Part 91 of the US Federal Aviation Regulations is the baseline rulebook for general aviation: owners flying their own aircraft, companies flying their own executives, flight departments moving their own people. Because nobody is buying transportation, the rules assume the person exposed to the risk is the one choosing it — so crew duty limits are largely absent, weather and runway margins are thinner, and maintenance can follow a lighter inspection regime than commercial rules require.
The differences are concrete. A Part 91 flight can be flown by a single private pilot with no commercial certificate; there are no regulatory duty-day caps, so an owner's crew can legally fly after a 16-hour day; and an instrument approach may be attempted in weather below the minimums a charter crew would be permitted to accept. None of this makes private flying reckless — well-run flight departments voluntarily hold themselves to charter standards or stricter — but the regulatory floor is much lower, because the government's interest in protecting a paying public isn't triggered.
For the charter customer, Part 91 matters mostly as a boundary. Part 91 flights cannot legally be sold to the public; when they are anyway, that is gray charter, and the practical consequences — void insurance, no commercial crew standards, no audited maintenance — land on the passenger. The recurring confusion is that the same physical aircraft often operates under both rule sets: Part 91 when the owner flies, Part 135 when it is chartered out through a certificated operator. What determines your protection is not the jet but the rules the specific flight operates under, which is why the operator's name and certificate belong in your contract.
A few legitimate structures live near the line and are worth recognizing. Part 91 Subpart K governs fractional ownership programs (NetJets-style shares) — commercial-grade oversight under a private-rules umbrella. Genuine dry leases, where a lessee truly takes operational control and hires crew independently, are legal but heavily abused as gray-charter cover. Timesharing and interchange agreements allow limited cost reimbursement between related parties under strict conditions. The test that cuts through all of it: if you are simply paying money to be flown somewhere, and the answer to "who holds the certificate for this flight?" is vague, walk away. A legitimate provider names the certificated operator immediately — and if the aircraft is managed, the owner's private use is also why charter availability sometimes waits on owner approval.
