Annex K: Additional Considerations for PBN Design Changes to Legacy Military Aircraft

1. General

1.1 This annex provides guidance on certifying PBN design changes to legacy military avionics systems.

2. Rationale and Guidance

2.1 Military aircraft are often developed using military avionics design standards that may differ from the civil aviation standards assumed by the authors of AC 20-138 (reference 3.2.2.a). In particular, this is the case for legacy military navigation and flight management systems.

2.2 The addition of a PBN capability to legacy military avionics systems may result in certification issues that have not been anticipated in AC-20-138D. The following are examples of issues that may be encountered:

1. Certification Basis. The certification basis examples shown in Annex B use the civil certification requirements.  For legacy military aircraft that were designed to military standards and not civil certification standards, there is the option to develop the PBN certification basis using military airworthiness codes, such as U.S. DoD Mil-Hdbk-516, or European Military Airworthiness Certification Criteria (EMACC), as described in Part 1, Chapter 2 of the ADSM, (regulatory reference 3.2.1.b).
2. Non-TSO Systems. Most of the criteria used in AC 20-138D assume the use of GPS receivers and Flight Management Systems (FMS) that have been certified to FAA Technical Standard Orders (TSOs), or equivalent.  A TSO is a minimum performance standard published by the FAA for specified materials, parts, processes, and appliances used on civil aircraft. The equipment tables in Part 3, Chapter 3 of regulatory reference 3.2.1.b provide a listing of these TSOs that apply to GPS receivers, FMS and other navigation systems.  The absence of a TSO for the navigation and flight managements systems on DND/CAF aircraft may be problematic.  Examples of such potential problems include the following:
   1. Lack of the required equipment redundancy, e.g., dual GNSS receivers;
   2. Military avionics software not developed to civil software or complex electronic hardware development standards, such as DO-178 or DO-254 listed in Part 2, Chapter 4 of the ADSM (regulatory reference 3.2.1.b);
   3. FMS functions embedded in aircraft mission computers that provide an FMS-like capability, but may not include all of the functionality required for PBN;
   4. Flight instrument display systems that do not provide the resolution or information display capability specified in AC 20-138D; and
   5. Avionics systems that do not have the data validation and verification processes required by the PBN criteria to maintain the NavDB Integrity throughout all phases of the data handling process. This may be an issue where the NavDB consists of Digital Aeronautical Flight Information File (DAFIF) formatted data, in full or part, from the U.S. National Geospatial-Intelligence Agency.
3. Additional Requirements for Non-TSO Systems. Avionics equipment that has not been certified to the FAA TSOs can be replaced with Military Standard Order (MSO) certified equipment.  For equipment that has not been certified to either a TSO or a MSO, the following will apply:
   1. AC 20-138D makes provisions for the use of non-TSO systems.  The process described in AC 20-138D Change 2, Section 7-1.b (1) may be used to achieve airworthiness approval for PBN capabilities when the navigation equipment does not have a TSO/MSO-approval.  The design must be evaluated to show compliance with the requirements of RTCA DO-236C, Change 1 (reference 3.2.2.f). In addition, the TAA requires that the design must also be assessed against the requirements of RTCA DO-283B (reference 3.2.2.e), as modified by TSO C115d (reference 3.2.2.g), and any deficiencies are documented.
   2. When the design incorporates non-TSO/MSO sensors, or a non-TSO/MSO navigation computer (that provides flight management functions), the AC-20-138D criteria may need to be augmented or replaced with criteria from one or more of the following three documents:  RTCA DO-236, RTCA DO-283 and RTCA DO-229 (reference 3.2.2.m).  Note that the RTCA documents do not refer to particular navigation specifications, and must therefore be addressed in their entirety.
   3. In some instances, the lack of TSO/MSO systems may only be addressed by one of the following measures:
      1. Replacing the avionics system with TSO/MSO compliant systems;
      2. Reducing the approved PBN capability to align with the existing avionics system capabilities; or
      3. Employing operational procedures to compensate for the missing avionics system capabilities.