15-3. Failure Indications.
a. It is important to install the equipment such that all failure conditions result in appropriate displays.
b. For installations not incorporating an external navigation display (CDI, HSI, etc.), complete blanking of the CDU display is acceptable.
c. Presentation of a failure/status annunciation (flag or integrity annunciation) does not require removal of navigation information from the navigation display.
d. A unique annunciation of the loss of integrity monitoring should be provided.
Consideration may be given to the aircraft’s overall cockpit alerting philosophy to reduce the number of independent indicators.
Note: Paragraph 15-3.d does not apply to GPS/SBAS Class Delta-4 LPV operations because this equipment only functions during the final approach segment (or its extension) where loss of integrity monitoring is defined as a loss of navigation. A normal loss of navigation indication is sufficient.
15-4. Documenting Effects of GNSS Outage.
a. GNSS outages are a normal operating condition in areas with routine interference testing, and can occur anywhere in the NAS due to unintentional interference. The aircraft-level effect from losing GNSS positioning, velocity, and timing is a complex problem that depends on the GNSS equipment design, the degree of integration with other systems, and where the installations take place. GNSS outputs are being integrated into a variety of functions beyond traditional navigation data. For example, GNSS PVT is being used for TAWS/HTAWS, synthetic vision systems, ADS-B, and micro-electro-mechanical system (MEMS) accelerometers/sensors in air data attitude heading reference system (ADAHRS) inputs to electronic primary flight displays. There are a wide range of integrations from legacy aircraft with self-contained GNSS navigation units and a simple autopilot interface to new production aircraft with digital cockpits using integrated modular avionics providing the advanced functions mentioned above and more (including potential GNSS time applications).
b. TC and STC applicants must document, in the AFM(S)/RFM(S) normal section, the aircraft-level effects if GNSS PVT is not available or the outputs are disrupted. The AFM(S)/RFM(S) must describe the effects on any affected system from losing GNSS outputs;
the indications that should be expected if GNSS outputs are disrupted; and the flight crew or pilot procedures. TC and STC applicants should not use GNSS for both primary and back-up critical systems that would result in a single point of failure due to a GNSS outage.
Additionally, installers should be alert for equipment upgrade combinations that may result in GNSS as a single point of failure.
c. TSOA applicants must include, in their required operating guide, a description of the effects of GNSS outage on the functions in the article. The installation instructions must 116
include a description of the GNSS outage effects on the outputs from the article so the installer can evaluate the effects at the aircraft level.
15-5. Installed Performance Capability.
a. It is essential that procedures or optional functions an aircraft is not capable of performing are either removed from the GNSS equipment database, or otherwise inhibited, even if the avionics do support the function (RNP AR procedures or procedures with RF legs for example). The AFM(S)/RFM(S) must contain an appropriate entry for any limitations (see paragraph 12-7 and 12-8).
b. Helicopter-specific RNAV(GPS) approach procedures to LNAV, LNAV/VNAV, LP, or LPV minima can have unique characteristics. For the most part, helicopter-specific RNAV(GPS) procedures that contain unique features are published as Special Instrument Approach Procedures. Examples of unique criteria are: steep descent gradients, speed restrictions, and turns at the FAF greater than allowed in public approach procedures. The GNSS equipment standards generally support the TERPs criteria for helicopter-specific RNAV(GPS) procedures, but that does not ensure installed performance. It is incumbent upon the airworthiness approval applicant to demonstrate the rotorcraft GNSS integration is capable of meeting the desired performance for the intended function, or provide appropriate RFM(S) limitations. Particular attention should be directed toward the autopilot integration and whether or not the autopilot, coupled to all flight axes, is able to support the intended function.
c. Data Quality Requirements (DQRs) for installed databases are normally addressed at the equipment design level. The DQRs applicable to the intended function must be confirmed at the aircraft level during airworthiness approval. A Type 2 LOA may be used to ensure the database requirements, including tool qualification, is appropriate for the intended function of the installed equipment. A Type 2 LOA also provides evidence the data quality characteristics are described and valid for the compatible equipment listed on the LOA (see paragraph 19-8).
