AC 20-191

U.S. Department of Transportation Federal Aviation Administration Advisory Circular Subject: Airworthiness Approval of Airborne Date: AC No: 20-191 Systems used for Takeoff, Precision Initiated by: AIR-626 Change:

Approach, Landing, and Rollout in lowvisibility conditions.

This advisory circular (AC) provides an acceptable means of compliance for airworthiness criteria for low visibility takeoff, final approach, landing, and rollout in Category (CAT) II and CAT III weather minima. While the airworthiness criteria for low visibility takeoff is generic in nature, the criteria for the precision approach, landing, and rollout are tailored to using Instrument Landing System (ILS), and Ground Based Augmentation System (GBAS) and it is airplane centric. Applicants seeking airworthiness approval for rotorcraft can add appropriate guidance to tailor their application. This AC does not address Microwave Landing System (MLS).

This new AC is derived from the airworthiness criteria previously contained in AC 120-28D, Criteria for Approval of Category III Weather Minima for Takeoff, Landing, and Rollout, and AC 120-29A, Criteria for Approval of Category I and Category II Weather Minima for Approach. This AC includes updated GBAS standards, issue paper findings from GBAS Landing System (GLS) and low visibility takeoff certification programs.

Daniel J. Elgas Aviation Safety Director, Policy and Standards Division, Aircraft Certification Service

AC 20-191

ParagraphPage

Cha

pter 1. General1-1

1.1 Purpose1-1

1.2 Audience1-1

1.3 Cancellation1-1

1.4 Where to Find This AC1-2

1.5 Related Materials1-2

1.6 Definitions of Key Terms1-2

Chapter 2. Scope2-1

2.1 Introduction2-1

2.2 Applicability. Explanation of Change2-1

2.3 Contents of this AC2-2

2.4 Related Regulations2-2

Chapter 3. Takeoff Guidance System3-1

3.1 Introduction3-1

3.2 Airworthiness Criteria3-1

3.3 Airborne Systems for Takeoff3-8

3.4 Takeoff Guidance Evaluation3-9

3.5 AFM3-13

Chapter 4. Category (CAT) II4-1

4.1 Introduction4-1

4.2 Airworthiness Criteria4-2

4.3 Airborne Systems for CAT II Operations4-12

4.4 Approach System Evaluation4-15

4.5 AFM4-18

Chapter 5. Category (CAT) III5-1

5.1 Introduction5-1

5.2 Airworthiness Criteria5-1

5.3 Airborne Systems for CAT III5-26

5.4 Landing and Rollout System Evaluation5-31

5.5 Aircraft Flight Manual5-46

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AC 20-191

Appendix A. Definitions and AcronymsA-1

A.2 Definitions of TermsA-4

Appendix B. Wind model for approach and landing simulationB-1

B.1 Wind ModelsB-1

B.2 Wind Model AB-1

B.3 Wind Model CB-10

B.4 Wind Model BB-10

B.5 Acronyms and Definitions for the Wind ModelsB-13

Appendix C. GBAS Performance Model for Approach and Landing SimulationC-1

C.1 GBAS Performance AnalysisC-1

C.2 Alternative Method for Calculating and Using NSE Model Scale FactorsC-15

C.3 References for GBAS Performance ModelC-22

C.4 Acronyms and Definitions for GBAS Performance ModelC-23

Appendix D. AFM Provisions and Example AFM WordingD-1

D.1 Example Provision: AFM "Certificate Limitation" SectionD-1

D.2 Example Provision - AFM “Normal Procedures” or “Normal Operation” SectionD-2

D.3 Example Provision - AFM “Normal Procedures” or “Normal Operation” SectionD-4

FIGURES Number Page

Figure 5-1 AFM Elevation Value From Flight Test And Validated Simulation5-38

Figure B-1 Headwind-TailwindB-4

Figure B-2 CrosswindB-5

Figure B-3 Annual Percent Probability of Mean Wind Speed Equaling or Exceeding Given

ValuesB-6

Figure B-4 Wind Direction Relative to Runway HeadingB-7

Figure B-5 Selected Description for Variances of Horizontal Turbulence ComponentsB-8

Figure B-6 Selected Integral Scale DescriptionB-9

Figure B-7 Cumulative probability of reported Mean Wind, and Head Wind, Tail Wind Cross

