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  2. Military Commercial Derivative Aircraft (MCDA): Selecting a Part 91 or Part 121 Maintenance Program

Military Commercial Derivative Aircraft (MCDA): Selecting a Part 91 or Part 121 Maintenance Program


Alternate Definition

Military Commercial Derivative Aircraft (MCDA) - A commercial off-the-shelf produced aircraft with a Federal Aviation Administration (FAA) Type Certificate (TC). The aircraft may be modified for use as a military aircraft. Military modifications may be fully or partially FAA-approved to civil statutes for the purpose of achieving type design certification as an input to overall airworthiness certification and to develop continued airworthiness requirements.

Title 14, Code of Federal Regulations Part 91 (14 CFR Part 91) - General Operating and Flight Rules, Subpart E - Maintenance, Preventive Maintenance, and Alterations, prescribes rules governing the maintenance, preventive maintenance, and alterations of U.S. registered civil aircraft operating within or outside of the United States.

14 CFR Part 121 - Operating Requirements: Domestic, Flag, and Supplemental Operations, applies to each person who holds or is required to hold an Air Carrier Certificate or Operating Certificate rules that refer to the scheduled air carriers. Subpart L - Maintenance, Preventive Maintenance, and Alterations, prescribes requirements for maintenance, preventive maintenance, and alterations for all certificate holders. Each certificate holder must provide for surveillance of facilities and practices to assure that all work performed is accomplished in accordance with the certificate holder's manual.

General Information


The purpose of this article is to:

  • Provide a brief overview of Military Commercial Derivative Aircraft (MCDA) policy
  • Provide an overview of 14 CFR Parts 91 and 121 requirements, including safety aspects
  • Address MCDA culture impacts for the Military Services
  • Identify decision points for determining a maintenance program path
  • Discuss considerations for adopting a baseline 14 CFR Part 91 strategy


Air Force (AF) Policy Directive (AFPD) 62-6, Department of the AF Instruction (DAFI) 62-601, and Military Handbook (MIL- HDBK)-516C all state that when a military mission is compatible with a certified civil usage, the AF will utilize FAA type certified MCDA to the maximum extent practical and that the USAF will leverage Airworthiness (AW) approvals issued by the FAA and other military AW authorities as a basis for AF approval. Department of the Navy policy is located in Commander, Naval Air Forces Instruction (COMNAVFORINST) 4790.2D, Naval Aviation Maintenance Program (NAMP), Chapter 11. MCDA are initially approved for safety of flight by the FAA and may have an FAA approved Type Certificate (TC) of AW.  One important acquisition decision in a program regarding AW is the decision to development the maintenance program under 14 CFR Part 121 or 14 CFR Part 91 (hereafter, "Part 121" and "Part 91").

Part 121 is generally considered the most restrictive and highest commercial air travel safety standard of three distinct parts (Part 121, Part 91, and Part 135), with Part 91 the least restrictive. All Part 91 requirements apply to a Part 121 operator, but there are more restrictive aspects under Part 121 that must be followed.  14 CFR Part 135, which covers commuter aircraft, is not addressed as it is not a consideration for an MCDA program.

Table 1 and Table 2 below highlight key aspects and elements of Part 91 vs Part 121 maintenance programs. Part 91 Subpart E and Part 121 Subpart L contain the maintenance requirements for each specific category. FAA Advisory Circular (AC) 91-91Maintaining Public Aircraft, describes responsibilities for an owner/operator who engages in the operation and maintenance of public aircraft as part of a governmental function. AC 91-91 also outlines the minimum requirements for maintaining a Part 91 maintenance program.  AC 120-16GAir Carrier Maintenance Programs, describes 10 elements that should be included in a Part 121 air carrier maintenance program. These elements encompass AW responsibility, maintenance manuals, maintenance organization, accomplishment and approval of maintenance and alterations, maintenance schedule, Required Inspection Items (RII), maintenance recordkeeping system, contract maintenance, training, and Continuing Analysis and Surveillance System (CASS).  Even if a military program selects Part 91, the FAA highly encourages developing increased program standards under Part 121 and AC 120-16G.

Part 121 and Part 91 requirements.

