The Crash of Flight 3407: Better Deicing Needed?

The passengers on Continental Airlines Flight 3407 from Newark, N. J. to Buffalo last Thursday night probably felt pretty confident about air travel, especially after hearing about Chesley Sullenberger's successful splash landing of his U. S. Air flight in the Hudson River less than a month ago after birds apparently clogged both engines. But a few minutes before the planned landing in Buffalo, the crew radioed that they were encountering noticeable icing on the wings. As soon as they attempted to lower the flaps for landing, the Bombardier Q400 commuter turboprop began to pitch and roll violently. The extended landing gear was retracted just before the plane crashed flat on top of a house in Clarence, New York, killing all 48 passengers and crew as well as one person on the ground.

Ice has been a problem for aircraft ever since the first airplane flew high and long enough to accumulate freezing rain on the wings. It tends to build up on the leading edges of airfoils. Besides its additional weight, ice can distort the airflow around the wing in unhelpful ways and even interfere with the mechanical movement of control surfaces such as ailerons and flaps.

There are three main approaches to deicing of aircraft. First, many airports are equipped to spray deicing solutions (basically a kind of antifreeze) on a plane's wings before takeoff. This can prevent ice buildup soon after takeoff, but it eventually wears off. In addition, larger aircraft use heated deicing strips that warm critical surfaces so that ice is less likely to form. Finally, smaller aircraft such as the two-engine Q400 usually use pneumatic deicing boots. If you imagine partially blowing up a balloon, spraying freezing water on it until it has a coating of ice, and then blowing it up more until it cracks out of its ice shell, you have pictured the essentials of a deicing boot.

At a news conference after the crash, federal investigators said that icing was a possible cause. The performance of pneumatic deicing boots has been of enough interest to inspire the Federal Aviation Administration to commission a lengthy investigation into how well they work. In 2006, the investigators published a 160-page report, which I have not had time to read since I found it this morning. However, the executive summary points out that in an actual flight test of an instrumented aircraft using the boots, enough so-called "intercycle ice" built up to produce a loss in lift of 25% or more. This loss became worse when the aircraft was close to a stall angle of attack.

The translation from aerodynamic engineering speak into layman's language goes something like this. Lift, the force that keeps an airplane in the air, can be reduced as much as one-fourth by ice that builds up between the times that pneumatic deicers operate (they work on an automatic cycle once they're turned on). If the airplane is trying to climb rapidly (as may well have happened, since a decision to retract landing gear is consistent with deciding to miss an approach and gain altitude), every airplane eventually reaches a "stall angle." When an airplane stalls, it nearly stops in the air and literally falls out of the sky. Recovering from a stall is hard, even in clear air in the daytime with a normally functioning aircraft when you have plenty of altitude left. The pilots of Flight 3407 were flying what was probably a plane with heavily iced control surfaces, in the dark, in freezing rain, with only about 2,000 feet between them and the ground. And it wasn't enough.

Speaking as a non-mechanical engineer, I have to confess that the picture of little rubber boots inflating and de-inflating to knock ice off a wing has its weirder aspects. Ice is not very strong mechanically, but it can be pretty sticky. And rubber exposed to the kind of life led by the leading edge of an aircraft wing is bound to get roughened and porous sooner or later, which will make ice stick to it that much better. I don't know any details such as how much the boots really inflate to blow off the ice. But the whole situation seems like it's a kind of empirical solution to a problem that is very hard to analyze theoretically, or even to model in the laboratory, as the FAA report itself admits. While the last accident where ice was implicated in the crash of this type of aircraft occurred fifteen years ago, even one life lost that could be prevented is too many.

We will have to wait for the final report of the crash investigation before we can draw any substantiated conclusions about what this accident says with regard to pneumatic deicing gear. As with many crashes, there may be a human factor involved. According to the Wikipedia website article "deicing," pilots used to believe that pneumatic deicers did not work well unless you allowed a certain minimum ice buildup to occur. Later studies reportedly revealed that this idea was false. We don't know what the late pilots of Flight 3407 believed, but we do have a record in the flight recording of what they did. Examination of that record plus engineering studies may show in more detail what went wrong and how accidents like this can be prevented in the future.

Sources: I used the MSNBC report from Associated Press currently available at http://www.msnbc.msn.com/id/29173163/, and the Wikipedia article "deicing." The FAA report "Investigation of Performance of Deicing Boots, Surface Ice Detectors, and Scaling of Intercycle Ice" is available for free download at http://www.tc.faa.gov/its/worldpac/techrpt/ar06-48.pdf.