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AIM 

6/17/21 

comfortable focal distance which may range from 
10 to 30 feet. For the pilot, this means looking 
without seeing, which is dangerous. 

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7.  Aerobatic Flight 

a. 

Pilots planning to engage in aerobatics should 

be aware of the physiological stresses associated with 
accelerative forces during aerobatic maneuvers. 
Many prospective aerobatic trainees enthusiastically 
enter aerobatic instruction but find their first 
experiences with G forces to be unanticipated and 
very uncomfortable. To minimize or avoid potential 
adverse effects, the aerobatic instructor and trainee 
must have a basic understanding of the physiology of 
G force adaptation. 

b. 

Forces experienced with a rapid push-over 

maneuver result in the blood and body organs being 
displaced toward the head. Depending on forces 
involved and individual tolerance, a pilot may 
experience discomfort, headache, “red-out,” and 
even unconsciousness. 

c. 

Forces experienced with a rapid pull-up 

maneuver result in the blood and body organ 
displacement toward the lower part of the body away 
from the head. Since the brain requires continuous 
blood circulation for an adequate oxygen supply, 
there is a physiologic limit to the time the pilot can 
tolerate higher forces before losing consciousness. 
As the blood circulation to the brain decreases as a 
result of forces involved, a pilot will experience 
“narrowing” of visual fields, “gray-out,” “black-
out,” and unconsciousness. Even a brief loss of 
consciousness in a maneuver can lead to improper 
control movement causing structural failure of the 
aircraft or collision with another object or terrain. 

d. 

In steep turns, the centrifugal forces tend to 

push the pilot into the seat, thereby resulting in blood 
and body organ displacement toward the lower part of 
the body as in the case of rapid pull-up maneuvers and 
with the same physiologic effects and symptoms. 

e. 

Physiologically, humans progressively adapt to 

imposed strains and stress, and with practice, any 
maneuver will have decreasing effect. Tolerance to 
G forces is dependent on human physiology and the 
individual pilot. These factors include the skeletal 
anatomy, the cardiovascular architecture, the nervous 
system, the quality of the blood, the general physical 
state, and experience and recency of exposure. The 

pilot should consult an Aviation Medical Examiner 
prior to aerobatic training and be aware that poor 
physical condition can reduce tolerance to accelera-
tive forces. 

f. 

The above information provides pilots with a 

brief summary of the physiologic effects of G forces. 
It does not address methods of “counteracting” these 
effects. There are numerous references on the subject 
of G forces during aerobatics available to pilots. 
Among these are “G Effects on the Pilot During 
Aerobatics,” FAA

AM

72

28, and “G  Incapacita-

tion in Aerobatic Pilots: A Flight Hazard” 
FAA

AM

82

13. These are available from the 

National Technical Information Service, Springfield, 
Virginia 22161. 

REFERENCE

 

FAA AC 91

61, A Hazard in Aerobatics: Effects of G

forces on Pilots. 

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8.  Judgment Aspects of Collision 

Avoidance 

a.  Introduction. 

The most important aspects of 

vision and the techniques to scan for other aircraft are 
described in paragraph 8

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6, Vision in Flight. Pilots 

should also be familiar with the following informa-
tion to reduce the possibility of mid-air collisions. 

b.  Determining Relative Altitude. 

Use the 

horizon as a reference point. If the other aircraft is 
above the horizon, it is probably on a higher flight 
path. If the aircraft appears to be below the horizon, 
it is probably flying at a lower altitude. 

c.  Taking Appropriate Action. 

Pilots should be 

familiar with rules on right-of-way, so if an aircraft is 
on an obvious collision course, one can take 
immediate evasive action, preferably in compliance 
with applicable Federal Aviation Regulations. 

d.  Consider Multiple Threats. 

The decision to 

climb, descend, or turn is a matter of personal 
judgment, but one should anticipate that the other 
pilot may also be making a quick maneuver. Watch 
the other aircraft during the maneuver and begin your 
scanning again immediately since there may be other 
aircraft in the area. 

e.  Collision Course Targets.

 Any aircraft that 

appears to have no relative motion and stays in one 
scan quadrant is likely to be on a collision course. 
Also, if a target shows no lateral or vertical motion, 
but increases in size, 

take evasive action. 

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Fitness for Flight