AC 20-138A

Historical

Advisory circular · Airworthiness Approval of Global Navigation Satellite System (GNSS) Equipment

A newer version is currentThis is a historical version. AC 20-138D is the version in force, approved March 28, 2014.
Approved
December 22, 2003
Last reviewed
February 25, 2011
Cancels
AC20-138
Cancelled by
AC20-138B
Family
AC 20-138 · 5 versions
Revision
A
14 CFR parts
21, 23, 25, 27, 29, 43, 91, 121, 135

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

Show pages 49–60

Appendix 3

EXAMPLE FORMAT FOR A QUICK REFERENCE GUIDE (Continued) Holding Patterns

1. Press GPS CRS switch to display “OBS”.

2. Press D

3. Press ENT

4. Set course for intercept into HSI.

Procedure Turns/Holding Patterns 1 of 1 Messages 1 of 3 Direct To 1 of 1 Procedure turns

1. Prior to the FAF outbound, press GPS CRS switch to display “OBS”.

2. Fly outbound course and procedure turn as specified.

3. Set inbound course into HSI.

4. Prior to the FAF inbound, press GPS CRS switch to display “LEG”.

Direct to the FAF

1. Rotate left outer knob to select flight plan pages.

2. Rotate left inner knob to select desired flight plan.

3. Press left CRSR

4. Rotate left outer knob to place cursor over the FAF.

5. Press D

6. Press ENT

2. Press ENT

3. Pull out right inner knob

4. Rotate right inner knob to select desired airport.

5. Push in right inner knob

6. Press D

7. Press ENT Direct to the nearest waypoint

This function available only for airports.

Direct to the nearest airport

1. Press MSG Direct To 1 of 1 Messages 1 of 3 Page A3-7

Appendix 3

EXAMPLE FORMAT FOR A QUICK REFERENCE GUIDE (Continued)

RAIM NOT AVAILABLE

APR MODE INHIBITED PREDICT RAIM ON STA 5

1. Rotate left outer knob to STAT

2. Predict RAIM to determine when approach can be flown.

3. Rotate left inner knob to “STAT 5” page.

RAIM NOT AVAILABLE

CROSS CHECK POSITION

1. Cross-check with other means of navigation.

RAIM POSITION ERROR

CROSS CHECK POSITION

1. Cross-check with other means of navigation.

Messages

ADJ NAV IND CRS TO _____

1. Set course into HSI.

2. Press MSG

ARM GPS APPROACH

1. Press GPS APR switch

BAD SATELLITE GEOMETRY

1. See EPE on “STS 2” page, or

2. Use other means of navigation

BAD SATELLITE GEOMETRY and

RAIM NOT AVAILABLE

1. Appears only when receiver is in approach active mode.

2. Use other means of navigation.

PRESS ALT TO SET BARO

1. Press ALT

2. Rotate left inner and outer knobs to input altitude.

3. Press left CRSR

4. Press ALT

Messages 1 of 3 Messages 3 of 3 Messages 2 of 3

SATELLITE COVERAGE

INADEQUATE FOR NAVIGATION

1. Use other means of navigation.

Page A3-8

Appendix 4

APPENDIX 4. SAMPLE AIRPLANE/ROTORCRAFT FLIGHT MANUAL

SUPPLEMENT

Installation Center/Repair Station Model XXX GPS 123 Fourth Street Navigation System Anytown, USA

FAA APPROVED AIRPLANE FLIGHT MANUAL SUPPLEMENT

ABC MODEL XXX GPS NAVIGATION SYSTEM

AIRPLANE MAKE:

AIRPLANE MODEL:

AIRPLANE SERIAL NO.:

REGISTRATION NO.:

This document must be carried in the airplane at all times. It describes the operating procedures for the ABC Model XXX GPS navigation system when it has been installed in accordance with <manufacturer's installation manual number and date>.

For airplanes with an FAA Approved Airplane Flight Manual, this document serves as the FAA Approved ABC Model XXX GPS Flight Manual Supplement. For airplanes that do not have an approved flight manual, this document serves as the FAA Approved ABC Model XXX GPS Supplemental Flight Manual.

The information contained herein supplements or supersedes the basic Airplane Flight Manual dated <insert date> only in those areas listed herein. For limitations, procedures, and performance information not contained in this document, consult the basic Airplane Flight Manual.

