Flicker Vertigo - Know It to Avoid It
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
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
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.