Did you say the spark plug "flew out"? If you meant that literally, you may have the additional problem of stripped threads on your head.
And I can only think of two reasons a plug would try for a pilot's license: air-fuel mix is wrong wrong wrong, or the plug is the wrong heat range and is long enough that the piston gave it an uppercut (sending it over the ropes in round two and winning the championship).
If it's the stock plug, I would try the fuel mix suggestions above. But I would also look at the threads on your head.