FAA APPROVED

_____________________ Title Office Federal Aviation Administration City, State

FAA Approved Page <> of <> Date: ________

Page A4-1

Appendix 4

SAMPLE AIRPLANE/ROTORCRAFT FLIGHT MANUAL (Continued)

Installation Center/Repair Station Model XXX GPS 123 Fourth Street Navigation System Anytown, USA

Table of Contents

SectionPage

1 General. ............................................< >

2 Limitations........................................< >

3 Emergency/Abnormal Procedures....< >

4 Normal Procedures ...........................< >

5 Performance......................................< >

6 Weight and Balance..........................< >

7 System Description...........................< >

FAA Approved Page <> of <> Date: ________

Page A4-2

Appendix 4

SAMPLE AIRPLANE/ROTORCRAFT FLIGHT MANUAL (Continued)

Installation Center/Repair Station Model XXX GPS 123 Fourth Street Navigation System Anytown, USA

SECTION 1 - GENERAL <Include the appropriate statement to describe the equipment capability:>

GPS TSO-C129a Class A1: The installed ABC system complies with AC 20- 138A for IFR navigation using GPS for en route, terminal area, and nonprecision approach operations (including “GPS”, “or GPS”, and “RNAV” approaches).

GPS TSO-C129a Class A2: The installed ABC system complies with AC 20- 138A for IFR navigation using GPS for en route and terminal area operations.

GPS/WAAS TSO-C146a Class 3: The installed ABC equipment complies with AC 20-138A for navigation using GPS and WAAS (within the coverage of a Space-Based Augmentation System complying with ICAO Annex 10) for en route, terminal area, non-precision approach operations (including “GPS”, “or GPS”, and “RNAV” approaches), approach procedures with vertical guidance (including “LNAV/VNAV” and “LPV”), and precision approaches (including “GLS” approaches).

GPS/WAAS TSO-C146a Class 2: The installed ABC equipment complies with AC 20-138A for navigation using GPS and WAAS (within the coverage of a Space-Based Augmentation System complying with ICAO Annex 10) for en route, terminal area, non-precision approach operations (including “GPS”, “or GPS”, and “RNAV” approaches), and approach procedures with vertical guidance (including “LNAV/VNAV”).

GPS/WAAS TSO-C146a Class 1: The installed ABC equipment complies with AC 20-138A for navigation using GPS and WAAS (within the coverage of a Space-Based Augmentation System complying with ICAO Annex 10) for en route, terminal area, and non-precision approach operations (including “GPS”, “or GPS”, and “RNAV” approaches).

GPS/WAAS TSO-C146a Class 1/TSO-C161: The installed ABC equipment complies with AC 20-138A for navigation using GPS, WAAS (within the coverage of a Space-Based Augmentation System complying with ICAO Annex 10), and LAAS (within the coverage of a Ground-Based Augmentation System complying with ICAO Annex 10) for en route, terminal area, nonprecision approach operations (including “GPS”, “or GPS”, and “RNAV” approaches), approach procedures with vertical guidance (including “LNAV/VNAV” and “LPV”), and precision approaches (including “GLS” approaches).

G PS TSO-C129/TSO-C161: The installed ABC system complies with AC 20- 138A for navigation using GPS and LAAS (within the coverage of a groundbased augmentation system complying with ICAO Annex 10) for en route, terminal area, and non-precision approach operations (including “GPS”, “or GPS”, and “RNAV” approaches). The installed ABC system complies with AC 20-138A for navigation using GPS and LAAS for precision approach operations (including “GLS” approaches).

FAA Approved Page <> of <> Date: ________ Page A4-3

Appendix 4

SAMPLE AIRPLANE/ROTORCRAFT FLIGHT MANUAL (Continued)

Installation Center/Repair Station Model XXX GPS 123 Fourth Street Navigation System Anytown, USA

<The following statement applies to all types of stand-alone equipment.> Navigation information is referenced to WGS-84 reference system, and should only be used where the Aeronautical Information Publication (including electronic data and aeronautical charts) conform to WGS-84 or equivalent.

<For TSO-C146a equipment and TSO-C129(AR) equipment that complies with the requirements in Appendix 1 of this AC:> The ABC equipment as installed has been found to comply with the requirements for GPS Oceanic/Remote Navigation, when used in conjunction with the ABC prediction program. This does not constitute an operational approval.

SECTION 2 - LIMITATIONS

1. The ABC Model XXX GPS Quick Reference Guide, P/N <insert part number>, dated <insert date> (or later appropriate revision) must be immediately available to the flight crew whenever navigation is predicated on the use of the system.

2. The system must utilize software version <insert version identification>.

3. A valid and compatible database must be installed and contain current data.

<Limitation for TSO-C129(AR) equipment (whether or not LAAS is also provided):>

4. The aircraft must have other approved navigation equipment installed and operating appropriate to the route of flight.

5. <Specify any additional limitations applicable to the particular installation.>

SECTION 3 - EMERGENCY/ABNORMAL PROCEDURES

EMERGENCY PROCEDURES No Change

ABNORMAL PROCEDURES

1. If ABC Model XXX GPS navigation information is not available or invalid, utilize remaining operational navigation equipment as appropriate.

