Positive G effects
Acceleration displaces blood and tissue by inertia, opposite to the acceleration force (TC AIM AIR 3.15.1).
G thresholds (TC AIM AIR 3.15.2):
- Above +3G: escape from aircraft in uncontrolled flight almost impossible.
- About +6G: weight of crash helmet alone flexes head onto chest.
Positive G effects, in order of severity:
| Effect | G level | What happens |
|---|---|---|
| Grey-out | begins around +2G | First symptom. Blood drains head to feet: stagnant hypoxia of brain. Vision dims, loses colour from periphery inward. |
| Black-out | usually complete around +4G | Blood flow at back of eye completely interrupted. Vision temporarily lost. Pilot stays conscious. |
| Loss of consciousness | usually above +6G, unprotected pilot | Returns quickly once G declines. Always brief confusion on waking. |
Timing and onset (TC AIM AIR 3.15.2):
- Very high rate of onset: G-loss of consciousness (G-LOC) with no other warning symptoms.
- Delay of 5-7 s between G onset and visual changes.
- If G then stabilizes, visual symptoms may improve after 10-12 s as reflexes raise blood pressure.
Trap: black-out is vision loss only, pilot stays conscious; G-LOC is unconsciousness, and at very high onset rate can come with no visual warning.
References
- TC AIM AIR 3.15