2 Equipment with existing approvals can continue to be installed within the provisions of their original design and airworthiness certification.
i. The guidance in this AC cannot ensure compliance with all 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. Mandatory terms such as “must” used in this AC are only applicable when following the compliance method outlined.
j. 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.
1-2. Audience.
This AC is for aircraft m anufacturers, 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.
This AC supersedes the ACs listed b elow. Equipment previously approved per the guidance in the superseded ACs is still valid for the operations and conditions stated in their approvals.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:
a. AC 20-129, Airworthiness Approval of Vertical Navigation (VNAV) Systems for use in the U.S. National Airspace System (NAS) and Alaska.
b. AC 20-130A, Airworthiness Approval of Navigation or Flight Management Systems Integrating Multiple Navigation Sensors.
c. AC 20-138A, Airworthiness Approval of Global Navigation Satellite System (GNSS) Equipment.
d. AC 25-4, Inertial Navigation Systems.
Chapter 2. T echnical Standard Order Authorization (TSOA) and Airworthiness Approval Considerations - General.
2-1. Guidance for TSOA/Letter of DesignApproval (LODA) and Air worthiness Approvals.
Title 14 CFR part 21; subparts D, E, and O provide the regulatory basis defining changes to type certificates (TC) (major/minor changes to aircraft type design), supplemental type certificates (STC), and TSOAs. The regulations and technical standard orders (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 TS Os have been written for positioning and navigation-related equipment and components (see appendix 8 of this AC). New TSO’s will be written to address modernized GPS and other GNSSs once appropriate equipment standards are in place. It is beneficial 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) The design of positioning and navigation equipment has become increasingly more complex and workload intensive with no standardized interface with the pilot. The human factors evaluation of the equipment is subjective.
(3) There is not 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.
For example, a decision by the positioning and navigation equipment manufacturer to develop system software to RTCA, Inc. document (RTCA)/(DO)-178B, Software Considerations in Airborne Systems and Equipment Certification, dated December 1, 1992, Level C, may limit installation eligibility if that equipment contributes to failure conditions of other systems that perform critical functions to a higher software level.
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 positioning and navigation STCs for installation solely to avionics manufacturers.
c. The installation manual and other data accepted during the TSO process can be used as a source of acceptable data to support an installation airworthiness compliance finding.
2-3. STC-Approved Model List (AML).
a. All STC applican ts 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. The 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.
Position ing and navigation equipment manufacturers are also encouraged to work with TC holders. This has also proven to be an effective method to aid installations.
Chapter 3. TSO Authorization.
3-1. TSO-C129(AR)/C196.
a. TSO-C129a, Airborne Supplemental Navigation Equipment Using the Global Position ing System (GPS), and TSO-C196, Airborne Supplemental Navigation Sensors for Global Positioning System Equipment Using Aircraft-Based Augmentation, both define an acceptable standard for GPS equipment without ground-based or space-based augmentations.
Both TSO-C129 all revisions (AR) and TSO-C196 equipment require other positioning and navigation systems appropriate to the operation on-board the aircraft due to equipment limitations.
b. TSO-C196 defines GPS sensor equipment that incorporates many processing improvements found in the GPS/SBAS equipment minimum operational performance standards (MOPS); but, without the GPS/SBAS requirements. Two improvement examples are: a requirement for the receiver to properly account for satellite range error if it is reflected in the User Range Accuracy index (commonly referred to as being “Selective Availability aware”);
and requirements to ensure performance is not degraded due to an increasing radio frequency noise environment as other satellite systems become available.
(1) TSOA/LODA applicants may wish to have a previously approved TSO-C145b/c, Airborne Navigation Sensors Using the Global Positioning System (GPS) Augmented by the Wide Area Augmentation System (WAAS), sensor approved as a TSO-C196 sensor. It is acceptable for these TSO-C196 applicants to take certification credit by re-using data developed during a previous TSO-C145b/c TSOA/LODA approval. RTCA/DO-316, Minimum Operational Performance Standards for Global Positioning System/Aircraft-Based Augmentation System Airborne Equipment, dated April 14, 2009, implemented by TSO-C196 was developed from RTCA/DO-229D, Minimum Operational Performance Standards for Global Positioning System/Wide Area Augmentation System (GPS/WAAS) Airborne Equipment, dated December 3, 2006, implemented by TSO-C145b/c. RTCA/DO-229D meets or exceeds the performance requirements for TSO-C196. The original RTCA/DO-178B documentation can be re-used if there are no software changes.
