AC 20-138B

Historical

Advisory circular · Airworthiness Approval of Positioning and Navigation Systems

A newer version is currentThis is a historical version. AC 20-138D is the version in force, approved March 28, 2014.
Approved
September 27, 2010
Last reviewed
February 25, 2011
Cancelled by
AC20-138C
Family
AC 20-138 · 5 versions
Revision
B
14 CFR parts
23, 25, 27, 29

Guidance, not mandatory unless a rule or AD requires it. DRS record last changed November 22, 2025.

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Appendix 6

(2) If a ‘non-standard’ method of installation is used, the equipment manufacturer’s recommended procedures should be followed.

(3) The GPS mounting should be tested to ensure that it will be restrained throughout the flight envelope.

(4) Where a modification is required to the aircraft structure to show compliance with paragraph A6-4.b(3) of this appendix, the modification is to be carried out in accordance with the aircraft maintenance manual and the guidelines in the latest revisions of AC 43.13-1 and AC

43.14-2.

c. If required, install a remote GPS antenna as follows:

(1) Determine a suitable location for the GPS antenna on the upper fuselage.

Particular care should be taken to ensure that at least the minimum separation is maintained between the GPS antenna and VHF communications and ELT antennas.

(2) Install the equipment manufacturer-approved GPS antenna in accordance with the equipment manufacturer’s installation manual/instructions, the aircraft maintenance manual and the guidelines in AC 43.13-2 (latest revision). The GPS antenna installation should be tested to ensure that it would be restrained throughout the flight envelope.

(3) If using an internally mounted, equipment manufacturer-supplied remote antenna, install the antenna in accordance with the equipment manufacturer’s installation manual/instructions. Ensure that the antenna and associated cable will not interfere with pilot’s vision, aircraft controls or displays, or pilot/passenger egress.

d. Install and placard an appropriately rated circuit breaker or fuse.

e. If required, install a remote GPS indicator as follows:

(1) Determine a suitable location for the GPS indicator. The indicator should be located in the instrument panel and readily viewable to the pilot from his/her normally seated position.

(2) Install the indicator in accordance with the equipment manufacturer’s installation manual/instructions, the aircraft maintenance manual and the guidelines in AC 43.13-2 (latest revision).

(3) Interconnect the GPS, power supply, indicator and antenna (as required) in accordance with the equipment manufacturer’s installation manual/instructions.

f. Unless the equipment automatically displays a message on start-up and pilot action is required to clear the message, install the following placard in the vicinity of the GPS and in clear view of the pilot: GPS APPROVED FOR VFR USE ONLY.

A6-2

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Appendix 6

A6-3

g. Close the GPS circuit breaker or install the fuse.

h. Carry out a functional test of the GPS installation in accordance with the equipment manufacturer’s installation/operation instructions. Checks are also to be carried out to determine satisfactory operation of all disturbed systems, that there is no interference between GPS and other systems, no effect on the aircraft compass system and that there is no interference with the aircraft flight controls.

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Appendix 7

Appendix 7. Definitions and Acronyms.

A7-1. Definitions.

a. Advisory Vertical Guidance. Vertical path deviation guidance indications generated by any means, but for no operational credit. See chapter 4.

b. All Revisions (AR). An all-inclusive reference for TSOs with multiple versions that remain applicable.

c. Aircraft-Based Augmentation System (ABAS). A generic term for an algorithm on the aircraft that verifies the integrity of the GNSS position output. That algorithm can use RAIM or RAIM-equivalent methods; and in the future may use advanced techniques to provide an integrity-assured position solution.

d. Approved Vertical Guidance. Vertical path deviation guidance indications generated by certified means to use for operational credit on charted approach procedures that contain a TERPS-protected glidepath (e.g., LNAV/VNAV, LPV, GLS, ILS).

e. Area Navigation (RNAV). A method of navigation that permits aircraft operation on any desired flight path within the coverage of station-referenced navigation aids; within the capability limits of self-contained aids; or a combination of these. For the purposes of this AC, the specified RNAV accuracy must be met 95% of the flight time.

f. Display. Within the context of this AC, a display is a device mounted in the aircraft instrument panel to convey course guidance navigation information to the aircrew for controlling the aircraft along the intended route. Displays may also be used to convey other information as well.

