I read that very high g forces could kill a pilot, brain pushing into
Not really. Humans who have been killed by very high accelerations (in the order of tens to hundreds of gs for a fraction of a second) tend to look perfectly fine on the outside, but are a mess internally. The most likely cause of death is rupture of the coronary arteries, followed by massive internal bleeding. Think Evelyn McHale (picture below, source):
These brief but very strong decelerations are typical for aviation accidents. Moving over into the survivable range you will again find tens of gs, but they could only be survived with plenty of restraints. Think Dr. John Stapp (picture below, source).
Now you will argue that you look at accelerations in a different direction. That's my point: There is not a single magic number, but a wide range, depending on several factors:
- Duration of exposure. Look at Eiband diagrams to get an idea. At 1 second exposure a well-trained sitting human will survive 10g unharmed and will suffer severe injuries above 30g.
- Direction of exposure. Again, look at Eiband diagrams. In a prone position the same pilot who could only tolerate 10g while sitting will survive 40g for one second.
- Physical condition: Older and infirm people will tolerate maybe a third of what a young and fit person can suffer through.
- Technology: Restraints, anti-g suit, anti-g straining maneuver (a physical technique where the aviator pushes air out of the lungs against a closed glottis, while simultaneously contracting the muscles in the calves, thighs, and shoulders).
A combination of all three enables a trained pilot to sustain 9 gs sitting for several seconds. Proof:
"Centrifuge training for qualifying to Gripen at Sweden" (YouTube)
Additional evidence: When the F8F Bearcat was introduced, Grumman told the pilots not to pull more than 7.5 gs, because at that load the wingtips would come off in order to protect the rest of the wing from overloading. Mysteriously, lots of F-8F returned with their wingtips clipped. Those Navy pilots simply wanted to find out if the protection worked.
When you want to go beyond that, switch to a prone position. That effectively quadruples the limit, on the other hand the straining maneuver will lose most of its effectiveness.