I’m not rated for a specific interval with only one leg, though. I’d really like to know how an aircraft gets certified for six hours and not, say, six and a half.
Proof of the required reliability, basically. Both showing that you've done it at a lower certification level, and actually doing demonstration flights where you run around with only one engine. Certification authorities check and make sure the required reliability is available (because having your second engine fail 5 1/2 hours from the nearest diversion airport sucks).
