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The Differences Between Parts 91, 121, and 135

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In this post I’ll cover the differences between Part 91 vs 121 vs 135.

It’s not uncommon to hear a DPE say, “Here is your ticket to learn more,” as they hand you your shiny new pilot certificate. A career as a pilot is one in which you are perpetually learning. 

While it’s just one aspect of flying, one thing you will keep learning more about is the Federal Aviation Regulations. 

Here’s a look at the big three FARs from the pilots’ point of view—FARs Part 91, 121, and 135. 

Rules for Flying — Parts 91, 121, and 135

The FARs, or Federal Aviation Regulations, are the rules set out in Titles 14 and 49 of the Code of Federal Regulations (CFR). These are the rules pilots learn and follow throughout their careers. 

Each chapter of the FARs is referred to as a “part.” Pilots learn the few key parts of a part that apply to what they’re doing as they go through ground school. Pilot licenses are issued under Part 61, and general operating rules are found in Part 91. 

But as you go on, you’ll learn more parts. While you might not know all the details, you will start to pick up on who each FAR Part applies to. 

Part 121 operator

There are rules for pilots, mechanics, aircraft manufacturers, flight instructors, flight schools, airlines, charter operators, and many more. 

In a nutshell, these three rules are for:

  • Part 91 — Everyone (flight rules for non-commercial operations)
  • Part 121 — Regional and major airlines
  • Part 135 — Charter and commuter operators

What Is Part 91?

Part 91 is one of the first FARs you learn because it deals with the basic flight rules. If you aren’t operating under any other section, you must be operating under Part 91. 

Part 91 lays out some basics everyone needs to know to fly. This includes what counts as VFR vs. IFR weather, minimum and cruising altitudes, and who has the right of way. 

And, of course, there are the rules of what a pilot must do, like preflight actions that must be taken. 

What’s not covered with Part 91? 

Commercial operations—that is, flying passengers or cargo for compensation or hire. 

Commercial operations are limited under Part 91 because the FAA wants commercial operators to be licensed—meaning they should operate under either Part 121 or Part 135. Those regulations are more restrictive and specifically created to protect the flying public with great safety. 

One confusing thing about Part 91 is that all aircraft operate under it. So a Part 121 or 135 pilot must abide by both regulations. 

Part 91 vs part 121 vs part 135

In effect, those more restrictive rules are built on the framework created by Part 91. So you start flying under Part 91, and if it’s a commercial operation, you must also abide by Part 121 or 135. 

What Is Part 121?

Part 121 is for scheduled carriers, what we non-Feds would call an airline. 

If an airline operates flights regularly between two destinations and sells the tickets, then it is licensed to do so under FAR Part 121. 

These are the most visible airlines that the general public uses. They range from small regional operators flying small piston twins (think Cape Air) to the biggest flag operators like United, American Airlines, and Delta. Large cargo operators like UPS and FedEx also operate under Part 121.

Since these are the carriers that most of the flying public uses, it is the most restrictive of the FARs. It’s considered the gold standard for safety in aviation.

What Is Part 135?

In contrast, Part 135 is for non-scheduled carriers or charter operators. 

Charters are booked on a case-by-case basis, and there’s no set schedule involved. We usually think of operators of corporate jets, like Learjets and Gulfstreams. 

Additionally, nearly all commercial helicopter flights are operated under Part 135. The rule can apply to any type of operation if the aircraft has fewer than 30 seats and a payload of less than 7,500 pounds.

Part 135 operators may be certified to do on-demand or commuter flights. On-demand flights are what we’d typically think of as charter flights, where someone calls and asks for a private plane flight to fit their schedule.

Commuter flights are more like small airlines that don’t meet the FAA’s definition of scheduled service, which would fall under Part 121. If an airline operates aircraft small enough to fall under Part 135 and doesn’t operate more than five round-trip flights between two destinations every week, then it is a commuter carrier and licensed under Part 135.

What Are the Key Differences Between Parts 91, 121, and 135?

Now that you know who uses which regulations, let’s look at some specific differences between them. 

Part 119

So, you own an airplane and wonder how to make money. What exactly can you do under Part 91 as a commercial operator? 

Part 119 is the regulation you seek. 

Part 91 FARs

This rule identifies the differences between 91, 121, and 135 operators and says who must abide by which rule. 

It primarily defines common carriage, which is forbidden under Part 91. 

Common carriage is the holding out to carry people or cargo from one place to another for compensation. Part 119 also lists a few things that aren’t common carriage, which are allowed under Part 91. Examples include flight instruction, ferry flights, banner towing, sightseeing flights, and crop dusting. 

More Restrictive For Safety

There’s a slew of things required under Parts 121 and 135 that are not required under Part 91. Here are a few examples.

  • Safety management systems and safety officers
  • Employee drug testing programs
  • FAA flight checks for pilots
  • Takeoff weather minimums apply
  • Cannot shoot an approach if reported weather is below minimums
  • Mandated crew rest periods and maximum duty times (per day and month)
  • Photo identification is required for all passengers

Pilot Licensing

In addition to those rules, there are minimum experience requirements. To be the Pilot-in-Command (PIC) of a Part 121 or 135 flight, you must possess an ATP (Airline Transport Pilot) certificate

To be Second-in-Command, you must possess at least a Restricted ATP

Part 121 vs Part 135

Parts 121 and 135 require that the operator designate a chief pilot in charge of flight operations. 

That person must meet experience requirements to be eligible. Larger operators might also have assistant chiefs or check airmen. Pilots in these positions must pass an FAA checkride.

Part 121 operators must have two pilots on board, whether or not the aircraft requires it. They will also work with a flight dispatcher. Part 135 operators flying single-pilot aircraft can be approved for single-pilot operations.

Ops Specs

All of these rules fashion what a Part 121 or 135 operator looks like, but the biggest difference is what is known in the industry as Ops Specs. 

The Operational Specifications (Ops Specs) is a document that an operator puts together when applying for their Part 121 or 135 certificate. 

It lays out which aircraft can be used and which airports they can fly out of, who the chief pilots is (and other key staff positions), what types of IFR approaches the pilots can use, and what other procedures and checklists the airline will use. 

The FAA goes through the Ops Specs document and approves each item. The operator cannot deviate from their approved Ops Specs without getting new approval. 

Where a Part 91 operator could purchase a new airplane, hire a new pilot, or land at a new airport, a Part 121 or 135 operator must add these to their Ops Specs and get the changes approved.  

To help each operator with all the details and approvals, the FAA assigns each certificate holder a POI (Principle Operator Inspector).

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