14 C.F.R. § 398.4 – Equipment.

(a) Except in Alaska, service will be provided by aircraft offering at least 15 passenger seats, unless:
1. (1) Average daily enplanements at the place did not exceed 11 passengers for any fiscal year from 1976 through 1986;
2. (2) The requirement would necessitate the payment of compensation in a fiscal year for service at the place when compensation would otherwise not be necessary; or
3. (3) The affected community agrees in writing to the use of smaller aircraft to provide service at the place.
(b) The aircraft must have at least two engines and use two pilots, unless scheduled air transportation has not been provided to the place in aircraft with at least two engines and using two pilots for at least 60 consecutive operating days at any time since October 31, 1978.
(c) The aircraft must be pressurized when the service regularly involves flights above 8,000 feet in altitude.
(d) All aircraft must meet the applicable safety standards of the Federal Aviation Administration.
(e) The aircraft must be conveniently accessible to passengers by stairs rather than over the wing.