15-6. G PS Equipment.
a. GPS equipment only uses a RAIM algorithm to provide integrity and has reduced availability of an integrity-assured position solution at the minimum, guaranteed satellite constellation configuration compared to equipment using GPS/SBAS or GPS/GBAS augmentation.
b. When installing GPS equipment that uses RAIM for integrity during instrument meteorological conditions (IMC), there must be a performance limitation in the installation manual (or equivalent installation documentation) for inclusion as a limitation in the AFM(S)/RFM(S). The limitation is to have other navigation equipment available appropriate to the operation.
c. There must also be a flight planning performance limitation in the installation instructions (or equivalent installation documentation) for inclusion as a limitation in the AFMS/RFMS. This limitation must indicate the equipment has a performance limitation necessitating an alternate airport flight planning operational mitigation (see appendix 5 for an 117
example). The Aeronautical Information Manual provides some operational flexibility regarding the flight planning operational mitigation for equipment that has FDE capability.
Refer to paragraphs 1-1-17b.5 and 1-2-3d in the Aeronautical Information Manual for specific operational guidance.
d. The limitation language can provide further reference to the Aeronautical Information Manual for additional information.
Note 1: See appendix 1 for methods to remove the limitation for other navigation equipment in oceanic and remote areas.
Note 2: The performance limitations in paragraphs 15-6.b and 15-6.c do not have to use the specific language above or the example in appendix 5. The intent is to indicate the equipment has a performance limitation and adequately capture that limitation in the AFM(S)/RFM(S).
15-7. GPS/SBAS Equipment.
This paragraph provides guidance related to the installation of GPS/SBAS equipment supporting LP/LPV approaches. This guidance applies to all installations, but is only expected to be specifically evaluated as part of the initial airworthiness approval (i.e., initial airworthiness approval may provide sufficient substantiation for follow-on installations).
15-7.1 Limitations.
a. When installing equipment that uses GPS/SBAS for integrity during instrument meteorological conditions, there must be a performance limitation in the installation instructions (or equivalent installation documentation) for inclusion as a limitation in the AFMS/RFMS. This limitation must indicate the equipment has a performance limitation necessitating an alternate airport flight planning operational mitigation (see appendix 5 for an example). The Aeronautical Information Manual provides some operational flexibility regarding the flight planning operational mitigation for operators that also have baro-VNAV capability. Refer to paragraphs 1-1-18c.9 and 5-4-5m.7(h) in the Aeronautical Information Manual for specific operational guidance.
Note: The performance limitation in 15-7.1a does not have to use the specific language above or the example in appendix 5. The intent is to indicate the equipment has a performance limitation and adequately capture that limitation in the AFMS/RFMS.
b. The limitation above can provide further reference to the Aeronautical Information Manual for additional information.
15-7.2 LP Approach.
a. It is not acceptable to install TSO-C145a/C146a equipment that hasn’t been certified with LP capability in such a way as to make the equipment perform LP approaches. To do so is forcing the equipment to perform an unintended, non-certified function that is a violation of the original certification basis.
b. TSO-C145a/C146a equipment manufacturers may provide LP capability through software changes. The guidance above in paragraph 15-7.2a is not intended for equipment updated by the manufacturer.
c. GPS/SBAS installations of Class 3 or Class 4 equipment that complies with TSO revision ‘b’ or later with a deviation to not include LP capability must have an appropriate limitation included in the AFM(S)/RFM(S).
d. There are no unique LP flight testing requirements for airworthiness applicants that have performed LPV flight testing. LP uses the same signal and horizontal performance capability that GPS/SBAS provides during LPV. Airworthiness applicants with LPV data may use similarity or analysis as the basis for approving LP capability on the aircraft. The only installation considerations are that LP is clearly and unambiguously annunciated, and the FGS is able to accept horizontal-only approach guidance inputs (see paragraph 21-2.2.f ).
Note: An LP approach is somewhat analogous to a localizer-only approach in that localizer-only uses the horizontal ILS component to provide horizontal guidance.
However, LP and Localizer are not operationally similar because LP is not published on the same approach chart as LPV.
15-7.3 Update Rate.
For sensors and stand-alone equipment that support an LP/LPV approach capability (Class 3 equipment of TSO-C145(AR), Class 3 and 4 equipment of TSO-C146(AR)), the position used for the approach should be computed and output at a minimum 5 Hz rate per the requirements in RTCA/DO-229D, Change 1. If the equipment provides a 1 Hz output, the installation must use inertial aiding or some other equivalent method to obtain deviations at a minimum 5 Hz rate.
