The two lights that appeared to be wingtip landing lights were actually the landing lights on two separate aircraft. The Cherokee pilot was never in any real danger, but the incident illustrates how the relative motion of lights can create strong visual illusions, even on a clear night.
In a similar incident reported to the ASRS (Aviation Safety Reporting System), the pilot was flying on a clear, moonless night with excellent visibility. About 15 miles from his destination the pilot began a descent when, he writes in his report, "I noticed another plane directly ahead of me, strobes flashing. When I was 9.5 miles from [the airport], the other plane called the tower for transition, saying he was nine miles out and was a helicopter. I immediately realized there was a problem. My eyes were telling me he was still miles ahead of me, yet his position report indicated he was closer to me than I had thought, possibly only a half mile away. I passed about 100 to 200 feet under the helicopter, which scared us both."
The pilot went on to report that his misjudgment of distance stemmed from his initial assumption that the other aircraft was an airplane, and that the distance between the aircraft's strobes made it appear to be further away. As he notes, the strobes on the helicopter were only about five feet apart, rather than about 35 feet apart, as they are on a small airplane.
These are just two examples of the visual illusions that can cause pilots to misjudge distances between aircraft, and to fly unusual maneuvers at night. But the night sky can create many other illusions.
Sparse lighting on the ground, or from boats on the water, can cause the sky and ground to meld. This eliminates any visual horizon and forces pilots to rely on instruments for attitude control. A row of lights along the ground can create a false horizon, and an unwary pilot can easily put the aircraft in an unusual attitude by trying to fly by this false visual cue. Even on a clear night, pilots must cross check their instruments to ensure they are maintaining the proper attitude and altitude.
Lights can also create disorienting illusions on the ground. In an ASRS report, a pilot aborted his night takeoff roll when he saw unidentified traffic crossing the centerline at the end of the runway. It turned out to be a ground service vehicle on another runway that didn't intersect the departure runway.
One of the better known and more dangerous visual illusions occurs when an aircraft is making a night approach to an airport surrounded by little or no ground lights. Called a "black hole" approach, pilots don't have the visual cues needed for depth perception and they may have trouble assessing their altitude and/or distance from the airport.
In a black-hole approach, a 4,000-foot long, 150-foot wide runway viewed from a distance of four miles at 2,000 feet above the ground may look the same as a 2,000-foot by 75-foot runway viewed from 1,000 feet on a two-mile final. As a result, when approaching a large runway at night pilots might make a premature descent to an unsafe altitude.
The black-hole illusion is especially pronounced when pilots make a straight-in approach over water or featureless, unlighted terrain, or on an overcast or moonless night. The illusion can also occur with brightly lit runway lights and approach lighting systems. The black hole illusion may have played a part in the following accident.
In early December 1993, the pilot of a Piper Arrow was making a night flight from Nantucket Island in Massachusetts to New Haven, Connecticut. The destination weather included scattered clouds at 500 feet, an overcast at 1,100 feet, and four miles visibility in rain and fog. With surface winds at 10 knots from 120 degrees, ATC vectored the Arrow for the ILS instrument approach to Runway 2. This approach would bring the aircraft straight to the airport over Long Island Sound - and ATC advised the pilot to circle to Runway 20. ATC radar last saw the aircraft at 100 feet in a descending left turn over the water about 2.5 miles from the airport. The pilot and his three passengers didn't make it.
Other possible factors in this accident were the atmospheric illusions that rain, haze, and fog cause. Rain on the windscreen can create the illusion of greater height. Likewise, atmospheric haze creates the illusion of being farther away from the runway. Finally, entering fog can create the illusion of pitching up. These illusions may have prompted the pilot to initiate his descent and circling maneuver prematurely, instead of continuing on the instrument approach.
Surprisingly, an illusion similar to the night black hole approach can occur during daylight hours, as the following accident report suggests.
After a training flight, the pilot of a Cessna 150 and her instructor were making a long, straight-in approach to Runway 1R in Fairbanks, Alaska. It was late in the day, and with flat lighting and a snow-covered landscape, the pilot didn't have much depth perception. Unfortunately, the instructor wasn't paying attention to the approach, and the aircraft touched down 124 feet short of the runway. The pilot and instructor escaped injury, but the aircraft didn't.
Sloping runways also can create visual illusions during daylight landings and cause pilots to fly a higher or lower than normal approach. An up-sloping runway or up-sloping terrain can create the illusion that the aircraft is higher than normal, causing pilots to fly a lower-than-normal approach. Likewise, down-sloping terrain and runways create the illusion that the aircraft is low, causing pilots to fly a higher-than-normal approach.
Other lesser-known visual illusions can occur in bright, sunny weather, and pilots wearing sunglasses can be a victim of several visual problems. Polarized sunglasses can reveal strain patterns in the windscreen and distract pilots. Some commercial carriers prohibit their pilots from wearing polarized sunglasses for this reason.
Sunglasses, either polarized or simply tinted, can alter the appearance of warning and indicator lights, as well as electronic displays. In one reported incident, a pilot didn't see a dimly lit annunciator light because he was wearing sunglasses. Fortunately, ground crews alerted the pilot to an open baggage compartment door before takeoff, and a potential incident or accident was avoided.
Another little-known visual illusion results from the "mirage" effect, which can occur when hot air rises from sun-baked pavement. The rising hot air can distort vision enough to obscure the presence of another aircraft, as the following ASRS report suggests.
It was a hot, hazy day, and traffic was using Runway 6. The pilot monitored the common traffic advisory frequency (CTAF) during engine run-up and preflight check at the approach end of the runway. With his checks completed, the pilot looked at both ends of the runway and saw no traffic, so he made his departure call and started his takeoff roll.
"Unknown to me, an airplane had approached and landed on Runway 24 while I did my run up," the pilot writes. "Just before rotation speed, I noticed the other airplane on the runway. I had enough room to rotate and sideslip to the right, and missed the other airplane by a good margin." As he notes at the end of his report, heat rising from the runway may distort your view of objects at the other end.
Pilots are trained to believe what they see - when you fly in the clouds you believe what the instruments indicate, not what your senses feel - but in the case of visual illusions, seeing shouldn't mean believing. The problem of visual illusions is a serious one, and it underscores the need for pilots to understand, trust, use, and cross-check their instruments and other resources, such as the radio, during all phases of flight, even while flying VFR in good conditions.
