Part 121 vs 135 vs 91: Differences & What You Should Know

Are you trying to understand the differences between Part 91, 121, and 135 regulations? Each part has its unique rules for aviation in the U.S. Part 91 generally governs non-commercial flights, offering flexibility and lower compliance costs, though some specialized operations require enhanced safety and regulatory oversight. This blog will help you navigate these regulations easily. Keep reading!

Key Takeaways

Part 91 covers non-commercial flights, offering more flexibility and lower compliance costs but less stringent safety oversight.
Part 121 regulates scheduled airlines with strict safety standards, requiring pilots to have an Airline Transport Pilot (ATP) certificate.
Part 135 applies to charter and commuter operations, demanding higher qualifications for pilots and rigorous maintenance standards to ensure passenger safety.
Safety protocols under Parts 121 and 135 are stricter than under Part 91, ensuring enhanced aircraft reliability but increasing operational costs.

Overview of FAA Parts 91, 121, and 135

The Federal Aviation Administration (FAA) regulates different types of flight operations under FAA Parts 91, 121, and 135. Each part has unique rules and guidelines that pilots must follow to ensure safe flying conditions.

Aspect	Part 91	Part 121	Part 135
Applicability	General aviation (non-commercial).	Scheduled airlines (commercial).	Charter, commuter, and air taxi operations.
Examples	Private flights, corporate jets (non-commercial), hobbyists.	Major airlines (e.g., passenger and cargo).	Charter flights, on-demand air taxis.
Crew Requirements	Single pilot allowed.	Two-pilot crew mandatory.	Single pilot allowed (depending on operation).
Aircraft Size	Any size (non-commercial use).	Typically 30+ seats or large cargo capacity.	Up to 30 seats or 7,500 lbs payload max.
Safety Standards	Fewer requirements, operator discretion.	Strict FAA oversight (highest safety level).	Rigorous, but less strict than Part 121.
Maintenance	Flexible, owner/operator responsible.	FAA-approved, strict protocols.	Regular inspections, FAA-mandated checks.
Operational Control	Managed by the pilot or owner.	Managed by the airline (dispatch oversight).	Operator oversees flights; FAA-approved Ops Specs.
Passenger Limits	No compensation for passengers allowed.	Commercial ticket sales allowed.	Passengers allowed under commercial rules.
Flexibility	High flexibility in scheduling and weather.	Fixed schedules and strict limitations.	Moderate flexibility, especially on-demand.
Compliance Costs	Low.	High (due to safety, training, and oversight).	Moderate (higher than Part 91, less than Part 121).

Part 91

Applicability: Regulates general aviation operations, including private pilots, corporate flight departments, and some small charter operations.

Requirements: The least restrictive of the three, with fewer regulatory requirements for maintenance, operational control, and pilot certification. However, pilots must still comply with certain safety standards and airworthiness requirements.

Operations: Used by private pilots, corporate aviation, and some small commercial operations not meeting the Part 135 or Part 121 criteria.

Examples: Private flying, corporate jets, and small-scale flying clubs operate under Part 91.

Part 91 covers basic non-commercial flight rules. Pilots flying under Part 91 cannot receive compensation. Rules set conditions on landing distances, weather checks, and pilot qualifications.

Training is also crucial for operating under these standards.

Liability insurance requirements for Part 91 operations depend on individual operators and insurers, unlike Part 135 operators who often opt for higher coverage due to commercial considerations. Cost-sharing between passengers and pilots is allowed under certain conditions, but no direct compensation or reimbursement can occur.

This makes it a useful choice for flying, both personal and hobby, without commercial pressure.

Part 121

Applicability: Governs commercial air carrier operations for scheduled airlines.

Requirements: Requires higher standards for certification, including specific maintenance, crew training, and operational procedures.

Operations: Applies to airlines operating large aircraft (more than 30 seats or payload capacity of 7,500 pounds) in scheduled air service.

Examples: Major and regional airlines operate under Part 121.

Part 121 applies to businesses with scheduled passenger or cargo flights. These operations require FAA certification and strict adherence to flight schedules. Pilots must hold an Airline Transport Pilot (ATP) certificate.

This ensures they meet high standards of skill and experience.

Two pilots are mandatory for Part 121 flights, working closely with a flight dispatcher. Safety is paramount; hence the maintenance standards are rigorous. Aircraft reliability remains a top priority.

Operating under Part 121 demands consistency, reliability, and high safety measures at all times.

Part 135

Applicability: Covers commuter and on-demand operations, including charter flights, air taxi services, and certain cargo operations.

Requirements: Less stringent than Part 121 but still requires specific operational and maintenance standards. It allows for a mix of small and larger aircraft.

