Part 91 vs Part 135: Operating Rules, Costs, and Revenue Implications

What is the core legal difference between Part 91 and Part 135?

Part 91 covers private operations; Part 135 covers commercial on-demand charter. The distinction turns on "compensation or hire" and "holding out." Under Part 91, the aircraft moves the owner and the owner's guests, and any cost reimbursement is tightly limited by §91.501 (for large or turbine multi-engine aircraft) or prohibited outright for smaller aircraft. Under Part 135, the operator holds an Air Carrier Certificate issued by the FAA, can advertise the aircraft for charter, and must comply with a separate, materially stricter regulatory regime governing crew, maintenance, training, and operational control.

The line between the two is enforced aggressively. The FAA's compliance and enforcement bulletins on "illegal charter" have driven multiple six- and seven-figure civil penalties since 2019, particularly against dry-lease arrangements that the agency views as disguised charter. If the lessee does not have genuine operational control — meaning the authority to choose the crew, dispatch the flight, and direct the mission — the FAA will treat the flight as Part 135 conducted without a certificate.

How do pilot duty and rest rules differ?

Part 135 pilot duty limits are significantly tighter than Part 91. A single-pilot Part 135 operation is capped at 8 hours of flight time in any 24-hour period under §135.267, with mandatory rest of 10 consecutive hours before duty. Two-pilot crews get 10 hours flight time. Cumulative limits run 500 hours per quarter and 1,400 hours per calendar year.

Part 91 imposes no federal flight or duty time limits on the pilots of non-commercial flights — the only constraint is the §91.13 careless-and-reckless standard and whatever the insurance policy requires. In practice, sophisticated Part 91 flight departments adopt internal duty rules that mirror Part 135 or NBAA guidelines, but they are not regulatory. Part 135 also requires recurrent training every 6 or 12 months depending on the program, an FAA-approved check airman regime, and §135.293/.297 competency and instrument proficiency checks. Part 91 requires only the §61.58 pilot-in-command proficiency check for turbojets every 12 months.

What maintenance obligations attach to each?

Part 135 requires an FAA-approved maintenance program; Part 91 generally does not. A Part 135 operator must follow either the manufacturer's program, an approved Continuous Airworthiness Maintenance Program (CAMP), or an Approved Aircraft Inspection Program (AAIP) under §135.419. Inspections, component time limits, and reliability tracking are all subject to FAA oversight, and the operator must maintain detailed records of every airworthiness action.

A Part 91 operator outside Subpart F can comply with the basic annual inspection regime under §91.409, follow the manufacturer's recommended schedule, or enroll in a maintenance tracking program voluntarily. Part 91 Subpart F (for large airplanes, turbojets, and turbine multi-engine airplanes) tightens this with §91.409(e) requirements for a current inspection program selected from approved options. The practical cost differential: a Part 135 maintenance program typically runs 15-20% higher in direct labor and parts cost than the equivalent Part 91 program on the same airframe, driven by shorter inspection intervals, mandatory component overhauls, and the administrative overhead of FAA-approved record-keeping.

How does insurance pricing differ between Part 91 and Part 135?

Part 135 hull and liability premiums run materially higher than Part 91 on the same aircraft. Underwriters price Part 135 exposure based on charter hours, passenger seat counts, and the operator's safety record. A super-midsize jet on a pure Part 91 policy with a $100 million liability limit might run $95,000-$130,000 annually in 2024 market conditions; the same aircraft on a Part 135 certificate generating 400 charter hours per year typically runs $160,000-$220,000 for comparable limits.

The differential reflects both higher claim frequency in commercial operations and the broader passenger liability exposure when strangers — not the owner's family — are in the cabin. Owners pursuing a hybrid model (aircraft on a 135 certificate but flown predominantly Part 91 by the owner) negotiate split-rate policies that price the two utilizations separately.

What is the federal excise tax exposure under each?

Part 135 charter triggers the 7.5% federal excise tax under IRC §4261, plus the $5.20 domestic segment fee per passenger per flight segment (2024 rate, indexed). Part 91 owner flights do not. This is the single largest tax distinction between the two regimes and the reason FAA and IRS coordinate enforcement against illegal charter — the government loses 7.5% of every dollar that moves through a non-certificated commercial flight.

The FET applies to "amounts paid for taxable transportation," which the IRS interprets broadly. Management company arrangements where the owner is charged for flights on their own aircraft can trigger FET if the management company exercises possession, command, and control — this was the central issue in the IRS's aircraft management company audits that culminated in the 2020 FET exemption clarified under IRC §4261(e)(5), which exempts owner flights on owner-provided aircraft from FET even when a management company is involved. International flights and certain small aircraft (6,000 lbs MTOW or less, not on established lines) are also exempt.

How does the choice affect depreciation and §280F?

Part 135 use counts as qualified business use under §280F; personal Part 91 use generally does not. To claim §168(k) bonus depreciation — 60% in 2024, 40% in 2025 — the aircraft must be predominantly used in a qualified business use, meaning more than 50% of flight hours support a trade or business. Charter revenue hours under Part 135 count toward that threshold without limitation; Part 91 hours count only if they serve the owner's active trade or business and survive §274 entertainment disallowance.

The §280F(d)(6)(C) "qualified business use" test also imposes a 25% floor of non-leased, non-5%-owner-related business use, which complicates aircraft held in a single-purpose LLC leased back to the operating business. A Part 135 charter program supplying third-party revenue hours is the cleanest path to satisfying both the 50% predominant use test and the 25% core business use floor.

When does it make sense to put an aircraft on a 135 certificate?

Put the aircraft on a 135 certificate when owner utilization is below roughly 200-250 hours per year and the airframe can absorb another 150-300 charter hours without disrupting owner availability. Below that owner-use threshold, the fixed cost of ownership — crew, hangar, insurance, management — is poorly amortized by owner flights alone, and charter revenue at typical 2024 rates ($6,500-$11,000 per hour for midsize jets, $12,000-$18,000 for heavy jets) meaningfully offsets carrying cost.

Above 400 owner hours per year, the operational friction of Part 135 — scheduling conflicts, maintenance downtime from tighter inspection intervals, crew duty limits constraining owner trips — usually outweighs the revenue. The middle case, 250-400 owner hours, is where most professional management decisions are made on a deal-by-deal basis weighing the aircraft type, base location, and the owner's tolerance for shared use.