Do you happen to have the Forestry or Foam Expansions to your Airport Station? And if so, Are they also turned on?
In some cases, This will randomly tamper with the Expansions and no Specialty Missions will spawn.
If not, Then this is a totally different issue that needs resolving.
Thanks
JB
I will run through all 3 of the stations I have that have the airport extension:
1. Airport extension, foam extension, fire investigation extension
2. Airport extension, wildland extension, fire investigation extension
3. Airport extension, foam extension
All of them are turned on. Why would it tamper? Other than spawning other missions instead, shouldnt it be split based on how many extensions i have active?
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
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Also to give you more insight, i have 26 fire stations, 17 ambulance stations (4 of my fire stations count as ambulance), 3 water rescue extensions, 10 police stations, 5 hospitals, 1 MEDEVAC station, 3 police helis, 1 game warden extension, 1 water police extension, 2 federal police stations with 1 of my stations having an extension (so 3), 2 bomb squads extensions, 1 DEA, 1 ATF, 5 forestry extensions, 4 foam extensions, 11 fire marshals offices, 3 airport extensions, 1 wildland MCC, 2 fire boat docks, 1 rescue boat docks, 3 flood control extensions, and 3 disaster management extensions.
You would think that with all of this I would get more of the airport/wildland/foam missions and less large cannabis grow operations (i got at least 5-10 per day for like a week straight once)
The interesting thing is, I had 20 stations for the longest time because I was busy building extensions rather than building new stations, and I would get consistent airport missions with only 1 airport expansion, then one of the updates came out (this was over a year and a half ago) and then i stopped getting those missions. Within the past year i bought the other 2 airport extensions and 4 crash tenders, all 4 of them have 0 miles on them and have never gone out because I havent gotten a mission since i bought them nearly a year ago.