Probably what his instructor was saying was in context to the type of sailplane involved which was likely a Schweizer SGU 233 rather than the 232. Since kinetic energy is all you have to get you through the loop in a sailplane the loop can go to 0 or even a slight negative. If you spend too much energy in the climb portion the over the top gets pretty weak. I used to watch the tabs on my shoulder harness flip straight up on my shoulders when things got too slow over the top.

As far as load ratings on airframes most sailplanes are rated +/- 6gs or more while most production light aircraft, Cessna, Piper etc. are 4.8 positive and 3.5 negative.
Sailplanes are an order of magnitude cleaner in parasitic drag and as such will accelerate from stall to red line at rates appproaching that of gravity.
[/quote]
Your instuctor didn’t have his thinking cap on that day. In a loop the g-forces are always positive. The fact that the aircraft is upside down is not relevant. During a loop the stress at that point is no more than in normal level flight.

[/quote]