Part 135, and why it matters.

Part 91 is the general operating rulebook: an owner flying their own aircraft for their own purposes. Part 135 is the rulebook for commercial on-demand carriage — someone paying someone else to fly them. Charter is Part 135. It is more restrictive in ways that matter on exactly the days you would rather they did not.

Runway margins

Under Part 135 a turbojet must be able to stop within 60% of the available runway on a dry surface, with further reduction required for a wet one. Part 91 has no such requirement. The same aircraft, the same runway, the same day: the private owner may legally depart where the charter operator may not. When an operator tells you a field is unavailable that your friend flew into last month, this is usually why.

Crew duty and rest

Part 135 caps flight time and duty periods and mandates rest. A crew that has been on duty since 06:00 cannot simply keep going because your dinner ran long. This is the most common cause of a departure being refused late in the day, and it is not negotiable — an operator who bends it is an operator whose insurance may not respond.

Maintenance and oversight

A Part 135 certificate brings a maintenance programme approved by the FAA, named personnel, recurrent crew training and check rides, and drug and alcohol testing. Part 91 maintenance is the standard annual and progressive inspections. Both are lawful; only one is audited on the assumption that strangers are paying to be aboard.

Illegal charter

The grey area to know about is a Part 91 aircraft being sold as if it were charter — sometimes through a dry lease that does not survive examination. It is cheaper because it skips everything above. It also voids the insurance in a way that reaches the passenger: after an accident, the policy may not respond, and the person who booked it can find themselves personally exposed.

Two questions settle it. Which certificated air carrier is operating this flight, and what is its certificate number? A legitimate broker answers both before you sign, because 14 CFR Part 295 requires the operating carrier to be identified before booking. If the answer is vague, walk.

What a broker is, in this picture

A broker holds no certificate and operates nothing. It arranges the flight with a carrier that does, and is required to disclose that it is acting as an agent. That is a feature rather than an apology: an operator can only sell you what is in its own hangar, so it will always tell you its aircraft is the right one for your trip. A broker can put the right aircraft on the trip from any operator in the country — and is obliged to tell you which one, before you commit.