2. If Loss of Integrity Monitoring message is displayed, revert to an alternate means of navigation appropriate to the route and phase of flight or periodically cross-check the GPS guidance to other, approved means of navigation.

SECTION 4 - NORMAL PROCEDURES

1. Normal operating procedures are outlined in the ABC Model XXX GPS Pilot's Guide.

2. <Describe approach mode sequencing.> FAA Approved Page <> of <> Date: ________ Page A4-4

Appendix 4

SAMPLE AIRPLANE/ROTORCRAFT FLIGHT MANUAL (Continued)

Installation Center/Repair Station Model XXX GPS 123 Fourth Street Navigation System Anytown, USA

3. System Annunciators <applicable to installations with external annunciators>

a. Waypoint - <describe each annunciator>

b. Message - <describe each annunciator>

c. Approach - <describe each annunciator>

d. <describe any other annunciators>

4. System Switches <applicable to installations with external switches>

a. Nav/GPS - <describe switch use and function>

b. RMI Switch - <describe switch use and function>

c. <describe any other switches>

5. Pilot's Display <describe the pilot's GPS display(s)>

6. Flight Director/Autopilot Coupled Operation <describe the procedures for coupling GPS to the flight director and/or autopilot system(s)>

7. < include any other normal operating procedures necessary>

SECTION 5 - PERFORMANCE

No Change

SECTION 6 - WEIGHT AND BALANCE

<Refer to revised weight and balance data, if applicable.>

SECTION 7 - SYSTEM DESCRIPTION

<Provide a brief description of the system, its operation, installation, etc.>

FAA Approved Page <> of <> Date: ________

Page A4-5 (and A4-6)

Appendix 5

APPENDIX 5. INSTALLATION OF EN ROUTE GPS EQUIPMENT FOR VFR

USE ONLY

1. GENERAL. This appendix provides guidelines fo r the installation of stand-alone GPS systems used for en route, VFR navigation only.

2. APPLICABILITY. All un-pressurized aircraft less than 6000 pounds where the modification is classified as a minor alteration.

3. TECHNICAL INSTRUCTIONS. Install the GPS as follows:

a. The GPS equipm ent should be lo cated in a position which is viewable and accessible to the pilot from his/her norm ally seated p osition. The installed GPS equipment should not:

(1) Restrict access to or view of any control, display or indicator;

(2) Restrict movement of the flight controls;

(3) Interfere with the pilot’s vision along the flight path; or

(4) Restrict pilot/passenger egress.

b. The installation should be carried out in accordance with the m anufacturer’s installation instru ctions, the airc raft m aintenance m anual and th e guideline s in AC43.13-2A. The installation should conform to the following requirements:

(1) Manufacturer-supplied or standard parts should be used.

(2) If a ‘non-standard’ method of instal lation is used, the m anufacturer’s recommended procedures are to be followed.

(3) The GPS mounting sho uld be te sted to ensur e that it will be res trained throughout the flight envelope.

(4) Where a modification is required to the aircraft stru cture to sh ow compliance with paragraph 3b(3), the modification is to be carried out in accordance with the aircraft maintenance manual and the guidelines in AC43.13-1B and AC 43.14-2A.

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 leas t the m inimum separation is maintained between the GPS antenna and VHF communications and ELT antennas.

(2) Install the m anufacturer-approved G PS antenna in accord ance with the manufacturer’s installation instructions, the aircraft m aintenance manual and the guidelines in AC 43.13-2A. The GPS antenna inst allation should be test ed to ensure that it would be restrained throughout the flight envelope.

Page A5-1

Appendix 5

(3) If using an internally mounted, m anufacturer-supplied rem ote antenna, install the antenna in accordance with the manufacturer’s installation instructions. Ensure that th e antenna and associated cable will not interfere with pilo t’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 G PS indicator. The indicator should be located in the instrum ent panel and readily viewable to the pilot from his/her normally seated position.

(2) Install the indicator in accord ance w ith the m anufacturer’s installation instructions, the aircraft maintenance manual and the guidelines in AC 43.13-2A.

(3) Interconnect the GPS, power supply, i ndicator and antenna (as required) in accordance with the manufacturer’s installation instructions.

f. Unless the equipm ent autom atically disp lays a m essage 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.

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

h. Carry out a functional test of the G PS installation in a ccordance w ith th e manufacturer’s ins tallation/operation instru ctions. Checks ar e also to be carr ied out to determine satisfactory operation of all disturbe d system s, that there is no in terference between GPS and other system s, no effect on th e aircraft compass system and that there is no interference with the aircraft flight controls.

Page A5-2