We are not creatures of the night, however, and we can easily become its prey. Illusions can arise from the eyes alone, such as an after-image from a bright light-or the brain alone, as when one is fooled by a false horizon. Illusions can also occur from the innocent cooperation between the eyes and the brain, as when one focuses on a single light in an area with no other references; minute involuntary ocular motions cause it to seemingly move in irregular arcs (autokinesis), which can induce vertigo.
One of the worst nighttime "eye/brain" illusions can occur during the so-called black hole approach.
Toward the end of a night flight, we're usually a little less alert and a good deal busier when landing, so the last thing we'd want to have to do is run the gauntlet of illusion. Although it usually doesn't get much worse than an imitation of a carrier landing, some very capable crews have deftly flown right into the ground during seemingly safe night visual approaches.
The black hole approach was given its name after a number of early commercial jet accidents occurred during night visual approaches made over featureless or unlighted terrain or water. The phenomenon can exist whenever a pilot can't see anything much beyond runway lights (or focuses on the lights exclusively), particularly when the horizon isn't available as a reference.
When you're approaching any runway, you initially see what can basically be described as a very skinny trapezoid, with the bottom or "fat" end being the runway threshold and the top being the far end. When you get closer, that trapezoid appears bigger-and disproportionately squatter. However, when the runway edge, end, and threshold lights are all that you have to delineate a runway, our ground-bound brains interpret the increase in "vertical" length (or angle) between the near and the far ends of the runway as an indication that we are getting too high on the approach. This can happen even when you hold the landing zone rock-steady in your windscreen.
Even though the rate of increase in the horizontal angle subtended at the eye by the nearer end of the runway (its width) is greater than that of the farther end, for some reason, our inclination is to keep that vertical angle constant. (You can calculate the numbers for a situation where you might be 2,500 feet from the nearest end of a 4,000-foot runway-compared to being 5,000 feet away, say 20 seconds earlier-and by that point, the near end would seem to have widened by more than 40 percent.)
Just what are the ramifications of keeping that vertical angle constant? Well, if there ever was a time when high-school geometry could not only improve your life but also save it, this is it. Do you remember the theorem stating that if any two inscribed angles intersect the same chord inside a circle, they'll be equal? Imagine that the runway is one small chord near the bottom of a huge circle with a radius of several miles, with its center right over the runway. If we're not careful, our approach path can become an arc of that circle, the size of which is determined only by where you are when you start your approach.
Assume that the angle between the runway threshold and the other end remains constant from vantage points one, two, and three (see figure, p. 42). In a black hole approach, this is what your brain expects to see, but the resultant flight path would become dangerously low as a result. In maintaining the same angle between the threshold and the far end of the runway, you wind up with a descent that gets progressively shallower as you move closer to the runway, resulting in a concave approach, curving toward the ground. In the daytime, relative motion begins to give us other cues, but at night, our depth perception is compromised (as is our ability to sense wind drift) and fewer objects remain visible enough to allow us to sense relative motion. The straight-line path from the first point to the runway threshold results in progressively larger angles (a, b, and c) as seen from the cockpit.
This is a good thing!
An up-sloping runway can deceive you into flying too low; it makes you feel like you're coming in too steeply. The same is true for narrow runways, which can give the impression that you're higher than you really are. Watch out when the airport is located on the near side of a city, and certainly any time an airport is at a lower elevation than surrounding terrain. Also, atmospheric obscuration such as haze, which increases apparent distances (including your height above terrain), can produce detrimental visual cues and can add to the danger of a black hole approach.
Before you get to a runway, other things can trick you into flying too low at night. One of the most basic cautions concerns higher terrain: Even though you might keep the lights in the same spot on the windshield, with the slightest descent, any intervening rising terrain (especially if it is a steep ridge, uninhabited, and therefore probably dark) could be right in your path. Beware especially when the only lights that you can see are the ones toward which you are flying. At night an airport can seem much closer than it really is, which could trick you into descending too early. And unless you are familiar with the areas you fly over, you'll need radio navigation because pilotage over unfamiliar territory at night becomes difficult, especially if it is unpopulated. It's too easy to want to descend, to "see better." Don't. You'll see more, have better obstacle clearance, and have more options if your engine quits if you don't give up altitude prematurely.
However, if you plan to fly above 5,000 feet, bring supplemental oxygen; your retinas are the first things that will go hypoxic. Be aware that although only 10 percent of VFR flights occur at night, this is when 30 percent of VFR-into-IMC accidents occur. Get a thorough preflight briefing, carefully monitor en route weather, and be prepared to fly on instruments-at least for a short time-should you inadvertently enter clouds. (See "Elementary Instrument Flying," August AOPA Flight Training, and "Four-Step Instrument Scan," p. 34.)
Anything else you can do to stay out of the weeds is fair game. Obviously, use whatever ground-based visual aids that might be available, such as a visual approach slope indicator (VASI) or even the low-tech "alignment of elements" system fashioned from plywood panels. Play it extra safe when utilizing VASI to help you get on the proper glideslope-if at any point you see too many red lights, do a go-around and try again.
If there aren't any glideslope aids, and if runway length allows, don't try to land on the numbers; give yourself some wiggle room by crossing the threshold at about 100 feet instead. Instrument-rated pilots know they can't rely on their senses, but the usual gauges won't help you here.
There are others that can, though. If your distance from the runway in nautical miles is three times your height above your touchdown zone in thousands of feet, that's an angle of just over 3 degrees. Another rule of thumb is that for a 3-degree approach, your vertical speed indicator should show a descent in hundreds of feet per minute that is 5.3 times your groundspeed (from your loran or GPS) in knots.
However, these alone aren't enough. Obviously, it's nice to know your distance from the runway, and how high you are above it, but what's critical is knowing where the ground is at from where you are to where you're going.
Safe night flying should always be preceded by thorough flight planning and flight plan filing (see "Getting from Point A to Point B," September AOPA Flight Training, and "Paper Trail," p. 28), and it is enhanced by using instrument skills, off-center viewing, supplemental oxygen, and radio navigation. If you must look at something bright, close one eye-the same one, each time. (Clearly, this "pirate" approach doesn't apply on short final.) Don't fly when you're tired or anxious, and if you have an instrument rating, use it.
If you want to remember just one thing, though, it's this: If the runway isn't getting taller, something's wrong. Begin your go-around immediately.
Jeff Pardo is an aviation writer in Maryland with a commercial private pilot certificate for airplanes, and instrument, helicopter, and glider ratings. He has logged about 1,100 hours in 12 years of flying. An AirLifeLine mission pilot, Pardo has also flown for the Civil Air Patrol.
Want to know more?
The following resources offer additional information on topics discussed in this article.
- Review night operations in the October 2001 AOPA Flight Training.
- Test your knowledge of airport lighting systems.
- Links to these resources are available on AOPA Online.
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