Proficient Pilot

False horizons

October 1, 2007

Author Barry Schiff retired from TWA in 1998 after a 34-year career. He has flown more than 300 types of aircraft.

One of my early instructors warned me about flying in visual conditions above a solid cloud layer. This is because of illusions that can mislead a pilot operating visually, especially one lacking current instrument-flying skills.

When "on top," a pilot tends to use the undercast as a horizon. A cloud layer sloping downward to the right, for example, can cause an unsuspecting pilot to unwittingly turn in that direction. A gradually rising undercast that slopes left can result in an inadvertent climbing left turn, and so forth. Using an autopilot at such a time is wise; it cannot be misled by optical illusions.

There are other types of false horizons that are so insidiously misleading that they can result in fatal accidents.

Determining the position of the actual horizon — the one used to visually establish a safe attitude — can be difficult if not impossible when operating over mountainous terrain. Ridgelines cannot be used as a horizon when flying below the peaks because this can result in excessively nose-high attitudes.

A classic example of the type of accident that can result from a false horizon involved a pilot departing at night from Runway 12 at Bishop, California, a few years ago. This runway heads directly toward the White Mountains, which rise steeply to more than 14,000 feet along the east side of the Owens Valley, only a few miles from the departure end of the runway.

If it had been an overcast, moonless night, the pilot might have recognized the need to go on instruments immediately after liftoff. There are absolutely no lights or other ground references that can be used to visually establish a safe aircraft attitude on the east side of the airport, a classic example of VFR conditions requiring IFR skills.

On this night, however, the moon was almost full. This enabled the pilot to see the solid black, featureless silhouette of the White Mountains against the lighter backdrop of the eastern sky. The upper limit of this silhouette was formed by the towering ridges of the White Mountains. Such conditions create a false horizon that is dangerously misleading to someone unaware of the potential consequences.

The pilot departed toward this illusion and began a climbing left turn (to avoid the mountains) during which he was lured into an excessively nose-high attitude and stalled. A witness on the ground saw the airplane roll into a 90-degree bank angle (the lights on the wings were aligned vertically) and then plummet earthward. There were no mechanical anomalies or powerplant failures discovered during the subsequent investigation.

Interestingly, the perceived horizon actually rises during a left turn after takeoff because the mountains ahead of the aircraft become closer and taller as the turn progresses. This undoubtedly exacerbated the effect of the illusion. If the pilot had turned right instead of left after takeoff, the perceived horizon would have lowered and the more southerly heading would have allowed the pilot to head safely down the valley along U.S. Highway 395. The left turn took him nowhere except deeper into difficulty.

Compounding the problem was that the pilot had never before flown that type of airplane, a Cessna 340, at night and had flown it only once during the day. He most likely did not know what target attitude to use during initial climb. Even if he did know, he might not have recognized the need to diligently establish and maintain it while ignoring outside visual cues.

The antidote to being misled by such optical illusions is to ignore them, which is easier said than done. Transition to instruments as soon as you are safely airborne and continue to fly the aircraft that way until out of harm's way. Pretend as though you are departing into a fog bank. Such a departure probably should not be attempted by noninstrument-rated pilots even though it was severely clear at the time.

False horizons also can mislead instrument pilots. During initial simulator training in the Lockheed 1011, I found myself tending to push the nose down somewhat when breaking out of a 200-foot overcast during ILS approaches. I could not understand why I kept dipping below the glideslope until I discussed the problem with a retired TWA captain.

"That's a very typical reaction to breaking out," he said. "It's called 'ILS tuck.'"

He explained this phenomenon by telling me that what a pilot initially sees of the ground is limited to what is directly in front of and below him, and well below the natural horizon. The bottom of the cloud layer still blocks his view of the natural horizon and forms a false horizon that appears to be lower. This creates the illusion that the nose of the aircraft is too high (with respect to the perceived horizon, not the actual horizon).

Consequently, pilots tend to push the nose down and dip below the glideslope when first breaking out of a low overcast, a dangerous reaction. Although this has the potential to cause an undershoot, it more often leads to a hard landing (especially in large, heavy aircraft). If a given pitch attitude and power setting keeps the airplane on the glideslope before breaking out, then the same combination of pitch and power should be maintained while breaking out. A conscious effort often is required to combat the temptation to tuck.

These examples clearly demonstrate that seeing is not always believing.