Major Portion Requirement
Amateur Built Aircraft

A determination if the builder meets the major portion requirement is made by evaluating the amount of work accomplished by the amateur builder(s) against the total amount of work necessary to complete the aircraft, excluding standard procured items.

The major portion of the aircraft is defined as more than 50 percent of the fabrication and assembly tasks, commonly referred to as the “51-percent rule.”

An aircraft is not eligible for an experimental amateur-built certificate under FAR 21.191(g) if the major portion of the aircraft fabrication and assembly tasks are not completed by an amateur builder(s).


The FAA uses a document called the "Amateur-Built Aircraft Fabrication and Assembly Checklist (2009)" as an aid in determining compliance with the major portion requirement of FAR 21.191(g).

The current checklist is for fixed-wing aircraft, but the FAA plans to develop other checklists for other types of aircraft in the future.

Click here to download a copy of Advisory Circular (AC) 20-27. The checklist is contained in Appendix 8 of the AC.

Instructions for completion are included on the form. The Amateur-Built Aircraft Fabrication and Assembly Checklist (2009) must be used when -

- Commercial assistance was used by the amateur builder(s) during construction.

- The amateur builder made modifications to an aircraft kit included on the FAA List of Amateur-Built Aircraft Kits that potentially affects the major portion determination.

- The aircraft was built from prefabricated major components that are readily available from aircraft parts suppliers.

- The aircraft was built using any salvaged components or used parts from aircraft that have been type certificated.

- The aircraft was built from a kit that has not been evaluated or found eligible by the FAA.

- Providing guidance to a kit manufacturer to determine if a proposed amateur-built kit may meet the major portion requirement.

- There are questions that arise as to the determination of major portion.


Amateur builders may contract for commercial assistance, but should notify the FAA if they intend to use commercial assistance. Amateur builders may also receive commercial educational assistance in the fabrication or assembly of specific parts, and the completion of tasks or processes involved in the construction of an aircraft.

In some cases, this commercial assistance may be provided by kit manufacturers. The FAA may credit commercial assistance provided for educational purposes toward the major portion determination. However, this educational assistance cannot exceed a demonstration on how to perform the task.

The amateur builder needs to submit a notarized FAA Form 8130-12 , Eligibility Statement, certifying the major portion was fabricated and assembled for educational or recreational purposes. The form specifies that an amateur builder identify if commercial assistance was used in the construction of the aircraft, and identify the source of the assistance.

In addition, the amount of commercial assistance needs to be annotated on the Amateur-Built Aircraft Fabrication and Assembly Checklist (2009) for the specific make and model of aircraft. Evidence and records should be available to support these statements and provided to the FAA/DAR upon request.

Any fabrication or assembly tasks contracted to another party (for hire) or provided by a commercial assistance center, including commercial assistance provided by a kit manufacturer, must not prevent the amateur builder(s) from meeting the major portion requirement.

Fabrication knowledge is necessary for the FAA to issue the amateur builder a repairman certificate as the primary builder of the aircraft. This allows the amateur builder to maintain their aircraft as provided in FAR 65.104.

The FAA may request to observe fabrication and assembly activities at any commercial assistance facility to determine whether the project can meet the major portion requirement.

Go from this Major Portion Requirement Page back to the Experimental Amateur Built Certification Page

Share this page:
Enjoy this page? Please pay it forward. Here's how...

Would you prefer to share this page with others by linking to it?

  1. Click on the HTML link code below.
  2. Copy and paste it, adding a note of your own, into your blog, a Web page, forums, a blog comment, your Facebook account, or anywhere that someone would find this page valuable.

Aircraft Registration

Experimental Amateur Built Certification