How They Fit Together: Part 91 Inspection vs. Part 135 Maintenanc

Over recent months there have been many discussions about 14 CFR Part 135.411(a)(1). This is the rule that specifically defines what constitutes the infamous (nine or less) Part 135 aircraft. What we will discuss this month is all of the things that make the nine or less operation different than Part 91 operations and those things that are different about other Part 135 operations (10 or more). I will be referring exclusively to requirements applicable to large and multi-turbine powered aircraft for the purposes of this article.

For starters there is a logical transition flow from Part 91 to Part 135 nine or less and on to Part 135 ten or more. It's almost as if God created Part 91, rested then created Part 135 nine or less, rested and then created Part 135 ten or more.

First let's take a close look at Part 91 inspection requirements.

CFR Part 91.409(e)

(e) Large airplanes (to which Part 125 is not applicable), turbojet multiengine airplanes, turbopropeller-powered multiengine airplanes, and turbine-powered rotorcraft. No person may operate a large airplane, turbojet multiengine airplane, turbopropeller-powered multiengine airplane, or turbine-powered rotorcraft unless the replacement times for life-limited parts specified in the aircraft specifications, type data sheets, or other documents approved by the Administrator are complied with and the airplane or turbine-powered rotorcraft, including the airframe, engines, propellers, rotors, appliances, survival equipment, and emergency equipment, is inspected in accordance with an inspection program selected under the provisions of paragraph (f) of this section, . . .

This language refers to the replacement of life-limited parts contained in FAA approved documents such as the aircraft specifications and TCDS's. For clarity, aircraft specifications and TCDS's have basically the same purpose. Aircraft specifications were used in the past and apply to older, less complex aircraft. In those cases, life-limited parts are actually listed in the aircraft specifications. For newer aircraft which have a TCDS, the document(s) that contain a list of life-limited parts is referenced in the notes section of the TCDS itself. The TCDS points us to these additional FAA approved documents and refers to them by name and part number as well. Regardless of how you get to the list of parts, the life-limited parts are also commonly referred to as Airworthiness Limitations.

Now, the above language states that the airplane must be inspected in accordance with an inspection program selected under paragraph (f) of the same rule. And it goes on to include reference to all of the parts of the airplane — the airframe, engines, propellers, appliances, survival equipment, and emergency equipment as needing to be included in the inspection program selected. Here is what paragraph (f) looks like.

CFR Part 91.409(f)

(f) Selection of inspection program under paragraph (e) of this section. The registered owner or operator of each airplane or turbine-powered rotorcraft described in paragraph (e) of this section must select, identify in the aircraft maintenance records, and use one of the following programs for the inspection of the aircraft:

  1. A continuous airworthiness inspection program that is part of a continuous airworthiness maintenance program currently in use by a person holding an air carrier operating certificate or an operating certificate issued under Part 121 or 135 of this chapter and operating that make and model aircraft under Part 121 of this chapter or operating that make and model under Part 135 of this chapter and maintaining it under Sec. 135.411(a)(2) of this chapter.
  2. An approved aircraft inspection program approved under Sec. 135.419 of this chapter and currently in use by a person holding an operating certificate issued under Part 135 of this chapter.
  3. A current inspection program recommended by the manufacturer.
  4. Any other inspection program established by the registered owner or operator of that airplane or turbine-powered rotorcraft and approved by the Administrator under paragraph (g) of this section.

As you can see, paragraph (f) gives the Part 91 operator four choices to choose from to establish how aircraft will be inspected. Option (f)(1), references a continuous airworthiness inspection program and option (f)(2) references an approved aircraft inspection program. The two options are different than (f)(3) and (f)(4). Both option (f)(1) and option (f)(2) inspection programs must currently be in use by an operator who is operating under the requirements of an air carrier certificated under Part 135 or Part 121.

