The Black Hole Approach: Don’t Get Sucked In!

Whether you fly a piston single or a heavy jet, a long straight-in approach at night over featureless terrain is a well-proven prescription controlled flight into terrain. AVweb's Linda Pendleton examines the optical illusions involved, and offers suggestions for making sure that you don't become a thing that goes bump in the night.


The Black Hole Approach

AirmanshipOne of the major tenets of instrument flying is that you cannot rely upon your body -or kinesthetic senses – to keep you upright. Repeatedly, your CFII has pounded into yourhead the premise that you can only believe what your eyes tell you. Just watch thosegauges and all will be well. And it’s true – if what you are looking at IS thegauges. When the view is out the windscreen, however, you cannot always believe yourvisual perceptions.

Great! First you can trust your eyes, then you can’t. What’s the deal here? Well, it’sall in being human. This marvelous species you’re so proud to be a part of has evolvedover millions of years as a land-based animal moving at a normal speed of about three tofour miles per hour. You can also manage occasional short bursts of up to 15 mph. Anytime you go faster than that or put your eyes higher than eye level above ground you’resubject to misperceptions.

Okay. So, you know that you can’t always trust our perceptions. What is a black holeapproach and what makes it so dangerous? The term “black hole” refers to theterrain below the approach to the airport, not the airport itself. Simply put, a blackhole approach is a long, straight-in approach at night to a brightly lit runway overfeatureless and unlit terrain. Over the years, the black hole approach has claimed thelives of many pilots – both novice and experienced. Night flying has always been moredangerous than daylight flying principally because of the lack of perceptual clues we alldepend on to keep the shiny side up. You’re all familiar with the false perceptions youcan fall prey to caused by using a sloping cloud deck for a level horizon and theunsettling ambiguity caused by mistaking sparse ground lights for stars. You can overcomethese visual traps, however, by simply referring to the flight instruments on the panel.The black hole approach is different in that a glance at the flight instruments won’talways clue you in to the danger.

Length Illusion
Figure 1: Which line is longer? (Mouse-over to check.)

Parallel Illusion
Figure 2: Are lines parallel? (Mouse-over to check.)

Color Illusion
Figure 3: Which gray square is darker? (Mouse-over to check.)

Optical illusions

Before we talk about black hole approaches, let’s explore some of the ways yourperceptions can mislead you. That will give us a basis for a better understanding of theillusions you experience during a black hole approach.

Your eyes really don’t do the seeing – your brain does. Your eyes simply transmitelectrical pulses and your brain does the work of making sense of those spikes ofelectricity. It perceives what it “sees” in the setting in which it is viewed.The surrounding objects and colors – or lack of them – will have a big effect on what senseyour brain makes of the electrical impulses sent to it by the retinas. Look at Figure 1.Decide which line is longer and then roll your mouse over the figure and see the change.Now you’ve probably seen this a million times before and you know the answer, but payattention to how fooled you brain is by the surroundings of the two vertical lines. Nowlook at Figure 2 and decide which lines, if any, are parallel. Again, you probably knowthe answer, but notice again the overwhelming perception that the three long lines are notparallel. Even when you know the answer, the false perception is overpowering. Figure 3shows the effect surrounding color or intensity has on perception. Note the relativebrightness of the smaller gray squares in the center of the black and white squares. Nowroll your mouse over the figure and note the “change” in the gray squares. Thesmall square surrounded by black seems brighter and closer than when it is surrounded bywhite, but both gray squares are in the same place and are the same color.

Seeing is not believing! But why would the brain play such tricks on you? It’s all apart of how you make sense of the world around you. The visual surroundings of an objectgive you valuable clues about its size and distance from you. Lines can show perspective,which is an indicator of distance. The brightness of an object is another attribute thebrain takes into account when determining the nearness of an object. We perceive dimmerobjects to be farther away than bright ones.

Night perils

Pilots have recognized since the early days of aviation that flying at night is moredangerous than flying in the daylight. In fact, flying at night in good weather is closerto a flight in IMC than it is to VMC. The low level of light means that the rod cells inthe retina of the eye are going to be doing most of the work since they are more sensitiveto very weak light energy. Unfortunately, the rods permit seeing only black, white, andgrays. Since you base much of your perception of size and distance on color variation, youhave a handicap already. Terrain and clouds can be almost impossible to see at night untilit’s too late and as was said earlier, ground lights can be mistaken for stars andhorizons.

But what makes the black hole approach so different and so lethal? Well, first,referring to the attitude indicator, altimeter, and turn coordinator won’t immediatelyalert you to the problem. Pilots who succumb to the black hole illusion are convinced,sometimes until it is too late, that they are on the proper glide path and all is goingwell. Second, although you may know intellectually that the illusion is taking place, youwill still have an overwhelming urge to believe your false impressions. You can’t take anytraining to keep from experiencing this illusion. Like hypoxia, it WILL happen to you andyour best defense is knowledge and avoidance.

