AC 20-199 CHAPTER 1. GENERAL INFORMATION
1.1 Purpose.
This advisory circular (AC) provides guidance for the installation design approval of a radio altimeter (RA) system that is intended for showing compliance with the requirements of title 14, Code of Federal Regulations (14 CFR) 91.220(b). An RA system consists of an RA transceiver, RA antenna(s), coaxial cabling, and display(s) or indicator(s). This AC provides guidance for all components of the RA system, as well as for the use of the existing RA antenna(s) and display(s) or indicator(s). This AC includes guidance for system design (chapter 2 of this AC) and testing (chapter 3 of this AC) to assure proper functioning of the installed RA system, including external radio frequency interference (RFI) tolerance, as well as proper functioning of integrated avionics and flight control systems that utilize the RA. When the RA system is properly installed in an aircraft, in accordance with applicable installation and operational approval guidance and regulations, it is expected that all aircraft-level functional and operational performance criteria will be met.
1.2 Applicability.
This AC is for RA system design organizations, type certificate (TC) applicants (including amended TC and supplemental type certificate (STC) applicants), installers, operators, foreign regulatory authorities, Federal Aviation Administration (FAA) engineers, and the Administrator’s designees.
1.3 Intent of this AC.
1.3.1 This is a guidance document. Its content is not legally binding in its own right and will not be relied upon by the Department as a separate basis for affirmative enforcement action or other administrative penalty. Conformity with the guidance document is voluntary only. Nonconformity will not affect rights and obligations under existing statutes and regulations.
1.3.2 This AC establishes an acceptable means, but not the only means, of compliance. The FAA will consider other methods of demonstrating compliance that an applicant may elect to present. Since the guidance material presented in this AC is not regulatory, terms having a mandatory definition, such as “shall” and “must,” apply either to the reiteration of a regulation itself, or to an applicant who chooses to follow a prescribed method of compliance without deviation. If the FAA becomes aware of circumstances in which following this AC would not result in compliance with the applicable regulations, the FAA may require additional substantiation as the basis for finding compliance. This material does not change or create any additional regulatory requirements or authorize changes in or permit deviations from regulatory requirements.
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1.4 Related Material.
See appendix C of this AC for related material that applies to this AC.
1.5 Definition of Key Terms and Acronyms.
See appendix D of this AC for terms and definitions, and appendix E of this AC for acronyms that apply to this AC.
1.6 Background.
1.6.1 RA System Approval for Operations in the U.S.
1.6.1.1 Aircraft with RA systems must meet the interference tolerance requirements in § 91.220(b) by the deadlines indicated in §§ 91.220(a), 14 CFR 121.326, and 14 CFR 129.16. Operators can comply by operating aircraft equipped with a next generation RA system. Next generation RA are new or modified RA systems that comply with the performance requirements in § 91.220(b), described in table 1 of § 91.220 as an interference tolerance mask (ITM). These performance requirements are also presented for reference in section 2.4 and table 2-1 of this AC.
1.6.1.2 Pilots or operators of aircraft with a special airworthiness certificate with an installed RA should evaluate the RA and determine if it is compliant with the ITM. If the RA meets the performance requirements of this final rule, no separate or specific FAA approval is required for operation of an aircraft that has a special airworthiness certificate.
1.6.2 RA System Description.
1.6.2.1 An RA system consists of the functional components necessary to transmit and receive radio frequency (RF) signals, process radio altitude, and present that data to the pilot(s) and downstream systems. These elements include:
• A transmitter that generates and amplifies a modulated RF signal.
• A transmitting antenna that radiates this RF signal from an aircraft toward the terrain surface beneath the aircraft.
• A receiving antenna that receives the reflected RF signal from the terrain surface and any surface features.
• Coaxial cabling or RF transmission lines.
• A receiver that demodulates the received RF signal.
• A signal processor that measures the round-trip RF signal propagation time, converts this into a radio altitude value, and outputs this value to an indicator or display or other downstream systems.
