On two occasions the landing gear has refused to extend before landing, and I have no idea why. I don't fly with failures and have checked to see if it had been activiated, but no, it seems to be the systems logic that prevents the landing gear from extending no matter how many times I try to extend them. Is there something in the system logic that can prevent them from extending, if so, what? It's not a big deal but it ruined a otherwise perfect flight....