Visual angles on approach
Figure 4: The visual angle subtended by the runway during a normal three-degree approach should get larger and larger as you continue the approach.

Many researchers have studied the black hole illusion. Two Boeing engineers, Dr. ConradL. Kraft and Dr. Charles L. Elworth, conducted a study in a specially developed nightvisual approach simulator flown by Boeing’s senior pilot-instructors and came to somesurprising conclusions. As you are aware, pilots flying a normal three-degree glide pathsee a constantly changing view of the runway. While the aiming point on the runway willremain stationary in the field of view, the visual angle occupied by the runway isconstantly changing. Figure 4 illustrates how this visual angle changes during theapproach. (I exaggerated the angles to make the illustration clearer, but the conceptremains valid.)

When black isn’t beautiful

Equal angles subtend equal arcs
Figure 5: Equal angles (ACB and ADB) inscribed in a circle subtend equal arcs (AB).

Angles on approach
Figure 6: If the pilot keeps the visual angle subtended by the runway constant, the approach path will be an arc.

What Kraft and Elworth discovered is that pilots conducting an approach overfeatureless terrain at night tend to keep the visual angle of the runway constant. Now,I’m going to ask you to think back to high school geometry. Do you remember the theoremthat says that if two inscribed angles intercept the same arc of a circle, the angles arecongruent? Whoa! That was a mouthful – and worthy only of a high school geometry teacher.Let’s look at another picture. Figure 5 shows a circle with an arc AB. Angles ACB and ADBare inscribed angles that intercept the same arc, AB, and therefore they are congruent, orequal. Do you see where I’m going here? It follows that if this theorem is true then youcan turn it around to say that if two angles intercept the same arc of a circle and arecongruent, then those two angles are inscribed on the circle, meaning that their verticesare on the circumferences of the circle.

Now let’s look at Figure 6. This shows (although exaggerated for clarity) what happenswhen a pilot flies an approach to a runway and keeps the visual angle of the runwayconstant. The approach path will be on the circumference of a large circle centered overthe approach area. This means that the descent to the runway will be too steep at firstand will flatten out as it gets closer to the runway. As a matter of fact, the Boeingresearchers found that the typical descent on a black hole approach, if continued totouchdown, would result in a landing (impact?) two to three miles short of the runway.Although the circular path is clear in the illustration, it is imperceptible to the pilotflying the approach.

Although research has not yet discovered why pilots tend to keep the visual angle ofthe runway constant under black hole conditions, they have discovered that the conditionis universal. You WILL be fooled if you try to conduct a long, straight-in approachover featureless terrain using only out-the-window references. There is no amount oftraining or practice that will make this illusion go away. Just like the visual illusionswe looked at earlier, you know what the answer is, but your perceptions lie to you repeatedly.As you have seen, these false perceptions can be overwhelming. The only defense you haveis awareness and avoidance.

Some conditions make the black hole effect more pronounced. Be alert for the illusionwhen you observe these conditions:

  • An airport that is on the near side of a brightly lit city with few or no terrain features or lights between you and the airport. The brightness of the city lights will give the impression that they are closer than they are.
  • An airport that is on the coast or in very sparsely settled terrain such as deserts and wilderness areas. This is the classic black hole scenario. Los Angeles International landing to the east and Salt Lake City landing to the south are classic examples.
  • A night with extremely clear air and excellent visibility. One of the things we use to judge distance is the normal hazing that distance provides. When the air is extremely clear, this lack of hazing makes things appear much closer than they really are.

Coping with the black hole illusion

Since you know what sets you up for the black hole illusion, what can you do to keepfrom being sucked in? The most obvious is to avoid long, straight-in approaches. The blackhole illusion disappears within two to three miles of an airport so the most obvious thingto do is to fly to the airport at a known safe altitude and then descend and fly a normaltraffic pattern.

We said earlier that reference to the flight instruments will not help in a black holesituation and that is true for a quick reference to the attitude indicator, airspeedindicator or altimeter. Nothing there will be immediately suspicious. If you study theVSI, however, you may notice a larger than normal rate of descent, but that may not beapparent. You need to do a little analysis to see the whole picture. A three-degreedescent – 300 feet per nautical mile – is the normal landing descent. If you see morethan that, you should be suspicious. However, what in the cockpit measures descent angles?Your airspeed indicator and VSI do. For that three-degree descent, your rate should be fivetimes your ground speed. If you’re doing 120 knots across the ground, your rate of descentshould be about 600 fpm. If you don’t know your ground speed, using your indicatedairspeed will be close enough to keep you out of trouble. Of course, to use this formulafor a descent to the runway, you have to know how far you are from the runway. DME, GPS,or good old-fashioned pilotage should be able to tell you that.

There are many other theories about factors that may contribute to the black holeillusions. Some are more believable than others, but the thing you MUST believe is that ifthe conditions are right, you can be fooled by the black hole illusion and the only way tokeep from getting sucked in is to analyze what you see out the windscreen and be awarethat you, too, can be fooled. Seeing is not believing.