Winter is coming, so I thought it a good time to touch on an optical illusion called flat light. Though it is more prevalent during winter months, it can occur any time of year.
For VFR flight, we need to see enough of the ground as a reference to control the aircraft and to avoid terrain, which is the problem with this illusion. Those of us who fly in Arctic regions take flat light very seriously, but it can also occur at lower latitudes.
If you haven’t experienced it personally, flat light can be difficult to appreciate. While horizontal visibility may often be very good–like being able to see a mountain range 50 miles away–when looking down one is unable to focus on the ground. Imagine being able to see the ground, without having the depth perception necessary to determine exactly how far away it really is. In a flat light condition your height above the ground determination may be off by as much as 2,000 feet!
The problem stems from the limitations of how we perceive our world. Our brain acts as a video processor and models an image based on raw data received from the retina via the optical nerve. We only see .0035 percent of the electromagnetic spectrum, visible light in the near ultraviolet class, and that data is badly pixilated with a hole in it. The hole, commonly referred to as the blind spot, is due to a lack of light receptors where the optic nerve attaches to the retina. Even when we close one eye we don’t see the blind spot because our brain is very good at interpolating data. It simply fills in the picture with what it calculates should be there. An interesting experiment demonstrating the brain’s imaging capability is when people are fitted with special glasses, turning the images they see upside down. After a time, the brain makes the correction and everything is right side up. That is until the glasses are taken off, when the image once again goes upside down until the brain can once again adapt.
If that wasn’t problematic enough, the best part of our field of view with good resolution is very narrow. Based around the retina center, it is about 1 degree, or about an inch using the distance from the pilot to the aircraft instrument panel. Now you know why our instructors always stressed a proper scan! As humans, we are stuck with these sensory capabilities, which unfortunately don’t serve well flying in a flat light environment.
Flat light typically occurs during winter with overcast skies and a snow-covered ground. The combination of a very reflective white surface and a lack of direct sunlight turns our 3-dimensional world into one that looks 2-dimensional. There are no shadows or contrast, which are necessary for depth perception. Rock, trees, rivers, buildings, and roads can all provide the pilot with a much needed depth reference. Knowing this, a prudent pilot flying over a large flat white valley may opt to fly along an area with objects providing contrast, such as a rocky ridgeline.
One of the things that makes flat light so dangerous is its insidious nature. The pilot thinks he can see the ground and judge the altitude. Others may be convinced that if it’s daytime and there isn’t a ground obscuration, such as fog or blowing snow that they will be able to see the ground well enough to avoid crashing into it.
The closer one is to the ground the more dangerous the situation, as during takeoffs and landings. You may have just landed on snow covered terrain with the sun shining, only to find 15 minutes later the sun has dipped below a ridge or been covered by a passing cloud. You are now enveloped in a shadow of flat light where an attempted takeoff could be very dangerous. This is a case where you are better off being on the ground wishing you were in the air, rather than being in the air wishing you were on the ground.
There was an incident in 1999, when a company crashed three helicopters in one day and all on the same glacier due to flat light. The first helicopter encountered flat light on the glacier and experienced a hard landing, injuring the pilot and passengers. With the first aircraft overdue, a second helicopter was dispatched to search, which also crashed on the same ice field. A third helicopter began to search for the two missing aircraft, which also ended up crashing on the same glacier. The pilot of the third helicopter reported that he thought he was 500 feet above the ground when the aircraft impacted the ground.
These were experienced pilots who had been flying tours over this glacier day after day. They didn’t become less experienced in a day and the glacier didn’t change. What changed were the lighting conditions. It can be hard to accept that at times one can see the ground without enough depth perception to know how far below it really is. Without instrumentation such as a radar altimeter or TAWS (terrain avoidance warning system), the pilot won’t even realize it’s happening.
Vermilion Bay, on the shores of Louisiana, is so notorious with Gulf of Mexico helicopter pilots that it is commonly referred to as “Vertigo Bay.” The bay’s water has a reddish brown color, and when coupled with an overcast cloud layer, low visibility, and no wind it presents a significant hazard to VFR flight. It is the same effect you get in a room with a full-sized wall mirror when it gives the illusion of the room being much bigger than it really is. Vertigo Bay is so large that with visibility less than 5 miles you can’t see land, and without any wind the highly reflective mirror-like water provides no contrast, but instead reflects the cloud layer from above. When these adverse conditions exist, VFR helicopter pilots circumnavigate the bay sticking close to the contrast of the shoreline.
Avoidance is the certainly the best remedy for flat light. Understanding the environmental conditions where flat light can exist helps the pilot in early recognition and avoidance. Study the terrain along the planned route of flight, including possible areas where you may divert. Review weather reports and forecasts to determine what lighting conditions will exist on the flight. Avoid flying over large expanses of water without wind to ripple the surface and direct sunlight to provide contrast. Stay clear of takeoffs or landings or any low-level flight over large areas of white snow without some direct sunlight. Flat light is a condition where a conservative approach is best, using your superior judgment to avoid the necessity of using your superior skill.
(These views and opinions are my own and do not necessarily reflect the views of Era.)