We like to call option (f)(4) the "owner's program." The reason is to prevent confusion with an approved aircraft inspection program (AAIP) approved under Part 135. The "owner's program," option (f)(4) is provided to allow a Part 91 operator to create his/her own inspection program. In all cases, the "owner's program" is to be based on information, including intervals, provided in the aircraft manufacturer inspection program. Once created, it must be approved by the FAA before it can be used. Part of an "owner's program" must be a description of how the program will be maintained and kept current.

Option (f)(3) is the most common choice and in our opinion the most logical. Option (f)(3) is the inspection program currently offered in the aircraft manufacturer's maintenance manual. The reason we prefer the manufacturer's inspection program is because it is continuously under evaluation by the manufacturer and pertinent revisions to the program are issued by the manufacturer on a regular basis.

Now, moving backward for just a moment, I call your attention to the latter portion of Part 91.409 (e). A fairly common oversight is the requirement to include inspection requirements issued by the engine, propeller, appliances, survival equipment and emergency equipment manufacturers. The FAA has issued the following guidance to help prevent oversight:

FAA Order 8300.10 Volume 3, section 26 Page 3

(a) Section 91.409(f)(3) refers to "A current inspection program recommended by the manufacturer." No reference is made to the aircraft manufacturer specifically. Section 91.409(e), however, requires inspection of the airframe, engines, propellers, appliances, survival equipment, and emergency equipment.

(b) Therefore, a complete manufacturer's recommended program consists of the program supplied by the airframe manufacturer and supplemented by the inspection programs provided by the manufacturers of the engines, propellers, appliances, survival equipment, and emergency equipment installed on the aircraft.

NOTE: Because this program addresses inspections only, it does not include service bulletins, service letters, service instructions, and other maintenance documents, unless they require an inspection to be performed.

So, basically we need to be aware that the airframe manufacturer does not always include all of the inspection requirements needed to make up a complete inspection program. Also, for Part 91 operations, the only mandatory replacement items are the airworthiness limitations called for by 91.409(e) (". . . replacement times for life-limited parts specified in the aircraft specifications, type data sheets, or other documents approved by the Administrator . . ."). There are generally other "recommended" replacement items as well as recommended overhaul times called for by the manufacturer in the maintenance manual. The reason they are recommended is that the replacement time or overhaul interval is simply a recommendation from the manufacturer, and is not called for in any FAA approved document. Periodic inspection of these parts may be sufficient to allow them to continue service if they are inspected (as part of the inspection program) and the inspector finds them to be in an airworthy condition.

Knowing now what makes up the inspection program for a Part 91 operator, it is time to introduce the Part 135 nine-or-less operator. Part 135.411(a)(1) is the regulation that defines the maintenance requirements (by reference) for Part 135 aircraft operating in the nine or less passenger seating configuration.

Part 135.411(a)(1)

(a) This subpart prescribes rules in addition to those in other parts of this chapter for the maintenance, preventive maintenance, and alterations for each certificate holder as follows:

(1) Aircraft that are type certificated for a passenger seating configuration, excluding any pilot seat, of nine seats or less, shall be maintained under parts 91 and 43 of this chapter and Sec. 135.415, 135.416, 135.417, 135.421 and 135.422. An approved aircraft inspection program may be used under Sec. 135.419.

Notice that the regulation states that if the aircraft is "type certificated" for a passenger configuration of nine passengers or less then the aircraft shall be maintained under Parts 91 and 43. A key here is the difference between inspected, as seen above under Part 91.409(e) and maintained as defined in 135.411(a)(1). The difference is that according to the definition of maintenance, inspection is part of maintenance, thus indicating there is more to do under Part 135. This rule also directs us to Part 135.421 (among others) which calls for "additional maintenance requirements" and further underscores the fact that under Part 135 the rule is talking about a maintenance program and not just an inspection program. Here is how Part 135.421 reads:

Part 135.421 – Additional Maintenance Requirements

a) Each certificate holder who operates an aircraft type certificated for a passenger seating configuration, excluding any pilot seat, of nine seats or less, must comply with the manufacturer's recommended maintenance programs, or a program approved by the Administrator, for each aircraft engine, propeller, rotor, and each item of emergency equipment required by this chapter.

