AC 20-174 is the FAA advisory circular titled “Development of Civil Aircraft and Systems,” and it recognizes SAE ARP 4754A (dated December 21, 2010) as an acceptable method for establishing a development assurance process when certifying an aircraft or aircraft system. In plain terms, it gives manufacturers a structured way to show the FAA that a system’s design, from requirements through verification, was built with enough rigor to meet airworthiness regulations. It was written primarily against 14 CFR Part 25 for transport-category aircraft, but it may also apply to aircraft certified under Parts 23, 27, 29, 33, and 35.
One point to fix in place before going further: AC 20-174 is not mandatory. It describes an acceptable means, not the only means, of showing compliance with 14 CFR. The FAA has said the more structured techniques in ARP 4754A are not intended to replace traditional approaches where those have already proven acceptable for conventional designs. The more complex or novel the system, though, the more likely the FAA will expect to see the ARP 4754A framework in a certification plan.
How Development Assurance Levels Work
The framework starts with a system-level safety assessment that identifies how each function could fail and what the consequences would be. From that assessment, each function receives a Functional Development Assurance Level (FDAL). The FDAL then flows down to the individual hardware and software items that implement the function, and each of those receives an Item Development Assurance Level (IDAL).
Assurance levels run from Level A, for functions whose failure could be catastrophic and which therefore demand the most rigorous development process, down to Level E, for functions with no safety effect and the least required rigor. Your certification plan should document the assigned levels, the justification for each, and the specific ARP 4754A processes and objectives you intend to use.
The safety assessment that feeds these levels typically follows SAE ARP 4761, the companion standard that provides the analytical methods for identifying failure conditions and rating their severity. Where AC 20-174 tells you how rigorous development must be, ARP 4761 tells you how to figure out what level of rigor is warranted.
A caveat for Part 23 work: AC 23.1309-1 may differ from AC 20-174 on assurance levels, and where the two conflict, AC 23.1309-1 takes precedence. ARP 4754A should not be used to assign a lower assurance level than AC 23.1309-1 would require.
How AC 20-174 Connects to Software, Hardware, and Environmental Standards
AC 20-174 operates at the system level. Once assurance levels flow down to components, separate industry standards govern the detailed work, and the FAA recognizes each through its own advisory circular.
Software
Airborne software is developed and verified under RTCA DO-178C, “Software Considerations in Airborne Systems and Equipment Certification.” The FAA recognizes DO-178C through AC 20-115D. The IDAL assigned to a software component determines which DO-178C objectives must be met and how much independence is required in verification. Higher levels demand more extensive testing, code review, and structural coverage analysis.
Hardware
Complex airborne electronic hardware follows RTCA DO-254, “Design Assurance Guidance for Airborne Electronic Hardware,” recognized by the FAA through AC 20-152A. As with software, the IDAL sets the objectives and verification activities. Simple off-the-shelf components with well-established service history draw less scrutiny; custom programmable devices such as FPGAs get the full DO-254 treatment.
Environmental Qualification
All airborne equipment must also pass environmental qualification testing under RTCA DO-160, “Environmental Conditions and Test Procedures for Airborne Equipment.” DO-160 covers temperature extremes, vibration, humidity, altitude, power input variations, and electromagnetic interference. The FAA recognizes versions D through G of DO-160 through AC 21-16G as acceptable environmental qualification procedures.
Approval Paths That Use AC 20-174 Data
AC 20-174 defines the development assurance methodology. The actual airworthiness approval to install a system on an aircraft comes through mechanisms in 14 CFR Part 21, and the right path depends on the scope of the change and how many aircraft are involved.
Supplemental Type Certificate
A Supplemental Type Certificate (STC) is the usual path for a major design change, especially when the same system will be installed across a fleet. The applicant works with the FAA Aircraft Certification Office responsible for the product type through familiarization meetings, establishment of a certification basis, design evaluation, conformity inspections, and ground and flight testing. The development assurance work documented under AC 20-174 forms a key part of the data package submitted for FAA evaluation. The process concludes with the FAA approving the flight manual supplement and issuing the STC.
