AC 20-138D

Table 6. Vertical Path Performance Limit for Baro-VNAV88

Table 7. Vertical Path Deviations89

Table 8. Typical Hazard Classifications93

Table 9. RNP FTE Performance132

Table 10. INS/IRU Accuracy After Loss of Radio Updating148

Table 11. Tailwind Component (VKTW) for Turn CalculationsA2-9

Table 12. Platform Dynamics for Horizontal Velocity Accuracy TestA4-3

Table 13. Platform Dynamics for Vertical Velocity Accuracy TestA4-6

Table 14. Alpha Departure WaypointsA7-4

Table 15. Bravo Departure WaypointsA7-5

Table 16. Arrival WaypointsA7-7

Table 17. Approach 1 WaypointsA7-9

Table 18. Approach 1 Vertical Error BudgetA7-9

Table 19. Approach 2 WaypointsA7-11

Table 20. Approach 2 Vertical Error BudgetA7-11

Table 21. Approach 3 WaypointsA7-13

Table 22. Approach 3 Vertical Error Budget7-13

Table 23. Approach 4 Waypoints7-15

List of Figures

Figure 1. RNAV Depiction6

Figure 2. RNP Depiction7

Figure 3. Minimum Straight Path Before DAA2-20

Figure 4. Example Procedure ProfilesA7-2

Figure 5. Alpha DepartureA7-4

Figure 6. Bravo DepartureA7-5

Figure 7. ArrivalA7-6

Figure 8. Approach 1A7-8

Figure 9. Approach 2A7-10

Figure 10. Approach 3A7-12

Figure 11. Approach 4A7-14

ix

-- Chapter 1. General Information.

1-1. Purpose.

This advisory circular (AC) provides guidance material for the airworthiness approval of installed positioning and navigation equipment. Positioning and navigation equipment may be used for a variety of functions such as navigation, automatic dependent surveillance, and/or terrain awareness and warning systems. This AC addresses the following equipment:

a. Global positioning system (GPS) sensors or stand-alone navigation equipment, including those incorporating aircraft-based augmentation system (ABAS), satellite-based augmentation system (SBAS), or ground-based augmentation system (GBAS).

Note: For standardization within this document, the acronyms GPS/SBAS and GPS/GBAS are used to indicate SBAS augmenting GPS and GBAS augmenting GPS respectively.

b. Area navigation (RNAV) integrating data from multiple navigation sensors such as global navigation satellite system (GNSS), inertial reference units (IRU), and distance measuring equipment (DME).

c. RNAV intended for required navigation performance (RNP) operations, including advanced functions and RNP authorization required (AR).

Note: RNP AR was previously named RNP special aircraft and aircrew authorization required (SAAAR). The name was changed to RNP AR for international harmonization, but might not yet be standardized throughout all documents.

d. Barometric vertical navigation (baro-VNAV) equipment.

e. Appendix 8 provides preliminary information for manufacturers that wish to add new satellite constellations or additional frequencies to their GNSS capability. This AC will be updated as new documentation is developed for multi-constellation equipment.

f. This AC supersedes the airworthiness considerations contained in all revisions of AC 90-101, Approval Guidance for RNP Procedures with AR, and AC 90-105, Approval Guidance for RNP Operations and Barometric Vertical Navigation in the U.S. National Airspace System.

Equipment previously approved per the airworthiness guidance in the 90-series ACs is still valid for the operations and conditions stated in their approvals. This AC does not replace the operational guidance found in the RNP 90-series ACs, but does replace the airworthiness guidance in the 90-series ACs. Also, this AC replaces the ACs listed in paragraph 1-3 by incorporating and updating the information they contained.

g. This AC is not mandatory and does not constitute a regulation. This AC describes an acceptable means, but not the only means, to install positioning and navigation equipment.

However, if you use the means described in the AC, you must follow it in all important respects.

h. Mandatory terms such as “must” used in this AC are only applicable when following the compliance method outlined. Other terms such as “should” or “recommended” are acceptable compliance methods applicants are encouraged to use. The term “may” is used for optional methods.

i. This AC revision is not intended to modify, change or cancel equipment design or airworthiness approvals previously in existence. However, new installation approvals (TC, ATC, STC or ASTC) should follow the guidance in this AC.

j. The guidance in this AC cannot ensure compliance with Title 14 of the Code of Federal Regulations (CFR) parts 23, 25, 27, and 29 airworthiness regulations for every possible installation configuration. The applicant remains responsible for regulatory compliance and should work closely with their geographic aircraft certification office (ACO) to ensure regulatory compliance.

k. In lieu of following this method without deviation, the applicant may elect to follow an alternate method, provided the alternate method is also found by the Federal Aviation Administration (FAA) to be an acceptable means of complying with the requirements of the federal aviation regulations 14 CFR. This approval is typically achieved through a project specific issue paper.

