Routings are rarely back and forth (when flying to/from/between hub and/or focus cities). In a small regional airport, 99% of the time, the aircraft that flew from the hub to that city turns around and goes back to the hub, but once it gets to the hub it goes somewhere else (doesn't sit around and wait to go back to that small regional airport again).
Every airline and route is different. As an example, the 772 that flies IAH
as CO34 returns as CO29 LHR
. Now there's also a 772 that comes into LHR
, along with a couple 752s. CO
has that nifty "where's my airplane coming from" feature on their website that makes it a lot easier to figure some of that out (if your EWR
example happened to be on CO
Long story short, there's lots of reasons for having the routings the way they are. Specific aircraft are rarely route specific, and in the scenario where there's multiple daily flights on a given route they're not necessarily all of the same type. Depending on how the slots and the turnarounds are scheduled, an aircraft coming in from one city may be turned to go somewhere else. You need this to minimize wasted time on the ground, keep fleet utilization as efficient as possible, and allow for rotating of aircraft to facilitate maintenance. If you had one specific frame dedicated to fly back and forth between EWR
, taking that frame out for maintenance (preventative or otherwise) get's much more difficult. You also run the risk of it sitting in FRA
for an extended (wasteful) period of time until the next departure back to EWR
. In addition, in the scenario of the ugly winter we've had this year, you run the risk of your metal being stuck somewhere else in the event of a major weather event (potentially crippling the fleet depending on how utilized everything is).
There's a real science that goes into fleet and route planning, and I can't even begin to explain it all. I think that's a high enough summary that answers your question though.
Work Hard. Fly Right. Continental Airlines