December 2006Departments

Accident Analysis

Dashing through the snow


But not 'laughing' all the way

Ah, winter! Short days; long, cold nights. See how the lovely morning frost coats the working airplanes whose idle time is spent tied down on a flight line rather than pampered inside a toasty hangar. Snow--yes, the time has come for that too. Driving to the airport, holiday music streaming from the car radio, you're delayed by traffic backups caused by the rash of fender-benders that always follows the first snowfall. This phenomenon appears even in "snow country" as drivers, distracted by gadgets and haste, remember how to drive on snow and slush. Some of their mishaps are also attributable to trying to drive while peering through a tiny patch of windshield hastily scraped free of the thick white frost that covers the rest of the vehicle.

You can't do that with an aircraft, you understand, thanks to the aerodynamic hazards frost can present. As a pilot you know that the unpreparedness and impatience are unacceptable in flying. There's enough to think about without taking unnecessary risks.

Even more so now. Add snow and ice to your calculation of risk factors when coming out to fly on winter days, even nice winter days. This is the time of year when braking-action reports start showing up in ATIS broadcast and pireps (see the AOPA Air Safety Foundation's Cold Facts: Braking Action Reports Safety Brief). It's when runways too rough or too expensive to plow close for a few months, making crosswind takeoffs and landings on the remaining runways a seasonal fact of life. It's the time of year when the abbreviation "PPR" starts showing up in notams, especially after big storms. You'll find PPR in the list of abbreviations in your airport/facility directory. It stands for prior permission required, as in, plowing operations are under way on Runway 17-35, 15 minutes PPR if you are inbound to land.

A runway newly plowed, or in the process of being plowed, can be an interesting place to land or take off an aircraft. Show up immediately after the storm has passed and you may find that it bears little resemblance to the spacious, hard-surfaced runway that you use in warmer weather. Plowing may not yet have been completed to the runway's full width.

From a half-mile out on final, it may not be immediately obvious just where the dividing line between plowed and unplowed lies. Even if the ATIS declares that "the runway is plowed full length and width," patches of snow or ice that remain can send you skidding or veering (see "ASF Safety Spotlight: Aviation Ice Capades," p. 80). On a clear sunny day after a storm, runway ice on pavement might melt, only to refreeze at dusk. Assume ice is present and save braking for slow-speed operations. Even then, be very cautious and remember that taxiway or runway snow can freeze one or both brakes, causing a loss of directional control or even difficulty getting moving again after a full stop.

A more insidious snow hazard is found at runway intersections, where plowing may have created a drift across the runway you plan to use. If your airport has

no automated information broadcast and you can't get the information you want over the common traffic advisory frequency or from the FBO, watch out! Local notams often don't keep pace with plowing operations. Remember that plowing or "snow removal" operations can continue long after the storms have passed, affecting aircraft movement. Why? Airport are big places, and snow may have to be moved more than once, first to get it off the runways and later, to get it out of the way for good.

Just as on the roads and highways, winter aircraft accidents change character as befits the season in light of the hazards mentioned here. Another commonality is that problems that might not have become accidents on a dry road become similarly aggravated on a slick or snowy runway. Nor are there reliable performance parameters for operations on snow, leaving us to guess at what works and what doesn't. How much is a takeoff roll lengthened by an inch of snow? Two inches? Powder, as compared to granular snow? A crust? Slush? What technique should you use when you land? What is the effect of crosswind?

If you weren't piloting a sled, Christmas season 2005 was a tricky period for aircraft negotiating the transition from ground to air, and back. A December 21, 2005, entry in the National Transportation Safety Board accident summaries from Spirit Lake, Iowa, shows that snow season is a slippery slope even when the person who plowed the runway and the person who landed on it are the same individual. "The airplane encountered snow on the runway during landing on a private airstrip and subsequently nosed over. The day prior to the accident the pilot reported that he removed snow from the runway. The pilot stated, 'After setting down the left wheel got into the snow on runway edge. Braking and correcting the aircraft caught in the right edge of snow on runway. This pulled the nosewheel into the snow also. At this time the aircraft was slowing down fast. All of a sudden the left wing and nose came over and that was 'all over.'" The probable cause, as released on April 25, 2006, was "The pilot using an unsuitable runway and not maintaining directional control during the landing, leading to the nose over on impact with snow. A factor was the snow that remained on the runway after plowing."

