What happened
On a flight from Chicago to Warsaw, a Boeing 767-300, registration SP-LPA, was cruising near North Bay, Ontario, when the captain's airspeed indicator suddenly jumped, triggering an overspeed warning. The flight crew responded by reducing thrust to idle and initiating a climb. However, as the aircraft' and altitude fluctuated, a second overspeed warning occurred, followed by the activation of the stick shaker. The aircraft's altitude shifted between 33,000 and 35,400 feet before descending to approximately 27,900 feet.
During a subsequent holding pattern for fuel dumping, the crew failed to monitor airspeed while the autopilot attempted to maintain altitude at idle thrust. This resulted in an increased angle of attack that triggered the stick shaker again. During the recovery, the aircraft climbed through its assigned altitude, leading to a loss of separation with another aircraft. The flight eventually diverted to Toronto/Lester B. Pearson International Airport, where all 206 passengers and 10 crew members landed safely without injury.
The investigation
Investigators examined the aircraft's Air Data Computer (ADC) and found a significant buildup of dust and dirt inside the unit. Testing revealed that a fault in the phase locked loop (PLL) circuitry caused the ADC to produce erroneous data at high altitudes and temperatures. This specific hardware failure had also caused a similar incident on the same aircraft earlier that month.
The investigation also reviewed the airline's training and documentation. It was noted that the airline's Flight Crew Operations Manual (FCOM) contained incorrect information regarding EICAS messages for this specific aircraft. Furthermore, the investigation found that the crew did not cross-check the captain's erroneous readings against the first officer's instruments, which were displaying normal data.
Findings
- A hardware fault in the phase locked loop (PLL) circuitry of the ADC caused sudden, incorrect airspeed and altitude indications.
- The flight crew did not compare the captain's instruments with the first officer's or standby instruments, leading them to believe the erroneous data was accurate.
- The airline's training syllabus lacked practical, specific training for responding to overspeed warning events.
- Inaccurate documentation in the FCOM regarding airspeed disagree messages increased the risk of crew misidentification of the problem.
- The use of a Cockpit Voice Recorder with only 30 minutes of capacity prevented investigators from analyzing the crew's specific decision-making during the initial event.