(2) Applicants seeking a TSO-C196 TSOA/LODA based on a previously approved TSO-C145b/c sensor must maintain separate part numbers for each unit. It is not acceptable to have both a TSO-C145b/c and TSO-C196 TSOA/LODA simultaneously (see paragraph 3-5.b(4)).
Note: Maintaining separate part numbers might only require different software part numbers.
(3) It is acceptable for TSOA/LODA applicants with TSO-C145b/c sensors to simply disable the GPS/SBAS tracking loops in their equipment to qualify as a TSO-C196 sensor.
(4) TSOA/LODA applicants with TSO-C145a sensors will need to show requirements traceability between RTCA/DO-229C and RTCA/DO-316 along with data applicability to receive appropriate certification credit. In particular, applicants will need to 5
show com pliance to any RTCA/DO-316 requirements not in RTCA/DO-229C; and requirements that are more stringent such as enroute/terminal mode accuracy and the broadband external interference noise environment.
c. There is no plan to withdraw TSO-C129(AR) authorizations or TSO-C129a.
Manufacturers can continue production according to their TSOA/LODA or submit new designs for approval according to TSO-C129a and the equipment is still eligible for installation in accordance with the guidance in this AC.
Note: TSO-C129/C129a does not provide sufficient performance for all operational applications.
d. It is possible to receive a TSO-C129a Class A incomplete system TSOA/LODA for displays or navigation computers provided they meet the applicable requirements in RTCA/DO-208, with Change 1, Minimum Operational Performance Standards for Airborne Supplemental Equipment Using Global Positioning System (GPS), dated September 21, 1993, as modified by TSO-C129a and are linked by part number to specific GPS sensors in the TSO application documentation.
e. TSO-C129a will take precedence when adding an incomplete system TSO-C129a Class A TSOA/LODA to an FMS with an existing TSO-C115b, Airborne Area Navigation Equipment Using MultiSensor Inputs, TSOA/LODA if there are any conflicts between the two TSOs.
Note: Paragraph 3-1.e is intended for new applications.
There is no intent to invalidate older equipment may have received an incomplete system TSOA/LODA per TSO-C129.
3-2. TSO-C145(AR)/C146(AR).
a. TSO-C145c/C146c defines an acceptable standard for GPS/SBAS equipm ent.
There is no equipment limitation for GPS/SBAS-equipped aircraft to have other positioning and navigation systems on-board. Additionally, RTCA/DO-229D referenced by the GPS/SBAS TSOs will remain as the MOPS for single-frequency GPS/SBAS equipment. Future multifrequency/multi-system equipment will have a separate MOPS and TSO.
b. There is no plan to withdraw authorizations to the earlier TSO revisions, so manufacturers of TSO-C145a/C146a or TSO-C145b/C146b equipment can continue production according to their TSOA/LODA; and the equipment is still eligible for installation in accordance with the guidance in this AC.
c. The difference between revisions ‘b’ and ‘c’ to the GPS/SBAS TSOs is eliminating a 3 decibel (dB) broadband intra-system noise credit for operational Class 1 and Class 2 equipment in RTCA/DO-229D. TSO-C145b/C146b operational Class 3 and Class 4 equipment fully complies with the requirements in TSO-C145c/C146c and is automatically eligible for a revision ‘c’ authorization, i.e., no further evaluation is necessary.
d. TSO-C145b/C146b operational Class 1 and Class 2 equipm ent that did not claim the 3dB noise credit is also eligible for a revision ‘c’ authorization. The TSOA/LODA holder only needs to provide substantiation that they did not take the noise credit during the original certification.
e. Displays or navigation computers (such as flight management systems (FMSs)) may receive a TSO-C146c Class 1, 2, or 3 (functional Class Gamma) incomplete system TSOA/LODA provided they meet the applicable Class Gamma requirements in RTCA/DO- 229D and are linked by part number to specific GPS/SBAS sensors in the TSO application documentation. This is an effective method to aid GPS/SBAS installation approvals to make a complete system (sensor, navigation computer, display) at the aircraft level.