Wind Components when landingB-12

Figure C-1 GBAS Signal ModelC-2

Figure C-2 GLS NSE GeneratorC-2

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AC 20-191

Figure C-3 NSE Step GeneratorC-5

Figure C-4 Limit and/or Malfunction GeneratorC-7

Figure C-5 Malfunction TransientC-8

Figure C-6 Maximum Error (E) and Monitor Threshold (TBAC) as a function of Alert Limits for

Various Numbers of Ground Reference Receivers (M)C-10

Figure C-7 Example of the Satellite Ranging Source Pmd in the Range domain and Position

DomainC-12

Figure C-8 Example Assessment of Landing Short PerformanceC-14

Figure C-9 Explicit Calculation of PUL for the Land Short Example AboveC-15

Figure C-10 Sigma Vertical SamplesC-16

Figure C-11 Worst Horizontal Sigma SamplesC-17

Figure C-12 Rise/set Sigma Step Vert. Error SamplesC-18

Figure C-13 Rise/set Worst Sigma Step-horizontal Error SamplesC-19

Figure C-14 Signal Loss- Sigma Step Vert Error SamplesC-20

Figure C-15 Signal loss worst sigma step hor. SamplesC-21

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AC 20-191

TABLES Number Page

Table 5-1: Example AFM Elevation Values5-39

Table C-1: GAST Dependent Parameters for KvertC-3

Table C-2: GAST Dependent Parameters for Kxtrk and KatrkC-4

σTable C-3: GAST Dependent Parameters for step _ vertC-5

Table C-4: GAST Dependent Parameters for σ step _ xtrk or σ step _ atrkC-6

Table C-5 Malfunction Transient Characteristics in the Vertical DirectionC-9

Table C-6 Malfunction Transient Characteristics in the Lateral DirectionC-9

Table C-7 Pmd_limit ParametersC-11

Table C-8 Sigma_vert Values and Number of OccurencesC-16

Table C-9 Sigma_Worst_Horizontal Values and Number of OccurrencesC-18

Table C-10 Rise/Set Sigma_step_vert and Number of OccurrencesC-19

Table C-11 Rise/Set Worst Sigma_Step_Hor and Number of OccurrencesC-20

Table C-12 Signal loss Sigma_Step_Vert and number of occurrencesC-21

Table C-13 Signal Loss Worst Sigma_Step_Hor and number of occurrencesC-22

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AC 20-191

CHAPTER 1. GENERAL

1.1 Purpose.

1.1.1 This Advisory Circular (AC) provides guidance for the airworthiness approval of airborne systems used for takeoff, precision approach, landing , and rollout in low visibility conditions. You may use this guidance as a basis for Title 14 of the Code of Federal Regulations (14 CFR) Parts 23, 25, 27, and 29.

1.1.2 This AC provides criteria for an acceptable means of compliance with performance, integrity and availability criteria for low visibility takeoff, approach, landing, and rollout systems to accomplish takeoff, landing, and rollout in low-visibility conditions. The contents of the document do not have the force and effect of law and are not meant to bind the public in any way. This document is only intended to provide the public with information regarding existing requirements under the law or agency policies .

1.1.3 The applicant is responsible for regulatory compliance and should work closely with their applicable Certification Branch to ensure regulatory compliance . Mandatory terms such as “must” used in this AC are only applicable when following the compliance method outlined in this AC.

1.1.4 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 .

1.2 Audience.

This AC is for those applicants applying for an initial type certificate (TC), supplemental type certificate (STC), amended TC, or amended STC for the installation and airworthiness approval of airborne systems used for takeoff, precision approach, landing, and rollout in low visibility conditions.

1.3 Cancellation.

1.3.1 This AC updates and incorporates the airworthiness guidance formerly offered in AC 120-29A, Criteria for Approval of Category I and Category II Weather Minima for Approach, and AC 120-28D, Criteria for Approval of Category III Weather Minima for Takeoff, Landing, and Rollout . This AC and AC 120 -118, Criteria for Approval/Authorization of All Weather Operations (AWO) for Takeoff, Landing, and Rollout, cancel both AC 120 -29A and AC 120-28D.

1.3.2 Aircraft types and systems approved previously to issuance of AC 120- 29A, AC 120- 28D, or equivalent criteria remain valid . Applicants may continue to produce, install , 1-1

AC 20-191

and retrofit those systems in existing aircraft, new production aircraft , variants, or future derivatives of those types or variants ; however, applicants seeking any additional credit permitted in this AC must meet the provisions of the applicable section in this AC. New airworthiness demonstrations should use the criteria in this AC.

