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Winter operations bring challenges, limitations

 

Whether we like it or not, winter is coming. The colder temperatures will provide us with improved aircraft performance and (mostly) smooth flying conditions. Winter also brings some challenges.

Aircraft stored outside will collect snow and frost. The wings must be completely free of snow, ice or frost before take-off and not just because the CARs say so.

Even the slightest amount of frost can disrupt the boundary layer of air over the wings and cause a stall. Wind tunnel and flight tests indicate that frost, ice or snow on the leading edge or upper surface of a lifting surface having a thickness and texture of medium to course sandpaper can reduce lift by as much as 30 per cent and increase drag by as much as 40 per cent.

After sweeping snow off the wings and horizontal stabilizer, check for a thin layer of ice on the wing and tail surfaces. The sun could have melted some of the snow, which will form a thin layer of ice on aircraft surfaces that is difficult to detect visually. Run your fingers over the surfaces and feel for the ice.

Even after cleaning all of the snow and frost off the wing and tail surfaces, frost can form again fairly quickly under certain temperature conditions. This thin re-formed layer of frost may be difficult to detect visually. Use your fingers to feel for newly formed frost.

A Cessna 208 Caravan was left overnight on the ramp at Abbotsford, B.C. where it collected about 3/16 to 1/4 inch of frost. The pilot believed the sun would melt it off before take-off. He did not check to see if the ice was gone before take-off, one hour later.

It was not gone. When the pilot retracted flaps from 10 degrees to 0 degrees at 400 feet, the aircraft banked left, descended rapidly, and crashed into a field 1/2 mile south of the runway threshold. The aircraft was destroyed and the pilot and five passengers were all injured.

Confirm the runway surface is suitable for take-off or landing. Where there is no control tower or FSS in operation, this means inspecting the runway. Do not assume the entire runway is in the same condition as the runway immediately ahead of you. Ridges left by snowplows may be difficult to see in the distance ahead, as are snow drifts formed by crosswinds.

Snowdrifts may form in as little as 20 minutes and may be dense enough to cause directional control problems. Snow that is hard packed where you are may be soft or slushy part way down the runway.

A pilot of a C-172 flew over the destination airport at 1,000 feet, but failed to notice that the snowplow had left a ridge of snow across the runway on which he intended to perform a touch-and-go.

The nose gear struck the snow ridge as the aircraft became airborne. The pilot returned to the departure airport for landing, where the nose gear collapsed on touch down.

A pilot of a C-172 returned for landing after a 20 minute sightseeing flight. He did not notice that snow had drifted back onto the cleared portion of the runway. On landing, the right main gear hit some drifted snow, causing the aircraft to veer to the right. The left wing struck a snow bank.

Flight in wet snow or rain in near freezing conditions can result in a rapid accumulation of ice all over the airframe.

A C-170 with an instructor and student on board were returning to the airport after a local training flight when a light rain started to fall, the rain froze on impact with the aircraft. They descended as low as possible in the hopes of reaching an above freezing temperature and performed a straight in approach to landing.

A descent to increase the outside air temperature, or a turn out of the airframe icing conditions, must be done immediately to stop the ice from accumulating. Forward visibility may be impaired, making the final approach to landing more challenging.

Fly the approach faster than normal; you have no idea what the aircraft stall speed is under these conditions. Do not lower the flaps. The aircraft is flying flapless, so don’t change anything. Adding flaps may suddenly increase the aircraft angle of attack beyond the critical angle.

Do not flare normally. Raising the aircraft nose for a flare may also result in a sudden stall. Fly the aircraft onto the runway at a faster than normal speed and then use brake to slow down.

Snow showers can also reduce the visibility and cause a rapid loss of horizontal reference. A whiteout condition may occur making it impossible to tell where the ground is. Disorientation and loss of control usually follow.

As soon as you realize forward visibility is being reduced or your horizontal reference is being lost, you must get on the instruments and get out of the poor weather conditions by descending or turning around.

The Graphic Area Forecast will indicate if snow or rain showers are forecast. Snow showers can lower the ceiling more than 2,000 feet and the visibility more than two miles almost instantly.

The pilot of a Helio Currier reported that he had encountered heavy snow and he was flying at 5,000 feet on instruments. There were no further communications from the pilot. Witnesses saw the aircraft in a vertical descent as it started to break up.

Low light conditions will also create whiteout conditions. It may be difficult to tell where the beginning and sides of the runway are.

The pilot of a PA 22-108 Colt touched down 30 metres short of the runway threshold in about 50 cm of snow. The nose wheel struck the ground hard and bent backward. The aircraft slid on the nose cowling until it came to a rest on the runway threshold.

The pilot reported that the lack of contrast on the ground short of the runway prevented him from judging his altitude correctly.

Prepare for the challenges of winter flying and take your time. Watch for changeable and changing weather conditions and plan your flight accordingly. Enjoy your winter flying.

Dale Nielsen is an ex-Armed Forces pilot and aerial photography pilot. He lives in Abbotsford, B.C., and currently flies air charters. He still freelances as a flying instructor and seminar facilitator. Nielsen is also the author of seven flight training manuals published by Canuck West Holdings.

Got an aviation safety story to tell? Dale Nielsen would like to hear from pilots who have educational aviation experiences to relate. Excerpts from these stories will be used in upcoming safety articles. Dale can be contacted via e-mail: dale@flighttrainingmanuals.com.