(1) FMSs are not required to have an incomplete system TSO-C146c Class Gamma TSOA/LODA to provide or process GPS/SBAS navigation functions such as flight path deviation data and GPS/SBAS alerts. It is sufficient to use TSO-C115b plus an alternate method to demonstrate compliance with the applicable RTCA/DO-229D performance requirements. However, an incomplete system TSO-C146c Class Gamma TSOA/LODA provides an established means for the installed FMS to demonstrate navigation computer compliance for credit as a GPS/SBAS system.
Note 1: FMSs that do not meet the applicable RTCA/DO-229D Class Gamma performance requirements can still receive Position/Velocity/Time (PVT) inputs from a TSO-C145(AR) sensor and apply them consistent with their existing approvals. Using a TSO-C145(AR) sensor can remove certain performance limitations (see paragraphs 5-5.2.b and 5-5.2.c).
Note 2: TSOA/LODA applicants may request deviations to TSO-C146c or RTCA/DO-229D performance requirements for the FMS if they demonstrate an equivalent level of safety.
(2) TSO-C146c will take precedence when adding an incomplete system TSO-C146c Class Gamma TSOA/LODA to an FMS with an existing TSO-C115b TSOA/LODA if there are any conflicts between the two TSOs.
(3) FMSs already incorporating GPS and baro-VNAV capability may qualify as TSO-C146c Class Gamma-2 navigation computer systems when receiving inputs from a TSO-C145c Class Beta-2 or Class Beta-3 sensor. The incomplete system TSO-C146c Class Gamma approval would be added to the TSO-C115b approval or replace the TSO-C129(AR) approval if one exists. Applicants are encouraged to seek an incomplete system Class Gamma- 2 navigation computer approval for these FMSs (or Class Gamma-1 for FMSs that don’t process vertical deviations) to provide full SBAS-based capability for enroute through lateral navigation (LNAV)/vertical navigation (VNAV).
Note: Several issues must be considered for FMSs integrating both baro-VNAV and GPS/SBAS to provide 7
LNAV/VNAV capability. For example; unambiguous navigation source and approach service level annunciation, discontinuities or jum ps when switching from baro-VNAV to GPS/SBAS vertical guidance, and prioritizing when to use which source (see paragraphs 6-3.c, 15-2.a), and 17-5).
(4) Localizer performance without vertical guidance (LP)/ localizer performance with vertical guidance (LPV) is considered a severe major/hazardous failure condition for misleading information (see paragraph 11-2.a). Applicants may consider a Class Gamma-3 incomplete system TSOA/LODA to provide LP/LPV navigation computer functions if the FMS incorporates level B software. Otherwise, applicants may consider a Class Delta-4 configuration when integrating LP/LPV capability if the FMS does not incorporate level B software since this configuration doesn’t require the FMS to process LP/LPV data (see paragraphs 3-2.e(5) and 3-2.e(6) below). The FMS could still operate as a Class Gamma-2 navigation computer for GPS/SBAS functions other than LP/LPV; or according to its TSO-C115b/C129(AR) authorization.
(5) For Class Delta-4 integrations, FMSs in typical federated avionics architectures do not need an incomplete system TSO-C146c Class Delta-4 TSOA/LODA since the FMS does not provide or process flight path deviation data or GPS/SBAS alerting functions during the final approach segment. The complete system (sensor, Class Delta-4 LP/LPV final approach segment capability, and display) will be confirmed at the time of installation.
(a) The FMS may provide an interface for pilot control of the Class Delta-4 equipment and may host database information such as the Final Approach Segment (FAS) data block for input to the Class Delta-4 equipment. The Class Delta-4 TSOA/LODA holder must list the specific compatible FMS(s), or provide a detailed description of the database information needed and the data format to establish FMS compatibility as a limitation in the installation manual.
(b) Non-federated FMS architectures, such as integrated modular avionics, may have the Class Delta-4 LP/LPV final approach segment capability residing within the FMS functions or components. These FMSs will need an incomplete system TSO-C146c Class Delta-4 TSOA/LODA and appropriate software level for the Class Delta-4 performance requirements listed in RTCA/DO-229D that they provide.