15-7.4 Performance Monitoring.
Sufficient information must be provided in the flight deck to allow the pilots to monitor the approach operation progress and safety. This in-flight performance monitoring must include:
a. Unambiguous indication of the intended path for the approach (e.g., LP/LPV approach type, approach identifier, active waypoint); and 119
b. Unambiguous indication of the position of the aircraft with respect to the intended path (e.g., cross track and vertical deviation information).
15-7.5 System Status.
The flight crew must have a means to evaluate the aircraft equipment’s capability to accomplish the approach operation. The flight crew must be advised of failed aircraft systems or equipment affecting the decision to continue to the destination or divert to an alternate.
Note: System status considerations also apply to IFR departure operations such as standard instrument departures. The flight crew must be advised of failed aircraft systems or equipment affecting the decision to depart.
15-7.6 Annunciations.
a. For installations containing more than one approach navigation source, the navigation source (for example, ILS, GPS/SBAS, baro-VNAV, etc.) selected for the approach must be positively indicated in the primary field of view. Consideration should be given to the overall aircraft-level annunciation philosophy.
Note: Aircraft requiring two pilots must have this
annunciation at each pilot station.
b. The approach type (LP/LPV, LNAV/VNAV, LNAV) must be clearly annunciated to the pilot prior to and throughout the entire approach in the primary field of view.
(1) It is acceptable for a primary flight display to annunciate localizer/glideslope using an abbreviation such as LOC/GS when the aircraft integration includes GPS/SBAS LPV, GBAS GLS Category I, and ILS. There must be an unambiguous indication the navigation source is provided by GPS/SBAS, GPS/GBAS, or ILS, as appropriate, in the primary field of view.
(a) LNAV/VNAV and LNAV approach types must still be annunciated, when applicable, if the applicant uses LOC/GS for LPV, or GLS Category I, or ILS.
(2) It is acceptable for the primary flight display to annunciate azimuth/elevation using an abbreviation such as AZ/EL when the aircraft integration includes GPS/SBAS LPV, GPS/GBAS GLS Category I, and MLS. There must be an unambiguous indication the navigation source is provided by GPS/SBAS, GPS/GBAS, or MLS, as appropriate, in the primary field of view.
(a) An unambiguous navigation source annunciation is still necessary if there is no MLS capability on the aircraft and AZ/EL is being used for a combination of only LPV and GLS Category I.
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(b) LNAV/VNAV and LNAV approach types must still be annunciated, when applicable, if the applicant uses AZ/EL for MLS, LPV, or GLS Category I.
c. Changes in the approach type annunciations should be very clear and distinctive to quickly catch the flight crew’s attention. This is so the flight crew knows and understands that there has been a change in service level and will make adjustments as needed to fly the appropriate approach minimums.
d. Aircraft installations containing both GPS/SBAS-provided vertical guidance and baro-VNAV vertical guidance capability should be carefully evaluated to ensure the annunciations clearly indicate the approach service provided and the sensor driving the vertical display (see paragraph 18-5).
15-7.7 Accessibility.
a. Single equipment installations must be accessible and usable by the pilot. For aircraft requiring two crew members, the equipment should be accessible and usable by either pilot located at a pilot or copilot crew station.
b. Dual (or more) equipment installations must have a convenient and expedient way to update data, information and programs, and a means to ensure consistency between the equipment (e.g., a “cross-load” capability can ensure identical data).
15-7.8 GPS/SBAS Glidepath Low Deviation Alerting Function.
a. To help prevent controlled flight into terrain, manufacturers and operators of aircraft that install a GPS/SBAS LPV and LNAV/VNAV approach capability are highly encouraged to install a glidepath deviation alerting function.
Note: There is no differentiation between LPV approaches using a 35m VAL or 50m VAL in the FAS datablock. The guidance above is intended for all LPV approaches.
b. If implemented, GPS/SBAS vertical performance monitoring and alerting based on deviation guidance outputs (i.e., FTE) can be used to provide a glidepath deviation alerting function. The alert must be prominent and easily recognized by pilots and may be either aural or visual or both. Manufacturers may propose other alternatives.
