Understanding Part 91 and 135 Rules

You wouldn’t put your loved ones in an unlicensed taxi or car service for a $20 ride, but some people booking $20,000 charter flights can be very blasé about the legal status of the jets they fly on. FAA’s Part 91 and 135 regulations defining the differences between aircraft operated purely for private flights and those flown for-hire can seem dry, but understanding the key points could save you from a costly error.

In plain English, the devil-in-the-detail between Part 91 and 135 operations differentiate the rules that apply when money changes hands for a trip, from when it does not. These differences matter: in terms of safety, legality, liability, and transparency. 

To be clear, this is not to say that Part 91 flights are unsafe. What matters though is that if a Part 91 aircraft and crew, knowingly or unknowingly, conduct a charter flight and something goes wrong, everyone is going to be on thin ice in terms of insurance implications and much more.

If you want to see this important paperwork—and you should—the Parts and other relevant FARs are located among the documents charter aircraft must carry, and within the databases of many digital flight apps and electronic flight bags pilots use. But before you even get close to the aircraft you plan to charter, you should be demanding to confirm the Part 91/135 status of the operation from the operator itself or from a broker if you are using one. No good broker would propose any sort of off-the-books Part 91 flight in the first place.

Based on different regulatory approaches to risk, these two sets of rules permeate the world of business aviation, so anyone who flies aboard business aircraft must understand the differences between and within Parts 91 and 135 so they can adequately assess flight safety issues.

The balance between operational freedom and public safety allows those using aircraft for their own transportation flexibility to fly as they wish, while holding commercial providers to more rigorous standards for safe operations.

Related Article

One-minute Explainer: Part 91 vs. Part 135

Avoid serious problems with your charter flight by doing your research beforehand.

Part 135 aircraft flown by commercial charter operators must be registered with the FAA on an Air Operator Certificate (AOC). As such, they have more rigorous maintenance, inspection, equipment, insurance, and record-keeping standards than do aircraft operated under Part 91 rules.  In the Part 91 world, a Private Pilot license and appropriate aircraft type rating or sign-off are all that’s needed to fly passengers.

Pilot licensing, experience, training, and medical certification requirements are also appreciably higher for charter than private pilots. Charter pilots require an Air Transport Pilot or Commercial Pilot license, and are subject to flight and duty time limitations, rest time rules, and mandatory drug and alcohol testing. These limits are non-negotiable, so please don’t apply pressure on these professionals to bend the rules. They are in the interests of your safety.

The Safety Perspectives

Critically, the operational limitations and differences between Part 91 and Part 135 primarily come into play when conditions put flights at their highest risk, such as when weather is unfavorable, crews are tired, or some flight deck equipment isn’t working properly. Deviations between Part 91 and Part 135 include required visibility for takeoffs and landings, onboard equipment and fuel, runway lengths, and minimum weather conditions for making instrument approaches.

It seems counter-intuitive that less experienced, non-professional operators – be it a private pilot or the owner sitting in the back of a Part 91 aircraft – are allowed to take greater risks than a professional operation and crew, but there’s logic behind it. A private operator may be more familiar with a particular airport and have experience with its missed approaches, for example, or simply have heard real-time weather from pilots on frequency that contradict a dire automated report that a Part 135 crew may be mandated to use for their operations. 

Aviation rule makers also assume people aboard private aircraft are more knowledgeable about operations and risks, and cede them greater risk management responsibility. On the commercial side, the greater limitations protect everyone onboard, including flight crews who might otherwise feel pressure to take risks that their passengers may not fully appreciate.

There is another key parallel between taxis and airplanes to keep in mind. Road vehicles and business aircraft alike can be operated privately on some trips and commercially on others. The Uber fleet is a prime terrestrial example; owners may drive their cars for hire, or personal use. 

Similarly, most aircraft registered on an AOC are privately owned, but managed and controlled by the charter operator. Aircraft owners typically regard charter revenue as simply an offset for fixed costs; coming out ahead via charter income has proved elusive at best. When paying passengers aren’t onboard Part 135 aircraft—such as when in use by an owner, or during “empty leg” repositioning flights between charter flights—the aircraft may and often do operate under Part 91 rules, taking advantage of their greater operational flexibility. 

Differences Within the Parts

Whatever their distinctions, consider both Parts 91 and 135 as minimums from a safety standpoint. Many charter, private operators, and corporate flight departments adhere to higher standards established by third-party auditors. These audits evaluate an operator’s history and safety practices, pilot qualifications, maintenance, and other operational benchmarks, and place the bar above the minimums set by regulation. Argus, with its  Gold, Gold Plus, and Platinum ratings, and Wyvern, offering its Registered Operator and Wingman programs, are among the most-utilized audits. 

Several non-profit organizations also provide safety audits used by both Part 135 and Part 91 operators. These include IS-BAO (International Standards for Business Aviation Operations), developed by the International Business Aviation Council; the Air Charter Safety Foundation’s Industry Audit Standard; and the Flight Safety Foundation’s BARS (Basic Aviation Risk Standard). Charter operators participating in these programs typically tout their audit ratings in promotional materials to show their commitment to safety. Anyone who flies privately should know the safety levels each of these audits represents and adopt standards they’re comfortable with for their flights.

Additionally, many operators have developed a Safety Management System (SMS), a protocol for continuous safety improvement. The FAA required U.S. airlines to establish an SMS in 2015, and in 2024 mandated air charter operators have such a process for their businesses as well, setting the deadline in 2027. Though not required, a growing number of Part 91 operators are proactively creating their own SMS programs.

Another avenue for enhanced Part 91 safety: Many owners and pilots of turboprop and light business jets belong to manufacturer-sponsored “type clubs” for their aircraft that provide training programs and promote best safety practices.

Before flying on a charter or Part 91 aircraft, know the operator’s safety standards and protocols. Any safety-focused operator should be happy to tell you about theirs.
Note that aircraft in shared ownership programs like NetJets and Flexjet are governed by Part 91K, a blend of Parts 91 and 135 created for this access model. 91K leaves operational control in the individual owners’ hands (whether they’re onboard or not), while also imposing higher safety, maintenance, training, and other standards for aircraft and crews than Part 91 requires. 

Shared Expense vs. Charter Revenue

One last note on the private auto/private airplane analogy: The venerable tradition of “gas money” is alive and well in Part 91. Passengers may contribute their fair share of a flight’s direct operating expenses, but the permissible reimbursements are spelled out in detail (see (FAR) 14 CFR 61.113). If the operator asks for, or a passenger pays more, the flight could be deemed revenue producing - an illegal charter in the FAA’s eyes - and possibly result in an enforcement action and fines, back taxes, voiding of the aircraft’s insurance coverage, and other penalties.