Circumstances surrounding the 12 February crash of a Colgan Air Bombardier Q400 on approach to Buffalo Niagara airport have raised issues “well beyond the widely discussed matter of airframe icing”, says the US National Transportation Safety Board, which has voted to conduct a public hearing into what it calls “the deadliest US transportation accident in more than seven years”.
The crew’s reaction to the stick shaker stall warning that operated just before the aircraft went out of control is in the spotlight, the NTSB report says. The aircraft stalled and went out of control during an instrument landing system approach to Runway 23 and crashed into a house about 9km (5nm) from the airport at 22.17, killing all 49 on board and one person on the ground.
Icing was initially high on the list of potential causes of the loss of control of the Q400, which crashed into a house while on an instrument landing system approach to Runway 23 about 5nm (9.25km) from the airport at 22.17 that night, killing all 49 on board and one person on the ground.
But while icing and cold weather operations will be under scrutiny when the NTSB convenes its hearing on 12-14 May in Washington DC, so too will the Colgan flight crew's experience and stall recovery training, as well as sterile cockpit rules that require pilots to refrain from non-essential activities during critical phases of flight, and fatigue management.
These safety issues are under the spotlight after preliminary examination of the Q400's systems "revealed no indication of pre-impact system failures or anomalies", and review of the carrier's maintenance records resulted in "no significant findings" at this time, according to the NTSB.
© PA photos
The agency says that while weather conditions on the night of the accident revealed the presence of variable periods of snow and light to moderate icing, and examination of the flight data recorder (FDR) and aircraft performance models shows some ice accumulation was likely present on the aircraft prior to the initial upset, the Q400 "continued to respond as expected to flight control inputs throughout the accident flight".
The FDR data also shows that the stall warning and protection system, which includes the stick shaker and stick pusher, "activated at an airspeed and angle-of-attack consistent with that expected for normal operations when the de-ice protection system is active".
Referring to the dramatic pitch-up that occurred just before control was lost, the NTSB report comments: “Flight data recorder data indicate that when the stick shaker activated, there was a 25lb [11.3kg] pull force on the control column, followed by an up elevator deflection and increase in pitch, angle of attack, and gs. The data indicate a likely separation of the airflow over the wing and ensuing roll two seconds after the stick shaker activated while the aircraft was slowing through 125kt [230km/h] and while at a flight load of 1.42 gs.”
Furthermore, preliminary modelling and simulation efforts indicate that icing had a "minimal impact" on the stall speed of the airplane. "The FDR data indicates that the stick shaker activated at 130 knots, which is consistent with the de-ice system being engaged," says the agency.
Airplane performance work is continuing. So too is the work of the agency's operations and human performance group, which continues to investigate and review documentation associated with the flight crew's flight training history and professional development during their employment at Colgan as well as prior to joining the company.
The NTSB is also examining several other areas "potentially related to the accident", including the circumstances of a recent event involving a Colgan Q400 in which the airplane's stick shaker activated during approach to Vermont's Burlington airport.
Questions over whether a safety hazard exists with the instrument landing system (ILS) for runway 23 have been highlighted. However, to date, the NTSB's investigation into reports of airplane deviations resulting from distortion of the ILS signal "has not revealed any connection to the accident flight", says the agency.