While commercial airlines have a fleet of x aircraft, not all x aircraft will be in the air at any given time. Typically, several aircraft are in the air, some are on the ground (turnaround, loading, unloading, etc.), and some are in maintenance (not active).
What is a typical ratio of aircraft in air vs. total aircraft of an airline? I would be interested in airlines like Southwest, Hawaiian, Alaska Airlines, or FedEx. Can anyone provide some ballpark numbers on such a ratio?
(I assume this heavily depends on the type of airline, ratio of long- to shorthaul flights, budget vs. full-service vs. cargo vs. regional airline, number of total aircraft, route network, and obviously time of day (as some airlines do not operate flights during the night, or at least reduce flights during the night), etc.)