Note: Applicants with non-federated FMS architectures may also consider pursuing incomplete system TSO-C146c Class Gamma approval for the navigation computer functions/components.
(6) An FMS that is part of an aircraft-level TSO-C146c Class Delta-4 integration normally provides the RNAV capability for the initial and intermediate approach segments to place the aircraft in proper position for the Class Delta-4 GPS/SBAS equipment to conduct the LP/LPV final approach segment. The FMS can also provide RNAV capability for the missed approach segment.
Note: RNAV capability for the initial, intermediate, and missed approach segments can be accomplished by the FMS according to its TSO-C115b and/or TSO-C129(AR) approval. However, the FMS will be limited to its approved functions per those TSOs or as modified if receiving PVT inputs from a TSO-C145(AR) sensor (see paragraphs 5-5.2.b and 5-5.2.c).
3-3. TSO-C161a/C162a.
a. TSO-C161a, Ground Based Augmentation Sys tem Positioning and Navigation Equipment, defines an acceptable standard for GPS/GBAS equipment that provides precision approach capability and PVT information to navigation management equipment. TSO-C162a, Ground Based Augmentation System Very High Frequency Data Broadcast Equipment, defines an acceptable standard for GPS/GBAS equipment designed to receive a very high frequency data broadcast (VDB) and output the VDB messages to GPS/GBAS positioning and navigation equipment.
b. TSO-C161a and TSO-C162a invoke only the requirements in RTCA/DO-253C, Minimum Operational Performance Standards for GPS Local Area Augmentation System Airborne Equipment, dated December 16, 2008, that are needed to support Category I (CAT I) precision approach and, if applicable, the GPS/GBAS positioning service. The equipment may output PVT based on either the GPS/GBAS positioning service, or PVT from GPS or GPS/SBAS. The PVT based on GPS/GBAS is only available when the ground station supports the positioning service. TSO-C161a equipment must also obtain either TSO-C129a, TSO-C145, TSO-C146, or TSO-C196 TSOA. Requirements in RTCA/DO-253C needed to support precision approaches beyond CAT I are not invoked in these TSOs as these requirements have not been validated. Once validated, the FAA will either publish TSO revisions or new TSOs.
c. TSO-C161a and TSO-C162a compliant equipment does not address any RNAV capability. A partial missed approach capability is an option in the GPS/GBAS equipment but is not required (see note). RNAV capability for the initial, intermediate, and missed approach segments must be addressed during the aircraft GPS/GBAS integration (see paragraph 14-8.1).
Note: Equipment meeting the RTCA/DO-253C precision and navigation (PAN) requirements can include an optional capability to provide guidance for the initial, straight-out portion of the missed approach segment. However, this capability doesn’t provide guidance for the complete missed approach segment.
3-4. RNAV Multi-Sensor Equipment.
a. TSO-C115b defines an acceptab le certification standard for obtaining design and production approval for multi-sensor navigation systems or FMSs integrating data from multiple navigation sensors. There is no plan to withdraw TSO authorizations granted under TSO-C115a. Manufacturers of TSO-C115a equipment can continue production according to their TSOA/LODA. However, integrating GNSS into the multi-sensor navigation equipment is 9
10 considered a major change and requires an application for a TSO-C115b TSOA/LODA.
(1) TSO-C115b only addresses TSO-C129 Class B and C sensors because it was published prior to TSO-C129a, TSO-C145(AR), TSO-C161(AR), and TSO-C196. It is acceptable to integrate any GNSS sensor with TSO-C115b equipment.
(2) Applicants with equipment that has an existing TSO-C115b approval may add an incomplete system TSO-C129a Class A or TSO-C146c Class Gamma TSOA/LODA for the navigation computer functions. The navigation computer requirements in TSO-C129a or TSO-C146c will take precedence if there are any conflicts with the requirements in TSO-C115b (deviations to TSO-C115b need to be approved). See paragraphs 3-1 and 3-2 respectively for additional information on TSO-C129a and TSO-C146c incomplete system TSOA/LODAs.
b. Multi-sensor systems must also comply with the appropriate TSO requirements for the individual sensor; for example, TSO-C66c, Distance Measuring Equipment(DME) Operating Within the Radio Frequency Range of 960 to 1215 Megahertz, for DME equipment.
