The Chinook had a tendency to cause Flicker Vertigo with the right sun direction beaming down through the Forward Rotor System and reflecting off the Attitude Indicator.
Flicker vertigo, sometimes called the Bucha effect, is "an imbalance in brain-cell activity caused by exposure to low-frequency flickering (or flashing) of a relatively bright light."[1] It is a disorientation-, vertigo-, and nausea-inducing effect of a strobe light flashing at 1 Hz to 20 Hz, approximately the frequency of human brainwaves.[2][3] The effects are similar to seizures caused by epilepsy (in particular photosensitive epilepsy), but are not restricted to people with histories of epilepsy.
This phenomenon has been observed during helicopter flight; a Dr. Bucha identified the phenomenon in the 1950s when called upon to investigate a series of similar and unexplained helicopter crashes. Flicker vertigo in a helicopter occurs when the pilot or front passenger looks up through the blades of the main rotor as it turns in the sun causing the light to strobe. The strobe light effect causes persons who are vulnerable to flicker vertigo to become disoriented, lose control of the aircraft or have a seizure. A similar situation can occur in fixed wing flight whenever flickering light conditions exist. An example would be looking through a slowly spinning propeller while the airplane is landing facing the sun.[4]
According to The US Naval Flight Surgeons Manual, flicker vertigo is a rare occurrence.[5]
Flicker vertigo has been considered as a principle for various forms of non-lethal weapon.[6][7] A related crowd-control device was invented by Charles Bovill, which "employed a combination of ultra-sonic waves and strobe lights to induce acute discomfort, sickness, disorientation and sometimes epilepsy."[8]