This is story related and unmissable--pops at the end of the first chapter of the campaign.
Most missions are quite easy, but I did find something useful near the end of the game that I wish I had known at first--the jets and helicopters seem to respond MUCH better if you use a very light and gentle touch on the controls. Rapid controllers movements seems to make them respond poorly!
Each mission generally takes between 5-15 minutes--if you focus only on the objectives and speedrun, you can probably complete most missions in 5 minutes or less but will be slightly riskier for whatever you're flying.
If you would like the mission names and aircraft/helicopter options: