DALMD88 From United States of America, joined Jul 2000, 2365 posts, RR: 15 Reply 1, posted (8 years 2 months 2 weeks 6 days 2 hours ago) and read 1519 times:
It would be a ratio to the number of engines in the fleet. I don't know what the standard is so I won't guess. Most smaller airlines don't even keep spares. They change engines so infrequently it wouldn't make sence to own a spare. If one was needed they could lease one until theirs is repaired. We always did this at a commuter airline I worked at. Due to airframe lease agreements we had to turn back the plane with the same engines so the extra engine change would have to be made anyways.