1.3.3 Airworthiness demonstrations may use equivalent European Union Aviation Safety Agency (EASA) criteria as agreed upon by the FAA through the FAA/EASA criteria harmonization process.

1.4 Where to Find This AC.

You may find this AC at FAA Advisory Circulars or at Dynamic Regulatory System (DRS).

1.5 Related Materials See Chapter 2, section 2.4 for all materials related to this AC.

1.6 Definitions of Key Terms See Appendix A for terms and definitions that apply to this AC.

1-2

AC 20-191 CHAPTER 2. SCOPE

2.1 Introduction.

2.1.1 This AC provides an acceptable means of compliance for airworthiness criteria for the approval of aircraft to conduct takeoff in low -visibility conditions, and final approach, landing, and rollout in CAT II and CAT III weather minima. It includes equipment such as Head Up Displays (HUD), Ground Based Augmentation System (GBAS) , Landing System (GLS) , Instrument Landing System (ILS) , and low-visibility takeoff guidance systems. This AC uses the generic term HUD to refer to HUD or any other equivalent display.

2.1.2 Criteria for operational approval for low-visibility takeoff and CAT II/III precision approach and landing as well as special authorization (SA) CAT 1 and SA CAT II are in AC 120-118.

Note: This AC does not provide airworthiness criteria for SA CAT I and SA CAT II.

2.2 Applicability. Explanation of Change.

This AC incorporates and revises the airworthiness criteria previously contained in AC 120-29A and AC 120-28D. This includes low-visibility takeoff and CAT II and CAT III precision approach, landing, and rollout using ILS and GBAS; and integrations that use multi-mode receivers supporting both ILS and GBAS.

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AC 20-191

2.3 Contents of this AC.

2.3.1 Chapter 1 provides the purpose of this AC and other general information .

2.3.2 Chapter 2 provides general airworthiness criteria and references for low visibility takeoff, and CAT II and CAT III precision approach, landing, and rollout.

2.3.3 Chapter 3 provides airworthiness criteria for equipment used to conduct low -visibility takeoff operation s.

2.3.4 Chapter 4 provides airworthiness criteria for equipment used to conduct CAT II precision approaches .

2.3.5 Chapter 5 provides airworthiness criteria for equipment used to conduct CAT III precision approaches .

2.3.6 Appendix A provides Acronyms and Definitions .

2.3.7 Appendix B provides Wind Models for Approach and Landing Simulation.

2.3.8 Appendix C provides a GBAS Performance Model for Approach and Landing Simulation.

2.3.9 Appendix D provides an Example Aircraft Flight Manual (AFM).

2.4 Related Regulations.

2.4.1 Title 14 of the Code of Federal Regulations (14 CFR) .

Title 14 CFR parts and sections can be found at eCFR and on the DRS.

• Sections 21.3, 23.1529, 23.2135, 23.2220, 23.2500, 23.2510, 23.2600, 23.2605,

23.2610, 23.2620, 25.143, 25.473, 25.479, 25.773, 25.1309, 25.1322, 25.1329,

25.1523, 25.1529, 25.1581, 27.143, 27.473, 27.479, 27.1309, 27.1322, 27.1523,

27.1529, 27.1581, 29.143, 29.473, 29.479, 29.1309, 29.1322, 29.1523, 29.1529,

29.1581, and 135.385,

• Parts 91, 121, 125, and 129.

2.4.2 Advisory Circulars.

Unless this AC makes a specific reference to a particular version of an AC, applicants should use the current editions of the following ACs. You can find the following ACs at FAA Advisory Circulars and on DRS.

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

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

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AC 20-191

• AC 23.1309-1, System Safety Analysis and Assessment for Part 23 Airplanes.

• A C 23.1311-1, Installation of Electronic Display in Part 23 Airplanes.

• AC 23.1523, Minimum Flight Crew.

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

• AC 25-11, Electronic Flight Displays.

• AC 25.773-1, Pilot Compartment View Design Considerations.

• AC 25.1309-1, System Design and Analysis.

• AC 25.1322-1, Flightcrew Alerting.

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

• AC 25.1523-1, Minimum Flightcrew.

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

• AC 29-2, Certification of Transport Category Rotorcraft.

• AC 120-40, Airplane Simulator Qualification.

• AC 120-118, Criteria for Approval/Authorization of All Weather Operations (AWO) for Takeoff, Landing, and Rollout.

2.4.3 Technical Standard Orders.

The following TSOs are related to this AC; you can find them on DRS.