Illinois and Wisconsin are snowy places. Two days after the Iowa mishap, both of those states were scenes of trouble for pilots that led to the appearance of the word "snow" in accident reports. In Morris, Illinois, that proverbially opportunistic snowdrift scored a hit on a Cessna 172R. The source of the distress, said the NTSB, was the pilot, who lost control and struck a side-of-the-runway snowdrift while trying to land in a quartering tailwind. The pilot recounted various omissions and failures in a written statement in the report: "Going to Morris I tried to get the weather, but could not figure out how to get the weather [frequency] in and went ahead and entered the pattern for [runway] 36. I ran a normal pattern and ended up [too] long and [too] fast and executed a go-around. I went around the pattern again and the conclusion ended up the same [too] high and [too] fast but I decided to land this time and the wind blew me off the right side of the [runway]. If I would not have gone off the side of the [runway] I would have gone off the end." The pilot offered a safety recommendation: "Know the airplanes radios, get weather, look for the wind sock, land into the wind, do not force the airplane to land, be prepared to do a go-around no matter what, and look for other signs of wind direction." (The NTSB report stated that "at 0905, the recorded weather at C09 was: wind 220 degrees at 14 knots; visibility five statute miles; present weather haze; sky condition clear; temperature 3 degrees Celsius; dew point 1 degree C; altimeter 29.71 inches of mercury.")

The Platteville, Wisconsin, accident happened when a student pilot soloing a tailwheel- equipped Champion 7EC bounced during landing. A ground loop began, and the snowbank took over from there. The pilot's account was included in the NTSB file: "Had perfect approach, centered on Runway 25, wings level. Touched down, plane did a little hop and then landed straight. Plane began to swerve left--I corrected, not enough tailwheel response to get back straight. Went off runway at intersection of 25-33 into snow. Hit snow bank and plane nosed over and then (went) on(to its) back."

It was mentioned earlier that a runway you know in summer as spacious and reassuring can become an ambush waiting to happen in winter. But start out with a runway only 25 feet wide, then constrain it further because half the center is ice-covered and one side isn't. Add an aircraft (a Piper PA-22) to the mix, and this was the result on December 28, 2005, in Jamestown, New York: "During the rollout, the right main landing gear wheel 'bumped' the ice. The airplane then veered left, and the left main landing gear tire went off the runway, and into 'soft grass.' The airplane then nosed over. The National Transportation Safety Board determines the probable cause of this accident as follows: The pilot's failure to maintain directional control during the landing rollout. Factors included the ice on the runway and the soft terrain."

Doesn't take much, does it? Then how about unplowed snow? What procedure should be used here? A Toronto, Ohio, accident on December 17, 2005, involved a PA-22 departing on unplowed, crusted snow--a condition suggesting that the snow may have been there awhile, or had partly melted and refrozen. The risks include not knowing how the aircraft will perform on an uncertain surface, what lies under the crusted snow, and whether drifting or compacting of the crusted snow has occurred. Reported the NTSB: "The airplane swerved during takeoff roll on a snow-covered runway and subsequently nosed over after dragging its left wing tip. The pilot reported that the turf runway was covered with approximately two inches of 'crusted snow.' The pilot stated that during the takeoff roll he attempted to follow the tire tracks left in the snow by a previous airplane. The pilot reported that about 75 yards into the takeoff run he decided that the 'aircraft was not going to fly.' The pilot stated he 'was about to close throttle and shut down, when the airplane veered to the left.' The airplane subsequently dragged the left wing tip and nosed over. The pilot stated that the accident could have been prevented if prior to takeoff he had 'inspected the runway surface for degree of snow crusting.' The pilot also stated that the runway was 'prone to drifting snow in various areas at times.'"

Fortunately, the relatively low speeds of general aviation takeoffs and landings keep most of these accidents non-tragic. But the GA accidents are plentiful, as illustrated by the fact that those mentioned here all occurred in the second half of December 2005.

So when the highways become skating rinks again this year, remember that the runways probably are, too. Add the snow factor to all the other risks you consider when weighing how--and whether--to conduct flight operations. Let these accident summaries convince you that all the useful lessons about this subject have already been learned by others. There's no point reinventing the wheel.

Dan Namowitz is an aviation writer and flight instructor. A pilot since 1985 and an instructor since 1990, he resides in Maine.


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