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Flicker Vertigo - Know It to Avoid It

by Charlie Spence, Aviation Writer and IFA Member

It was my long solo cross-country flight before taking a test for my private pilot's license. The first landing was at an airport with a single runway where the wind meant making an approach directly into the sun. As I rolled out on final, the propeller produced rapid flashes of bright sunlight directly in my face. Instinctively, I added power to change the RPM and the flashing stopped. I didn't know it then, but I had experienced my first approach to flicker vertigo.

Flicker vertigo can be unpleasant and produce dangerous reactions for the pilot. A light flickering at the rate of 4 to 20 cycles per second can produce this illusion. It can result in nausea, vomiting, or, on rare occasions'unconsciousness. It affects helicopter pilots more than those in fixed wing, propeller-driven aircraft, but both are susceptible. During the day, flicker vertigo can be caused by the sunlight flickering through the rotor blades or propeller, as I experienced on that landing approach. This can be especially true when flying on instruments. At night, anti-collision lights reflecting off the clouds can produce the effect. Flicker vertigo can develop when viewing rotating beacons, strobe lights, or reflections of these off water or the clouds.

Usually, symptoms are mild and will stop when the source of the flickering goes away. Sometimes the individual is unaware of flicker vertigo. It can, however, cause a pilot to have spatial disorientation causing inaccurate perceptions of altitude, speed or attitude.

Flicker vertigo should not be confused with vertigo, which is a disorder of the inner ear. An individual suffering from vertigo has a sensation of spinning or believing the surroundings are spinning.

The Flight Safety Foundation (FSF) describes flicker vertigo as 'an imbalance in brain cell activity caused by exposure to low-frequency flickering (flashing) of a relative bright light.'

FSF says the eye and the brain act together to perceive flickering light. The activities of the retina (at the back of the eye where final images are formed) and the brain are synchronized as part of the visual process. If the flicker frequency is high enough, the system will perceive the light as steady. The critical speed of flickering will vary from person to person.

Flicker vertigo has been reported as the cause of some aviation accidents. An early study of incidents between 1956 and 1971, noted by the FSF, revealed patterns of light through rotor or propeller blades caused flicker and associated symptoms in 26 percent of the disorientation incidents involving helicopter pilots and 13 percent of incidents involving pilots of transport airplanes, training airplanes and high altitude airplanes. Twenty two percent of helicopter pilots and 30 percent of airplane pilots said flight through fog with a rotating beacon had caused flickering light in the cockpit.

In flight, the propellers of most fixed wing aircraft rotate at speeds outside the frequencies that could cause flicker vertigo but this does not eliminate the possibility of experiencing it from beacons or navigation lights. During taxiing, some propeller rotations might fall into the danger range. FAA regulations set the flash frequency range for anti-collision light systems just below the range of susceptibility for flicker vertigo. Although these steps are taken to reduce the potential for flicker vertigo, the ranges are in some cases just above or just below the danger zones. As mentioned above, the critical speed will vary from person to person so it is wise to take precautions.

When it can be done, turn away from the light source. Changing the RPM even slightly can often eliminate the symptoms. When entering instrument meteorological conditions turning off strobe lights eliminates reflections off clouds and rain. This should be done, however, only after notifying air traffic control.

In fixed-wing single engine propeller driven airplanes, avoiding landing on runways directly into sun or bright lights is a practical way to avoid flicker vertigo if facilities and conditions permit. Changing RPM when on approach or while taxiing can reduce potential symptoms as I discovered lo, these many years ago.

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