Part 91 vs Part 135: the rules that change your costs

The United States: when a certificate is required

14 CFR 119.1 applies to each person operating or intending to operate civil aircraft as an air carrier or commercial operator in air commerce, and, where noncommon carriage is involved, except as provided in 91.501(b), or in private carriage for compensation or hire. Part 119 sets the certification requirements for operating under part 121, 125 or 135. Part 119 does not govern operations conducted under part 91 subpart K where common carriage is not involved.

Put plainly: a company flying its own people on its own aircraft, with no charge, is normally a part 91 operation. Carrying passengers for compensation or hire usually pulls you into part 135 and the certificate, operations specifications, maintenance and crew requirements that come with it.

The part 91 arrangements people actually use

14 CFR 91.501(b) lists operations that may be conducted under subpart F instead of parts 121, 129, 135 and 137 when common carriage is not involved. Three of them matter commercially, and 91.501(c) defines them.

Time sharing agreement

An arrangement whereby a person leases his airplane with flight crew to another person, and no charge is made for the flights other than those specified in 91.501(d).

Interchange agreement

An arrangement whereby a person leases his airplane to another person in exchange for equal time, when needed, on the other person's airplane, with no charge except a charge not exceeding the difference between the cost of owning, operating and maintaining the two airplanes.

Joint ownership agreement

An arrangement whereby one of the registered joint owners of an airplane employs and furnishes the flight crew, and each registered joint owner pays a share of the charge specified in the agreement.

The list of recoverable expenses in 91.501(d) is closed and specific: fuel, oil, lubricants and other additives; crew travel expenses including food, lodging and ground transportation; hangar and tie-down costs away from base; insurance obtained for the specific flight; landing fees, airport taxes and similar assessments; customs, foreign permit and similar fees directly related to the flight; in-flight food and beverages; passenger ground transportation; flight planning and weather contract services; and an additional charge equal to 100 percent of the fuel, oil, lubricants and additives.

That last item is the one people misremember. It is 100 percent of the fuel line, not of everything, and the whole list is a cap on what may be charged, not a menu for pricing a flight.

Fractional ownership: part 91 subpart K

A fractional ownership program under 14 CFR 91.1001 is a defined structure, not a marketing term. It requires a single program manager providing management services on behalf of the owners, two or more airworthy aircraft, at least one aircraft with more than one owner, a minimum fractional ownership interest held by each owner, a dry-lease aircraft exchange making the aircraft available without crew on an as-needed basis, and multi-year program agreements covering all of it.

The United Kingdom and Europe

UK Air Operations Regulation (EU) No 965/2012 Part-NCC applies to non-commercial flights in complex motor-powered aircraft. Instead of holding an air operator certificate, the operator submits a declaration to the CAA detailing aircraft type, operational and continuing airworthiness arrangements and approvals held, completed by the accountable manager (CAA).

A complex motor-powered aeroplane is one with a maximum certificated take-off mass exceeding 5,700 kg, or a maximum passenger seating configuration of more than nineteen, or a minimum crew of at least two pilots, or turbojet engines or more than one turboprop engine. Operators of complex aeroplanes at or below 5,700 kg MCTOM equipped with turboprop engines, on non-commercial operations, may instead operate under Part-NCO.

The cost consequence is direct. A declared or certificated operation carries a compliance overhead in manuals, training, oversight and record-keeping that a simple private operation does not, and it belongs in the fixed side of your model. Take legal advice on which regime applies before you build a budget on the answer.