Why Twilight Can Be a Complicated Time for Pilots

Do you know the difference between civil twilight, nautical twilight and astronomical twilight?

Twilight is partitioned into three zones: civil, nautical and astronomical. Pexels

Twilight is a complicated thing. It is partitioned into three zones. There is civil twilight, which lasts from sunset to when the center of the sun is 6 degrees, or about 12 solar diameters, below the horizon and things can no longer be clearly seen.

Then comes nautical twilight, which ends when the center of the sun is 12 degrees below the horizon and a seaman no longer discerns the break between the western sea and the sky. Finally, astronomical twilight ends when the sun is 18 degrees below the horizon, leaving the sky black enough for the inspection of distant galaxies.

The sun travels 15 degrees across the sky each hour, but because its path — the ecliptic — is usually tilted with respect to the horizon, it takes more than six-fifteenths of an hour, or 24 minutes, to traverse each zone of twilight. The angle of the ecliptic is particularly shallow in wintertime. On December 7, 2015, the sun set at 1651 at Greenville, Pennsylvania, near the Ohio-Pennsylvania border, but civil twilight did not end until 32 minutes later, at 1723. Extrapolating, nautical twilight ended at 1755 and astronomical twilight at 1827. The moon would not rise until 0340 the next day. A nearby airport reported a 1,000-foot ceiling. It was pitch dark. Furthermore, the National Weather Service had issued a dense fog advisory for the area.

Nevertheless, two men, both pilots, took off from the Greenville Municipal Airport (4G1) at 1814 in a Carbon Cub — a homebuilt analogue of a Piper Super Cub — for a 13-minute flight to an unlighted 1,800-foot turf airstrip (PA01).

Presumably, the airplane’s 600-hour pilot-builder, 59, thought he would locate the private strip — which many pilots would have difficulty spotting even in broad daylight — with the help of his GPS and perhaps the configuration of nearby roads and houses.

He didn’t.

The owner of a farm a mile past the airstrip heard the sound of an impact, but saw nothing, since, she said, it was “dark and foggy.” The GPS history, recovered from the wreckage, showed that the airplane had initially flown at 2,000 feet agl, descending a few minutes before reaching PA01. It passed over the strip at 300 feet and continued northward for a mile before striking 50-foot trees and crashing into an isolated patch of woods, killing both men.

Not surprisingly, the National Transportation Safety Board identified poor judgment as the precipitating factor in the accident. It came to a similar conclusion about another night flight that had taken the life of an Illinois chiropractor, 36, and his father a few months earlier. The father was not a pilot; the son had only a student permit. He could not legally carry a passenger, but perhaps he felt that rule ought not to apply when pilot and passenger were, so to speak, one flesh. The pilot’s total time could not be found. He had reported 30 hours on his medical application four months earlier, but investigators did not determine how much flying he had done since then, if he had received further instruction or whether he had any night-flying experience.

The two men took off in their Grumman Cheetah a few minutes before the end of astronomical twilight. The moon had set two hours earlier. About 15 minutes later, the pilot sent a message to his wife saying that he intended to return to the airport. Possibly he saw lightning in the distance; satellite data placed the airplane, which was heading northwestward, “ahead of a large mesoscale convective system.” Nevertheless, the airplane was still flying in the same direction a few minutes later when it struck treetops and plunged into a dense forest.

Why the Cheetah was flying low enough to collide with treetops is anybody’s guess. It’s common sense that the first thing you want to do at night is get high above any obstacle in or near your path. The NTSB speculates that the weather conditions may have been cloudy, though an airport 15 miles north of the accident site was reporting clear skies. Still, no matter how cloudy it is, you don’t want to be flying 100 feet above the ground in total darkness.

Trees seem to be the earth’s way of bringing pilots back down to it. A little before midnight on the same night as the Cheetah accident, a few hundred miles to the north, in Michigan, the pilot, 58, of a Piper Lance was approaching Runway 28 at Harbor Springs (KMGN) after a flight from Detroit. The Lance’s left wingtip struck a tree less than half a mile from the runway threshold and the airplane crashed, killing the pilot.

The pilot’s logbook reflected about 400 hours total time, of which 80 were in the accident airplane. He had logged 20 hours at night, but the last entry had been made nearly two years earlier.

The area under the approach to Runway 28 is sparsely lit, and the town of Harbor Springs lies beyond the end of the runway. An FAA circular on spatial disorientation describes an illusion such an arrangement can produce:

“A particularly hazardous black-hole illusion involves approaching a runway under conditions with no lights before the runway and with city lights or rising terrain beyond the runway. Those conditions may produce the visual illusion of a high-altitude final approach. If you believe this illusion, you may respond by lowering your approach slope.”

The words high-altitude are odd; it should be simply “high.” But you would not think visual illusions could have been a problem, because Runway 28 has a four-light precision approach path indicator (PAPI) with a 4-degree — slightly steeper than normal — slope, and it was working on the night of the accident. In principle, it should have provided accurate glidepath guidance.

When a runway has a displaced threshold, as this one does, it is not uncommon for pilots to drop below the official glidepath in order to touch down at or before the threshold line. I do this all the time at my home field, which has a two-light PAPI, a 3.8-degree approach slope and obstacles near both ends of the runway. In daylight, I nearly always fly a straight-in approach, remaining on or above the PAPI glidepath until I am close to the runway, at which point I drop below it in order to bleed speed and flare before reaching the displaced threshold. As a result, I am used to seeing red on the PAPI during the final portion of the approach.

Possibly this pilot had done likewise. If he experienced a “black-hole illusion” of being high and so dropped down, then, when the lights went red as he neared the end of the runway, he might have felt no particular alarm.

There were no witnesses to the accident, and the pilot had no two-way radio communications before landing. It’s possible, even likely, that he made a straight-in approach. While there is nothing illegal about a straight-in approach to a deserted airport at midnight, it might be preferable to fly a standard pattern, staying at least 300 feet above the field elevation until the last turn. Doing so may reduce the danger of getting too low on short final.

Peter Garrison taught himself to use a slide rule and tin snips, built an airplane in his backyard, and flew it to Japan. He began contributing to FLYING in 1968, and he continues to share his columns, "Technicalities" and "Aftermath," with FLYING readers.

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