Refer to AC 90-100 (latest revision), U.S. Terminal and Enroute Area Navigation (RNAV) Operations, for operational approval of terminal and enroute area navigation systems for use in the United States national airspace system.
Note: The multi-sensor navigation system must comply with TSO-C115b (or a later revision) when integrating GNSS sensors. It is not acceptable to use equipment certified to earlier revisions (see paragraph 3-5.b(1)).
3-5. Multiple TSO Authorizations.
a. Multip le TSO authorizations may be granted for equipment that accomplishes multiple functions. However, the number of TSO markings on positioning and navigation equipment should be minimized to avoid confusion during installation and approval.
b. The following combinations of TSO authorizations must not be granted, as the TSOs either contradict each other or are superfluous to a TSO with broader scope:
(1) TSO-C115/C115a with TSO-C129(AR), TSO-C145(AR)/C146(AR), TSO-C196, or TSO-C161a/C162a).
(2) TSO-C129(AR) Class A with TSO-C146(AR) Class Gamma.
(3) TSO-C129(AR) Class B or Class C with TSO-C145(AR), TSO-C146(AR) or TSO-C196.
(4) TSO-C196 with TSO-C145(AR) or TSO-C146(AR).
Note: The intent is that combinations not specifically prohibited by paragraphs 3-5.b(1) through 3-5.b(4) are acceptable.
Chapter 4. Equipmen t Performance - Advisory Vertical Guidance.
4-1. Introduction.
Position ing and navigation equipment may provide vertical path deviation guidance indications on a non-essential, not-required basis as an aid to help pilots meet barometric altitude restrictions. Equipment that has this capability typically uses GNSS or baro-VNAV, but may use any method to generate the vertical path information. Advisory vertical guidance does not provide approved vertical guidance deviation indications for operational credit. Only vertical guidance deviation indications for LNAV/VNAV or LPV approach procedures are approved for operational credit.
Note 1: Advisory vertical guidance is an optional capability implemented at the equipment manufacturer’s discretion for enroute, terminal, and/or approach operations; not a requirement for positioning and navigation equipment.
Note 2: Paragraph 4-1 does not apply to instrument landing system (ILS).
Note 3: We recommend that equipment manufacturers provide a means for pilots to inhibit advisory vertical guidance to support nonprecision approach training requirements.
4-2. Implementations.
a. It is accep table to provide advisory vertical guidance as a descent aid during oceanic/remote, enroute, and terminal operations.
b. GPS equipment without GPS-provided approved vertical capability (TSO-C129(AR), TSO-C196, TSO-C145/C146(AR) Operational Class 1) and no baro-VNAV integration in the aircraft may provide advisory vertical guidance deviation indications on any approach using the LNAV line of minima.
c. Some aircraft integrations may use GPS equipment without GPS-provided approved vertical capability (TSO-C129(AR), TSO-C196, TSO-C145/C146(AR) Operational Class 1) to provide lateral path deviation indications and an approved baro-VNAV installation to provide vertical path deviation indications for an approach to the LNAV/VNAV line of minima.
(1) This integration constitutes approved vertical guidance to conduct a charted instrument approach procedure to the LNAV/VNAV line of minima and should not be confused with advisory vertical guidance applications.
(2) There may be occasions where it is operationally advantageous to use the LNAV line of minima rather than the LNAV/VNAV minima during an instrument approach procedure. It is acceptable for approved baro-VNAV installations to provide advisory vertical 11
guidance when using the LNAV line of m inima. However, during these operations, the flight crew must use the primary barometric altimeter as the primary reference for compliance with all altitude restrictions associated with the instrument approach procedure; including compliance with all associated step-down fixes.
Note 1: Baro-VNAV integrations may use non-GPS RNAV position sources to generate lateral path deviations for approaches that do not require GPS.
