Is it normal for a trainee to have an instantaneous G-force induced loss of consciousness (GLOC) at 4-5 G's in a light military trainer aircraft like Socata TB 30 Epsilon, after doing a manoeuvre sequence?
That sounds really unlikely to be a G-LOC. That usually happens at much higher loadings after 3-4 seconds of straining. The counscious and cognitive centers of the brain take a few seconds to shut down once the systolic and diastolic pressures hit zero in the brain. It may be an indication of another underlying medical issue.
I agree that doesn't sound normal. I didn't strain unless I was pulling in excess of 4 and never experienced any ill effects. I also never had a student GLOC and we would pull north of 6g's pretty routinely. They should definitely be checked out medically.
I will caveat this because you used the word instantaneous. It would not be unusual in my opinion for your student to go to sleep if they were pulling a sustained 5g's and were not correctly using their g-strain. Every person's resting g-tolerance is different, and can be higher or lower on any given day based on a lot of factors.
Normally, there should be signs of oxygen deprival before full loss of consciousness. In my experience, first the field of sight narrows down as if someone drew a curtain in the eyes (tunnel vision), and then you go totally blind. This would happen to me at maybe 1 g less than usual when preceded by inverted flying, but still only above 4 g. Hearing and reasoning are not yet affected - for this it needs more gs.
This is 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. is resistance inhibits the blood from owing away from the brain, and it simultaneously increases the pressure in the carotid arteries.
Instantaneous G-LOC is only possible with very rapid rates of g increase to a level above 6 to 8. At 4-5 gs this should not happen to a healthy person. I would recommend to see a physician about this.