Parts Manufacturer Approval
A Parts Manufacturer Approval (PMA) is a combined design and production approval that lets a manufacturer produce and sell modification or replacement articles for installation on type-certificated products. PMA is not a path for approving a brand-new design from scratch; it lets a manufacturer produce parts that conform to a design already approved under a Type Certificate or STC. The PMA holder maintains a quality system that keeps production conforming to the approved data.
Field Approval
A field approval lets the FAA approve technical data for a major alteration or major repair on a single, specific aircraft, documented on FAA Form 337 and governed by FAA Order 8900.1. Contrary to a common misconception, field approvals are not for minor changes; the applicant must first determine that the change qualifies as a major alteration or repair under 14 CFR 1.1 and 14 CFR Part 43, Appendix A. Because a field approval covers only one serial-numbered aircraft, it is generally impractical for complex avionics that need the full development assurance treatment described in AC 20-174.
Documentation the FAA Expects
Whichever approval path applies, the data package for a system developed under AC 20-174 must cover design, operation, installation, and long-term maintenance.
Instructions for Continued Airworthiness
Instructions for Continued Airworthiness (ICA) describe how to maintain, inspect, and overhaul the system across its service life. For Part 23 aircraft, the requirement appears in 14 CFR 23.1529 (older Part 23 certifications) or 14 CFR 23.2529 (current performance-based standards). The ICA must be acceptable to the FAA Administrator and may be incomplete at type certification as long as a program is in place to finish them before first aircraft delivery or issuance of a standard airworthiness certificate, whichever comes later. Comparable ICA requirements exist under Parts 25, 27, and 29.
Flight Manual Supplement
A flight manual supplement gives the crew the operational procedures, limitations, and performance information specific to the installed system. For an STC, the FAA approves the supplement as one of the final steps before issuing the certificate, and it becomes a required part of the aircraft’s approved flight manual.
Engineering and Installation Data
The engineering data package is the legal basis of the approved design change. It includes installation drawings, wiring schematics, structural modification data, and system interface documentation. Installation must follow standard aircraft practices and show non-interference with existing systems, typically demonstrated through electromagnetic compatibility testing and High-Intensity Radiated Fields (HIRF) testing. Under 14 CFR 25.1317, for example, electrical and electronic systems whose failure could prevent continued safe flight must be designed and installed to withstand prescribed HIRF environments without adverse effects. Similar HIRF protection requirements exist in the other certification parts.
Working With a Designated Engineering Representative
Most certification projects rely on Designated Engineering Representatives (DERs) to carry the technical review workload that would otherwise overwhelm FAA engineering staff. A DER is an individual appointed under 14 CFR 183.29 who holds an engineering degree or equivalent and has the technical background to evaluate compliance data. DERs can approve, or recommend approval of, technical data to the FAA.
Company DERs work inside the applicant’s organization and can approve or recommend approval of technical data only for their employer. Consultant DERs are independent and can work with any applicant. The FAA maintains a Designee Locator tool for finding consultant DERs by specialty. On a complex project following AC 20-174, engaging a DER early is one of the most effective ways to keep detailed engineering review moving in parallel with development instead of waiting on FAA staff availability.
Where AC 20-174 Fits for NORSEE
AC 20-174 applies broadly to system development assurance and is not limited to any particular category of equipment, but it often comes up around Non-Required Safety Enhancing Equipment (NORSEE): equipment not mandated by regulation but installed to improve safety, such as angle-of-attack indicators, synthetic vision displays, and enhanced GPS navigators for general aviation.
The FAA has adopted separate policies to streamline NORSEE installations in the general aviation fleet, reducing cost and approval burden for equipment whose failure would not create a hazardous condition. For simple NORSEE installations, the full ARP 4754A process described in AC 20-174 may not be necessary. For more complex non-required systems, particularly those involving sophisticated software or tight integration with existing avionics, the structured methodology in AC 20-174 remains the most reliable way to show the system works as intended and does not introduce new risks to the aircraft.