g. Design Eye Position. The position at each pilot's station from which a seated pilot achieves the required combination of outside visibility and instrument scan. The design eye position is a single point selected by the applicant that is normally a point fixed in relation to the aircraft structure (neutral seat reference point) at which the midpoint of the pilot’s eyes should be located when seated at the normal position.

h. Fault Detection and Exclusion (FDE). A receiver autonomous integrity monitoring algorithm that can automatically detect and exclude a faulty satellite from the position solution when a sufficient number of redundant satellite measurements are available.

i. Galileo. Galileo is a European satellite-based radio navigation system being developed that will provide a global positioning service. Galileo will provide two levels of service: an open service for non-critical applications; and a fee-based service for safety-of-life applications. However, the two-level structure hasn’t been irrevocably set. The Galileo open service will provide two signals that are interoperable with, but will be independent from, GPS for aviation applications.

j. Global Navigation Satellite System (GNSS). GNSS is a generic term for A7-1

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Appendix 7

sate llite-based navigation, including GPS, GPS/SBAS, GPS/GBAS, GLONASS, Galileo and any other satellite navigation or augmentation system suitable for aviation use within the Aeronautical Radio Navigation Service (ARNS) frequency band.

k. Global Positioning System (GPS). GPS is a United States satellite-based radio navigation system that provides a global positioning service. The service provided by GPS for civil use is defined in the September 2008 GPS Standard Positioning System Performance Standard, 4th edition available at http://pnt.gov/public/docs/2008/spsps2008.pdf.

l. GLONASS. GLONASS is a Russian Federation satellite-based radio navigation system providing a global positioning service. GLONASS is not fully operational, and currently, no operational credit can be taken for its use in the United States National Airspace System.

m. Ground-Based Augmentation System (GBAS). GBAS is a localized coverage augmentation system to GPS that calculates integrity and correction data and broadcasts the data to GPS/GBAS users directly from a ground-based transmitter. In the United States, this is referred to as local area augmentation system (LAAS). International GBAS standards are defined in ICAO Annex 10.

n. Letter of Design Approval (LODA). An FAA design approval that is issued only to a foreign (non-United States) manufacturer of an appliance that the FAA has found meets a specific TSO. A LODA is not a production approval and does not confer an installation approval. The civil aviation authority for the manufacturer’s country approves and oversees the quality control system and production of the appliance under the provisions of the bilateral agreement with the United States.

o. Local Area Augmentation System (LAAS). LAAS is the United States implementation of GPS/GBAS.

p. Navigation Computer. Within the context of this AC, navigation refers to any function used to direct the course of an aircraft. A navigation computer as addressed in this AC provides the functions of position estimation, path definition, path steering, and situation indications and alerting to the flight crew.

q. Normal Line of Sight. Established at 15 degrees below the horizontal plane referenced to the design eye position for fixed-wing aircraft (see AC 25-11 latest revision). For rotorcraft, reference the visibility requirements defined in the latest revisions of AC 27-1 and

29-2.

r. Primary Field of View. The vertical and horizontal visual fields relative to the design eye reference point that can be viewed with eye rotation only using foveal or central vision. The values for the horizontal (relative to the normal line of sight) are +/-15 degrees optimum, with +/- 35 degrees maximum. The values for the vertical (relative to normal line of sight) are +/-15 degrees optimum, with + 40 degrees up and -20 degrees down maximum (see AC 25-11 latest revision). The primary field of view definition should be broad enough to include the center radio stack on 14 CFR part 23 airplanes with “classic”, analog basic ‘T’ A7-2

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Appendix 7

instrum entation. For rotorcraft, reference the visibility requirements defined in the latest revisions of AC 27-1 and 29-2.

s. Primary Optimum Field of View. For the purpose of this AC related to RNP AR, the primary optimum field of view is within +/-15 degrees horizontal and vertical relative to the pilot’s normal line of sight.

t. Receiver Autonomous Integrity Monitoring (RAIM). Any algorithm that verifies the integrity of the position output using redundant GPS measurements, or using GPS measurements and barometric aiding, is considered a RAIM algorithm. An algorithm that uses additional information (e.g., multi-sensor system with inertial reference system) to verify the integrity of the position output may be acceptable as a RAIM-equivalent.

u. Required Navigation Performance (RNP). RNP is a statement of the total aircraft navigation performance necessary for operation within a defined airspace.

