Fabricated parts and alterations

Making a part instead of waiting

Not every maintenance task can wait for a replacement part to arrive from the manufacturer. In some cases, a repair station or maintenance organization may fabricate a replacement part as part of an approved repair or alteration. Because these parts become part of a certificated aircraft, their design, manufacture and installation must provide a level of safety equivalent to the original approved configuration.

The key distinction: a fabricated part is made to support a specific repair or alteration and is consumed during that work. It is not manufactured for sale as a replacement aircraft part.

Fabricating parts during maintenance

Fabrication is often faster and more practical than waiting for a replacement component, particularly when the part is simple and its design is already defined by approved data. Maintenance organizations commonly fabricate items such as:

• Doublers
• Brackets
• Clips
• Shims
• Gussets
• Reinforcement plates
• Formed sheet-metal parts
• Non-complex structural details

Fabrication still demands airworthiness

Fabricating a part does not remove the requirement to ensure its airworthiness, conformity, traceability and structural suitability. The fabricated part must conform precisely to the approved design data.

What is a fabricated part?

The authority to fabricate a part comes from the approved repair or alteration data, not from the act of manufacturing the part itself. According to the guidance of AC 43-18, a fabricated part:

• Is manufactured during maintenance.
• Is installed by the organization that fabricated it.
• Is used only in the repair or alteration for which it was made.
• Is not produced as a spare for inventory or sale.

Three categories of fabricated parts

AC 43-18 categorizes fabricated parts according to the consequences of their failure. The more severe the potential consequences, the more stringent the approval requirements become.

Category 1 parts

Category 1 represents the highest level of criticality. Failure of the part could prevent continued safe flight and landing. These parts are associated with major repairs or alterations involving highly critical structures or systems, such as primary structural components, flight-control load-path components and critical system supports.

The design data must receive FAA approval. Although a designated engineering representative may develop or recommend the data, final approval authority rests with the FAA. Because these parts affect the aircraft's highest safety levels, they receive the greatest scrutiny.

Category 2 parts

Category 2 parts occupy the middle level. Examples may include certain secondary structural components, non-primary system brackets and equipment installation structures. Failure would not prevent continued safe flight and landing but could:

• Reduce aircraft capability
• Increase crew workload
• Reduce safety margins
• Affect system redundancy

Category 2 approval

The design data may be approved by the FAA, or by a properly authorized designated engineering representative or equivalent approval authority. The approval path is less restrictive than Category 1 but still requires authorized engineering oversight.

Category 3 parts

Category 3 parts have the lowest safety impact. Failure would have no effect on continued safe flight and landing. Examples may include non-structural supports, interior attachments and limited secondary hardware.

Acceptable data is generally sufficient. Approved engineering data is not typically required if the applicable regulations and guidance permit the fabrication and installation. Even so, the part must still conform to the specified design and workmanship requirements.

What the fabricator must have

Fabrication is not simply cutting metal to fit. A proper fabrication process requires complete technical information.

• Design data: drawings, dimensions and specifications defining shape, configuration, material, thickness, finish and installation method. Without complete design information, conformity cannot be assured.
• Material specifications: the correct alloy, temper, composite material and heat treatment condition must be known and verified. Substituting materials without approval can invalidate the repair or alteration.
• Process requirements: some fabricated parts require controlled processes such as heat treatment, forming operations, composite curing, welding and surface treatments. The fabrication instructions must identify these requirements.

Inspection and testing

Additional testing may be required depending on the complexity and criticality of the part. Fabricated parts must be inspected to verify:

• Dimensional accuracy
• Material conformity
• Manufacturing quality
• Compliance with design requirements

Quality control requirements

A maintenance organization fabricating parts should have a documented quality system. The objective is to ensure repeatable and verifiable fabrication quality. It controls:

• Materials
• Tooling
• Fabrication methods
• Inspection
• Documentation
• Training

Control of subcontractors

If portions of the fabrication are subcontracted, responsibilities must be defined, quality oversight must be maintained, and the final product must still conform to the approved design. The repair station or maintenance organization remains responsible for the airworthiness of the finished part.