1-2. Audience.

This AC is for aircraft manufacturers, avionics manufacturers, installation shops, or other applicants seeking design and/or airworthiness approval for the positioning and navigation systems described in this document.

1-3. Cancellation.

a. This AC supersedes the ACs listed below. Equipment previously approved per the guidance in the superseded ACs is still valid for the operations and conditions stated in their approvals. The following documents are cancelled:

(1) AC 20-129, Airworthiness Approval of Vertical Navigation (VNAV) Systems for use in the U.S. National Airspace System (NAS) and Alaska.

(2) AC 20-130A, Airworthiness Approval of Navigation or Flight Management Systems Integrating Multiple Navigation Sensors.

(3) AC 20-138D Change 1, Airworthiness Approval of Positioning and Navigation Systems.

(4) AC 25-4, Inertial Navigation Systems.

b. This AC supersedes the airworthiness considerations in AC 90-45A, Approval of Area Navigation Systems for use in the U.S. National Airspace System. Equipment previously approved per the airworthiness guidance in AC 90-45A is still valid for the operations and conditions stated in their approvals.

1-4. Frequently Asked Questions.

This paragraph contains a list of questions concerning the guidance material in this AC.

The responses to these questions are intended to help applicants interpret the guidance contained in this AC.

a. Ground Proximity Warning System/Terrain Awareness and Warning System (GPWS/TAWS) Mode 5 alert.

(1) Question: Is a GPWS or Class A TAWS Mode 5 alert a hard requirement to add localizer performance with vertical guidance (LPV) or GBAS landing system (GLS) capability?

(2) Answer: No. It is not the FAA’s intent to burden manufacturers and operators by changing the GPWS or TAWS functions. The intent is simply to recommend a glidepath alerting function to help prevent controlled flight into terrain during LPV, LNAV/VNAV, and GLS approaches. See paragraphs 15-7.8 and 15-9.13 for clarification and additional guidance.

b. Class B TAWS and LPV or GLS Capability.

(1) Question: Is a low glideslope alerting function for LPV or GLS required with Class B TAWS for compliance with the guidance in paragraphs 15-7.8 and 15-9.13?

(2) Answer: No. It is not the FAA’s intent to burden manufacturers and operators by changing the GPWS or TAWS functions. The intent is to simply recommend a glidepath alerting function to help prevent controlled flight into terrain during LPV, LNAV/VNAV, and GLS approaches.

c. Using the term vertical navigation (VNAV).

(1) Question: Doesn’t RTCA/DO-229D assign a very specific meaning to the term ‘VNAV’ and dictate the term’s specific uses for SBAS-based vertical navigation?

(2) Answer: No. RTCA/DO-229D, section 2.2.1.1.7 provides a recommended usage for the ‘VNAV’ label, but only requires that the label be used in a consistent manner.

‘VNAV’ can be used for an LNAV/VNAV approach procedure generated by GPS/SBAS-based or barometric-based vertical navigation capability (GPS/GBAS does not provide an LNAV/VNAV approach capability). The ‘VNAV’ label can also be used for an advisory vertical navigation function that provides vertical path deviation guidance indications on a 3

non-essential, not-required basis. Refer to the following AC 20-138D, chapters for additional information:

• Chapter 4, Advisory Vertical Guidance;

• Chapter 11, Baro-VNAV;

• Chapter 12, section 12-11, General Human Factors Considerations;

• Chapter 15, section 15-7.6, GPS/SBAS Equipment Annunciations; and,

• Chapter 18, section 18-5, Baro-VNAV integration with GNSS-Provided Vertical Guidance.

d. Tightly-coupled inertial navigation system/global positioning system (INS/GPS).

(1) Question: AC 20-138D, section 6-7 c. includes the following statement: "The FAA assumes that there is a limitation on the use of the GPS/inertial position data output to support enroute through lateral navigation (LNAV) approach only." Can you please explain the basis of this assumption? It appears to exclude the use of tightly coupled INS/GPS for oceanic use. Is this the intent? Or, is the intent to exclude it for use for LPV?