v. RNAV Multi-Sensor Equipment. This type of navigation system computes and displays one independent or blended position and other navigation data combined from one or more navigation sensors, which provide independent positions.

w. Satellite-Based Augmentation System (SBAS). SBAS is a wide coverage augmentation system to GPS that calculates integrity and correction data on the ground and uses geostationary satellites to broadcast the data to GPS/SBAS users. In the United States, this is referred to as Wide Area Augmentation System (WAAS).

x. Sensor. Within the context of this AC, a sensor is a device on the aircraft that receives signals from an appropriate antenna to calculate an output supplied to a navigation computer.

y. Stand-alone. Stand-alone as addressed in this AC refers to navigation equipment incorporating a GNSS position sensor and a navigation function, so that the equipment provides path deviations relative to a selected path.

z. Time of Arrival Control (TOAC). TOAC is a system capability that determines the necessary and available adjustments to aircraft speed and vertical profile necessary to satisfy a required time of arrival (RTA) at a fix.

aa. Wide Area Augmentation System (WAAS). WAAS is the United States implementation of GPS/SBAS.

A7-2. Acronyms.

14 CFR Title 14 of the Code of Federal Regulations ABAS Aircraft-Based Augmentation System AC Advisory Circular ACO Aircraft Certification Office ADAHRS Air Data Attitude Heading Reference System A7-3

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Appendix 7

ADS-B Automatic Dependent Surveillance - Broadcast AF/D Airport/Facilities Directory AFM Airplane Flight Manual AFMS Airplane Flight Manual Supplement AGL Above Ground Level AHRS Attitude Heading Reference System AIRAC Aeronautical Information Regulation and Control AML Approved Model List APCH Approach (AR) All Revisions ARNS Aeronautical Radio Navigation System ASI Aviation Safety Inspector ATIS Automated Traffic Information System atk Along Track baro-VNAV Barometric Vertical Navigation CA Course to Altitude termination per ARINC 424 CAT I Category I precision approach CDI Course Deviation Indicator CDU Control Display Unit CF Course to Fix per ARINC 424 CFR Code of Federal Regulations DMAX Maximum Use Distance DA Decision Altitude dB Decibel dBc Decibels Relative to Carrier Power dBm Decibels Referenced to one Milliwatt D/D/I DME/DME/Inertial Reference Unit DER Designated Engineering Representative DER Designated End of Runway (for RNP AR only) DF Direct to Fix per ARINC 424 DME Distance Measuring Equipment DOD Department of Defense DOT Department of Transportation DP Departure Procedure EHSI Electronic Horizontal Situation Indicator ELT Emergency Locator Transmitter EMC Electromagnetic Compatibility EMI Electromagnetic Interference ESV Expanded Service Volume EVS Enhanced Vision System FA Fix to Altitude termination per ARINC 424 FAA Federal Aviation Administration FAF Final Approach Fix FAS Final Approach Segment FD Flight Director FDE Fault Detection and Exclusion FGS Flight Guidance System A7-4

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Appendix 7

FHS Functional Hazard Assessment FM Fix to Manual termination per ARINC 424 FMS Flight Management System FOM Figure of Merit FRP Federal Radionavigation Plan FSTD Flight Simulation Training Device FTE Flight Technical Error GBAS Ground-Based Augmentation System GEO Geostationary Orbit Satellite GNSS Global Navigation Satellite System GLS GBAS Landing System GNSS Global Navigation Satellite System GPA Glide Path Angle GPS Global Positioning System GPWS Ground Proximity Warning System HAL Horizontal Alert Limit HAT Height Above Terrain HDOP Horizontal Dilution of Precision HFOM Horizontal Figure of Merit HFOMV Horizontal Figure of Merit (velocity) HMI Hazardously Misleading Information HPL Horizontal Protection Level HSI Horizontal Situation Indicator HUL Horizontal Uncertainty Level Hz Hertz ICAO International Civil Aviation Organization IF Initial Fix per ARINC 424 IFR Instrument Flight Rules ILS Instrument Landing System IMC Instrument Meteorological Conditions INS Inertial Navigation System IRS Inertial Reference System IRU Inertial Reference Unit LAAS Local Area Augmentation System LNAV Lateral Navigation LNAV/VNAV Lateral Navigation/Vertical Navigation LOA Letter of Acceptance LOC Localizer LODA Letter of Design Approval LP Localizer Performance without Vertical Guidance LPV Localizer Performance with Vertical Guidance LTP Landing Threshold Point MAG Magnetic MEMS Micro-Electro-Mechanical System MLS Microwave Landing System MMR Multi-Mode Receiver MNPS Minimum Navigation Performance Standards A7-5