Repairs vs. alterations

Repairs and alterations are not the same. Understanding the difference is important because the approval requirements often differ.

• Repair: restores an aircraft to its approved condition after damage or deterioration, such as structural repairs, corrosion repairs, composite damage repairs and replacement of damaged structural details. The goal is restoration.
• Alteration: changes the aircraft from its original approved configuration, such as installing new avionics, adding antennas, installing lighting systems, modifying interiors, installing surveillance equipment and adding performance-enhancing equipment. The goal is modification rather than restoration.

AC 43.13-1B and AC 43.13-2B

These two advisory circulars serve different purposes. Together, they form a foundation for much general aviation repair and alteration work.

• AC 43.13-1B primarily addresses inspection practices, repairs, structural restoration and standards of workmanship. When repairing an aircraft, this is often the first reference.
• AC 43.13-2B primarily addresses aircraft alterations, equipment installations, system modifications and structural changes associated with alterations. Typical subjects include radio, antenna and lighting installations and electrical-system modifications.

Engineering considerations for alterations

An alteration affects more than the equipment being installed. Every alteration must be evaluated as a complete aircraft change, not merely as a new piece of equipment. Engineers must consider:

• Structural effects: can the structure support the additional loads?
• Electrical effects: can the electrical system power the new equipment safely?
• Cooling requirements: will additional heat be generated?
• Electromagnetic compatibility: will the new equipment interfere with existing systems?
• Weight and balance: how does the installation affect aircraft loading and CG?

Major and minor alterations

Not all alterations require the same level of approval.

• Minor alterations typically do not appreciably affect weight and balance, structural strength, performance, powerplant operation or flight characteristics. They can be accomplished using acceptable data and the applicable maintenance procedures.
• Major alterations can significantly affect structure, performance, flight characteristics, systems or airworthiness. They require approved data, and documentation requirements are usually more extensive.

FAA Form 337

Major repairs and major alterations are commonly documented using FAA Form 337. The completed record becomes part of the aircraft's permanent maintenance history. The form records:

• Work performed
• Approved data used
• Aircraft identification
• Approval information

Supplemental type certificates (STCs)

When the same alteration is performed repeatedly on multiple aircraft, the preferred solution is often a supplemental type certificate. An STC approves a modification to the original type design. Instead of creating new engineering data for each aircraft, operators can use the existing STC package. An STC provides:

• Standardized data
• Repeatable installation procedures
• Regulatory approval
• Continued airworthiness instructions

Documentation and continued airworthiness

Both fabricated parts and alterations must be documented properly. Documentation typically includes fabrication records, material traceability, drawings, inspection results, installation records and maintenance entries.

Some alterations also create new inspection requirements, airworthiness limitations and continued airworthiness instructions. These become part of the aircraft's maintenance program.

Key takeaways

• Fabricated parts are made during maintenance and consumed in the repair or alteration for which they are created.
• They are not produced for inventory or sale as replacement parts.
• Category 1, 2 and 3 classifications are based on the consequences of part failure.
• The category determines the level of design approval required.
• Fabricators need complete design, material, processing and inspection data.
• A documented quality-control system is essential for fabricated parts.
• Repairs restore an aircraft; alterations change its approved configuration.
• AC 43.13-1B primarily addresses repairs, while AC 43.13-2B focuses on alterations.
• Major alterations require approved data and are commonly documented on FAA Form 337.
• STCs provide an efficient approval path for alterations repeated across multiple aircraft.

The bottom line

Fabricating a part during maintenance can be an efficient and completely acceptable way to support a repair or alteration, provided the part is built in accordance with the appropriate design data and quality controls. The critical question is not who made the part, but whether it conforms to approved data and provides the required level of safety. Likewise, alterations must be evaluated as complete aircraft changes, with proper engineering approval, documentation and continued-airworthiness considerations. In both cases, the goal is the same: maintaining the aircraft's approved level of airworthiness while ensuring safe and reliable operation.