Why doesn't G-force knock this pilot out twice? In this video an F-16 goes into a dive after the pilot loses consciousness. The Auto-GCAS corrects to avoid a crash, and the pilot regains consciousness and pulls up even harder. Why doesn't he pass out a second time?

According to this article and the G-force meter (at the top left) the pilot:


Goes into turn and loses consciousness at about 8g's.
The Auto-GCAS rolls and pulls up to a limit of 5g's.
The pilot recovers consciousness and pulls up to 9.1g's.
Pilot responds to radio calls to cancel the exercise and gain altitude.


It seems he held the recovery for quite a bit longer and at higher G-forces than the turn that made him pass out. I assume he also didn't have time to do anything breathing exercises or preparations for a high g maneuver. He is also experiencing the G-forces in the same direction both times (pulling back on the stick). 

So why didn't the pilot pass out the second time?
Your body responds to a lack of oxygen to the brain with physiological responses that can take up to ten seconds to be effective.

...The brain is highly sensitive to cellular hypoxia (lack of oxygen), so rapid impaired function and almost unconsciousness follows (A-LOC), and then ultimately total unconsciousness (G-LOC). To avoid this, your body will reflexively make adjustments in heart output and blood pressure to increase blood flow to the brain (by parasympathetic response–an adrenaline surge), but it takes 6-9 seconds for this reflex to initiate.  By this time, unconsciousness has likely occurred, which obviously means no one is conscious to fly the aircraft.

Raptor 9.0g turn

In addition, the pilot likely made greater effort to avoid a second blackout.


Breathing: Rapid (&lt; 1 sec) exhalation/inspiration cycles every 3 seconds.  This maintains oxygen content and decreases carbon dioxide in blood, while also relieving increased pressure of chest, and allowing the heart to refill with deoxygenated blood from the rest of the body.

Isometric Contraction:  Flexion of skeletal muscles of legs and abdomen.  When muscles contract, the small blood vessels in them constrict, making them smaller with less room for blood.  This step increases pressure in chest and displaces blood away from these contracted muscles into the upper body and brain.



Together, some combination of these measures would prevent a repeat blackout.