Ask the pilot

Was ice the culprit in last week's deadly plane crash? Or some other unlikely chain of events?


Patrick Smith
February 16, 2009 4:26PM (UTC)

On Thursday evening, Colgan Air Flight 3407, a 74-seat, twin-engined turboprop, slammed into a house shortly before it was scheduled to land in Buffalo, N.Y. Fifty people were killed,  including one person on the ground, making it the worst aviation crash on U.S. soil in over seven years.

Colgan Air, based in Manassas, Va., flies on behalf of Continental Airlines as a Continental Connection code-share carrier, using its own aircraft and crews.

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Ice. According to the early speculation, ice was the likely culprit.

Before we get to if and how icing can bring down a plane, I'd like to begin by dismissing the idea, now making the rounds in some media circles, that the airplane itself, a Bombardier DHC-8-400, was somehow old-fashioned or unsafe because it happened to be powered by propellers. The Canadian-built DHC-8 is known to pilots as the Dash-8, and the -400 is the newest and most advanced variant, nicknamed the Q400.  The Q stands for "quiet"; the plane's state-of-the-art turboprop engines and high-tech soundproofing are engineered to produce jetlike comfort in the passenger cabin. Something about propellers implies quaint, but remember that a turboprop engine is really no different from a jet engine. For all intents and purposes, it  is a jet engine, except that the compressors and turbines are geared to a propeller instead of a cowled turbofan.

Turboprops are highly fuel efficient at low altitudes, and are therefore preferable to pure jets on a lot of short-haul routes. But there is nothing quaint or unsafe about them. Upfront, the cockpit of a Q400 has all the bells and whistles you'd find on an Airbus or Boeing. (For what it's worth, I have a few hundred hours as captain in an older model Dash-8, and the type is among my all-time favorite planes to fly.)

Icing, which can occur both aloft and on the ground, is a common phenomenon. As most passengers understand it, icing is something that affects airplanes before they take off. You know the routine: sitting on the tarmac for an hour waiting for a snow-covered airplane to be sprayed down with strangely colored fluid. Parked at the terminal, an aircraft collects precipitation the way your car does -- via snowfall, sleet, freezing rain or frost. (Thanks to supercooled fuel in the wings, frost can form insidiously even with temps above freezing.) The delicious-looking spray (apricot-strawberry) used to remove it is a heated combination of propylene glycol alcohol and water. It melts away existing snow or ice, and prevents the buildup of more. Different fluid mixtures, varying in temperature and viscosity, are applied for different conditions.

De-icing is complicated and expensive. The preflight de-icing checklist can take up several pages of a pilot's manual, and with fluid costing upward of $5 per gallon, airlines loathe snowstorms almost as much as strikes, wars and recessions. When handling and storage costs are considered, relieving a single jet of unwanted winter white can cost tens of thousands of dollars.

It is money well spent, however, because ice on an airplane is potentially hazardous, especially when adhering to the wings. The monster isn't the weight of the frozen material, but the way it disrupts airflow over and around a wing's carefully sculpted contours, robbing a plane of lift. There have been several accidents over the years in which planes attempted takeoff with  inadequately de-iced wings. Most recent of these was a 1991 USAir incident at LaGuardia. There was also the infamous crash of Air Florida Flight 90 in Washington, D.C., in 1982. In addition to buildup on the wings, frozen-over probes gave a faulty, less-than-actual thrust reading after the crew failed to run the engine anti-ice system.

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Potentially dangerous icing also occurs during flight. Under the right combination of moisture and temperature, it can form along the leading edges of the wings and tail, along engine intakes and propeller blades, as well as on windscreens, probes and various other surfaces. Left unchecked, heavy icing can damage engines, throw propeller assemblies off balance, and, just as it does on the ground, steal away precious lift. In a worst-case scenario it can induce a full-on aerodynamic stall -- the point when a wing essentially ceases to fly. Planes are most susceptible during takeoff and landing, when speed is slowest and the lift margins already slim.

The good news is that most icing encounters are brief and routine, posing little if any danger. And all modern commercial aircraft, the Q400 among them, are equipped with sophisticated de-icing equipment for the rare times when things become more serious. Propeller blades, probes and windscreens are kept clear electrically; engine intakes and wing leading edges are heated using air bled from the engines, or are de-iced through a series of pneumatically inflated boots that break away any accumulation. These systems use redundant sources and are separated into independently operating zones to keep any one failure from affecting the entire plane.

Thus, airliner crashes brought on by in-flight icing are exceptionally rare. Most well-known is the 1994 crash of American Eagle Flight 4184. Sixty-eight people died in what remains the deadliest-ever mishap involving a regional aircraft. The plane, an ATR-72, had made several circuits of a holding pattern in freezing rain, when suddenly it was thrown into an uncontrollable roll and plummeted from the sky, disintegrating in a soybean field near the town of Roselawn, Ind. A design flaw in the ATR's wing de-icing system was later discovered, and corrected.

Which brings us to Colgan at Buffalo. 

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Like the Roselawn ATR, the Colgan Q400 reportedly hit the ground at a radically steep angle, indicative of a stall or other loss of control. Was the Q400 victim to a design flaw similar to that of the ATR? It's possible. The hunch among pilots right now is that the plane may have suffered a tailplane stall due to ice buildup on the horizontal stabilizers. Horizontal stabilizers are the smaller, tail-mounted wings that help control a plane's nose-up/nose-down motion, known as "pitch." Normally, stabilizers are considerably less sensitive to icing than the main wings, but a prolonged and severe encounter could have, in theory, overwhelmed the aft de-icing boots. Then, as the aircraft was slowed and configured for landing, one or both stabilizers stalled entirely, resulting in a loss of pitch authority and the subsequent crash.

That's conjecture at this point, but it's certainly plausible. As is the chance that the tailplane de-icing system may have malfunctioned. Or maybe the icing event was so extreme that the entire plane was overwhelmed. Not likely but, again, plausible. Of course, nothing about a disaster is likely. Any plane crash, almost by definition, is the the rare exception to a long and reliable set of rules.

We'll eventually learn what happened, once the black boxes have revealed their sad secrets. The official findings are liable to point to not a single cause but rather a combination of causes -- a chain of unlikely events, survivable by themselves but deadly in combination. 

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Author's Postnote:

Just as this story was going to press, investigators revealed that the Colgan Q400 was  operating on autopilot at the time of the crash. This was an apparent violation of protocol; the airline and manufacturer recommend that the plane be flown manually during icing conditions. This is to allow the pilots to have a better awareness of any adverse effects of ice buildup. While the media seems to have seized on this news, I'm unsure how important it is. The suggestion is that had the aircraft been under manual control, the pilots may have realized the urgency of the situation before it was too late. While that's possible, I'm for now skeptical that the outcome would have been any different.

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Do you have questions for Salon's aviation expert? Send them to AskThePilot and look for answers in a future column.


Patrick Smith

Patrick Smith is an airline pilot.

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