I just opened a new base, and even though its a class 4 airport, it won't let me schedule routes on my BA11 aircraft that I got specifically to use this airport.
The speficifi error is "You cannot schedule routes to this aircraft at this base since the aircraft is too large. The chosen base airport is smaller than the size class of this aircraft. To use this aircraft at the selected base please upgrade the base airport."
However this is a Class 4 Airport, all aircraft are allowed, and it has a pretty long runway, enough for a BA11 any class.
Also, I operate BA11 to travel to this airport (SEQU) from my other base which is SEGU (pic attached).
I don't understand what the problem is. Must be a bug.