With half the country suffocating under 14 feet of snow, I should probably say something about the pains and perils of winter flying. This is something I touched on back in December, but the storms keep coming. Pardon the pun, but here's a more in-depth look.
Ice or snow on an airplane is potentially very dangerous, especially when adhered to the wings. The devil isn't the added weight, but the way it disrupts the flow of air over and around a wing's carefully sculpted contours, destroying lift. You've also got slick runways to contend with, and myriad other challenges.
On the ground
Snow piles up on a plane parked at the gate the way it piles up on your car. A cursory brushing or a sweep of your arm is a safe enough remedy for driving, but not for flying, when even a quarter-inch layer of frozen material can adversely alter airflow -- highly important during takeoff, when speed is slow and lift margins are thin.
Further complicating things, frost can sometimes build on a stationary wing even when there is no active precipitation. This happens when fuel carried in the wings becomes cold-soaked during prolonged flight at high altitudes. If the wing remains cold enough, for long enough after landing, moisture in the air can freeze onto the surface. On some aircraft, such as the MD-80 series, the crew must perform a pre-flight de-icing check on sunny days with the temperature well above freezing.
The fluid used to remove the stuff is a heated combination of glycol alcohol and water. Different fluid mixtures, varying in temperature and viscosity, are applied for different conditions, often in combination. A plane might be hit with so-called Type I fluid to get rid of the bulk of accumulation, then further treated with Type IV, a stickier substance that wards off any subsequent buildup.
Pilots follow a checklist to ensure their plane is correctly configured. It varies model to model, and whether the spraying will occur at the gate or remotely. Usually the flaps and slats will be lowered to the takeoff position, with the APU providing power and the main engines shut down. The air conditioning "packs" will be switched off to keep the cabin free of fumes.
When de-icing is complete, the ground crew will tell the pilots which types of fluids were used, as well as the exact time that treatment began. This allows us to keep track of something called a "holdover time." If the holdover time is exceeded before the plane has a chance to take off, a second round of de-icing may be required. The length of the holdover depends on the kind of fluids that were used, plus the rate and type of any active precipitation (dry snow, light snow, ice pellets? Light, moderate, heavy?). We have charts to figure it out. If, during a long taxi queue, you see a pilot come out of the cockpit and peer through a window at the wing, this is a good indication that holdover time has elapsed. He is checking to make sure the surface remains clear. (Or, he might be checking for the sake of checking, even if the holdover hasn't been exceeded.)
Thin coatings of contaminant are sometimes allowable for takeoff, depending on the plane, the type of material and where it's located. But what if you see something that makes you uncomfortable? Should you speak up? It's extremely unlikely that the crew would be unaware of any unacceptable condition, but sure, if something is making you nervous, you won't hurt anybody's feelings if you hit your call button and ask about it.
De-icing fluid isn't especially corrosive or poisonous, but neither is it the most environmentally friendly stuff in the world. And although it resembles apple cider, or an apricot-strawberry purée, I probably wouldn't drink it; certain types of glycol are highly poisonous. And at upward of $5 a gallon, it is also very expensive. When you add in handling and storage costs, relieving a single jet of winter white can cost several thousand dollars. At many airports the fluid is recycled. Another approach is to tow aircraft into specially built hangars equipped with powerful, ceiling-mounted heat lamps. Such hangars are in limited use at Newark and JFK, among other places. In some ways this is a greener method, though it uses hideous amounts of electricity.
In the air
Snow will not stick to an airplane during flight. Ice, however, is another story. Owing to aerodynamic forces, it tends to adhere to the thinner, lower-profile areas, and not to larger expanses. It will build on the forward edges of the wings and tail, for example, around engine inlets and on various antennae and probes. Left unchecked it can damage engines, throw propeller assemblies off balance, and rob the wings of precious lift. In a worst-case scenario it can induce a full-on aerodynamic stall -- the point when a wing essentially ceases to fly.
The good news is that all affected surfaces are equipped with devices to keep them clear. On propeller-driven planes, pneumatically inflated "boots" will break ice from the leading edges of the wings and horizontal tail. On jets, hot air bled from the engine compressors is used instead, plumbed to the wings, tail and engine intakes. Windshields, propeller blades, plus various probes and sensors, are kept warm electrically. De-icing systems use redundant power sources and are separated into independently functioning zones to keep any one failure from affecting the entire plane.
Airframe ice comes in three basic flavors: rime, clear and mixed. Rime is the common one, appearing as a sort of white fuzz. The rate at which ice accretes is graded from "trace" to "severe." Severe icing, most commonly encountered when flying through freezing rain, is a killer. But it's also rare, and tends to exist in thin bands that are easy to avoid or fly out of. On the whole, in-flight icing is considerably more of a threat to smaller noncommercial planes than it is to airliners -- jets especially. Even in the heaviest precipitation it is uncommon to see more than a trace amount of rime on a jetliner.
