score:10
The table on the Wikipedia page for G-force shows that WWI fighter planes could pull 4.5-7G's.
Typical human tolerance on the body's vertical (up-down) axis is said to be in the neighborhood of 5G (and 9G for modern pilots with G-suits). Much less for redouts. So yes, this is something an experienced WWI fighter pilot likely encountered. However, it would have meant not only their bodies but their airframes were reaching their tolerance limits.