(2) Answer: Paragraph 6-6 addresses loosely-coupled systems and paragraph 6-7 addresses tightly-coupled systems. There is no intent to exclude either loosely-coupled or tightly-coupled INS/GPS from oceanic use. However, un-augmented GPS, even with tightlycoupled INS, is not adequate for vertical approach capability, and hybrid inertial vertical approach capability is not addressed in RTCA/DO-229D, appendix R. As stated in this AC, a GPS-based vertical approach capability requires GPS/SBAS or GPS/GBAS augmentation.

e. Flight management system (FMS) limitations.

(1) Question: AC 20-130A, section 5.d.(2) contained a limitation on FMS use and required an alternate means of navigation for the route flown. AC 20-138D does not include this section or make a mention of this limitation. What was the reasoning behind not including a similar section in AC 20-138D? Is there some higher level guidance that addresses this or is it left up to the safety analysis to address the need for an alternate means of navigation?

(2) Answer: It is important to differentiate between operational limitations and equipment performance limitations. AC 20-138D addresses equipment performance limitations while the 90-series ACs produced by Flight Standards address operational limitations (for example, 90-100, 90-101A, 90-105 for RNAV, RNP AR, and RNP respectively) as well as the Aeronautical Information Manual and operational rules (parts 91, 121, 135).

(a) AC 20-130A was written prior to implementing performance-based navigation concepts and the evolving RNAV/RNP-based National Airspace System. An FMS incorporating multiple navigation sensor inputs by its very nature has multiple sources of navigation, but the individual sensors have performance limitations on their navigation capabilities. Accordingly, the FMS functions will meet different requirements depending upon the sensors involved and the intended navigation capability. This AC addresses the performance limitations of the individual sensors contained in the document. These performance limitations 4

must be considered for the overall intended function at the aircraft level during the airworthiness approval (i.e., during the type certificate (TC), amended type certificate (ATC), or supplemental type certificate (STC) approval) to determine any limitations on the aircraft.

(b) In summary, operational rules dictate the navigation equipage requirements. It is up to the operator to determine how to comply based on their specific operating rules and mission needs.

f. Satellite Navigation Terminology.

(1) Question: Why are so many different names and acronyms (such as GNSS, WAAS, SBAS, etc.) used for GPS?

(2) Answer: The different names and acronyms used depend upon whether the topic is a generic, international reference to satellite navigation, or a particular State’s specific implementation.

(a) GNSS is used internationally to indicate any satellite-based positioning system or augmentation system. The acronym ‘GNSS’ includes satellite constellations, such as GPS, GLONASS, Galileo, or Beidou, along with augmentation systems such as ‘SBAS’ and ‘GBAS’; all of which provide a satellite-based positioning service.

(b) The acronyms ‘SBAS’ and ‘GBAS’ are the respective international designations for satellite-based and ground-based augmentation systems complying with the International Civil Aviation Organization (ICAO) standards and recommended practices (SARPs). Several countries have implemented their own versions of ‘SBAS’ and ‘GBAS’ that have specific names and acronyms. For example, WAAS is the U.S. implementation of an ‘SBAS’ while EGNOS is the European implementation. Refer to Appendix 9 for definitions of terms and acronyms.

Note: This AC uses the acronyms GPS/SBAS and GPS/GBAS throughout for consistency and clarity to indicate augmentation systems for the GPS constellation.

g. RNAV versus RNP.

(1) Question: Are technical standard order (TSO)-C129(AR) Class A1 GPS and TSO-C146(AR) Class Gamma GPS/SBAS equipment RNAV or RNP systems?

(2) Answer: Both. RNAV systems conform to the ICAO performance-based navigation specification for total system error (TSE). RNAV total system error is the 95% probability that the navigation system accuracy remains within the limits defined for the RNAV operation. For example, during an RNAV-1 operation the TSE remains within one nautical mile of the desired path 95% of the time (see Figure 1 below). RNP systems conform to a performance-based navigation specification based on RNAV capability that also includes requirements for on-board performance monitoring and alerting. For example, during an RNP 5

1 Nautical Mile 95% of flight time --T-rack Centerline~ 1 Nautical Mile 95% of flight time 4/7/16 AC 20-138D Change 2

1.0 operation, the TSE remains within one nautical mile of the desired path 95% of the time, and on-board performance monitoring provides the pilot with an alert when the probability that TSE exceeds 2xRNP is greater than 10-5 (see Figure 2 below). RNP is an RNAV subset that also includes a requirement to provide on-board navigation system accuracy performance monitoring and alerting which means an RNP system is also an RNAV system. GNSS equipment provides accuracy performance monitoring and alerting which, by definition, makes it both an RNAV and RNP capable system.