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Appendix 7

MOPS Minimum Operational Performance Standards MSL Mean Sea Level NACV Navigation Accuracy Category (velocity) NAS National Airspace System NAVAID Navigation Aid NDB Non-Directional Beacon NM Nautical Mile NOTAM Notice to Airmen NSE Navigation System Error NTAP Notices to Airman Publication OBS Omni Bearing Selector ODP Obstacle Departure Procedure OEM Original Equipment Manufacturer PAN Precision and Navigation PAPI Precision Approach Path Indicator PHAC Plan for Hardware Aspects of Certification POH Pilot Operating Handbook PSAC Plan for Software Aspects of Certification PVT Position, Velocity, Time RAIM Receiver Autonomous Integrity Monitoring RF Radius to Fix per ARINC 424 RFM Rotorcraft Flight Manual RFMS Rotorcraft Flight Manual Supplement RNAV Area Navigation RNP Required Navigation Performance RNP APCH RNP Approach RNP AR . RNP Authorization Required RTA Required Time of Arrival SAAAR Special Aircraft and Aircrew Authorization Required SATCOM Satellite Communications SBAS Satellite-Based Augmentation System SFAR Special Federal Aviation Regulation SID Standard Instrument Departure STAR Standard Terminal Arrival Route STC Supplemental Type Certificate STC-AML Supplemental Type Certificate Approved Model List SVS Synthetic Vision System TAWS Terrain Awareness and Warning System TC Type Certificate TF Track to Fix per ARINC 424 TOAC Time of Arrival Control TOGA Take-Off Go Around TSE Total System Error TSO Technical Standard Order TSOA Technical Standard Order Authorization VA Heading to Altitude termination per ARINC 424 VAL Vertical Alert Limit A7-6

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Appendix 7

A7-7 VASI Visual Approach Slope Indicator VDB Very High Frequency Data Broadcast VDOP Vertical Dilution of Precision VFOM Vertical Figure of Merit VFOMV Vertical Figure of Merit (velocity) VFR Visual Flight Rules VHF Very High Frequency VI Heading to Intercept per ARINC 424 VM Heading to Manual termination per ARINC 424 VNAV Vertical Navigation VOR Very High Frequency Omni-Directional Range VPL Vertical Protection Level VSWR Voltage Standing Wave Ratio URA User Range Accuracy WAAS Wide Area Augmentation System xtk Cross Track

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Appendix 8

Appendix 8. Related Documents.

A8-1. FAA Advisory Circulars.

a. AC 20-107B, Composite Aircraft Structure.

b. AC 20-115B, Radio Technical Commission for Aeronautics, Inc. Document RTCA/DO-178B.

c. AC 20-152, RTCA, Inc., Document RTCA/DO-254, Design Assurance Guidance for Airborne Electronic Hardware.

d. AC 20-153, Acceptance of Data Processes and Associated Navigation Databases.

e. AC 20-159, Obtaining Design and Production Approval of Airport Moving Map Display Applications Intended for Electronic Flight Bag Systems.

f. AC 20-163, Displaying Geometric Altitude Relative to Mean Sea Level.

g. AC 20-165, Airworthiness Approval of Automatic Dependent Surveillance - Broadcast (ADS-B) Out Equipment for Operation in the National Airspace System (NAS).

h. AC 21-16F, RTCA Document DO-160 versions D, E, and F, Environmental Conditions and Test Procedures for Airborne Equipment.

i. AC 21-40A, Application Guide for Obtaining a Supplemental Type Certificate.

j. AC 23-8B, Flight Test Guide for Certification of Part 23 Airplanes.

k. AC 23-13A, Fatigue, Fail-Safe, and Damage Tolerance Evaluation of Metallic Structure for Normal, Utility, Acrobatic, and Commuter Category Airplanes.

l. AC 23-17, Systems and Equipment Guide for Certification of Part 23 Airplanes.

m. AC 23-18, Installation of Terrain Awareness and Warning System (TAWS) Approved for Part 23 Airplanes.

n. AC 23-21 with Change 1, Airworthiness Compliance Checklists Used to Substantiate Major Alterations for Small Airplanes.

o. AC 23-22, Guidance for Approved Model List (AML) Supplemental Type Certificate (STC) Approval for Part 23 Airplane Avionics Installations.

p. AC 23.1309-1D, Equipment, Systems, and Installations in Part 23 Airplanes.

q. AC 23.1311-1B, Installation of Electronic Displays in Part 23 Airplanes.

