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Thread: Dangerous optical illusion

  1. #1
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    Default Dangerous optical illusion

    With 1,800 hours' total time, I was hired for my first charter job with an air taxi company in Fairbanks, Alaska. I had been flying for the company for several months on the day I was to fly three round-trip flights between Fairbanks and Bettles. I would be transporting three Cessna 207-loads of construction workers who were finished for the season and glad to be heading home. Bettles is well north of the Arctic Circle in the southern foothills of the Brooks Range, about 200 miles northwest of Fairbanks. The weather was IFR single-engine IFR was still legal under Part 135 in the early 1970s but the ceiling had been gradually rising throughout the day. It was mid-autumn and getting dark early.My first two round trips were uneventful, solid IFR flights on NDB amber airways with an approach each time into Bettles. There was no icing or turbulence. The approaches, to well above minimums, were normal and relatively easy.On my third and final trip of the day, the sun had set. I shot the NDB approach, landed in Bettles, and picked up the last five men. I filed an IFR flight plan for our return to Fairbanks, and we took off.

    Fairbanks was reporting 4,000 feet overcast and 30 miles visbility actual visibility was probably more than 100 miles with light and variable winds. We climbed to the assigned altitude of 7,000 feet and headed for Fairbanks in the dark through solid stratus clouds.
    It had been a long day, and I was tired but still alert. After flying almost every day for months, including trips through winter blizzards and obscurations, I had become somewhat complacent about the important details of each flight. I was flying on instruments and not really paying much attention to our location over the ground. I couldn't see the ground at night in the clouds anyway, and besides I knew that at any given time we were a certain number of minutes from Fairbanks. The route, a straight line from Bettles to Fairbanks, passes over several ridge lines and small mountain ranges, but at 7,000 feet there was ample terrain clearance.

    About 20 minutes or so out of Fairbanks, we were cleared to descend to 4,000 feet. We broke out of the clouds right at that altitude and, at what appeared to be about 15 to 20 miles in the distance, there were the lights of Fairbanks and the airport. I was a little surprised to be so close in, but thought no more of it. I informed approach that we had the field in sight, and they cleared us to descend at pilot's discretion for a visual approach.


    I began a gentle cruise descent with the idea of arriving at the final approach altitude just a few miles out from the airport. I turned on the landing light, my usual practice when approaching Fairbanks from any direction at any time of the day. (The Alaskan pipeline was under construction at the time, and there was a lot of traffic in and out of the Fairbanks airport.) The lights of the city and the airport did not seem to be getting closer as quickly as I expected them to, but I dismissed this thought and continued my descent.
    Suddenly, with the lights of the city still visible in the distance, there were trees dead ahead illuminated by the landing light. I pulled up sharply and cleared the ridge top I had almost flown into. The guy in the right seat looked at me and said, "I'm glad you did that." Needless to say, I had not been expecting to fly into the ground with the lights of the city still showing in my windshield and my altimeter reading 2,700 feet, but it had almost happened. Had I been flying a faster airplane, I might not have had enough time to react and pull up.

    There were a number of factors at work leading me into that ridge top. I was fatigued. I was complacent from months of flying with almost no time off. But the optical illusion of the situation is probably the most significant factor. This illusion has two aspects to it, the first being the apparent closeness of the bright lights of the airport and the city. I later realized that my original estimate of 15 to 20 miles out was wrong, and that we were probably 40 or more miles from the airport when we descended out of the overcast. I have since noticed that distant lights appear to be closer than they really are on nights with really good visibility.
    There is a second and more insidious illusion. As the pilot descends toward the airport, the lights appear to remain at the same angle and in the same spot on the windshield. This visual reference, without the benefit of other visual cues, gives the pilot the false sense that there is nothing to run into between the aircraft and the lights. This dangerous illusion can be illustrated with a simple diagram:
    As you can see by the blue lines indicating the pilot's line of sight just before impact, he can still see the lights of the airport just as he is about to crash into the ridge top. In a fast airplane there wouldn't be enough time to react. When I saw the trees in the landing light, they were not more than 500 feet in front of me. Individual trunks and branches were clearly visible. I was probably descending at about 140 mph. I expected to find some spruce twigs in the gear after landing, but there were none

    .
    It is important when making a visual approach at night in good visibility to know of any terrain still in front of you that is higher than the lights you're descending toward, especially when the only lights you can see are the ones you're heading for. This kind of near disaster is less likely to happen in populated areas where there are lights on the ground along your route, but it is probably still possible. It would also be possible to fly into the ground on a flat level plain if you were not watching your altitude.

    In my near disaster, the landing light saved us. It is important to monitor the rate of descent and to correlate it with altitude and distance remaining to the airport. It is also important to be especially vigilant when tired. Flying for a living day in and day out doesn't relieve a pilot of the responsibility to be on top of the situation at all times. Complacency can spoil your whole day. I almost learned this lesson the hard way.

    Now days with GPS, this kind of disaster would be less likely to happen, but not out of the realm of possibility.

  2. #2
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    Clearly a close call, and yes - - - the landing lights were a saving grace. In the back of my mind I'm wondering if the C-207 didn't have DME? If so, that and your altiimeter might have told you (a) your distance from FAI and (b) your altitude. Given your recent experrience with that route, you might have remembered the hills you almost met. A lot of trips in one day . . . Did fatigue make you just a little bit complacent? I'm not being critical here, just wondering if being tired was a bit of a factor too.

    At any rate, glad you made it, and it surely was a close one. Thanks for the tip and the explanation of the visual.

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    This sort of scenario highlights the need stay on top of situational awareness. An accident in the Kobuk Valley, where, the pilot of a CE-207 hit the east end peak of the Waring Mtns., while decending into Shungnak, comes to mind... You should know the elevation of that peak and the distance it is from Shungnak. You should know the altimeter setting. You don't decend below a safe clearing altitude, until you are past the peak. You don't go by visual input, you use the knowledge of the terrain and where you are in relation to it, to guide your course and altitude on the approach to your destination. Take the optical illusion factor out of the equation....

  4. #4
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    Grizzly,

    Complacency and fatigue did contribute to this near accident. This happened in 1975, not recent. The plane was not equipped with DME.

  5. #5
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    I appreciate the story and good outcome. As a low time student pilot I would never of though about this. Not that I am flying IFR but it gives a good precspective of what can happen to the most experianced flyers.

  6. #6
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    Glad you made it !!!

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