From the 1.3 Changelog:
* Added a new method for calculating / reducing the effect of flight that depart at the same time:
- If you have several flights to the same destination (ie. same route-pair) departing at the very same time, or very near the same time, they are counted almost like a single flight.
- The "minimum departure interval" depends on how large the passenger demand of the route is. But rule of thumb is that on lower-demand sectors (less than ~400pax/day) the route departure times should be always at least 60 minutes apart. If the route is more busy the allowed interval drops linearily all the way to 5 minutes. On a 1000 pax/day route you can have the flights depart approx every 35 minutes, and on 4000pax/day route the min. interval is 15 mins between two flights. The interval is calculated so that players can keep decent frequencies during daylight hours.
- If the flights are too close to each others they are treated basically as a single flight (but with the total amount of seats available of course), so this leads to lower sales for those flights that overlap or nearly overlap on dep. times.
- But from now on, never make two flights depart to the same destination at the exactly same time.
The spacing that actually works has been a mystery since this feature was added and with demand figures given fluctuating a lot of trial and error is needed to ensure flights do not take a penalty. This is a major PITA when 7 day schedules are involved.
For the bug part of this post: Spacing on several flights, including LAX-ORD and LAX-DTW with newly added flights in GW1, seems to give mixed results. One day it seems ok, the next 20% loads. Has the code changed where each day is treated differently or is the "average" demand still the determination of minimum spacing?
And the feature request part of this post: Please add a flight spacing warning if a planned flight is too close to another flight. Not sure if this is feasible or not, but it would save a lot of stress and frustration.