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AC 20-199
1.6.2.2 These elements may be implemented as separate components or combined, integrated, or distributed across multiple units depending on the aircraft architecture. Generally, the transmitter, receiver, and signal processing functionality are all implemented in a single component, referred to as the transceiver, and the transmitting and receiving antenna are combined into a single antenna.
1.6.3 RA Integrated Systems.
1.6.3.1 The RA system is an essential aeronautical safety system that measures aircraft height above terrain and obstacles. The RA system is primarily used during takeoff, approach, and landing, and when terrain clearance is within range, but it is operational in all phases of flight. The height information supplied by RAs is utilized to support numerous navigation and safety functions and operational capabilities, including:
• Displays of the aircraft height above terrain.
• Autoflight systems, such as autopilot, flight director, autothrottle, autoflare, and alerting systems, that are critical during approach and landing operations in low-visibility conditions.
• Terrain avoidance systems, including terrain awareness and warning systems (TAWS) and enhanced ground proximity warning systems, which provide information about terrain and obstacles to help pilot(s) avoid controlled flight into terrain.
• Collision avoidance systems, including traffic alert/airborne collision avoidance systems (TCAS/ACAS), which detect nearby aircraft and provide guidance to help pilot(s) avoid mid-air collisions. The radio altimeter is used to ensure availability and provide proper inhibitions near the ground.
• Wind shear protection systems that detect, alert, and manage the flight path during wind shear conditions encountered on takeoff and landing.
• Vision systems and operations, including enhanced flight vision systems (EFVS), synthetic vision systems, and night vision goggle (NVG) operations, all of which increase pilot visibility and situational awareness.
• Other flight control systems, such as tailstrike avoidance systems and ground spoiler/speed brake deployment logic.
• Other avionics systems, such as transponders, automatic dependent surveillance-broadcast, and weather radars.
• Flight deck information, including alerts to aid pilot(s) during landing and awareness of height a bove terrain.
1.6.3.2 Refer to appendix A of this AC for standards and policy related to these functions, systems, and operations.
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AC 20-199 CHAPTER 2. RADIO ALTIMETER SYSTEM INSTALLATION DESIGN APPROVAL
2.1 Classification of Change and Areas Affected.
2.1.1 The approval of a change under the TC, or an amended TC or STC, considers the extent to which the modification of the previously approved RA transceiver affects the performance of the transceiver as installed in accordance with § 21.93. If the RA supplier approves a minor change to an RA manufactured under a previously issued TSO authorization (TSOA) and demonstrates that the ITM-related alterations do not have an appreciable effect on other performance parameters, the replacement of the RA can also be considered a minor change to the type design. For new RA systems, or if the ITM-related alterations to existing RA systems do appreciably affect other performance parameters, the effect of those changes must be assessed and compliance demonstrated for any areas that are affected as a major change to the type design, in accordance with § 21.97. Appendices A and B of this AC provide references and RA system-specific considerations that may apply depending on the scope of the change in the type design.
2.1.2 When demonstrating compliance with the ITM under a TC or STC, the TC or STC holder or applicant should consider the supporting data and assumptions from the RA supplier. If the installation is within the parameters defined by the RA supplier and the RA supplier has demonstrated compliance with the ITM, an additional aircraft-level showing of compliance is not necessary. If the installation is outside the parameters defined by the RA supplier, those parameters must be addressed and ITM compliance demonstrated as appropriate.
2.2 TSO Authorizations.
2.2.1 TC/STC applicants who are showing compliance for an installation may use the TSO authorization of a third-party supplier who manufactures the RA transceiver. RA transceiver manufacturers with authorization under TSO-C87(), Airborne Low-Range Radio Altimeter, must provide manuals containing installation procedures, maintenance instructions, and limitations that will allow the article to continue to meet TSO requirements after installation. The RA’s installation manual (IM) or design data should include the declared antenna and aircraft installation characteristics and provide any data necessary to support the TC/STC applicant’s demonstration of compliance.