15-8. GPS/GBAS VDB Receiver.
This paragraph provides guidance related to the installation of a VDB receiver, including those embedded within a multi-mode receiver (MMR). This guidance applies to all installations, but is only expected to be specifically evaluated as part of the initial airworthiness approval (i.e., initial airworthiness approval may provide sufficient substantiation for follow-on installations).
15-8.1 Interference – EMC.
a. The VDB receiver may be susceptible to interference from VHF communications equipment on the aircraft.
b. At least 53 decibels relative to carrier power (dBc) of isolation is required to protect the VDB receiver from desensitization for horizontally polarized VDB antennas operating in the presence of a +40 decibels referenced to one milliwatt (dBm) (effective isotropically radiated power) vertically polarized VHF communications transmission. This isolation may be achieved through a combination of 15 dBc of cross-polarization rejection and -38 dBc of antenna coupling.
c. The full 53 dBc must be achieved through antenna coupling for aircraft using a vertically polarized VDB antenna. It is not feasible to accomplish this isolation simply due to free space path loss. Special considerations will be required.
15-8.2 Compatibility with Other Systems.
The VDB receiver may use the localizer or VOR antenna. If this involves introducing additional splitters in the RF path, the performance of the localizer or VOR receivers must be re-assessed due to the degradation in signal strength resulting from the splitter. The latest revisions of AC 23-8, AC 25-7, AC 27-1, and AC 29-2 provide guidance material on these installations.
15-8.3 VDB Aircraft Implementation Loss.
The aircraft implementation loss should be less than 6 dB (gain) and greater than -15 dB (loss) for a horizontally polarized antenna, or less than 6 dB (gain) and greater than -11 dB (loss) for a vertically polarized antenna. The total implementation loss is the algebraic sum (in dBs) of the antenna gain (referenced to an isotropic radiator) and the attenuation (as a negative number) between the antenna and the VDB receiver. The attenuation includes line dissipation and voltage standing wave ratio (VSWR) mismatch losses. The implementation loss should be verified over the frequency range 108.00 to 117.95 MHz.
Note: Integrations that re-use VOR and localizer antennas may be problematic when trying to meet the -15 dB aircraft implementation loss allocation due to aircraft-level issues
(e.g., antenna patterns, cabling losses, splitters, connectors, etc.) and/or receiving the signal from any direction (see paragraph 5-4.1.b).
15-9. GPS/GBAS Navigation Computer Receiver.
This paragraph provides guidance related to the installation of GPS/GBAS equipment supporting GLS Category I approaches. This guidance applies to all installations, but is only expected to be specifically evaluated as part of the initial airworthiness approval (i.e., initial airworthiness approval may provide sufficient substantiation for follow-on installations).
15-9.1 G PS/GBAS Equipment Intended for Category I.
Category I approach operation must provide for the following:
a. The GPS/GBAS approach and landing systems should meet the guidance set forth in AC 120-29 (latest revision), Criteria for Approval of Category I and Category II Weather Minima for Approach, appendix 2.
Note 1: GPS/GBAS no longer requires proof of concept testing to establish appropriate criteria for operational approval and certification testing for Category I as stated in AC 120-29a, appendix 2.
Note 2: AC 120-29a will be replaced soon with a new 90 series AC.
b. For installations where the GPS/GBAS sensor is integrated with other landing systems, such as an MMR, the “ILS look-alike” implies that the MMR signal to the autopilot is scaled in terms of depth of modulation (DDM) units, with sensitivity as described in RTCA/DO-253C, paragraph 2.3.11.5.1.1.1 and paragraph 2.3.11.5.1.2.1.
c. Aircraft integrating GPS/GBAS must have an RNAV capability for the initial, intermediate, and missed approach segments.
d. FMSs that are part of GPS/GBAS integrations should provide the RNAV capability for the initial and intermediate approach segments to place the aircraft in proper position for the GPS/GBAS equipment to conduct the GLS final approach segment. The FMS should also provide RNAV capability for the missed approach segment.
(1) RNAV capability for the initial, intermediate, and missed approach segments can be accomplished by the FMS according to its TSO-C115b (or later revision) or TSO-C146(AR), approval. However, to provide the RNAV function, the FMS will need the ability to recognize a GLS RWY XX approach procedure, store the appropriate segment information in the database, and recall the information as part of the selected approach procedure.
