Spare car keys- Its a really bad feeling being the last one at the flying field and reaching into your pocket for your keys and......."hmmmm,, where are they?"
I am a habitual locker like you. It is for this reason that I always fly with my truck windows down about 4 inches
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I agree with all of the above and double agree with the 1st aid kit. Even if you think you won't need it (which you will), someone else will. I hand out lots of band aids.
Even though I take enough stuff to the field to scratch build a plane like homebrewer, I've learned that there's a definite time to cut your losses, stop fixing, and take it home to the bench. If you find yourself doing an above normal amount of repairs, then it should be done properly at home on the bench. I've seen too many quick field fixes (including my own) come crashing down on the next flight. If things start to go really wrong, stop flying and take it home.