Operations: Can include both scheduled and non-scheduled operations, typically with smaller aircraft (fewer than 30 seats).

Examples: Charter services like NetJets or local air taxi companies operate under Part 135.

Part 135 applies to commuter and on-demand services like air taxis. These operations require a management team, including a Chief Pilot, Maintenance Manager, and Operations Manager.

The FAA assigns a Principal Operator Inspector (POI) for oversight.

Stricter safety protocols under Part 135 enhance passenger safety. This includes more rigorous maintenance requirements and pilot licensing standards. Aircraft used must have no more than 30 seats or weigh less than 7,500 pounds in max payload.

Law enforcement agencies operating helicopters typically follow Part 91 rules but may qualify as public-use aircraft, exempting them from certain FAA requirements.

Key Differences Between Part 91, 121, and 135

Each FAA part governs specific types of aviation operations and sets different standards. Understanding these distinctions helps pilots choose the right operation type for their needs.

Operational Scope and Usage

Part 91 covers non-commercial operations. This includes private flying for personal and business use, such as a police helicopter pilot transporting officers. Common carriage under Part 91 is prohibited except for specific exceptions.

Part 121 regulates scheduled airlines, known as the "gold standard" for aviation safety. It sets strict rules to ensure safe, reliable service. Commercial pilots operating under this must meet high standards.

Part 135 pertains to charter and commuter operators. These flights offer more flexible schedules but still require stringent safety measures. Under Part 135, a Pilot-in-Command may require an ATP certificate for certain operations, though some flights may only require a Commercial Pilot Certificate.

Certification and Licensing Requirements

Pilots under Part 91 must showcase sound decision-making and comprehensive knowledge of regulations. They don't need a commercial pilot certificate for most operations, but they can't accept compensation for air transportation either.

Part 121 requires pilots to have an ATP (Airline Transport Pilot) license. It mandates two-pilot crews for all operations. Some operations under Part 135 allow single-pilot flights, depending on the aircraft and mission type..

However, both Parts 121 and 135 require Ops Specs approved by the FAA and a more rigorous selection process than Part 91. These include specific training and higher safety standards to ensure passenger protection.

Safety Standards and Requirements

Parts 121 and 135 mandate stringent safety standards. These parts require an Air Carrier Certificate from the FAA to operate legally. Strict regulations govern aircraft maintenance, crew training, and operational procedures.

For example, Part 121 has more demanding requirements compared to Part 135.

Part 91 offers greater flexibility but requires personal responsibility for safety. Unlike Parts 121 and 135, it does not demand a formal Safety Management System or drug testing programs for pilots.

This allows operators to adapt their practices based on specific needs without a bureaucratic framework.

Detailed Comparison of Part 91, Part 135, and Part 121

Learn about flight crew training standards, aircraft maintenance protocols, and operational flexibility by reading further.

Flight Crew and Training Standards

Part 121 requires the Pilot-in-Command to hold an Airline Transport Pilot (ATP) certificate. The Second-in-command must have a Restricted ATP certificate. Pilots under Part 121 undergo rigorous FAA flight checks and must meet stringent training standards.

Part 135 pilots also face intensive training and rest requirements set by the FAA. These pilots complete regular flight checks to ensure they can handle commercial operations safely.

Enhanced training standards are optional for Part 91 operators but are more common among Part 135 operations aiming to meet industry benchmarks.

Aircraft Maintenance and Inspection Protocols

Aircraft maintenance under Part 135 mandates rigorous standards. These include frequent inspections and detailed record-keeping to ensure safety. The protocols for these aircraft are stricter than those for Part 91, enhancing passenger safety. Pilots operating under Part 121 follow stringent maintenance requirements approved by the FAA. This ensures aircraft reliability and minimizes operational risks.

Part 91 has less stringent maintenance protocols compared to Parts 135 and 121, offering more flexibility but potentially less oversight. Knowing these differences can guide aspiring pilots in choosing the right operational scope.

Maintenance standards directly impact safety and flight performance, so understanding them is crucial for any pilot or operator aiming for excellence in their aviation career.

Operational Flexibility

Part 91 offers the most operational flexibility. Pilots can fly in various weather conditions and decide their schedules. There are fewer rules compared to Parts 135 and 121, making it ideal for private flying.

Part 135 imposes more restrictions due to commercial operation rules. For example, zero-visibility operations are not allowed under Part 135. This guarantees passenger safety but limits some flight options.

Both Part 121 and Part 135 mandate pilot rest and duty limitations, with Part 121 following stricter guidelines for scheduled operations. Commercial airlines follow these rules, ensuring high levels of security and reliability for passengers. The compliance costs increase significantly from Part 91 to Parts 135 and 121 due to these stringent requirements.

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