Note 2: Baro-VNAV is subject to temperature and pressure altitude (i.e., correct altimeter setting) performance limitations that could potentially cause advisory vertical path guidance to fall below step-down fixes on LNAV approaches.
d. During RNAV (GPS) instrument approach operations, TSO-C145/C146(AR) Operational Class 2 equipment may provide advisory vertical guidance when the procedure defines only the LNAV line of minima (i.e., procedures without a charted LNAV/VNAV and/or LPV line of minima). GPS/SBAS Operational Class 2 equipment is not capable of performing LP approaches, but may provide advisory vertical guidance for the LNAV line of minima on instrument approach procedures that define both an LNAV and LP line of minima. GPS/SBAS Operational Class 2 equipment may not provide advisory vertical guidance on instrument approach procedures with an LNAV line of minima published with LNAV/VNAV and/or LPV lines of minima. The reason is due to potential confusion over advisory versus approved vertical guidance and which line of minima applies. This human factors issue is even more critical when GPS/SBAS equipment “fails-down” to LNAV minima during the final approach segment if vertical guidance is lost during an LNAV/VNAV approach.
Note: There is no intent to preclude using baro-VNAV advisory vertical guidance for an LNAV approach provided it is selected prior to the final approach fix. See paragraph 17-5 when integrating baro-VNAV with GPS/SBAS vertical guidance.
e. During RNAV (GPS) instrument approach operations, TSO-C145/C146(AR) Operational Class 3 equipment may provide advisory vertical guidance when the procedure defines only the LNAV and/or LP line of minima (i.e., procedures without a charted LNAV/VNAV and/or LPV line of minima). GPS/SBAS Operational Class 3 equipment may not provide advisory vertical guidance on instrument approach procedures with an LNAV line of minima published with LNAV/VNAV and/or LPV lines of minima. The reason is due to potential confusion over advisory versus approved vertical guidance and which line of minima applies. This human factors issue is even more critical for GPS/SBAS equipment that has a “fail-down” mode to LNAV minima during the final approach segment.
Note 1: LP approach procedures will never be published with other lines of minima that contain approved vertical guidance (i.e., LNAV/VNAV or LPV). LNAV and LP lines of minima can be published on the same approach 12
chart; and, it is acceptable to provide advisory vertical guidance during approach operations using these lines of mi nima.
Note 2: There is no intent to prevent using baro-VNAV advisory vertical guidance for an LNAV approach provided it is selected prior to the final approach fix. See paragraph 17-5 when integrating baro-VNAV with GPS/SBAS vertical guidance.
f. TSO-C146(AR) Operational Class 4 (Functional Class Delta) equipment provides an “ILS look-alike” capability for LPV and LP only. GPS/SBAS Operational Class 4 equipment indicates LP by flagging the vertical guidance indications. It is acceptable for Class Delta equipment to provide advisory vertical guidance during approach procedures to LP minimums. However, it is not acceptable for Operational Class 4 equipment to provide advisory vertical guidance when the equipment “fails-down” from LPV to LNAV minima during the final approach segment due to potential confusion over advisory versus approved vertical guidance and which line of minima applies.
Note: Per RTCA/DO-229D it is not appropriate for GPS/SBAS Operational Class 4 equipment to “fail-down” from LP to LNAV.
g. Advisory vertical guidance may be provided during very high frequency omni-directional range (VOR) and non-directional beacon (NDB) instrument approach operations (including overlays with “or GPS” in the title) and also for older GPS instrument approach operations titled “GPS RWY XX”.
4-3. Limitatations.
a. There are no TSO requirem ents or MOPS standards regarding advisory vertical guidance minimum performance or how advisory vertical guidance is generated (see notes).
Positioning and navigation equipment manufacturers can generally use their discretion when choosing to implement this capability. However, the equipment manufacturers cannot claim any operational or certification credit for advisory vertical guidance. When implementing advisory vertical guidance, the equipment manufacturer must provide an equipment limitation in the installation/instruction manual for inclusion in the airplane flight manual supplement (AFMS)/rotorcraft flight manual supplement (RFMS) and may also include the information in the operations manual. The limitation should state the following or be equivalent:
“Advisory vertical guidance deviation information is only an aid to help pilots comply with altitude restrictions. When using advisory vertical guidance, the pilot must use the primary barometric altimeter to ensure compliance with all altitude restrictions, particularly during instrument approach operations.”
Note 1: RTCA/DO-229D, referenced by the GPS/SBAS TSOs, does provide some cursory requirements related to advisory vertical guidance.