Runway hazards
An icy runway is a slippery runway, needless to say. Airports issue so-called braking action reports for each runway -- even different portions of a runway -- which pilots make careful note of, along with the latest wind and weather reports. Together this data helps determine whether it's safe to arrive or depart. Because there must always be adequate rollout distance on landing, as well as adequate room to stop following an aborted takeoff, operations are prohibited when braking reports fall below a certain value, or when snow, ice or slush exceed certain depths. Takeoff and landing speeds, as well as the power and flap settings to be used, are often different in snowy weather than they are in dry weather.
If you've ever looked closely at a runway, you'll see it is cut laterally by thousands of grooves spaced inches apart. This helps with traction, as do the extremely sophisticated anti-skid systems found on modern planes.
I've made plenty of winter-weather landings. One thing that always jumps out at me is the way in which fresh snowfall can make a runway difficult to see and align yourself with. In normal conditions the runway sits in stark contrast to the pavement, grass or whatever else is around it. When it's snowing, everything is white. Runways are outfitted with an array of color-coded lighting -- approach, threshold, centerline and edge lights, VASI and PAPI, and so on. Most of the time you pay only cursory attention to these displays. That is, until the moment you break from a low overcast, just a few hundred feet over the ground with a half-mile of visibility, and find yourself confronted with a landscape of undifferentiated whiteness. All those lights and colors are suddenly very helpful.
Accidents and incidents
There have been several tragedies over the years in which planes attempted takeoff with iced-over wings. The most recent of these was a 1991 USAir incident at LaGuardia. Nine years earlier was the infamous Air Florida disaster in Washington, D.C., when in addition to ignoring buildup on the wings, the crew failed to run the engine anti-ice system, allowing frozen probes to give faulty, less-than-actual thrust readings. Iced-up and underpowered, the 737 slammed into the 14th Street Bridge seconds after takeoff killing 78 people.
In other cases, planes have gone skidding off the end of snowy runways. Culprits have included erroneous weather or braking data, an unstable approach continued when it shouldn't have been, and the occasional malfunction -- or any combination of such things. You might recall the American Airlines 757 that slid from a runway in Jackson Hole, Wyo., earlier this winter. Apparently the plane's speed brakes (panels that rise from the wing to aid in deceleration) did not deploy automatically on touchdown, and the crew failed to lift them manually. The jet's thrust reversers also were unusually slow to activate.
On Halloween night in 1994, 68 people died in the crash of American Eagle Flight 4184 near the town of Roselawn, Ind. (still the deadliest-ever mishap involving a regional aircraft). The ATR-72 turboprop had made several circuits of a holding pattern in freezing rain, when suddenly it plummeted from the sky and disintegrated in a soybean field. Investigators determined that a design flaw in the plane's de-icing system allowed a ridge of ice to form ahead of the plane's ailerons (hinged surfaces on the wings that control turning), ultimately resulting in a total loss of control. The ATR's leading-edge pneumatic boots were redesigned, and it has avoided trouble ever since.
Back in my cargo flying days, at the airport in Brussels, Belgium, one of our jets was inadvertently de-iced using an improperly prepared fluid. Unbeknownst to the crew or to the de-icing team, the liquid was mostly water. After seeping into the nooks and crannies around the flaps, ailerons and elevators, it promptly began to freeze. The plane took off, and within minutes the pilots began to experience serious control problems. They wrestled the jet back to Brussels, but in the words of the first officer, who told me the story later, "We didn't know for sure if we were going to make it."
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I can't tell you there will never be another ice-related accident. I can, however, assure you that airlines and their crews take icing a lot more seriously than they used to. We've learned a lot -- much of it the hard way -- and this has carried over into specific, formalized procedures that leave little to chance.
Winter storms aren't likely to be fatal so much as aggravating. As anybody who travels knows, when the white stuff falls, nothing seems to run on time. The trouble isn't flight-related so much as ground-related. Tarmac logistics slow to a crawl as snow and ice accumulate. Luggage loading, cargo handling, fueling -- everything slows to a crawl as personnel and ground vehicles get bogged down in the slush. Runways and taxiways, meanwhile, need to be plowed and chemically treated. And, of course, planes themselves have to be cleaned of snow and ice.
If it seems that delays are longer than they used to be, that's because they are. The number of planes in the sky has more than doubled over the last 25 years. Our airports and air traffic control infrastructure, on the other hand, have hardly changed. Shut down a runway for 30 minutes of plowing and literally hundreds of flights can be affected. For this reason airlines are known to preemptively cancel hundreds, even thousands of departures when storms take aim at their hubs. They're still losing millions of dollars, but they'd be losing a lot more by pushing ahead with a full schedule. And while inconvenient for passengers, most would prefer a preemptive cancellation over having to spend three nights sleeping on a terminal floor.
According to a report released this week by Bloomberg, disruptive weather will cost the U.S. airline industry $600 million this winter. Storms have triggered nearly 90,000 flight cancellations -- more than in any year since government tracking of such data began.
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Do you have questions for Salon's aviation expert? Contact Patrick Smith through his website and look for answers in a future column.
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