Note: TSO-C129(AR) Class B/C, TSO-C145(AR), and TSO-C196(AR) sensors provide both RNAV and RNP capability when interfaced to an appropriate navigation computer (such as TSO-C115d).

(a) By definition, TSO-C129(AR) Class A1 and all TSO-C146(AR) Class Gamma GNSS equipment provides an RNAV capability that includes on-board performance monitoring and alerting. This equipment has the capability to perform RNAV(GPS) approaches to at least the LNAV line of minima.

(b) RNAV(GPS) approaches require GPS, which includes on-board performance monitoring and alerting. Therefore, an RNAV(GPS) approach is an RNP procedure where the initial, intermediate, and missed approach segments are RNP 1.0. The LNAV final approach segment is RNP 0.3.

(c) None of the preceding statements should be confused with RNP AR that requires special aircraft and aircrew approval. Neither TSO-C129(AR) Class A1 GPS nor TSO-C146(AR) Class Gamma GPS/SBAS can qualify for RNP AR operations without additional aircraft and aircrew approvals (see appendix 2). No RNP AR procedures can be included in the navigation databases of equipment that is not approved for RNP AR operations.

RNP AR approach procedures are titled RNAV(RNP) and have an “authorization required” designation printed on the charts.

Figure 1. RNAV Depiction 6

RNP 1 RNP 1 Alert to Pilot ne Track Centerline 1 N ne 1 Nautical Mile 95% of flight time 1 Nautical Mile 95% of flight time Track Centerline 1 Nautical Mile 99.999% of flight time 1 Nautical Mile 99.999% of flight time The Key Difference:

On -board Performance Monitoring and Alerting Figure 2. RNP Depiction

h. Incomplete Technical Standard Order (TSO).

(1) Question: Is it possible to receive an incomplete TSOA for a GPS or GPS/SBAS receiver circuit card assembly providing the functional performance of a sensor unit?

(2) Answer: No. The basis for a TSOA is that the applicant has sufficient control over the design and production to assure the system meets the minimum performance standard published by the FAA. However, we do allow for incomplete systems to obtain an authorization, as explained in the latest revision of AC 21-46, Technical Standard Order Program.

(a) The incomplete authorizations are applicable to a complete unit that performs a subset of the required performance or functionality of the TSO (see AC 21-46 (latest revision), paragraph 5-3). A GNSS receiver circuit card assembly is not a complete unit; it is a part that must be incorporated into another box to function properly. AC 20-170 (latest revision) Integrated Modular Avionics Development, Verification, Integration, and Approval Using 7

RTCA/DO-297 and Technical Standard Order-C153, addresses a somewhat similar situation for integrated modular avionics (IMA). But, navigation and non-navigation avionics equipment that incorporate GNSS receiver circuit card assemblies are not an IMA. Therefore, the IMA guidance is not applicable.

(b) However, the FAA recognizes that GNSS receiver circuit card assemblies are more than “component parts” and fundamentally provide the GNSS sensor functional capability. In essence, the only difference between a complete GNSS sensor unit and a receiver circuit card assembly is the sensor unit includes a power supply and enclosure shielding. But this difference is significant relative to the performance tests in the minimum operational performance standard (MOPS) specified by the GNSS TSOs. The concern is that a bare receiver circuit card assembly could pass the MOPS performance testing in isolation, but not maintain that performance when integrated into another device where it might be exposed to significant radio frequency interference from other circuit card assemblies or devices. The FAA has created three new TSOs to address this concern and provide certification credit for the functional capabilities in GNSS receiver circuit card assemblies. But the GNSS receiver circuit card assembly’s end-user must repeat selected MOPS performance tests with the circuit card assembly installed in the end-use equipment and apply for the applicable GNSS sensor unit TSO (see paragraph 3-5).

i. Incorporating Step-down Fixes in Navigation Databases.

(1) Question: The latest ARINC 424 format has fields for both named and un­ named step-down fixes. Is it acceptable for equipment manufacturers to include approach procedure step-down fixes in the navigation database; particularly when LPV capability is included?

(2) Answer: Yes. But ARINC 424 is not a TSO-specified standard, so there is no “requirement” to include step-down fixes or use all the fields. Having step-down fixes in the navigation database can present challenges for the airworthiness approval when LPV capability is included since step-down fixes are not applicable to LPV and the LPV requirements are designed to mimic an instrument landing system (ILS). The airworthiness applicant must ensure there is no confusing or disparate information presented to flight crews due to the cockpit arrangement. This can be a significant issue in legacy cockpits that have limited display capabilities. See paragraph 12-8 for guidance when including step-down fixes in the navigation database.

j. Using RTCA Inc. document (RTCA)/(DO)-178C Software Considerations in Airborne Systems and Equipment Certification, dated December 13, 2011.

