During mergers, things get even more compressed as airlines end up having to codeshare with each other. During United’s merger with Continental, the numbers were carved up. Most of pre-merger Continental’s long haul flights used numbers between 1 and 199. Pre-merger United used 200-999 (with most Asian flights using the historic 800 series and European flights using the historic 900 series). Pre-merger Continental shorter haul flying fell into the 1000-1764 range (with a few cutouts in the middle). Above that? Express carriers (with each carrier having its own dedicated range) and then codeshare partners beyond that. You can follow it all the way up to United flight 9998, a flight from Brussels to Vienna operated by Brussels Airlines.

That’s a lot to pack into 9,999 flight numbers, and United decided to get creative. So it was back in late 2012 when United introduced a new system that would optimize flight numbers. It would randomly look at the schedule every day and, using some parameters, would spit out a list of flight numbers to be used. What were those parameters?

There were the usual things, like trying to keep prominent flight numbers on regular routes (mostly long haul). But it was more than that. Even though the need for segregating United and Continental operations by flight number range had passed, United opted to continue to do things along those lines for operational reasons. It’s not actually a strict Continental/United breakdown anymore. It’s more about fleet type. For example, there are some 737-900s that were technically delivered to the United side and those operate under the same flight number ranges as the other 737-900s on the Continental side.

The airline apparently liked this internally because it could know a lot about a flight just by looking at the flight number. But this didn’t consider that from a customer perspective, it was really confusing.

For example, let’s stick with Ft Lauderdale to Denver. Today, United has a 320p flight on an A320. That’s flight 780. But tomorrow, it’s a 630p flight on a 737-900. That’s flight 1571. On Saturday, it’s back to a 325p flight on an A320 but the flight number is 764.

But that’s not all. The morning flight leaves at 744a today as flight 1452 on a 737-900. Tomorrow it’s a 737-800 and moves to flight 1758. Sunday, it’s back to a 737-900 but it now leaves at 736a. It’s flight 1736. Monday it’s still a 737-900 but it leaves at 746a and it’s flight 1660. What gives?

Some of the problem lies in shifting daily schedules. These days, airlines optimize their schedules by day, so it’s easy to just re-optimize every day and not really worry about consistency during the week. That’s how United has run things for the past couple years.