Analysis of flight data points away from severe aircraft icing as a key factor in the Feb. 12 crash of Continental Connection Flight 3407 near Buffalo, NY in which 49 people onboard the Bombardier Dash 8 Q400 and one person on the ground perished.

The data “shows that some ice accumulation was likely present on the airplane (a Bombardier Dash 8 Q400 turboprop) prior to the initial upset event, but that the airplane continued to respond as expected to flight control inputs throughout the accident flight,” the National Transportation Safety Board (NTSB) said in it preliminary report on the atal accident..

Review of the weather conditions on the night of the accident revealed the presence of variable periods of snow and light to moderate icing during the accident airplane’s approach to Buffalo International Airport (BUF).

Investigators will perform additional examinations on the dual distribution valves installed in the airplane’s de-ice system. (The de-ice system removes ice accumulation from the leading edges of the wings, horizontal tail, and vertical tail through the use of pneumatic boots.) The dual distribution valves transfer air between the main bleed air distribution ducts and the pneumatic boots.

The Flight Data Recorder (FDR) shows that the stall warning and protection system, which includes the stick shaker and stick pusher, activated at an airspeed and angle-of- attack (AOA) consistent with that expected for normal operations when the de-ice protection system is active.

The airplane’s stick shaker will normally activate several knots above the actual airplane stall speed in order to provide the flight crew with a sufficient amount of time to initiate stall recovery procedures. As a result of ice accumulation on the airframe, an airplane’s stall airspeed increases.

To account for this potential increase in stall speed in icing conditions, the Dash 8-Q400’s stall warning system activates at a higher airspeed than normal when the de-ice system is active in-flight to provide the flight crew with adequate stall warning if ice accumulation is present.

Preliminary airplane performance modeling 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.

The FDR data further indicate that when the stick shaker activated, there was a 25-pound pull force on the control column, forcing the aircraft’s nose up.

The data indicates 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 125 knots and while at a flight load of 1.42 Gs. The predicted stall speed at a load factor of 1 G would be about 105 knots.

Early in the NTSB probe, there was immediate suspicion that severe icing may have played a critical role in the fatal accident. Some aviation safety experts suspect that a tailplane (horizontal stabilizer) stall prompted the crash of the Colgan Air Bombardier Dash 8 Q400. Experts say it is difficult to tell the difference between a tailplane stall and a wing stall. And the recovery from each is completely different. For a wing stall, you push the control yoke forward and increase airspeed. For a tailplane stall you do the opposite: pull the yoke back, reduce flaps and on some aircraft, ease off on power.

Captain Marvin Renslow, 47, of Tampa, FL, began flying the Dash 8 Q400 in December, accumulating 110 hours. But he had 3,379 flying hours, with much of that in the Saab 340, where tailplane stalls can be more of a problem. There is reason to believe that Renslow, who was in control of the Dash 8 when the stick shaker activated, reacted as if he were still flying a Saab 340, and in suspecting a tailplane stall pulled back hard on the yoke. But by then it was too late.

The Safety Board said it is also investigating a recent Colgan Air flight in which a Dash 8-Q400’s stick shaker activated during the plane’s approach to Burlington International Airport (BTV) in Burlington, VT. A preliminary review of the FDR data from that flight shows the momentary onset of the stick shaker during the approach phase of flight. That plane landed without incident.

In addition, investigators are examining the instrument landing system for Runway 23 at the Buffalo airport. There have been reports of airplane deviations resulting from distortion of the instrument landing system (ILS) signal for runway 23 at BUF. There is an existing Notice to Airmen (NOTAM) related to this distortion condition. To date, investigation into these reports has not revealed any connection to the accident flight.

The NTSB has scheduled a public hearing to be held May 12-14, 2009 at the NTSB’s Board Room and Conference Center in Washington, D.C. Acting Chairman Mark V. Rosenker, who will chair the hearing, said “the circumstances of the crash have raised several issues that go well beyond the widely discussed matter of airframe icing, and we will explore these issues in our investigative fact-finding hearing.”

Meanwhile, speculation over the March 22 crash of a Pilatus PC-12 (N128CM) in Butte, MT has shifted to aircraft wing icing after it became less likely that overloading was to blame. Thirteen passengers, including seven small children were killed along with the veteran pilot. Meteorologists say conditions in Butte that day was similar to conditions in Buffalo on Feb. 12.

Investigators will also be looking at whether the plane’s owner had complied with an FAA airworthiness directive issued March 10. The AD results from reports from operators that the rear stick-pusher cable clamp has shifted forward on the elevator cable. The FAA said this condition, if not corrected, might reduce the effectiveness of the stick-pusher and/or limit elevator control movement.

Former NTSB Chairman Jim Hall pointed to similarities between the Montana PC-12/45 crash and a March 26, 2005 crash near Bellefonte, PA, in which a pilot and five passengers were killed.

In the 2005 event, the PC-12/45 (N770G) was on an instrument landing system (ILS) approach to land, when witnesses reported seeing it spinning in a nose down, near vertical attitude before it collided with the ground.

The accident site was about three miles from the approach end of the intended runway. A review of radar data disclosed that the private pilot had difficulty maintaining altitude and airspeed while on final approach, with significant excursions above and below the glidepath, as well as large variations in airspeed.

Interviews with other pilots in the area just prior to and after the accident revealed that icing conditions existed in clouds near the airport, although first responders to the accident site indicated that there was no ice on the airplane. Postaccident inspection of the airplane, its engine and flight navigation systems, discovered no evidence of preimpact anomalies. An analysis of the airplane’s navigation system’s light bulbs, suggests that the pilot had selected the GPS mode for the initial approach, but had not switched to the proper instrument approach mode to allow the autopilot to lock onto the ILS.

The National Transportation Safety Board said the probable cause of the 2005 PC-12 crash was the pilot’s failure to maintain sufficient airspeed to avoid a stall during an instrument final approach to land, which resulted in an inadvertent stall/spin. Factors associated with the accident were the inadvertent stall/spin, the pilot’s failure to follow procedures/directives and cloudy conditions.