Officer's pitot probe heater during its preceding leg from Colombo to Dhaka.
Investigators analyzed flight data recorder (DFDR) information to reconstruct the sequence of events. The data revealed that during descent through cloudy conditions, the failure of the pitot probe heater caused the second airspeed reading to become invalid due to potential ice accumulation. Because the crew did not switch off the ADR2 as required by operational procedures, the system continued to provide erroneous data to the flight control computers, eventually leading to the loss of multiple flight control functions and a reversion to Direct Law upon landing gear extension.
Findings
- The primary cause of the incident was a loss of redundancy in the Air Data Reference System caused by the simultaneous failure of the First Officer's pitot probe heater and a faulty standby Air Data Module (ADM 3).
- The flight crew did not follow the required MEL operational procedure to switch off ADR2 during icing conditions, which allowed erroneous data to persist in the system.
- The crew failed to identify a speed discrepancy during the takeoff roll, as required by standard operating procedures.
- Maintenance personnel had previously identified and deferred similar defects via Acceptance of Deferred Defect (ADD) entries but had not successfully identified the underlying cause of the recurring faults.