8/15/19 AIM  comfortable focal distance which may range from 10 to 30 feet. For the pilot, this means looking without seeing, which is dangerous. 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. a. Introduction. The most important aspects of vision and the techniques to scan for other aircraft are described in paragraph 8 − 1 − 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. 8 − 1 − 7. Aerobatic Flight 8 − 1 − 8. Judgment Aspects of Collision Avoidance Fitness for Flight 8 − 1 − 8