2.2.2 Next Generation RA—TSO-C87() Minor Design Change (§ 21.619(a)).
2.2.2.1 For RA suppliers who are modifying an RA design that is approved under TSO-C87 or TSO-C87a, the new design is a minor change under the existing TSO authorization if the scope of the change is not extensive enough to require a substantially complete investigation (within the meaning of § 21.619(b)) into the compliance with the requirements of the applicable TSO. In this case, the changed article keeps the original model number (part numbers may be used to identify minor changes).
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2.2.2.2 Data establishing compliance with the ITM, or with transceiver-related requirements related to the ITM, are considered related data approved under the TSO and do not affect the eligibility of the change as a minor change. The TSOA holder should document the means of compliance with the ITM and retain the MOC and supporting data for use by the aircraft installation design approval applicant. The FAA recommends that ITM compliance be shown as addressed in section 2.4, section 3.1, and appendix B of this AC.
2.2.2.3 In this case, the RA supplier should also identify any changes to the performance of its RA, other than the demonstrated interference tolerance, to support TC and STC holders assessing the impact of replacing the original RA with the modified design.
Note: Refer to AC 21-46A, Technical Standard Order Program, for additional information on design changes and non-TSO functions, including substantiation data to support the installation.
2.2.3 Next Generation RA—New Products or TSO-C87a Major Design Changes (§ 21.619(b)).
2.2.3.1 These changes include new RAs and modifications to existing RAs approved under TSO-C87a by the manufacturer that are extensive enough to require a substantially complete investigation (within the meaning of § 21.619(b)) to determine compliance with the TSO. Before making a major change, the manufacturer must assign a new type or model designation to the article and apply for a TSO authorization under § 21.603.
2.2.3.2 Data establishing compliance with the ITM, or with transceiver-related requirements related to the ITM, are considered related data approved under the TSO and do not affect the eligibility of the change as a minor change. The TSOA holder should document the means of compliance with the ITM and retain the MOC and supporting data for use by the aircraft installation design approval applicant. The FAA recommends that ITM compliance be shown as addressed in section 2.4, section 3.1, and appendix B of this AC.
Note: Refer to AC 21-46A for additional information on design changes and non-TSO functions, including substantiation data to support the installation. TSO-C87a will not be revised to incorporate RFI requirements.
2.2.4 Next Generation RA—New RA Standard.
The FAA plans to issue a new TSO (TSO-C221, Airborne Low-Range Radio Altimeter Transceiver) to recognize a new industry standard expected in 2027. This TSO will include a demonstration of compliance with the ITM as a minimum requirement, as 2-2
AC 20-199 well as other minimum requirements arising from the voluntary consensus standard process to facilitate aircraft integration of a new RA design.
2.3 Aircraft Type Design Change.
2.3.1 Compliance with the ITM must be established for the installed RA system in a particular aircraft type design. If the TSOA holder for the RA transceiver has established compliance for the transceiver, that information can be used to establish compliance at the aircraft level. The TC/STC applicant should follow all sections of the AC applicable to the RA system installation change impact.
2.3.1.1 Design Approval Without a TSO or Outside the Scope of the TSO.
All airworthiness requirements not demonstrated at the equipment level must be demonstrated by the TC/STC applicant at the aircraft level. This may include installations with certain aspects that fall outside the scope of the RA transceiver manufacturer’s IM or installations requiring additional modifications outside the RA transceiver (e.g., antenna or cabling).
2.4 Radio Frequency Interference (RFI)—Reference § 91.220, Radio Altimeter Systems.
2.4.1 The power flux-density (PFD) levels for the RFI tolerance requirements represent the total exposure from all RFI sources within the relevant frequency bands. The RA system will be simultaneously exposed to both in-band and out-of-band RFI, and some sources of RFI may simultaneously present both types of interference at significant levels; however, it is not required to test both simultaneously (refer to appendix B of this AC). The interference tolerance is specified in § 91.220(b), table 1, and replicated in table 2-1 and depicted in figure 2-1 of this AC.
