Flight data recorder information shows the Colgan Air Q400 that crashed in Buffalo Thursday night pitched 31 degrees nose-up after stick shaker and stick pusher systems activated at the start of the instrument approach.
The events occurred as the crew began configuring the twin-engine turboprop for landing in light to moderate icing and snow conditions.
According to National Transportation Safety Board (NTSB) member Steven Chealander, data shows that the stick shaker and stick pusher activated roughly 34 seconds after the landing gear was deployed and as the wing flaps transitioned between 5 and 10 degrees down. The pilots had commanded the flaps to the 15 degree down position.
A “reference speed increase” switch that increases by 20kt the speed at which the stall warning system activates had been set to the ON position by the pilots as called for in icing conditions, says Chealander. The crew turned on the aircraft’s automatic de-icing system shortly after departure from Newark. Flight 3407, flying as Continental Connection, crashed into a house about 5mi from the airport, killing all 49 on board and one person in the house.
Chealander says aircraft had been flying in autopilot mode until the stick shaker activated, an action that automatically disengaged the system. The stick pusher follows the stick shaker if the aircraft continues to approach an aerodynamic stall, driving the control column forward to decrease the angle-of-attach and wing loading. The stick pusher is designed so that pilots can overpower it however.
While the NTSB has recommended that pilots hand flying their aircraft in icing conditions to get a better sense for trim changes, Chealander says the US Federal Aviation Administration has not mandated the practice, in part due considerations over pilot workload in such conditions without the use of autopilot.
Further, he says that Bombardier, the manufacturer of the Q400, recommends hand flying only if icing conditions are severe. “From what we’ve seen so far, we haven’t determined there was severe icing,” he says.
Investigators are still in the process of determining what the aircraft’s stall speed would have been for its weight and configuration, says Chealander. Flight data recorder (FDR) information shows the aircraft was flying at a calibrated airspeed of 134kt just before the landing gear was deployed.
Following the initial pitch upset to 31 degrees nose up, Chealander says the aircraft experienced a nose-down pitch of 45 degrees with a roll to the left of 46 degrees. The Q400 then rolled right to 105 degrees. Engine power was increased to full-power about 6 seconds after the upset. The pilots had commanded the flaps and landing gear to retract after the upset.
The last data point from the FDR, captured when the aircraft was approximately 250ft above the ground, showed a heading of 53 degrees (magnetic), a 26-degree right roll, 30 degree nose-down attitude and a speed of 100kt, says Chealander.
Radar data from air traffic control showed a descent rate of 9,600fpm between approximately 1,150ft and 350ft above the ground.
Chealander says the G-forces during the final minute of the flight ranged from 0.75G to 2G.