Section III: Images

Chapter 25: Optical Illusions: Flying's Subtle Traps

"See what I mean?" I said to my customer as we passed the rams at a fair distance from the ridge. It did not require a closer look to make the determination,

"Yeah, you were right -- there's nothing in here worth -- oh oh..."

In the valley below us, to one side, a large cloud of orange smoke blossomed by the side of the creek bed. It was obviously an emergency signal.

"...looks like somebody needs help."

I nodded as I came off the power and the Cessna 180 started down. Sheep season was due to begin in two days and, as always, there would be problems for some.

* * *

The airplane, a Super Cub, lay on the gravel of the creek bed, its gear torn off by a gully, propeller pretzeled, and one wing tip crumpled. In front of it some 60 feet or so began the landing strip, a narrow track between boulders, water channels, and driftwood lined with brush. Slightly wider than the width of an airplane's wheels, with just room enough for the wings to clear the brush, it was a one-way strip slightly less than 500 feet long with thick alder on the upstream end.

By the wrecked Cub one man was waving a white cloth tied to a stick; in front of a small tent a second lay in a sleeping bag with something tied around his head. He waved weakly.

"It doesn't look good," my customer said as we passed over.

"Yeah," I agreed. "We're going to have to land."

"Is there room enough? It looks pretty short for a 180."

"We're light enough. And the uphill slope will help both going in and coming out. But it will be one passenger at a time."

I made my turn, thankful the wind was calm and the early morning light would not be in my eyes, lined up with the strip, and began the approach. Passing over the Cub with full flaps, just a few feet above it, I leveled off about a foot above the terrain and raised the nose slightly, adding power and bringing the 180 to its minimum in-ground-effect speed. Then our main gear was crossing the threshold, and as it did I came off the power with brakes partially on, dumped the flaps, and we were rolling to a smooth easy stop with room to spare. Some strips, while usually only suitable for Super Cub class planes, will under certain conditions of wind and temperature be usable with the C-180/185, and this was one of them.

Then the hunter was at my door, telling us what had happened -- or what he thought had happened, for it turned out he was wrong -- and we were on our way to assist his partner to the C-180. The pilot had struck his head against the instrument panel (no shoulder harness), had only a bloody towel for a bandage, and was in considerable pain; it would be necessary to get him to a hospital as soon as possible.

My customer would have to wait here until my return, hoping the wind would not come up so I could no longer use the C-180, but have to return to base to get the Cub. As for my other customers, I would have to radio in and notify them there was going to be a slight delay. The problem, as so often in these undershoot cases, was the classic one of optical illusions, and in bush flying, as in instrument flying, we seem to have more than our fair share.

* * *

Some years ago, a subject in a medical experiment wore for some days an optical device that made everything he looked at appear upside down. One day, upon awakening, he discovered the world once again appeared right-side up -- his brain had adapted so he could function.

When we age and go to bifocals, we often get a pair of the "invisible" type, which look like normal glasses. When first trying these, every head movement makes the objects around us seem to swim and distort as in a movie depicting an LSD trip. Yet after a few days we too, like the fellow in the experiment, wake up to discover that everything is normal once again.

So, too, with runway widths. If you are used to flying into large airports with nice wide runways, then go into the bush and begin an approach to a short, very narrow strip, it seems as though you are higher than you actually are. If you fail to recognize this illusion for what it is, you will end up flying an approach that is too low, and risk landing short. This is called runway width illusion.

As with most things of this nature, there is another side to the coin, for if you are used to operating into very short, narrow strips, the opposite effect occurs: when presented with a large wide strip, you will have a tendency to flare high and drop it in. This is one of the reasons behind the dislike high-time bush pilots often exhibit for commercial airports, and it just goes to show that we all do best in our home territory.

In aviation, trouble rarely comes unaccompanied, and the injured pilot had more than runway width illusion to contend with. The runway also sloped upward, which exposed him as well to slope illusion. With an upsloping runway, or upsloping terrain, once again you get the illusion that you are higher than you actually are. The two combined are especially deadly, and account for a number of accidents each year, with the pilot, as in this case, often having no idea what actually happened.

And once again there is the coin effect, for a downsloping runway, or downsloping terrain, reverses the illusion.

Another illusion is one caused by featureless terrain. You see a lot of this in winter, or when landing on a strip whose approach end is on the edge of a calm, windless lake, and this too makes it seem you are higher than you actually are.

Heavy rain on the windshield also makes it seem you are higher than you actually are.

If, VFR, you attempt to go through a thin, patchy layer of fog, you can get the illusion of suddenly pitching up.

Approaching a bush strip lit by flares or lanterns over dark terrain gives the illusion of being closer to the runway than you actually are -- both in distance and altitude -- causing you to tend to fly a higher approach than is suitable. On a short bush strip this could easily lead to trouble.

Flying over sparsely scattered cabins on the outskirts of a small town at night, using their lights to judge height as you approach an unlit strip, makes it seem that you are higher than you really are; once again this is the coin effect.

All these illusions create problems when landing.

* * *

It was late, almost dark, by the time I made my last trip into the little strip to pick up my patient customer. Yet, before we left, he wanted to show me something. So we walked over behind some brush where a small pile of twisted metal and fabric lay. There was a constant-speed prop, and some gear, wing and window parts from a Scout.

"Seems our friend wasn't the first to come to grief here," my customer said.

"No," I agreed. "And probably not the last." And, as we walked towards the C-180, I thought of how I sometimes made a less than graceful landing at a big airport, even after so many years in the business, and how sneaky those optical illusions are, especially in strange places or when one is tired. Yet, knowing about them, and being aware of their nature, goes a long way towards pulling their fangs. And I guess that is basically what safe flying is all about: knowing what problems are likely, then making sure they don't occur. Especially the ones that go beyond embarrassment and into damage.

$Date: 2005/05/06 19:16:05 $ Copyright © 1993 by F. E. Potts CSS XHTML 1.0 Strict