Understanding the difference between Part 91 and Part 135 operations

Different restrictions apply to different types of flights, and they can often create confusion. In this article, we’ll explain the primary regulations that apply to aircraft operations in the business aviation industry: Part 91, and part 135.

Parts

The “parts” refer to the parts of the Federal Aviation Regulations (FAR). The FARs are found under Title 14 (Aeronautics and Space) and Title 49 (Transportation) within the Code of Federal Regulations (CFR), which is US Federal Law.

It is important to note that Title 48 of the Code of Federal Regulations (CFR) is titled “Federal Acquisitions Regulations” (also FAR). The two identical acronyms have created confusion, leading the Federal Aviation Administration (FAA) to refer to regulations as “14 CFR part XY.”

To summarize:

• The Code of Federal Regulations (CFR) is US Federal Law.
• The Federal Aviation Regulations (FAR) are included within the CFR under Titles 14 and 49 as federal law.
• The “parts” referred to are parts of the FAR.
• The FAA refers to the parts of the FAR as “14 CFR part XY,” not “FAR part XY,” as it is colloquially used.

Part 91 operations

A Part 91 operator is only permitted to provide flights for non-commercial purposes, and works under regulations defined by the US Federal Aviation Administration (FAA) for non-commercial operations. This means that Part 91 operators cannot under any circumstances receive compensation or even reimbursement for flights conducted.

Part 91 regulations are a set of defined conditions under which the aircraft must operate. These include required landing distances, safe weather requirements, in addition to pilot training. Part 91 operators are also insured under a more limited set of regulations than Part 135 operators, and so have a reduced liability.

General aviation operations fall into this category – Think of a private pilot flying with his friends or family. Part 91 is the least restrictive of the three parts (Part 91; Part 121*; Part 135) of the FAR concerning aircraft operations.

Part 135 operations

A Part 135 operator provides commercial, non-scheduled aircraft operations – such as private air charter and air taxi flights. Part 135 operations have to work within a much more detailed and strict operational and legal framework than a Part 91 operator.

A major difference between Part 135 and Part 91 requirements is the back-end structure and support that is required to obtain a Part 135 operating certificate.

The FAA requires a Part 135 operator to have a full team of management personnel to oversee all aspects of organisation. This includes a Chief Pilot, Maintenance and Operations Manager – establishing a clear chain of command in the company. Whereas under Part 91, the pilot-in-command is the final authority. 

Part 135 operators are also required to have a flight-following system, which tracks the status of all flights and ensures operational control of the companies aircraft.

The Part 135 regulations establish protocols for a series of air charter safety criteria, including:

• Aircraft maintenance requirements
• Pilot licensing
• Minimum insurance coverage
• Crew duty time limitations

Part 135 only applies to aircraft with 30 or fewer seats or a maximum payload capacity of 7,500 pounds, including commercial helicopter operations (other than external loading (i.e., a helicopter sling), which is covered by part 133).

Think of private jets, small turbo-propeller aircraft, and commercial helicopters – these are usually part 135 operation aircraft.

An important distinction must be made between the part under which an operation falls and the aircraft it uses. A charter company, for example, may be able to fly their turbo-propeller aircraft under part 91 for a repositioning flight with no passengers, allowing fewer restrictions on that specific flight. Then, when an aspect of part 135 is met (such as a paying passenger onboard), their ops spec for part 135 is enforced.

Differences between Part 91 and Part 135

The main difference between Part 91 and Part 135 operations largely has to do with the safety standards and regulations that go into the flight.

With charter flights operating under Part 135 operations, pilots are required to undergo more intense initial and recurrent training and are subject to specific flight duty-time and rest requirements. This is one of the most significant differences from Part 91, for when the duty day is over, it’s over, and it doesn’t start again until the rest requirement is satisfied. Part 135 also imposes a higher standard of pilot qualifications and requires that flight crew be subjected to drug and alcohol testing. An aircraft on a Part 135 certificate must be maintained to Part 135 standards, and in some cases, must have different equipment than would be mandated under Part 91, including fire-blocking of the interior.

Part 135 operational requirements also differ from Part 91’s. Under Part 91, but not Part 135, it is theoretically possible to take off under conditions of zero visibility. Aircraft operated under Part 135 cannot generally use airports that lack on-site weather reporting. Under Part 91, moreover, necessary runway length is determined by the aircraft’s performance limitations, while Part 135 requires a 40-percent “cushion” of additional runway length.

Part 91 is also more relaxed when it comes to security. Passenger identification is not required for domestic flights under part 91. For part 135 operations, passenger identities need to be verified by the operator, and passengers who are at least 18 years old will be required to provide photo identification, just as the commercial airlines.

Aircraft maintenance is highly scrutinized under Part 135 regulations where as Part 91 operations are mainly self-policed. Part 135 companies must also follow a robust Safety Management System that monitors all aspects of operations, requiring them to notify the FAA of any safety issues well before they become a potential hazard. Part 91 operations are not required to have any established safety programs.

*Part 121 refers to a scheduled operations or air carriers – Think of regional and major airlines such as Delta, Southwest, American, SkyWest, and large cargo aircraft operated by FedEx or UPS.