What is the Difference Between Part 91, 121 and 135?

I could write a book on the differences between Part 91, 121 and 135, but you and I don’t have that kind of time. Let’s keep it simple.

First of all, you need to understand, these “parts” refer to the different parts of the Federal Aviation Regulations, Titles 14 and 49 of the Code of Federal Regulations.

At a minimum every pilot should know the difference between these parts:

Part 25: Rules governing airworthiness standards

Part 61: Outlines requirements for getting licenses (also applies to small flight instructing schools)

Part 91: General operating and flight rules (general aviation)

Part 121: Rules for scheduled air carriers (ie. regional and major airlines)

Part 133: Rules governing external load operations for helicopters

Part 135: Rules for commuter and on-demand operations (ie corporate & government & all helicopter ops)

Part 141: Rules for flight schools (Embry-Riddle, or Hillsboro Aero)

Part 142: Rules for training centers (ie Flight Safety and CAE)

Note: The military does NOT operate under any of these parts which makes it difficult to transition from military to civilian flying. You have to learn a whole new set of regulations.

If you are in the military and you want to transition to the airlines, bust out the Part 121 regulations and start studying. If you are an Army helicopter pilot looking for a helicopter job, read Part 135.

Let’s get a little deeper into what Parts 91, 121, and 135 mean.

Part 91: Applies to general aviation and some corporate commercial operations

For small airplane pilots who fly around on the weekend, this is you. It will always be you. This regulation is your bread and butter. Know it in and out.

Some commercial operations will operate under Part 91 rules. They chose to stay Part 91 instead of Part135 to save money. Part 135 has strict training and maintenance requirements.

Part 91 does not mean commercial vs non-commercial or hire vs non-hire. You can get paid for your flights under Part 91.

Part 91 are the least restrictive regulations (besides sport regulations). For example, you can initiate an instrument approach when the weather is below the approach minimums. Not so for Part 121 and 135 operators.

However, if you want to fly passengers for hire then the FAA wants you to step up your game which is where the next two parts come in:

Part 135: Applies to commuter and on-demand operations

First, let’s get this out of the way: if you are a helicopter pilot, Part 135 (and Part 133) is almost always you.

I don’t care if you are flying tours or hauling passengers and cargo, you are working under an approved Part 135 Operation Specification.

Second, notice how I mentioned an “Operation Specification?” This is an official document from the FAA which establishes your company officially as a Part 135 operator. Parts 121, 141 and 142 also have Operations Specifications.

The Ops Specs are what allows companies to deviate from certain CFRs. For example, a Part 91 pilot has to have an alternate when the weather within one hour of their ETA is less than 2000 feet and 3 miles visibility. FAA approved Ops Specs will allow carriers to go lower than that.

Just be aware, Ops Specs are great until you try and change them. The FAA must approve any changes in writing. This includes checklists. This is one reason small corporate operators stay away from Part 135 operations.

Enough about OpSpecs!

Third, I know what you’re thinking: what are “commuter” and “on-demand” operations? Ugh…I have a hard time with these definitions. The FAA’s glossary doesn’t help. I’ll try my best to explain it in layman’s terms.

Commuter:

I think of a “commuter” as a super small airline. It’s a regional’s regional airline. Think island hopping in the San Juans, Hawaii, or flights in Alaska to small villages.

The official FAA definition is a scheduled operation of at least “5 round trips per week on at least one route between two or more points according to the published flight schedule.”

Here’s the kicker: it only applies to airplanes carrying 9 passengers or less (think King Air 350 or a Caravan), and cargo less than 7,500 pounds.

It applies to all regardless of passenger and cargo load.

On-demand:

I like to think of on-demand operations as flying a rich person wherever they want to go whenever they want to go.

Paul Allen does this all the time. He calls up any of his helicopters or airplanes at a moment and demand they take him anywhere in the world.

This is an extreme example, but you get the point. The schedule isn’t set per se. It’s at the request of the customer. Sure, there may be some consistency to the schedule, but as a general rule, it’s not a regular weekly thing.

Another example is an Air Ambulance operation out of Alaska. They go to the same places the majority of the time, and it can look like a set schedule, but technically the schedule depends on patient needs.

Part 121: the regional and major airlines

I don’t have much to say about this part. It’s pretty self-explanatory.

If you want to go work for Skywest, Horizon Airlines, United Expres, Delta, Alaska Airlines, United…etc, this part applies to you.

Airline pilots also need to pay attention to Part 117 which are the new regulations requiring certain rest requirements.

If you work for an airline, you will have to learn their “Flight Operations Manual.” When I worked for Horizon I don’t remember specifically seeing the official operation specification. I believe it gets blended into the FOM and for good reason. I work for a Part 135 operator now and the Ops Specs are not reader-friendly.

I hope that helped clarify the different federal aviation regulation parts.

Here’s the bottom line:

You need to know the different parts so you can tell whether a regulation applies to you.

Throughout your career, you will hear these terms thrown around. Just because you follow one regulation doesn’t mean other pilots operate the same way.

So, if you hear something a bit off from what you are used to doing, ask your fellow pilot what “Part” they are operating under.