(b) For the purpose of this section, a manufacturer's maintenance program is one which is contained in the maintenance manual or maintenance instructions set forth by the manufacturer as required by this chapter for the aircraft, aircraft engine, propeller, rotor or item of emergency equipment . . .

When the air carrier obtains its operation specifications, the operator will have satisfactorily defined to the FAA, as part of the certification process, how the aircraft will be "maintained." Generally this is by reference to the appropriate manufacturer's maintenance manuals. The maintenance program will, in that case, include the requirements contained within the airframe maintenance manual, as well as the maintenance manuals provided by the engine, propeller, emergency equipment, and safety equipment manufacturer's maintenance manuals, just like under Part 91. However, for Part 135 nine or less operators, the additional requirements are those items that are included in the definition of "maintenance," but are not considered "inspections." Some examples are overhaul and replacement recommendations listed in the appropriate manufacturer's maintenance manual that are not listed in the TCDS or any other document approved by the FAA.

As always, Part 135 nine or less operators may create their own inspection program for approval by the FAA under Part 135.419. When a Part 135 nine or less operator chooses to write and maintain their own inspection program it is officially referred to as an Approved Aircraft Inspection Program (AAIP). The FAA will require that the AAIP closely resemble the manufacturer's inspection program and it is often not worth the hassle of writing, getting approval, and maintaining. Even when an AAIP is used, the additional maintenance requirements required by Part 135.421 are still additional to the inspection program.

Additional note of interest: There has been much discussion about what qualifies an aircraft as a nine or less passenger configuration aircraft. As you can see, the definition of a nine or less aircraft states that it is "Aircraft that are type certificated for a passenger seating configuration, excluding any pilot seat, of nine seats or less." Any aircraft seating configuration must be an FAA approved seating configuration, approved by means of the TCDS or an appropriate STC.

One way to know if the seating configuration for your aircraft is approved is to look in the FAA approved flight manual and see that the configuration is listed as an option for your aircraft. The seating configuration approved by means of an STC will essentially have been installed and approved via the STC. Blocking of seats or removal of seats without an STC approving the configuration with the seat removed is not acceptable according to the FAA.

The scope and detail of aircraft maintenance requirements further expands when we get to the Part 135 ten or more type aircraft. These aircraft are placed into a whole new arena. The difference is seen if you look closely at Part 135.411(a)(2)
Part 135.411(a)(2)

(2) Aircraft that are type certificated for a passenger seating configuration, excluding any pilot seat, of 10 seats or more, shall be maintained under a maintenance program in Sec. 135.415, 135.416, 135.417, and 135.423 through 135.443.

Here we see that rather than calling out 135.421 (additional Maintenance Requirements) as 135.411 (a)(1) does, the rule for aircraft configured to seat 10 or more passengers requires the operator use 135.423 through 135.443 as a basis for their maintenance program. Essentially what this means is that the operator must create and maintain a detailed, operator specific maintenance program. This includes a continuous airworthiness maintenance program created by using the appropriate manufacturer's maintenance manuals but actually written and maintained by the operator. The 10 or more passenger-configured aircraft operator also must create a general maintenance manual for review and approval by the FAA. Much like a repair station's inspection procedures manual, the air carrier's general maintenance manual is a document that defines the business processes for how maintenance is to be performed. The general maintenance manual is not a replacement for the manufacturer's maintenance manual.

Hopefully this flow makes sense and will make it easier to understand the different requirements for aircraft operating under Part 91 vs. Part 135. Until next time.

Joe Hertzler is the CEO and co-founder of Avtrak Inc., provider of the industry's first Internet-based and compliance-focused maintenance tracking service — Avtrak GlobalNet. Avtrak has announced, within the past two years, alliances with Gulfstream, Raytheon, and Sikorsky that have each selected Avtrak's GlobalNet technology for the next generation of their factory-provided maintenance tracking services, Gulfstream CMP.net, Raytheon FACTS, and Sikorsky Helotrac II.

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