How Kits And The New Mosaic Rules Work
By Gene Yarbrough
In recreational aviation, a common point of confusion surrounds how the Federal Aviation Administration’s (FAA) Modernization of Special Airworthiness Certification (MOSAIC) rule interacts with the long-standing "51% rule."
While MOSAIC dramatically expands the capabilities, weight, and seating of light-sport aircraft, it does not subject those aircraft to the 51% amateur-built requirement. Understanding why requires examining the distinct regulatory frameworks governing Experimental Amateur-Built (E-AB) aircraft versus Light-Sport Category Aircraft (LSA) and Experimental Light-Sport Aircraft (E-LSA).
The Origin and Purpose of the 51% Rule
The 51% rule, formally known as the "major portion" requirement under Title 14 of the Code of Federal Regulations (14 CFR) § 21.191(g), applies exclusively to Experimental Amateur-Built aircraft. Under this regulation, an individual or group must fabricate and assemble more than 50 percent of the aircraft’s components solely for their own education or recreation. Because the FAA does not inspect or approve the engineering designs or factory manufacturing processes of E-AB aircraft, the 51% rule exists to prevent commercial companies from selling virtually complete, ready-to-fly airplanes as "kits" to bypass standard certification standards. In exchange for meeting this labor threshold, amateur builders gain immense freedom to modify designs, install custom avionics, and perform their own maintenance.
How MOSAIC Regulates Aircraft Certification
By contrast, the FAA MOSAIC final rule, which took effect across two phases in October 2025 and July 2026, governs Light-Sport Category Aircraft under 14 CFR § 21.190 and the newly established 14 CFR Part 22. Instead of relying on amateur labor to justify special airworthiness status, MOSAIC relies on industry consensus standards—such as those developed by ASTM International—and manufacturer accountability.
Under MOSAIC, a factory-built Light-Sport Category Aircraft (often referred to as S-LSA) is designed, manufactured, and flight-tested by a commercial entity. Rather than proving that an amateur built the major portion of the airframe, the manufacturer must sign a Statement of Compliance attesting that the aircraft meets FAA-accepted consensus standards for design, production quality, and continued operational safety. Because safety oversight is anchored in manufacturer compliance and consensus engineering rather than personal builder education, the 51% rule is entirely irrelevant to factory-built MOSAIC aircraft.
Kit-Built Light-Sport Aircraft (E-LSA) Under MOSAIC
Confusion often arises when builders purchase kits intended for Experimental Light-Sport Aircraft certification under 14 CFR § 21.191(k). Under MOSAIC, an individual can still assemble a kit-built LSA without adhering to the 51% rule, regardless of how much pre-fabrication the factory completed. However, this exemption comes with a strict trade-off. To qualify for an E-LSA certificate without meeting the major portion test, the kit must be a direct duplicate of an existing, consensus-compliant Light-Sport Category Aircraft produced by that same manufacturer. Furthermore, the builder must assemble the kit strictly according to the manufacturer's instructions without unapproved modifications. If a builder wishes to deviate from factory plans or build a design that lacks a consensus-compliant factory counterpart, they cannot use the MOSAIC E-LSA pathway and must instead satisfy the traditional 51% rule under E-AB regulations.
Distinct Pathways on the FAA Safety Continuum
Ultimately, the 51% rule does not apply to MOSAIC aircraft because the two frameworks occupy different rungs on the FAA's safety continuum. The E-AB category trades design oversight for builder labor and creative freedom. Conversely, MOSAIC trades builder freedom for consensus standards and manufacturer responsibility, allowing pilots to access faster, heavier, four-seat aircraft with modern features while keeping factory production distinct from homebuilding.