(2) FMS RNAV capability to support GPS/GBAS approaches should be addressed as part of the considerations for installation approval.
e. All installations should meet the constraints on the total differential group delay for the antenna, navigation computer receiver, and all cables/connections referenced in RTCA/DO-253C paragraphs 2.3.6.4 for GPS satellites and 2.3.6.5 for GPS/SBAS satellites.
15-9.2 GPS/GBAS Stand-Alone or MMR Equipment Intended for Category I.
When implementing GPS/GBAS to support GLS Category I operation either as stand-alone equipment or integrated as part of the MMR, the following criteria should be addressed:
a. The airplane’s system response to a loss of GPS/GBAS guidance should be no worse than a similar ILS event (minor) and should be free of objectionable transients and sustained oscillations.
b. The GPS/GBAS should be shown to be compatible with a GPS/GBAS service provision consistent with criteria contained in ICAO Annex 10 (as modified by amendment 84) or, accepted by ICAO for incorporation into Annex 10. It is noted that the GPS/GBAS ground facility approval process is outside of the airborne system approval process.
c. It should be possible to initiate a missed approach at any point during the approach down to the minimum altitude sought by the applicant for operational use. Safe go-around should be demonstrated at this minimum altitude.
Note: The presence of large navigation system errors (detectable only by pilot visual reference out-the-window) should not hinder the pilot’s decision to land or conduct a missed approach. Annex 10 integrity limits for Category I operations reference vertical errors as large as 35 m and lateral errors as large as 40 m.
15-9.3 Performance and Performance Monitoring.
Sufficient information must be provided in the flight deck to allow the crew to monitor the approach operation progress and safety. This in-flight performance monitoring must include:
a. Unambiguous identification of the intended path for the approach (e.g., approach type, approach identifier, frequency or channel number); and
b. Indication of the position of the aircraft with respect to the intended path (e.g., cross track and vertical deviation information).
15-9.4 Integrity.
The onboard landing system components, considered separately and in relation to the other associated onboard systems, should be designed to comply with the appropriate 14 CFR part 2X.1309, considering any specific safety-related criteria in the latest revision of AC 120-29, appendix 2 and the operating rules. Presenting misleading information to the flight crew on a GPS/GBAS approach should be considered a Hazardous failure condition (see Table
8).
Note: AC 120-29a will be replaced soon with a new 90 series AC.
15-9.5 Continuity of Navigation.
a. Verify continuity of navigation data during normal aircraft maneuvering on the GPS/GBAS approach.
b. Unlike ILS, where the ILS facility is always located in front of the aircraft on approach, a GPS/GBAS facility could be located anywhere with respect to the approach path.
Therefore, the flight test should verify VDB signal reception at all angles relative to the airframe.
15-9.6 Fault Detection and Annunciation.
The automatic means to detect and annunciate misleading guidance signals should have coverage and integrity similar to that of an ILS system when used in the approach flight phase and during go-around.
15-9.7 System Status.
The flight crew must have a means to evaluate the capability of the aircraft elements to accomplish the approach operation prior to and after departure. The flight crew must be advised of failed aircraft systems or components affecting the decision to continue to the destination or divert to an alternate airport.
15-9.8 Annunciation.
a. For installations containing more than one approach navigation source, the navigation source (for example, ILS, GPS/GBAS, etc.) selected for the approach must be positively indicated in the primary field of view. Consideration should be given to the overall aircraft-level annunciation philosophy.
Note: Aircraft requiring two pilots must have this
annunciation at each pilot station.
b. The approach type (GLS) must be clearly annunciated to the flight crew prior to and throughout the entire approach in the primary field of view.
(1) It is acceptable for a primary flight display to annunciate localizer/glideslope using an abbreviation such as LOC/GS, when the aircraft integration includes GPS/SBAS LPV, GPS/GBAS GLS Category I, and ILS. There must be an unambiguous indication the navigation source is provided by GPS/SBAS, GPS/GBAS, or ILS as appropriate in the primary field of view.
(a) LNAV/VNAV and LNAV approach types must still be annunciated when applicable if the applicant uses LOC/GS for LPV, GLS Category I, or ILS.
