Flight-data recorder information from the crashed West Atlantic Bombardier CRJ200 freighter indicates evidence of a spurious pitch reading immediately before the autopilot on the jet disengaged.
Swedish investigation authority SHK has stated that, while the aircraft was in cruise, the captain expressed sudden surprise before the aircraft’s autopilot disengaged and the jet entered a rapid descent.
SHK has not reached any conclusions on the event which prompted the captain’s comment.
But the flight-data recording suggests that – about 2s before his remark – the device registered a sudden pitch change to about 15° nose-up.
Other parameters, such as the engine thrust, and elevator and aileron positions, appeared to remain stable during this 2s period.
While the pitch record indicates a nose-up deviation, the inquiry believes the recording is inaccurate and does not reflect the true pitch experienced by the aircraft. But it has yet to assess whether the anomaly prompted a reaction from the crew.
SHK says it has embarked on research to validate the pitch angle. It says the recorded data is “not compatible” with the CRJ’s motion.
Its initial recalculation to show the “physically probable” pitch suggests that the aircraft did not experience a significant deviation in pitch in the moments before the captain’s expression of surprise, and that subsequent changes in pitch were actually nose-down.
SHK has not explained the difference between the recorded data and the aircraft’s actual behaviour. Nor has it explicitly stated whether the pilots would have been presented with a false impression of the aircraft’s attitude.
But it states that attitude information, including pitch, is generated by the inertial reference units, which then feeds this data to the pilots’ primary flight displays.
“The continue investigation will, among other subjects, focus on clarifying in which way this [discrepancy] might have affected the airplane’s instruments,” it says.
SHK notes that a failure in the attitude information would generate a flag warning on the primary flight display. Discrepancies between each pilot’s displayed data trigger caution messages. The inquiry adds that attitude information is available on standby instruments.
The pitch information is one of four spurious parameters on the flight-data recorder traces.
Investigators have disclosed that the aircraft’s autopilot disconnected – apparently automatically – and that the jet entered a rapid descent, with nose-down elevator and horizontal stabiliser trim.
Neither pilot survived the 8 January crash. SHK says it expects to release a final report into the accident in December.