After about 100 plus flights with my zdz 80, the engine died while I was about 20 feet from the ground right after I pulled out from a torque roll. I managed to pancake airplane on the bushes, bent the gear. Upon inspection we found out that the plug came loose. Tightened it, bent back the gear and up I went. A few minutes later, dead stick again. Plug came loose again. A friend told me to really tighten it up good, both spark plug and set screw. Did what he said, and now have about 30 more flights without it coming loose.