Stopping by the Airport on a Snowy Evening

Jan. 29, 2007 — -- We're getting fairly used to predicting the effects of a major snowstorm on aviation: airlines grounded, travelers stranded and sleeping on cots, schedules snarled and the whole mess directing TV correspondents and camera crews to the nearest terminal.

During the last several weeks, Denver's airport has been mentioned in countless stories after being shut down twice by major snowstorms. Other airports across the nation have had to scramble to stay ahead of record snowfalls and other winter-related challenges.

While it makes sense to think that snow or frigid weather equals outright cancellations (and maybe even heightened risk of accidents), what we've been experiencing is really more the exception than the rule.

Commercial aviation and most major airports are able to continue business as usual during most winter weather as long as the rules are followed and the airports themselves can stay ahead of the accumulation of snow.

The Denver episodes were an unusual case, partly because of the airport's vulnerable location on the prairies east of Denver, and partly because years ago the airport's designers -- in a stunning lack of foresight -- failed to build a commuter rail line between their distant, new airport and the city.

The result when bad snowstorms hit: a 10-mile strip of highway that's partially or totally shut down and a major airport essentially cut off from the rest of civilization, leaving passengers no way to get to or from the airfield.

A Perfect (Snow) Storm

This season, however, heavy snowfalls combined with inadequate snowplow capability -- and some very questionable planning -- to create a perfect storm. The result was disastrous for Frontier Airlines' flight operations, extremely disruptive for United and the cause of ruined holiday plans, stranded travelers and misplaced bags all across the nation. Snow can do that, but cold weather alone seldom shuts airports down.

Winter weather for aviators divides into several categories. First, there's cold weather without any snow or ice, and then there are the cold days when the air is full of various forms of frozen water. A further division involves the difference between ground operations and in-flight operations.

In the air, modern jets are routinely flown through temperatures nearing minus 50 degrees Fahrenheit high in the stratosphere, and even moderate amounts of snow on takeoff or landing are seldom a problem, except when there are visibility problems.

Icing in the air, however, is a problem, especially for smaller, lighter aircraft. When supercooled water droplets hit a metal wing and freeze into ice, the resulting accumulation can essentially redesign a wing, robbing the aircraft of lift and control.

In smaller airplanes -- especially those powered by piston engines and propellers -- clear ice that a jetliner can shrug off can be rapidly lethal, even with expandable, rubber de-icing "boots" on the front of the wing.

The ground, however, is where most of the cold weather problems get in the way. Slick runways and taxiways can make stopping a landing aircraft difficult, as a Southwest Airlines crew discovered a year ago when their 737 slid off the end of a short Midway Airport runway in Chicago and through a fence.

Sometimes the problem is simply what Denver's airport authorities were unable to handle: too great an accumulation of snow on the ground. When snowplows can't keep at least one runway reasonably clear, or an ice storm makes taxi-ing or stopping impossible, operations grind to a halt.

Airlines on the Rocks

Many of us have experienced that most worrying of winter delays, the loud and time-consuming process called de-icing.

The FAA rules demand that no snow or ice "adhere to the lifting surfaces" of an aircraft. Some terrible accidents in the past 20 years have insured that these rules are not bent in the slightest.

One of those accidents was the tragedy in Washington, D.C., on Jan. 13th, 1982, when a Boeing 737 operated by now defunct Air Florida tried to depart with snow and ice on its wings and leading edges.

The aircraft didn't make it beyond the adjacent 14th Street Bridge, and the rescue of the only survivors (five out of 83) was shown live on national television.

From that accident and others we learned to be even more conservative about when to take the time to wash the wings and tail with a hot solution of water and glycol, a de-icing fluid.

The de-icing not only washes all the snow and ice off; it also helps prevent more from accumulating -- at least for a short while.

Before the Air Florida disaster, pilots were given no real guidelines on how long such an de-icing wash would protect them if the snow or freezing rain continued to fall.

Today, pilots use very precise charts on the maximum amount of time that can pass between the de-icing and a mandatory return to the wash rack. Airline crews sometimes have to reluctantly break out of a long line waiting for takeoff, lose their slot and go back for another bath of de-icing fluid, even if it means further delays.

In 1982, most de-icing was done at or near the gates and terminals. Today, more and more airports are setting up de-icing "wash racks" near the end of the runway so that aircraft can taxi almost directly from the de-icing to takeoff.

Starting up a jetliner that's been sitting at a gate overnight in very cold weather brings some additional worries, even without snow or ice.

Blasts of Hot Air

Jet engines are fairly simple machines, but their moving parts still rely on lubricants that get increasingly thick and ineffective as the temperature heads below zero.

Even jets may need a considerable blast of hot air to stay ready for takeoff in places like Fairbanks, Alaska, or even Minneapolis during a deep freeze.

There are checklists of special steps both pilots and mechanics must follow in such temperatures, but otherwise - especially in the absence of snow or ice on the airport's runways and taxiways -- operations stay reasonably normal.

The bottom line? A true blizzard can still cause a shutdown, but the aviation industry has learned from the mistakes of the past, and in most cases, when the snow flies, we do too -- safely.