Hi all,
At IAD I have a fleet size of about 90 airplains now.
30 Small, 30 Medium and the rest is X and XL.
In the morning when I start the game, 30 small, 30 medium and the 30 other arive.
I always service them fully, including boarding, to get rid off the yellow tags.
After sending away a mix of 14 Small and Medium, there is a cooldown time. I understand and appriciate the cooldown time, don’t worry 
But… there are about 46 of my airplanes with passengers boarded also waiting for the cooldown time. Those passengers haven’t complained yet, but I can see the claims coming soon
I think it would be an idea to start boarding own airplanes only when a take-off slot is available or available within the time boarding takes.
Example:
There are 3 take-off slots available, so you can start 3 own airplanes boarding.
There are no take-off slots available, but 2 will be available within 1 minute, so you can start 2 own airplanes boarding.
There are no take-off slots available, and none available within 10 minutes, no own airplanes can be boarded.
Thanks for reading 