What happened
On August 20, 2004, an Airbus A310-300, registration CS-TEH, operating as TAP Air Portugal flight 1821, was en route from Lisbon to Lajes Airport. Simultaneously, a Beechcraft B200 King Air, registration CS-DDU, operated by Omni, was performing a positioning flight from Lajes to Cascais.
While operating near the Lajes airspace, the Air Traffic Controller (ATC) provided traffic information to both aircraft. During the exchange, the crew of the A3 .310-300 misinterpreted the controller's traffic information regarding the altitude of the opposing aircraft as an instruction to descend. This resulted in a level bust, where the aircraft descended below its authorized flight level of 170.
To avoid a potential conflict, the ATC instructed the TAP aircraft to turn right immediately. As the crew executed a steep turn with positive G-forces, the aircraft's Traffic Collision Avoidance System (TCAS) issued a Resolution Advisory (RA) commanding a descent. The transition from a climbing turn to an immediate descent caused the aircraft to experience negative G-forces of -0.92 Gs. Because the passengers and cabin crew were not wearing seatbelts, the sudden maneuver caused 36 passengers and 2 crew members to be thrown against the cabin ceiling and seats, resulting in 38 injuries.
The investigation
The GPIAAF investigation examined the flight data recorder (FDR) and cockpit voice recorder (CVR) to reconstruct the sequence of events. The investigation focused on the communication between ATC and the flight crews, the functionality of the TCAS RA, and the cockpit's interpretation of traffic information. Investigators also reviewed the cabin safety procedures and the impact of the unbuckled passengers during the high-G maneuver.
Findings
- The primary cause of the incident was the crew's misinterpretation of ATC traffic information as a descent instruction, leading to a level bust.
- The crew's error was influenced by "expectation," as descending to flight level 160 is a routine procedure for entering the Lajes control area.
- The aggressive nature of the evasive maneuver was induced by the combination of urgent ATC instructions and the simultaneous TCAS RA command, compounded by a lack of visual contact with the other aircraft.
- The severity of the injuries was significantly increased because the cabin crew had not yet instructed passengers to fasten their seatbelts, and the passengers were not wearing them at the time of the maneuver.
- The Beechcraft B200 was not equipped with TCAS, though the risk of collision was considered minimal as the Omni crew maintained visual contact with the Airbus.