Originally Posted by motormadness

This is very simple to figure out that DL1520 ATL-BNA is the same aircraft used on DL539 BNA-CUN. Delta flies to all its' hubs from BNA except SLC (CVG,SLC,JFK,ATL). Mainline aircraft are only used from ATL. By using process of elimination, we can see that on Saturday, 12/27 that the only aircraft flown in for DL539 is DL1520 and it does not return to ATL, thus being used on the BNA-CUN flight.

That aircraft probably sits in ATL overnight, but there is always a chance it comes in from a 0600 departure from some other southeastern city. There are a number of reasons why the on-time percentage is poor. That could be a new flight number on this route, thus giving it little time to prove that it is on-time more often than not. The MD-88 does encounter small tech delays more often than some of DL's newer a/c, or the inbound crews could be late often. These are just a few reasons of many why that percentage could be low. However, one thing to keep in mind is that this is a Saturday flight. Saturday schedules in the domestic system are drastically reduced generally, so if an a/c swap was required in ATL, that could be done quickly. Also, flow delays are much less likely obviously.