Okay,
Also another thing to take into perspective. Look at the amount of Stations you have total. Fire, Police, EMS and Specialty. Each Specialty Expansion/Stations holds a Small Percentage of chance that a Specialty Mission will pop up.
Say if you have 100 of each Station but only 10 Airport Expansions. Yes 10 is quite a bit of Expansions to have but in a Larger Set-up that's really not a lot.
In order to even remotely see enough Airport Missions for you to utilize your Crash Tenders you would need like half the amount of Airport Expansions compared to your Fire Stations.
I would say for every 10 Fire Stations, 5 of them would need an Airport Expansion. And to notice them even more, Getting the Airport Specialty will make that Station spawn only Airport Missions itself.
So if you had 10 Fire Stations with 5 Airport Expansions AND the Airport Specialty Expansion, Then you will notice Half of your Missions will be Airport Missions.
In other words, The More The Merrier!
It's taken me years to get things pat down for my expansions. It takes a lot of work. So if you want Airport Missions more often, Go for the gusto!
Hope this helps!
JB