(1) Question: Is it acceptable for equipment manufacturers to use RTCA/DO-178C in lieu of RTCA/DO-178B?

(2) Answer: Yes. In the U.S. it is acceptable for software development to use revision ‘C’ for TSOs that specify revision ‘B’ thru the deviation process. The project ACOs may approve deviation requests to use revision ‘C’ in lieu of revision ‘B’ without Headquarters 8

coordination. Refer to AC 20-115C for additional guidance on software qualification, including re-using software qualified to previous RTCA/DO-178 revisions.

Chapter 2. Technical Standard Order Authorization (TSOA) and Airworthiness Approval Considerations - General.

2-1. Guidance for TSOA/Letter of Design Approval (LODA) and Airworthiness Approvals.

Title 14 CFR part 21; subparts D, E, and O provide the regulatory basis defining changes to TC (major/minor changes to aircraft type design), STC, and TSOAs. The regulations and TSOs can be found at: http://rgl.faa.gov. This chapter provides general guidance on TSOA/LODA and airworthiness approval considerations specific to positioning and navigation equipment.

2-2. TSOA.

a. Several TSOs are available for positioning and navigation-related equipment and components (see appendix 10 of this AC). New TSOs will be written to address modernized GPS and other GNSSs once appropriate equipment standards are in place. It is benefi cial to

involve the ACO (both engineers and flight test pilots) in equipment evaluations as early as

possible for four primary reasons:

(1) Obtaining a TSOA/LODA does not ensure that the equipment will satisfy all of the applicable operational requirements and/or airworthiness regulations when it is installed.

(2) Positioning and navigation equipment has become increasingly more complex and workload intensive with no standardized pilot interface. The human factors evaluation of the equipment can be subjective.

(3) There is no one, single standard for interfacing positioning and navigation equipment to other equipment on-board the aircraft such as displays, autopilots, and terrain

warning systems. Positioning and navigation equipment manufacturers should develop compatibility data relative to other equipment for inclusion within the installation manual.

(4) The installed intended function for the equipment needs to be compatible with the original design for the system, including software and hardware design assurance levels and data process assurance levels. For example: a decision by the positioning and navigation equipment manufacturer to develop system software to RTCA/DO-178B Level C, may limit installation eligibility if that equipment contributes to failure conditions of other systems that perform critical functions to a higher software design assurance level.

Note: AC 20-115C provides guidance on using RTCA/DO-178C including re-using software qualified to previous RTCA/DO-178 revisions.

b. It is recommended that the positioning and navigation equipment manufacturer elect to obtain an STC for equipment installation concurrent with obtaining the TSOA/LODA. This has proven to be an effective method to aid installations.

Note: Any applicant may apply for an equipment installation STC. It is not this paragraph’s intent to limit 10

positioning and navigation installation STCs solely to avionics manufacturers.

c. The installation manual and other data approved during the TSOA process can be used as a source of approved data to support an installation airworthiness compliance finding provided the data is applicable to the installation airworthiness requirements (see AC 21-50 (latest revision), Installation of TSOA Articles and LODA Appliances).

2-3. STC-Approved Model List (AML).

a. All STC applicants are encouraged to use an AML for aircraft makes/models (including 14 CFR parts 23, 25, 27, and 29 aircraft) with similar characteristics and compatible equipment configurations to streamline the approval of subsequent installations. Applicants and ACOs need to contact the applicable directorate for specific policy and guidance information on obtaining an STC-AML.

b. Using the STC-AML method can simplify follow-on installations for makes/models on the AML. T he installer needs access to the STC approved data; apply the approved data;

and comply with the detailed installation instructions. The installer then needs only complete FAA Form 337, Major Repair and Alteration (Airframe, Powerplant, Propeller, Appliance), identifying the nature of the change to return the aircraft to service (see AC 43.9-1 (latest revision), Instructions for Completion of FAA Form 337).

c. GPS and GPS/SBAS equipment airworthiness approval is not considered novel or unusual per 14 CFR 21.16 and applicants can use designees during the STC process.

2-4. Type Certificate.

Positioning and navigation equipment manufacturers are also encouraged to work with TC holders. This has also proven to be an effective method to aid installations.