adverse weather conditions. Reductions in the level of illumination will
result from increasing cloud cover. Warning of impending cloud coverage or
meteorological change at night can be observed by the change in the level of
a. Obscuration. The obscuration of the moon or stars indicates the
formation of clouds; the greater the obscuration, the thicker the cloud
b. Ambient Light. Varying levels of ambient light along the flight
path indicate clouds are obscuring the source of light (moon or stars).
c. Halo. A halo effect around ground lights indicates moisture in the
air and the possibility of fog formation.
HAZARDS TO NIGHT VISION
The effectiveness of the human eye is dependent on the amount of stress
being experienced by the observer. The normal stresses of night flying will
be compounded by many things. Additional stress is created by the use of
drugs and alcohol, the level of exhaustion and dietary habits of the
individual and the use of tobacco.
a. Drugs. Even over-the-counter medication can severely impair visual
acuity both during the day and at night. Cold medicines can cause
drowsiness and blurred vision and some appetite suppressants can cause
nervousness and irritability. Therefore, a flight surgeon should be
consulted before flying, especially at night, if these types of medication
are being used.
b. Alcohol. The use of alcohol causes a loss of coordination and an
impairment in judgment. Under the influence of alcohol a person may not use
(or be able to use) proper vision techniques. Additionally, he may not be
able to recognize or interpret properly the monocular cues required in
determining distance. Therefore, the 12-hour rule for alcohol must be
c. Exhaustion. As an observer tires, his reaction time and ability to
concentrate begin to degrade. As the level of exhaustion increases, the
individual may develop tunnel vision and tend to ignore peripheral
conditions. For these reasons a crew endurance program must be adopted and
d. Hypoglycemia (Nutrition). A proper diet is essential for crew
members who fly during periods of reduced illumination. Vitamin A is
essential for the visual purple in retinal rods, which are in turn essential
for night vision, therefore, a deficiency of Vitamin A will cause a problem.
Vitamin A can be found in eggs, butter, cheese, liver, carrots and green
vegetables. Vitamin deficiency is not the only problem; hunger itself can
cause lapses in concentration that may be hazardous to night flight as well.