Mountain flying represents a real challenge for most flatland pilots. At the same time the rewards are tremendous, offering very spectacular sceneries and views. It also means that the pilot needs training, develop skills and knowledge to fly safely in such an environment. We provide some basic knowledge on this subject, but this is no substitute for real training in the actual environment.
We are going to discuss the different kind of illusions the pilot is likely to encounter when flying in the mountains, caused by light and snow.
Flying in the mountains with its large structures makes it very difficult to judge distance and scale. reason is that you look small compared to the mountains. Some glacier crevasses are capable of swallowing a complete aircraft without leaving any trace at all. To counter this scale illusion, look for familiar objects like trees or bush. It will help you judge how far your are from the large mountains. Being able to estimate your distance from any obstacle is important as you may have to make that reversal turn into the wind.
In snow covered areas it can be very difficult to see the different ranges as they can merge with each other due the the white color of the snow. This can happen when climbing towards a mountain range and a peak covered with snow is located in between you and that range. You will almost not see it due to the relative slow movement of that peak. It thus looks as if that peak is part of the range, until you come very close..
This situation is worsened when there is an overcast of clouds.
The lack of a good external horizon (due to snow, sloping mountains, haze) will sometimes create altitude and airspeed problems. Having a good artificial horizon and altitude indicator in the aircraft helps, due to the sloping of the terrain you must look outside and learn to assess your attitude. Instruments lag behind the real situation, looking outside gives the opportunity to anticipate attitude changes. A good example of a false horizon is a sloping ridge, causing the pilot to fly the aircraft with a bank angle.
When there are any trees in the area, the tree or bush line can be used to locate the horizon in steep areas. With the help of your instruments and outside indicators it remains difficult, even more so in hazy situations.
The light in the mountains can produce definition and depth illusions. When the sun is shining, deep dark shadows are present in valleys and when you are flying with sunglasses it can be very difficult to see any structure, let alone obstacles. The same can be said when a light haze is present and the sun is low as can be in the morning or in the afternoon. Do keep an eye on the position of the sun so you will not be surprised when flying around a peak or range and get blinded when turning.
Usually there will be more snow in the winter, changing the scenery in the mountains. And because the stuff is white the difference between clouds and snow is sometimes hard to see. Orientation will be difficult and certain features covered with snow are not so easy to detect so that familiar grounds during other seasons suddenly become unfamiliar during the cold season.
To see any depth on snow covered surfaces has always been difficult, especially so when the light from the sun is spread out by haze and/or a cloud cover. All objects seem to disappear into the background and have the same color, any horizon is lost and it will become very difficult to estimate altitude and distance even clouds below your altitude are difficult to see. This can happen very fast when flying over glaciers under a cloud cover, make sure to be prepared to turn around when visual indications are lost.
Disorientation can occur when flying over snow covered terrain under clear sky and bright sunlight. Shadows are not visible due to the reflection of the snow. Everything appears to be flat. Could be a real problem for low and slow aircraft.
An illusion of good contrast can be seen when a rock or ridges are visible above the snow. It gives the impression that you can perfectly estimate your height above ground, while in fact it is still very difficult. Pilots of slow flying aircraft like helicopters are prone to this illusion.