You probably aren't too concerned with the snow-covered terrain you're flying over. But what happens when there are overcast skies? It can create a hazardous meteorological condition called "flat light", and you should be ready for it when you're flying this winter.
What Exactly Is "Flat Light?"
It's a normal winter day, with snow-covered fields and an overcast sky. It's not hazy at all; in fact, the visibility is crystal clear at well over 50 miles. You fly a short cross-country flight. Without realizing it, you descend closer and closer to the ground below, until suddenly, your aircraft impacts a snow-covered field at cruise speed.
CFIT, or Controlled Flight Into Terrain, isn't limited to the mountains. A meteorological phenomenon called "flat light" is to blame, and it's not something we're often trained to recognize.
Flight light conditions have caused dozens of accidents around the world. An otherwise perfectly VFR day can turn into a challenging, if not hazardous, flight for pilots not operating on an instrument flight plan. According to the NTSB...
"Flat light occurs when the sky is overcast, especially over snow-covered terrain and large bodies of water. In flat light conditions, no shadows are cast and terrain features and other visual cues are masked, making it difficult for pilots operating under visual flight rules (VFR) to perceive depth, distance, or altitude. The photograph [above] shows how these conditions combine to create an environment where it is difficult, if not impossible, to distinguish the sky from the ground. As a result, pilots may have the illusion of ascending or descending when actually flying level."
This lack of contrast makes it extremely difficult to perceive aircraft closure rate, speed, and distance from objects outside.
The insidious nature of flat light is the real risk. Thinking they can see the ground, VFR pilots tend to rely on outside references to judge aircraft attitude. Normally, this is perfectly safe when flying VFR. But as visual references are slowly lost, the distinction between the ground, horizon, and sky becomes unclear.
Pilot Misses The Runway On Landing From Flat Light
On a snowy winter day, an ATP rated corporate pilot flying a Cirrus SR22 landed off-runway in flat light conditions. Read the report below, which we pulled from NASA ASRS.
I was shooting the ILS and at about 1,500 AGL, I began to pick up ground contact, and went back onto instruments. At the minimums callout, I picked up the Runway End Identifier Lights, and continued the approach. As I crossed the threshold, I saw that the runway was snow-covered by over 4 inches of snow and was unplowed, although 1 inch was reported by Approach as I was vectored onto the ILS. After crossing the threshold, I could see the runway lights, but they were not on as I expected they would be. I reduced the sink rate and started to flare, but because the light was so flat, I had a difficult time judging my distance over the runway. I considered a go around, but decided to do a soft-field landing in the snow. I brought the nose up to decrease my sink rate, and temporarily lost sight of the runway. I re-acquired the runway lights just as I contacted what I thought was the runway, and brought the stick full back to complete the soft-field landing. The airplane came to a very quick stop, and I knew something was wrong. I shut down the airplane and canceled my IFR flight plan.
When I exited the airplane, I realized I had landed north of the runway, between the runway and taxiway. While I crossed the threshold, apparently I had drifted to the left as I set up for the soft-field landing... Due to the falling snow and cloud cover, the light was also much flatter than I expected. I should have executed a missed approach as soon as I realized I was not seeing what I expected to see, but I wanted to please my clients. I thought a missed approach might scare the two non-pilots on board. I let up when I saw the HIRL's, assuming the landing was assured. Contributing factors were the unplowed runway and flat light. I should have maintained the same vigilance from the time I acquired the HIRL's that I had from the FAF to the MAP. And when I didn't see what I was expecting, I should have executed an immediate missed approach.
Why Flat Light Is Different Than A "Whiteout"
Whiteouts are different than flat light, though they can be just as hazardous. Whiteouts occur when pilots experience a loss of depth perception due to "being surrounded by blowing snow, dust, sand, mud or water. There are no shadows, no horizon or clouds and all depth-of-field and orientation are lost" (FAA Safety).
There aren't any visual references in a whiteout, so it's the same as flying through clouds. Whiteouts have been the cause of several aviation accidents in snow-covered areas. Helicopter pilots are especially susceptible when taking off from snow-covered ground when rotor wash envelopes the helicopter in swirling snow.
What You Can Do About Flat Light
If you find yourself flying VFR into flat light conditions, there are a few things you can do to minimize the hazards:
- Don't fly until you only have one visual reference left.
- Try not to lose sight of your reference point at any time.
- Plan your approach so that your reference is always on your side when flying.
- Never turn away from your reference point.
- Fly with your head straight, looking forward; believe what your flight instruments are telling you.
- Obtain an instrument rating and become proficient and comfortable with operating in instrument meteorological conditions (IMC). Trust the cockpit instruments and develop good cross-check practices.
- If you regularly fly in snowy conditions, become proficient and comfortable with taxiing, taking off, landing, and conducting en route maneuvers and go-arounds in areas with snow. If visibility drops, use your instruments and land at the nearest suitable airport.
Have you ever experienced flat light conditions? Tell us about it in the comments below.
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