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r. AC 23.1419-2D, Certification of Part 23 Aircraft for Flight in Icing Conditions.

s. AC 25-7A, Flight Test Guide for Certification of Transport Category Airplanes.

t. AC 25-10, Guidance for Installation of Miscellaneous, Non-Required Electrical Equipment.

u. AC 25-11A, Transport Category Airplane Electronic Display Systems.

v. AC 25-15, Approval of Flight Management Systems in Transport Category Airplanes.

w. AC 25-23, Airworthiness Criteria for the Installation Approval of a Terrain Awareness and Warning System (TAWS) for Part 25 Airplanes.

x. AC 25-1309-1A, System Design and Analysis.

y. AC 25.1329-1, Approval of Flight Guidance Systems.

z. AC 25.1419-1A, Certification of Part 25 Aircraft for Flight in Icing Conditions.

aa. AC 25.1529-1A, Instructions for Continued Airworthiness of Structural Repairs on Transport Airplanes.

bb. AC 27-1, Certification of Normal Category Rotorcraft.

cc. AC 29-2C, Certification of Transport Category Rotorcraft.

dd. AC 43.9-1F, Instructions for Completion of FAA Form 337.

ee. AC 43.13-1B, Acceptable Methods, Techniques and Practices – Aircraft Inspection and Repair.

ff. AC 43.13-2B, Acceptable Methods, Techniques and Practices – Aircraft Alterations.

gg. AC 90-96A, Approval of U.S. Operators and Aircraft to Operate under Instrument Flight Rules (IFR) in European Airspace Designated for Basic Area Navigation (BRNAV/RNP-5).

hh. AC 90-100, U.S. Terminal and Enroute Area Navigation (RNAV) Operations.

ii. AC 90-101A, Approval Guidance for RNP Procedures with SAAAR.

jj. AC 90-105, Approval Guidance for RNP Operations and Barometric Vertical Navigation in the U.S. National Airspace System.

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kk. AC 91-49 with Change 1, General Aviation Procedures for Flight in North Atlantic Minimum Navigation Performance Specification Airspace.

ll. AC 120-28D, Criteria for Approval of Category III Weather Minima for Takeoff, Landing, and Rollout.

mm. AC 120-29A, Criteria for Approval of Category I and Category II Weather Minima for Approach.

nn. AC 120-33, Operational Approval of Airborne Long-Range Navigation Systems for Flight within the North Atlantic Minimum Navigation Performance Specification Airspace.

A8-2. FAA Technical Standard Orders.

a. TSO-C66, Airborne Distance Measuring Equipment (For Air Carrier Aircraft).

b. TSO-C115, Airborne Area Navigation Equipment Using Multi-Sensor Inputs.

c. TSO-C129, Airborne Supplemental Navigation Equipment Using the Global Positioning System (GPS).

d. TSO-C144a, Passive Airborne Global Navigation Satellite System (GNSS) Antenna.

e. TSO-C145c, Airborne Navigation Sensors Using the Global Positioning System (GPS) Augmented by the Wide Area Augmentation System (WAAS).

f. TSO-C146c, Stand-Alone Airborne Navigation Equipment Using the Global Positioning System (GPS) Augmented by the Satellite Based Augmentation System (SBAS).

g. TSO-C151b, Terrain Awareness and Warning System.

h. TSO-C154c, Universal Access Transceiver (UAT) Automatic Dependent Surveillance-Broadcast (ADS-B) Equipment.

i. TSO-C161a, Ground Based Augmentation System Positioning and Navigation Equipment.

j. TSO-C162a, Ground Based Augmentation System Very High Frequency Data Broadcast Equipment.

k. TSO-C165, Electronic Map Display Equipment for Graphical Depiction of Aircraft Position.

l. TSO-C166b, Extended-Squitter Automatic Dependent Surveillance-Broadcast (ADS-B) and Traffic Information.

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