Flicker vertigo


Flicker Vertigo as defined by 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”[ Chippindale, R (2006 1)].

Another definition of Flicker Vertigo by U.S Air Force Flight Surgeon's Guide stated that Flicker Vertigo can be a result of exposure to sun or light passing through a rotor blade system, producing a modorate frequency flicker into the cabin, similiar to the flicker of a failing neon light tube[ U.S. Air Force ( 1991 7)].

Theoretical frame

Flicker vertigo is a phenomenon that is more prone to happen during helicopter flights, but it can occur in fixed wing flight (turbo propellers) whenever flickering light conditions exist. Flicker vertigo in the 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 [Rash, C. E. ( 2004 5)].

During daylight, flicker vertigo can be caused by the sunlight flickering through the rotor blades or propeller. During night flying, anti-collision lights reflecting off the clouds can produce the same effect. Flicker vertigo can develop when viewing rotating beacons, strobe lights, or reflections of these off water surfaces or the clouds.

Medical Explanation of Flicker Vertigo

The Perception of Flicker and the Interaction of Eyes and Brains
The phenomenon of Flicker Vertigo can be derived from the study of eyes and brains interaction [Rash, C. E. (2004 5)]. With the eyes and brain processing acting together it precieves the amount of flicking light. The activities of the retina and the brain are synchronized in order to create this visual process.

If the modualtion is adequate, the visual system will percieve the flicking continuous lights as STEADY, in which Flicker Vertigo will not occur. This "Critical Flicker Frequency" (CFF) will occur only with mutiple factors related to lighting sources, the indivdual viewing the lights and the enviroment/situation [Johannson, A. & Sandstorm, M. (2003 2)]. The CFF which varys from individual are ranged from 25hertz to 55 hertz.

Symptoms of Flicker Vertigo

The signs and symptoms of Flicker Vertigo can be various. However, the common symptoms are (Kern, 2002 [3]) and (Reinhart, 2008 [6]):

1. Drowsiness
2. Mild discomfort
3. Headache
4. Nausea

  • In extreme cases the follwing effects may also be experienced:

5. Unconsciousness
6. Body locked-up
7. Paralyses,
8. Violent convulsions

Supporting Evidence

A study conducted by the U.S. Navy based on EEG and stimulations on thier student naval flight officers, they have concluded that different individuals have differnt resistant levels of Flicker Vertigo, but it cannot be determine otherwise with other methods such as medical history or medical checkups. Therefore no one can predict who will have seizures from Flicker Vertigo. Epilepsy-prone people maybe more suspectible therefore people with a family history of epilepsy should take notice. Chances of Flicker Vertigo will increase as medical test demostrated that a healthy human body might escape with the worst symptoms but with physical exhaustion, fatigie, stress etc, it can turn a survivable encounter of Flicker Vertigo into more of fatal situation [ Chippindale, R (2006 1)].

In another research conducted by FAA, ten men were subjected to three different stimulations of intermittent phonic environment in a laboratory: strobe light, grimes red rotating beacon and propeller flicker. IFR conditions were created by using steam. Samples were studied 10 minutes before the stimulations and 10 minutes after the stimulation. None complained abut the strobe light. Three subjects experienced drowsiness du to Grimes light and six subjects experienced drowsiness due to propeller flicker (Melton, Higgins, Saldivar, & Wicks, 2002 [4]).

Risk for pilots and passengers

By exceeding the stated amount of CFF, Flicker Vertigo can be developed bring hazard to flights. What happens when an aircraft is flying with the sun shining through the rotor blades system will be a direct function of the revolutions per minute (RPM) and the number of blades each helicopter or aircraft are installed with.

As the pilot is subjected to 10 to 30 flicker of lights per second, this will create a highly potential area for Flicker Vertigo to happen. If the flicker rates are between 16 to 20 flickers per second, it can be considered as highly extreme. As a pilot recieve this highly extreme pattern of flicking lights, the pilot sees only flashing lights for 1 second or less and the light will be appeared or being percieved as a steady source. During this time, the brain might react violently in the form of instant unconsciousness, body locked-up, paralyses, or violent convulsions even if the individual is fully conscious. Normally Flicker Vertigo are further enhance with pilot error. With the consistent flicking, pilots may tend to close their eyes as the apparent natural defence mechanism. but this will further onset and speed up the process of Flicker Vertigo. As closing of eyes changes the lights to orange-red due to the blood contained in the eyelids, the intense colour of this reddish effect will further increase the rate of vertigo as well as creating a enhanced devastating effect [ Chippindale, R (2006 1)].

Refuting evidence

Way forward (to do list)

1. Chippindale, R. (2006). Flicker Vertigo-A Helicopter Factor. 190.321 Advanced Air Safety Investigation Study Guide. NZ: Massey University.
2. Johannson, A. & Sandstorm, M. (2003). Sensitivity of Human Visual System to Amplitude Modulated Light. National Insitute of Working Life. Arbresilverapport. Report:2003-4.
3. Kern, T. (2002). Controlling pilot error: Approach and landing. McGraw-Hill: New York.
4. Melton, C. E., Higgins, E. A., Saldivar, J. T., & Wicks, S. M. (2002). Exposure of men to intermittent phonic stimulation under simulation IRF conditions. Retrieved 14 January 2009 from http://www.dtic.mil/srch/doc?collection=t2&id=AD0646872.
5. Rash, C. E. (2004). Awarness of Cause and Symptoms of Flicker Vertigo can Limits Ill Effects. Flight Safety Foundation: Human Factors and Aviation Medicine. Vol: 51 No: 2. March-April 2004.
6. Reinhart, R. O. (2008). Basic flight physiology (3rd ed.). McGraw-Hill: New York.
7. U.S Air Force. (1991). Flight Surgeon's Guide. U.S.A.

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