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AC 20-199 Table 2-1. Interference Tolerance Mask (ITM) Frequency Range (MHz) Power Flux-Density, RMS (dBW/m2/MHz), 0–500 ft Height Above Ground Level (HAGL) 3000 ≤ f < 4000 9.5 4000 ≤ f < 4100 9.5 4100 ≤ f < 4150 9.5 4150 ≤ f < 4160 6.5 4160 ≤ f < 4170 -1 4170 ≤ f < 4180 -7 4180 ≤ f < 4190 -17 4190 ≤ f < 4200 -34 4200 ≤ f < 4400 -82 4400 < f ≤ 4410 -33 4410 < f ≤ 4430 -21 4430 < f ≤ 4440 -8 4440 < f ≤ 4450 -1 4450 < f ≤ 4460 6.5 4460 < f ≤ 4500 9.5 4500 < f ≤ 4600 9.5 4600 < f ≤ 5600 9.5 Note: The PFD levels define the maximum safely tolerable RFI environment at the surface of the RA antenna and are 6 dB higher than the expected, authorized environment to provide a safety margin. This safety margin aligns with the International Civil Aviation Organization (ICAO) recommendations in ICAO Doc 9718, Handbook on Radio Frequency Spectrum Requirements for Civil Aviation, which specify that a safety margin of 6−10 dB is to be applied for aeronautical safety systems.
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AC 20-199 Figure 2-1. Interference Tolerance Mask (ITM) Interference Tolerance Mask, Up to 500 ft HAGL 4140, 9.50 - 4470, 9.50
5.00 4150, 6.50 - 4460, 6.50 -5.00 4160, -1.00
- 4450, -1.00 Power Flux Density (dBW/m2/MHz) -15.00 4170, -7.00 4440, -8.00 4180, -17.00/ -25.00 - 4430, -21.00 -35.00 4190, -34.00-
- 4410, -33.00 -45.00 -55.00 -65.00 -75.00 4200, -82.00 — 4400, -82.00 -85.00 5600 Frequency (MHZ)
2.4.1.1 RA System Performance.
The RA system must meet all its performance requirements as installed in the aircraft when the RFI environment at the RA antenna(s) is at or below the PFD levels specified in table 2-1 of this AC. These requirements are derived from the aircraft-level and integrated system-level safety assessment process. For retrofit installation designs, the requirements from the original certification can be used without reevaluation, since replacing RA system components does not affect the higher-level requirements derived from the integration.
2.4.1.2 RA System Antenna Gain.
2.4.1.2.1 The PFD levels at the frequencies defined in table 1 of § 91.220(b) (replicated in table 2-1 of this AC) must be met with the maximum RA antenna gain. Refer to section 3.1 of this AC for additional details on how the RA antenna gain is considered when the RA transceiver is tested to verify compliance with the interference requirements. This ensures full- 2-5
AC 20-199 spectrum compatibility with RFI that may be directed into the RA at any angle, including at boresight.
2.4.1.2.2 Studies of previously-fielded RA antennas indicate that the maximum antenna gain is 11.5 dBi. Therefore, if the RA transceiver demonstrates compliance with the ITM for an antenna with a maximum gain of 11.5 dBi and without any frequency selectivity required from the antenna, data for the antenna selectivity is not required to show compliance with the performance requirements of § 91.220(b). In this case, previously approved and installed antennas may be used without further showing.
2.4.1.2.3 The accuracy of the integrated system may be degraded if the selectivity (and corresponding group delay) exceeds the design expectations. TC/STC applicants should consider the requirements of systems that use the RA output, and the demonstrated RA accuracy, to determine whether additional accuracy testing is appropriate for the integrated system. If additional testing is needed, comparative testing of the accuracy of a previously approved RA and the RFI-tolerant RA can be used to show acceptable performance. By using comparative analysis, the RF environment need not be considered in the accuracy test.
2.5 Aircraft Flight Manual (AFM).
In the Normal Procedures, Normal Operations, or equivalent section of the AFM, AFM supplement, or Pilot Operating Handbook, the applicant should include a statement that the aircraft has been demonstrated to meet § 91.220, Radio Altimeter Systems.
That statement may be used by pilots or operators for demonstrating compliance to the operating requirements of § 91.220, § 121.326, or § 129.16, as appropriate. For example, the AFM may state: “This aircraft has an RA system that operates in the interference environment described in 14 CFR 91.220(b).”
