What happened
While cruising in level flight at FL 330 during the approach briefing phase, the aircraft experienced a sudden and unexpected event. The incident occurred at night under dark conditions with no moonlight or turbulence. During this period, the crew was entirely reliant on flight instruments, including three independent attitude indicators.
Data recordings indicate that the left attitude indicator showed a rapid increase in pitch. This sudden change caused a surprise effect for the pilot in command, leading to spatial disorientation. Following this, the autopilot likely disconnected, accompanied by an aural warning and a chime resulting from a miscompare between the primary flight displays (PFDs) of the two pilots. The aircraft's elevators moved toward a nose-down position, and stabilizer trim was activated via the left control wheel.
As the aircraft descended, it experienced negative G-loads and an increasing angle of attack. Approximately 13 seconds into the event, the crew faced contradictory attitude indicators featuring red chevrons pointing in opposite directions. Due to a declutter function triggered by the unusual attitude, no comparator cautions were displayed on the instruments. The aircraft exceeded its maximum operating speed and Mach number 17 seconds after the initial event, triggering an overspeed warning. Although the crew transmitted a distress call and reduced engine thrust to flight idle, they struggled with conflicting perceptions of turn directions. One minute and twenty seconds after the initial loss of altitude, the aircraft struck the ground. The accident resulted in two fatalities and the total destruction of the aircraft.
Findings
Investigations determined that a malfunction in the Inertial Reference Unit (IRU 1) produced erroneous attitude indications on the first PFD. A critical issue was identified regarding the flight instrument system, as pitch and roll comparator indications were removed when the displays entered an unusual attitude state; notably, this specific behavior had not been encountered during the crew's simulator training.
The accident was caused by insufficient operational prerequisites for managing a failure in a redundant system. Other contributing factors included:
- The lack of an effective communication system for handling abnormal or emergency situations.
- The flight instrument system failed to provide adequate guidance regarding the ongoing malfunction.
- The initial maneuvers that produced negative G-loads likely impaired the crew's ability to manage the emergency effectively.