(2) It is acceptable for the primary flight display to annunciate azimuth/elevation using an abbreviation such as AZ/EL when the aircraft integration includes GPS/SBAS LPV, GPS/GBAS GLS Category I, and MLS. There must be an unambiguous indication the 125
navigation source is provided by GPS/SBAS, GPS/GBAS, or MLS as appropriate in the primary field of view.
(a) An unambiguous navigation source annunciation is still necessary if there is no MLS capability on the aircraft and AZ/EL is being used for a combination of only LPV and GLS Category I.
(b) LNAV/VNAV and LNAV approach types must still be annunciated, when applicable, if the applicant uses AZ/EL for MLS, LPV, or GLS Category I.
15-9.9 Accessibility.
a. Single equipment installations must be accessible and usable by the pilot. For aircraft requiring two crew members, the equipment should be accessible and usable by either pilot located at a pilot or copilot crew station.
b. Dual (or more) equipment installations must have a convenient and expedient way to update data, information and programs, and a means to ensure consistency between the equipment (e.g., a “cross-load” capability can ensure identical data).
15-9.10 Glidepath Capture.
Demonstrate that acceptable glide-path capture, followed by a stable approach, can be accomplished. This demonstration may include simulations and flight test evaluations.
Factors that influence an acceptable capture followed by a stable approach may include, but are not limited to, angular offset with the glide-path centerline, altitude at which the airplane acquires that centerline, airplane flying qualities, wind and turbulence conditions, flap settings and center of gravity conditions, and time interval required by the flight control computer algorithm to acquire stable signals.
Note: With some GPS/GBAS ground facilities, guidance may not be available until the aircraft is within 20 NM of the approach runway. This is because the differential corrections are used in the avionics only within a service region defined for the specific ground facility providing the approach. The ground facility VHF data broadcast indicates the limit of this service region. Although service regions smaller than 20 NM may be indicated, they would not meet the normal, minimum coverage required to support precision approach operations.
15-9.11 Equipment Qualification.
The GPS/GBAS positioning and navigation equipment should meet the minimum performance standard in TSO-C161a. The GPS/GBAS VHF data broadcast equipment should meet the minimum performance standard in TSO-C162a.
15-9.12 Interoperability.
Any GPS/GBAS ground installation provided or used by the applicant for the purpose of performance demonstration should be shown to be consistent with ICAO Annex 10, of Standards and Recommended Practices Applicable for Category I. The applicant must show that the GPS/GBAS ground installation complies with the requirements of ICAO Annex 10. If this compliance cannot be shown, the applicant should identify and evaluate differences between the GPS/GBAS Annex 10 requirements and the flight test site. The applicant should address the implications of those differences with respect to the validity of the flight test demonstrations and address any possible implications on aircraft interoperability when used with an ICAO Annex 10-compliant ground subsystem.
15-9.13 GPS/GBAS Glidepath Low Deviation Alerting Function.
a. To help prevent controlled flight into terrain, manufacturers and operators of aircraft that install a GPS/GBAS GLS approach capability are highly encouraged to install a glidepath deviation alerting function.
b. If implemented, GPS/GBAS vertical performance monitoring and alerting based on deviation guidance outputs (i.e., FTE) can be used to provide a glidepath deviation alerting function. The alert must be prominent and easily recognized by pilots and may be either aural or visual or both. Manufacturers may propose other alternatives.
15-10. Special Use Mission Equipment - Not for Navigation.
A special approval category for GNSS equipment is “Special Use -- Not for Navigation.” Systems that are installed for special use (agriculture, search and rescue, etc.) that will not be used for aircraft positioning or navigation can be approved on a non-interference basis using less strict airworthiness approval methods. Field approvals are normally acceptable for special use GNSS mission equipment. Inspectors should ensure the flight evaluation includes maneuvers that will normally take place while the GNSS equipment is in use and the equipment does not interfere with any required systems on the aircraft. AC 20-168 (latest revision), Certification Guidance for Installation of Non-Essential, Non-Required Aircraft Cabin Systems and Equipment and AC 20-169 (latest revision), Guidance and Certification of Military and Special Mission Modifications and Equipment for Commercial Derivative Aircraft (CDA) provides additional guidance on miscellaneous, non-required equipment installations.