Part 91 vs. Part 135: Understanding the Regulatory Divide

← Back to Resources

Why It Matters

The distinction between Part 91 and Part 135 operations is one of the most fundamental concepts in aviation regulation, and one of the most commonly misunderstood. Part 91 governs general operating and flight rules for all civil aircraft. Part 135 governs commuter and on-demand operations, which is the commercial side of aviation for smaller operators. The moment you start carrying passengers or cargo for compensation or hire, the regulatory landscape changes dramatically.

Understanding where that line falls, and what changes when you cross it, is critical for anyone considering commercial operations, transitioning from private to charter, or simply trying to understand what their certificate requires.

Operational Control

Under Part 91, the pilot in command has final authority over the operation of the aircraft. The owner or operator decides when and where to fly, and the PIC makes the go/no-go call.

Part 135 introduces a formal operational control structure. The certificate holder is responsible for operational control of every flight conducted under its authority. That means the company, not just the pilot, is accountable for flight planning, weather analysis, fuel requirements, and the decision to initiate, continue, divert, or terminate a flight. For on-demand operations, this responsibility typically falls on the Director of Operations or Chief Pilot, with clear documentation requirements for how that authority is exercised.

This isn't just a paperwork difference. It changes how decisions are made, who is accountable, and what documentation is required for every flight.

Maintenance Requirements

Part 91 maintenance is relatively straightforward. Aircraft must be maintained in an airworthy condition, with annual inspections required under §91.409 and 100-hour inspections for aircraft used for hire. The owner or operator is responsible for ensuring maintenance is performed, but the regulatory framework is flexible.

Part 135 maintenance requirements are significantly more structured. Certificate holders operating aircraft with nine or fewer passenger seats must comply with Parts 91 and 43 along with additional Part 135 requirements, including mechanical reliability reports, mechanical interruption summary reports, and additional maintenance documentation. Operators may use an Approved Aircraft Inspection Program (AAIP) under §135.419 as an alternative to the standard inspection requirements.

For operators with ten or more passenger seats, the requirements expand further with a dedicated General Maintenance Manual and a more comprehensive inspection and maintenance program. The record-keeping requirements under Part 135 are also more detailed, with specific retention periods and documentation standards that go well beyond what Part 91 requires.

Pilot Qualifications and Training

Under Part 91, pilot qualifications are governed by Part 61. A private or commercial certificate with the appropriate ratings is sufficient for the type of operation being conducted. There are no recurrent training requirements beyond a flight review every 24 calendar months and instrument proficiency checks for IFR operations.

Part 135 raises the bar considerably. Pilots must meet the experience requirements in §135.243 and §135.245, which include minimum total flight time, PIC time, and instrument time. Initial, transition, upgrade, and recurrent training are required, including competency checks and instrument proficiency checks at defined intervals.

For standard certificate holders, training programs must be formalized and submitted to the FAA for approval. The training curriculum covers indoctrination, aircraft-specific training, emergency procedures, crew resource management, and (where applicable) hazardous materials handling. This is a significant investment in time and documentation compared to the Part 91 world.

Weather Minimums and IFR Operations

Part 91 IFR operations are governed by the instrument flight rules in Subpart B and the equipment requirements in Subpart C. Pilots may file and fly IFR to published minimums with no additional operational restrictions beyond what the regulations and the aircraft's equipment allow.

Part 135 imposes additional weather limitations and operational requirements. Takeoff minimums, destination and alternate airport weather requirements, and visibility minimums are all more restrictive under Part 135 than under Part 91. For example, Part 135 operations require specific takeoff minimums that don't apply to Part 91, and the alternate airport requirements are more stringent.

These restrictions exist because commercial operations carry a higher standard of safety, and the FAA expects certificate holders to build additional margins into their weather decision-making.

Documentation and Manuals

Part 91 operators are not required to maintain operations manuals, training records, or formalized standard operating procedures. Many professional Part 91 operations choose to develop these documents voluntarily, but the regulation doesn't mandate it.

Part 135 changes this entirely. As covered in our article on the General Operations Manual, certificate holders (with the exception of single-pilot operators who may receive a deviation) are required to develop and maintain a GOM that covers every aspect of their operation. Training programs must be documented with formal curriculums. Maintenance procedures, crew qualifications, and operational control processes all require written documentation that is subject to FAA review and approval.

The documentation burden is one of the biggest differences operators notice when transitioning from Part 91 to Part 135. It's not just about having the right policies in place. It's about having them written down, approved, and accessible to every person in the operation.

Safety Management Systems

Under Part 91, there is no SMS requirement. Operators may voluntarily implement safety management practices, but the regulation doesn't mandate a formal system.

Part 135 operators are now required to implement a Safety Management System under 14 CFR Part 5 by May 28, 2027. This includes safety policy, safety risk management, safety assurance, safety promotion, and a Declaration of Compliance submitted to the FAA. The SMS requirement adds another layer of documentation and process management that Part 91 operators don't face.

The Bottom Line

The jump from Part 91 to Part 135 is not just a matter of getting a certificate. It's a fundamental shift in how you operate, how you're held accountable, and how much documentation is required to keep your operation in compliance. Maintenance standards increase. Pilot training becomes formalized and recurrent. Operational control moves from informal decision-making to a documented, structured process. Weather minimums get tighter. And the paperwork, from your GOM to your SMS to your training records, becomes a significant part of running the business.

None of that is meant to discourage anyone from pursuing a Part 135 certificate. Commercial aviation is a rewarding and viable business. But operators who go into it with a clear understanding of what changes, and what it takes to stay in compliance, are the ones who build sustainable operations.

If you're considering the transition from Part 91 to Part 135, or if you're already certificated and need help with your manuals, training documentation, or SMS program, we can help. Every document we build is tailored to your operation, delivered at a one-time cost, and designed to meet FAA standards.