What happened
On April 11, 1999, an Air Portugal Airbus A310/300, registration CS-TEW, was operating flight TP-1107 from Lisbon to Sal, Cape Verde, carrying 134 passengers and 10 crew members. During the descent, while passing through Flight Level 160, the autopilot (AP2) commanded a nose-down pitch attitude. This maneuver caused the aircraft's speed to increase rapidly, eventually exceeding the Maximum Operating Speed (VMO) and triggering the cockpit overspeed warning.
In an attempt to manage the speed, the pilot flying (PF) selected the 'LVL/CH' mode and a target speed of 290 knots. However, the acceleration was too great for the autopilot to arrest. In response to the overspeed alert, both the pilot flying and the pilot not flying (PNF) simultaneously applied manual forces to the control column. This uncoordinated intervention caused an irregular disconnection of the autopilot and transitioned the aircraft into manual flight. The resulting pitch oscillations were severe, with vertical acceleration peaks reaching +2.42g and -0.09g within a 15-second interval. These rapid changes in attitude caused several passengers and crew members to be thrown against the cabin ceiling and floor, resulting in one serious injury to a flight attendant and two minor injuries to crew and passengers.
The investigation
The GPIAAF investigation examined the flight data from the Quick Access Recorder (QAR) and analyzed the interaction between the flight crew and the aircraft's automation. The investigation focused on the autopilot's behavior during the descent, the flight management system (FMS) instructions, and the specific manual inputs made by the crew. Investigators also reviewed the crew's experience levels, noting that while the commander was highly experienced, the co-pilot had recently transitioned to the A310 type.
Findings
- The primary cause was the uncoordinated and simultaneous manual application of control column forces by both pilots, which disconnected the autopilot in a manner contrary to standard operating procedures (SOP).
- The autopilot's sudden nose-down command was driven by instructions from the FMS, which was struggling with degraded navigation accuracy due to poor radio aid coverage on the route.
- The pilot flying's recent transition to the A310 type influenced an instinctive reaction on the controls that deviated from established procedures.
- The pilot not flying attempted to assist by applying physical force to the control column without being requested to do so and without formally assuming control of the aircraft.
- The severity of the injuries was exacerbated by the airline's policy of not activating the 'fasten seatbelt' sign until below 10,000 feet, which left some passengers unseated during the vertical oscillations.