14 C.F.R. § 398.5 – Frequency of flights.

(a) Except in Alaska, at least two round trips each weekday and two round trips each weekend.
(b) In Alaska, a level of service at least equal to that provided in 1976, or two round trips each week, whichever is greater, except that the Department and the appropriate State authority of Alaska may agree to a different level of service after consulting with the affected community.
(c) An essential air service level may be set at more than that stated in paragraphs (a) and (b) of this section if:
1. (1) Historical traffic data and studies of traffic-generating potential for the place indicate that more frequent service is needed to accommodate passengers and accompanying baggage with the aircraft used at that place;
2. (2) More flights are needed because the capacity available to the eligible place is being shared with traffic destined for an intermediate stop or for a place beyond the eligible place;
3. (3) More flights are needed to accommodate passengers because smaller aircraft are being used at the place;
4. (4) More flights are needed in order to ensure adequate connecting opportunities as provided for by § 398.7; or
5. (5) For Alaska, the appropriate state agency agrees that more frequent service is needed to accommodate cargo traffic with the aircraft used at the eligible place.
(d) For eligible places where traffic levels vary substantially with the season, a two-tier level of essential air service may be established with required flight frequencies changing accordingly.