On May 22, 5:27*pm, Andrew Sarangan wrote:

Class D
tower is the highest I've seen at GA airports, and even there it
appears to be mostly due to historic reasons (ie it used to be a
military field or used to have airline traffic some time in the past).
Some airports operate their class D tower only when scheduled airlines
are expected to arrive and depart.

The need for a tower is based on the number or volume of A/C
movements. Also, keep in mind that it takes special authorization for
a 121 carrier to operate IFR in uncontroller airspace.