Mitigating the Hazard of Visual Illusions
by Dr. Leo Hattrup, FAA Medical Officer
A review of aircraft mishaps quickly reveals that visual illusions and/or poor visibility have been factors in the majority of aircraft accidents.
Unless you are actively instructing or preparing for a new certificate/rating, chances are that it has been a while since you last thought about the different types of visual illusions and the impact they have on flight safety. With few exceptions, these illusions make it appear that you are too high/too low and too close/too far from the runway. When pilots sense they are too high or too close, they tend to land short and/or hard. When illusions indicate a pilot is too low or too far, they tend to land long and risk overruns.
The illusion of being either too high or too low can result from a black hole effect, water refraction from rain on the windscreen, haze, narrow runways, upsloping terrain or runways, and bright approach lights.
Conversely, conditions that make pilots think that they are too low and risk landing long are caused by wide runways, down-sloping terrain or runways, very clear air (such as at high altitude airports), and low intensity lighting systems.
Many of us have experienced false horizons from sloping cloud decks or from ground lights on slopes. It’s important to recognize that entry into fog, even when the ground is visible, can induce a sensation of pitching up. The tendency to pitch down can be catastrophic if close to the ground, a tower, or building.
As with spatial disorientation, we are all susceptible to visual illusions. Illusions are the result of how we have learned to perceive the world around us. We can compensate with pre-flight preparation, as well as the use of aircraft instruments and navigation tools.
If you are instrument qualified, maintain proficiency. If not, work with an instructor to gain proficiency so you can correctly use flight instruments and instrument approach procedures to increase situational awareness throughout a visual approach. Title 14 Code of Federal Regulations (14 CFR) section 91.103 requires pilots to review all available information prior to flight, but consider more than just fuel, runways, and weather. Evaluate the potential for visual illusions based on runway configuration, runway lighting, forecast weather, and terrain (black hole potential, slopes, off-airport lights, etc.). Forewarned is forearmed. Preserve your night vision and consider the use of supplemental oxygen.
Another good practice that can help combat visual illusions is to use a flight training device or simulator to fly to your destination under a number of different scenarios (e.g., changing time of day, weather, and runways). Then use the tools available at your destination. As noted, an instrument procedure can provide valuable guidance, but only if you are trained and proficient in using it. Many airports have either a visual approach slope indicator (VASI) or precision approach path indicator (PAPI). These are typically set at a 3-degree descent angle, but can be greater. Even without these aids, GPS can help pilots maintain a safe altitude until close to the airport and provide guidance on an appropriate approach angle for a straight-in approach. A good rule-of-thumb for descent is 300 feet of altitude for each nautical mile from the runway.
In summary, visual illusions may be unavoidable, but you can mitigate the risk. No one plans to land short, to land hard, or to overrun the runway, yet we still do. Know what to expect before departure, maintain proficiency, and use the tools available to you.
Several FAA handbooks, including the Pilots Handbook of Aeronautical Knowledge, the Helicopter Flying Handbook, and the Instrument Flying Handbook describe visual illusions in detail along with accompanying illustrations.
The Flight Safety Foundation (FSF) has an excellent discussion of visual illusions in Approach and Landing Accident Reduction Task Force Briefing Note 5.3 (PDF) and in an FSF article (PDF).