Table 1. 14 CFR Part 121 and Part 91 Requirements

Maintenance and inspection requirements.

Table 2. Maintenance and Inspection Program Differences Between 14 CFR Part 121 and Part 91.



With the introduction of new ideas and changes to policy there comes a needed shift in culture. The ideas and concepts that need to be embraced to implement change have always been open to scrutiny.  Under current policy and regulations, it could be argued the Military Services are more suited to be Part 91 operators than Part 121 operators due to MCDA programs with unique mission sets. While some constructs of Part 121 have been embraced and implemented there has been some difficulty to adapt the more stringent airworthiness compliance requirements.  Appling a methodology of differentiating between FAA policy that will and will not be followed can be a roadblock to overall compliance. This approach results in a cultural mentality that can lead to confusion, which in turn can lead to undesired long-term impacts to maintaining AW. 

A decision to pursue Part 91 does not mean abandoning the Type Certification processes and the commitment to maintaining AW, nor capitalizing on the acquisition of CDA platforms and synergizing with Industry. Obtaining Supplemental Type Certification (STC) for modifications to the commercial baseline would still be accomplished.  The need for FAA Military Certification Office (MCO) support would remain but may be reduced based on levels of effort and requirements. 

As an example, as seen in Table 1, one key decision point to selecting a Part 121 strategy is the mandatory incorporation of a CASS program under the Continuing Airworthiness Maintenance Program (CAMP).  FAA AC 120-79ADeveloping and Implementing an Air Carrier Continuing Analysis and Surveillance System, provides greater detail as to what a CASS program entails. Under Part 91, a CASS program is NOT mandatory and the designated Lead Command can assume all responsibility for audit requirements without an officially sanctioned CASS office. In this case, taking a Part 91 approach allows for programs to make independent CASS office stand-up decisions. Currently, Military Service policies correlate well to CAMP required elements, and CASS have been evaluated under Meet the Intent (MTI) efforts and have gained FAA concurrence. Further information on CASS/CAMP and MTI can be found in the article Military Commercial Derivative Aircraft (MCDA) and Federal Aviation Administration (FAA) Approved Meet the Intent (MTI).

Program acquisition strategies can be more easily tailored and scaled under Part 91 to individual military program mission requirements while maintaining certification and AW requirements. The elements of CAMP/CASS as recommended by the FAA in AC 120-16G can be adopted and tailored under a Part 91 program and worked more freely within the unique military operational and mission constraints of those programs. The higher standards of safety that come with a Part 121 operation can be applied to Part 91 operations and is recommended by the FAA. A Military Service acquisition strategy and product support strategy to adopt a singular acquisition standard approach of pursuing only Part 91 AW certification could be cost-effective for MCDA platforms.  Seeking and gaining concurrence from the FAA MCO on a single Part 91 approach for current and future MCDA platforms in the form of a baseline AW acceptance would reduce individual program rework of MTI compliance efforts. This baseline standard could then be amended at the individual program level to establish requirements for the individual program unique mission sets.


Early in the formulation of acquisition and product support strategies for MCDA platforms the path for maintaining the platform must be determined. The decision to pursue a Part 91 or Part 121 maintenance program will impact program development as well as have lasting life cycle impacts. Since there is no substitute for safety, choosing either path will provide the necessary requirements to satisfy this imperative. The challenges of culture and training of MCDA program office, Warfighter, and product support integrator and provider workforces are being positively affected with recent DoD MCDA acquisitions, resulting in new lessons being learned. Updates to Military Service policy can establish the groundwork for a lasting commitment to the policy directives to maintain type certification and AW of MCDA platforms as closely to FAA regulations as possible. At the time of this article's publishing, there is still work to be done. Pursuing opportunities to streamline and baseline processes will aid in uniformity across the MCDA enterprise as well as potential manpower and cost savings.  Incorporating standard MCDA processes and determining the most effective maintenance sustainment strategies suited to the mission of a weapon system will ensure the overall success throughout the life cycles of all MCDA programs.

NOTE: This article was developed by a subject matter expert from the Air Force Life Cycle Management Center ISR/SOF Directorate (AFLCMC/WIL). It is approved for public release; distribution is unlimited.