Many highly instrumented aircraft and very capable crews have flown their aircraft right into the ground during seemingly safe night visual approaches — here’s why. The black hole approach was given its name after a number of early commercial jet accidents occurred as the result of night visual approaches that were made over featureless or unlighted terrain or water. The black hole phenomenon exists whenever crews are unable to see anything much beyond runway lights (or focus on them exclusively), particularly when the horizon wasn’t available as a reference. There’s something subtle — and deadly — going on here…
WHAT’S GOING ON: When you’re approaching any runway, illuminated or not, 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 — right? What happens when you get closer? That trapezoid gets bigger, of course, BUT when the runway edges (or lights) are all you see, research shows that the increase in ‘vertical’ length between the near and the far ends of the runway is interpreted by the brain as an increase in relative height on the approach. This will happen even though you may be holding the landing zone rock steady in your windscreen and even though the near end of the runway appears to grow faster (because it is closer) than the far end of the runway — which should make us realize we’re getting low. For some reason, our earth-educated brains want to keep the vertical length constant! Think about this a minute and refer to the picture for help.
Now, look at the second figure. For a pilot to visually maintain the same vertical length between the threshold and the far end of the runway, he must start out with a steep descent that gets progressively more shallow, as he moves closer to the runway.
Result: A ‘concave’ approach that curves toward the ground.
Note 1: In the figure, the angle between the runway threshold and end remains constant from vantage points i, b, and c. In a black hole approach, this is what your brain expects to see, but the resultant flight path will be low as a result.
Note 2: The straight line path from ‘i’ to the endpoint ‘f’ (shown by the darker line) all result in progressively larger angles between the runway’s threshold and end as seen from the cockpit. This is a ‘good’ thing!
Note 3: In case it’s not clear, Note 2 is the way things will look when you fly a proper approach…
COMPLICATIONS: Certainly, an upsloping runway makes things much worse. It makes you feel like you’re coming in too steeply. Caution: 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, be sensitive of visual conditions, both haze, which increase apparent distances, and very clear air, which decreases them, can produce detrimental visual cue and both can be very dangerous during a black hole approach.
DEFENSE: Use that VASI, ILS, or whatever ground aids might be there and trust them. If there aren’t any…
- Always know how far from the runway you are, as well as how high above it — and the height of any intervening terrain. If the runway distance is 19 times your height, that’s only three degrees. At night, I use six (which works out to about nine-to-one).
- Use your airspeed indicator (or better yet, ground speed data from your LORAN or GPS). If you want a three-degree approach, your VSI should read about five times your groundspeed. (For six degrees, it’s 10 times.)
- If the runway isn’t getting taller, something is wrong. Go around!