Last Thursday, Continental Express flight 3407 was just five miles (eight kilometers) short of the runway in Buffalo, N.Y., when it suddenly pitched, rolled, and plunged into a house outside of town, killing all 48 passengers and crew and one man inside the house.
Media reports have suggested that the pilot may have violated company guidelines—but no federal laws—by engaging autopilot on the propeller-driven Bombardier Dash 8 Q400 airplane under severe icing conditions. In December, the National Transportation Safety Board (NTSB) had warned that "using the autopilot can hide changes in the handling qualities of the airplane that may be a precursor to premature stalling or loss of control." The Federal Aviation Administration has not made such recommendations mandatory.
The pilot of flight 3407, which was operated by Colgan Air, had turned on the de-icing system just 11 minutes after taking off from Newark Liberty International Airport in New Jersey and had discussed "significant" ice buildup on the windshield and wings of the plane. One other plane on the same route landed after flight 3407 without incident, after reporting severe icy conditions. Still, NTSB member Steven Chealander warned yesterday against "jumping to conclusions" about icing being the primary cause of the crash, which occurred just 26 seconds after an antistall system disengaged the autopilot.
Ice typically builds up when tiny cloud droplets impact and freeze on the leading edges, or front surfaces, of the plane. The ice alters airflow over the wing and tail, reducing the lift force that keeps the plane in the air, and potentially causing aerodynamic stall—a condition that can lead to a temporary loss of control. The Dash 8 was fitted with pneumatic de-icing boots that inflate and deflate to break off the crust that forming on the wing's leading edge during flight, but if the plane is pelted with larger droplets, they may freeze farther back on the wing where ice cannot be effectively removed. (The IceController, a device not yet in use on planes, zaps ice off with a pulse of electricity).
To find out more about the dangers of icing, we spoke to Thomas Ratvasky, who has worked as an engineer at the Icing Branch of the NASA Glenn Research Center for the last 19 years.
[An edited transcript of the interview follows.]
How does ice build up on airplanes?
Ice builds up on aircraft in two ways: in flight or on the ground. On the ground, precipitation falls onto the airplane and freezes on upper surfaces much like what happens if you leave your car out overnight. On planes, ground icing forms on the upper surfaces of the wing and tail. That type of ice is managed by de-icing the plane with a fluid [typically propylene glycol] at the airport.
In flight icing is where the airplane is flying through clouds made up of small liquid water droplets. These liquid water droplets can be sustained as liquid below the freezing point. Everybody knows that 32 degrees Fahrenheit (0 degree Celsius) is where water freezes. It turns out that if the water is very pure—if it is condensed out of the atmosphere—and there is nothing for that water to freeze on, it can be sustained below the normal freezing point. What we find in the wintertime is clouds that are made up of small water droplets where the water temperature can be as low as negative 40 degrees C. Here comes this plane flying through the cloud, and the water droplets impact the airplane and then freeze because now they have a surface to freeze on. Ice builds up in flight on the frontal surfaces: leading edge of the wings, the nose and the tail surfaces. There are systems to prevent ice or to remove ice. The de-icing system works on the basis of allowing ice to form before being broken off [using pneumatic boots that inflate to crack the ice]. The anti-icing system prevents ice from forming by blowing hot air from within the compressor of the engine.
We have here at NASA an Icing Research Program with folks who do computer codes to predict ice growth, and we have a wind tunnel in which we can recreate icing conditions on a model.
Why is ice a problem for airplanes?
Ice reshapes the surface of the lift-producing parts of the airplane: the wings and the tail. That roughness is enough to change the aerodynamics of the wing such that there's more drag and less lift.
The amount of lift a wing creates depends on the relative angle that the airstream has to the airfoil. As you increase that angle—the angle of attack—you generate more and more lift. But at some point air cannot flow over upper surface, and you have aerodynamic stall. The point at which aerodynamic stall takes place has to do with the contour of the airfoil. If the surface is contaminated with slight roughness—sandpaper roughness—it will reduce the lift and change the point at which stall takes place.
For scheduled air carriers [including commercial passenger airlines] icing has been a contributing factor in 9.5 percent of fatal air carrier accidents.
How are pilots trained to handle aerodynamic stall?
As you go through pilot training—without icing involved—you practice wing stalls. You bring the nose up and the airplane shakes around because of separated flow. To recover from that, you push the nose down to reduce the angle of attack on the wing and recover. What happens with ice is same principle, but it is happening at a lower angle of attack.
Why would having a plane on autopilot interfere with a pilot's ability to prevent stall?
If a person is hand-flying an airplane and the airplane has a reversible control system, then for every action the pilot makes on the control there is some reaction on the control. There is the ability for the airplane to talk to you.
When the autopilot is engaged, that information isn't being passed onto the pilot. The National Transportation Safety Board recommends against flying with autopilot under icing conditions. [The Federal Aviation Administration recommends against autopilot only under severe icing conditions.] Companies make their own choices on how to present that information to the pilot.