GLOC

Gravity-induced loss of consciousness, or G-LOC, is a temporary incapacity that occurs when cerebral blood flow is insufficient during exposure to high sustained accelerations, most commonly in aviation and spaceflight. It is defined by a loss of consciousness that results from the brain not receiving enough oxygenated blood due to the effects of high gravity on venous return.

Mechanism and signs: As G-forces rise, blood pools in the lower body and pelvis, reducing blood flow to the brain. This can produce a sequence of perceptual changes—from grey-out or tunnel vision to a complete blackout and then loss of consciousness. Some pilots may first experience headaches, dizziness, or fainting, followed by rapid recovery once normal blood flow returns.

Typical factors and thresholds: G-LOC tends to occur at higher sustained or peak accelerations, with individual tolerance varying widely. Trained aviators often tolerate higher G levels than untrained individuals, but dehydration, fatigue, hypoxia, and improper breathing can reduce tolerance and hasten onset.

Prevention and training: Prevention relies on equipment and technique. G-suits apply pressure to the legs and abdomen to help maintain blood return to the core, while the anti-G straining maneuver (AGSM) involves coordinated muscle tensing and controlled breathing to raise thoracic pressure. Regular centrifuge training, physical conditioning, hydration, and adequate rest are important. Cockpit design and oxygen supply also play roles in mitigating G-LOC risk.

Management and consequences: In the event of G-LOC, recovery typically occurs within seconds to minutes as normal circulation returns. In aviation, repeated or unexplained episodes warrant medical evaluation to rule out underlying conditions. G-LOC is a safety concern in high-performance flight and is a key reason for rigorous G-tolerance training and monitoring in pilots.