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AC 20-199 CHAPTER 3. RADIO ALTIMETER SYSTEM TESTING
3.1 Radio Frequency Interference Testing.
3.1.1 The RFI injection test setup and requirements are specified in appendix B of this AC and may be done:
• At the RA transceiver line replacement unit (LRU), or
• By testing the RF Filter Subassembly (RFFSA) and RA transceiver separately.
3.1.2 For the LRU level, the conducted interference power levels associated with the system ITM (PFD levels) are directly injected into the RA transceiver RF ports. When testing the RFFSA and RA transceiver separately, test the RFFSA at the maximum level (direct conducted) and use the resulting data to reduce the level of interference into the RA transceiver. The equations that define the interference power levels are provided in equations B-1 to B-3 of appendix B in this AC, which assumes specific values for installation parameters, including antenna gain and cable loss. Under these interference power levels, the RA transceiver must meet the measurement validity, accuracy, and noise requirements. As stated in note 1 in appendix B, section B.4, of this AC, the assumed antenna gain must be the gain specified in the RA transceiver IM that is sufficient to maintain the required RA performance, and it may include engineering judgment to bound the antenna gain at all applicable frequencies in real aircraft installations.
3.1.3 Demonstration of compliance may be done using the RA transceiver manufacturer’s data, and an analysis showing that the data is applicable to the installation (i.e., the installed antenna gain and cable loss is within the RA transceiver assumed parameters) across all frequency bands specified in table 2-1 of this AC.
3.2 Installed RA System Testing (as applicable to the change impact).
Installed RA system testing should include testing with the aircraft on the ground using simulated or real inputs or with the aircraft in flight using real inputs. The TC/STC applicant should:
• Demonstrate that the RA system performs appropriately with real terrain returns for all integrated systems. Various types of complex terrain scenarios should be assessed. There is no need to perform this demonstration in the presence of RFI interference.
• Demonstrate that the accuracy and performance are not degraded by installing a new RA system. Negligible degradation may be acceptable if it is shown that all integrated systems continue to meet their intended function, safety, and performance requirements.
• Demonstrate compliance using the manufacturer’s data for the RA transceiver, antenna, or display, as applicable, and an analysis showing the data is applicable to the installation.
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3.3 Integrated System Testing (as applicable to the change impact).
The TC/STC applicant should perform regression and additional integrated system testing appropriate to the design change and its impact on the integrated systems. When replacing one RA with another, it is acceptable to demonstrate that any degradation in performance (e.g., accuracy) is within the integrated system’s performance requirements. Testing should be conducted as applicable to the systems, functions, and operations integrated into the aircraft. Demonstration of compliance may be accomplished using the RA transceiver, antenna, or by displaying the manufacturer’s data. Refer to appendix A of this AC for additional guidance.
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AC 20-199 Appendix A APPENDIX A. INTEGRATED SYSTEM REFERENCES Table A-1. Standards and Policy for Integrated Systems Operational Application System or Function Reference TSO Reference Advisory Circular Terrain Avoidance1 TAWS FAA or EASA TSO-C151d AC 23-18 AC 25-23 HTAWS FAA or EASA TSO-C194 EASA TSO-2C522 AC 27-1B AC 29-2C Collision Avoidance1 TCAS/ACAS I FAA or EASA TSO-C118a TCAS/ACAS II FAA or EASA TSO-C119e AC 20-131A AC 20-151C ACAS X FAA or EASA TSO-C219a Windshear Protection1 Weather Radar FAA TSO-C63g EASA TSO-C63f AC 25-12 AC 20-182A Predictive Windshear Reactive Windshear FAA or EASA TSO-C117b Flight Guidance Systems Autopilot (AFGCS/FD) FAA or EASA TSO-C198 AC 120-29A/120-28D AC 25.1329-1() AC 20-138() Autothrottle Flight Guidance Alerting Approach and Landing Systems (including Autoflare) 1 These safety systems are required by regulation for certain aircraft operations by multiple state aviation authorities.
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