• TSO-C34e, ILS Glide Slope Receiving Equipment Operating within the Radio Frequency Range of 328.6-335.4 Megahertz (MHz), 1/15/88.

• TSO-36e, Airborne ILS Localizer Receiving Equipment Operating within the Radio Frequency Range of 108-112 Megahertz (MHz), 1/25/88.

• TSO-C161b, Ground Based Augmentation System Positioning and Navigation Equipment, 12/17/2019.

• TSO-C162b, Ground Based Augmentation System Very High Frequency Data Broadcast Equipment, 12/17/2019.

2.4.4 Other Publications.

The following documents are related to this AC.

• European Union Aviation Safety Agency (EASA) AMC AWO 231, Flight Demonstration (Acceptable Means of Compliance), available at EASA.

• RTCA DO-253D Change 1, Minimum Operational Performance Standards for GPS Local Area Augmentation System Airborne Equipment, June 27, 2019, available at RTCA.

2-3

AC 20-191 CHAPTER 3. TAKEOFF GUIDANCE SYSTEM

3.1 Introduction.This chapter provides an acceptable means of compliance for airworthiness approval criteria to conduct takeoff in low -visibility weather conditions .

This AC defines a low-visibility takeoff as a takeoff made when runway visibility is below the standard weather minima . Takeoff guidance equipment is required under this AC when visibility conditions alone are inadequate.

Note: Chapter 3 may not be applicable to 14 CFR Part 23 certificated airplanes operating under Part 91; applicants may apply Chapter 3 guidance during Part 23 certification.

3.1.2 Takeoff guidance system s include position sensors, cockpit displays, indicators , and controls as a minimum. The takeoff guidance system provides directional command display guidance to the pilot during takeoff and rejected takeoffs .

3.2 Airworthiness Criteria.

3.2.1 An approved takeoff guidance system must enable the pilot to track and maintain the runway centerline during takeoff from brake release to liftoff, or from brake release through deceleration to a stop for a rejected takeoff. In addition to enabling satisfactory centerline tracking during the takeoff roll , the system must enable the pilot to maintain ground track aligned with the runway heading through the transition to the appropriate mode for post takeoff climb. In addition to enabling satisfactory centerline tracking on the ground, th e takeoff guidance system should also lead to a combination of the airplane’s position and track at the point of liftoff that would not result in significant deviations after liftoff. At the point of liftoff, the airplane track should neither be diverging from the runway centerline nor be at such an angle from the runway heading that would cause significant deviations during the climb. After liftoff, the takeoff guidance system must enable the pilot to detect divergence of the airplane’s track from the runw ay centerline, and at least to be able to arrest further deviation until the system transitions to the appropriate mode for post takeoff climb.

3.2.2 The applicant’s proposal for type certification approval for the equipment, system installations, and test methods should include factors such as the intended function of the installed system, its accuracy, reliability, and fail -safe features, as well as the operational concepts contained in AC 1 20-118.

3.2.3 The following sections identify the performance and workload provisions for the lowvisibility takeoff roll through liftoff or for a rejected takeoff .

3.2.3.1 The FAA considers the entire takeoff operation, through completion of the en route climb configuration, an intensive phase of flight from an airworthiness perspective. The use of the takeoff system must not require exceptional skill, workload or pilot compensation. The takeoff guidance system must provide an appropriate transition from lateral takeoff guidance 3-1

AC 20-191 to en route climb guidance for a takeoff. It must also provide lateral gui dance from brake release through deceleration to a stop for a rejected takeoff. The pilot must be able to continue the use of the same primary display(s) for the airborne portion as well as for the runway portion.

Changes in guidance modes and display formats must be automatic and apparent to the pilot regarding the guidance being utilized.

3.2.3.2 The applicant must show the relationship and interaction of the aircraft elements of the takeoff system with non-aircraft elements (e.g., ground station, satellites, etc.).

3.2.3.3 The applicant may establish the performance of the aircraft elements with reference to an approved flight path (e.g., localizer, GBAS lateral guidance) provided the budgeting between aircraft and non-aircraft elements does not compromise the overall performance.

3.2.3.4 When international standards exist for the performance and integrity aspects of any non-aircraft elements of the takeoff system, the applicant can assume member states of the International Civil Aviation Organization (ICAO) apply these standards.

3.2.3.5 When international standards do not exist for the performance and integrity aspects of any non-aircraft elements, the applicant must address these considerations as part of the airworthiness process. The applicant must provide a means to inform the operator of the limitations and assumptions necessary to ensure a safe operation.

3.2.3.6 Takeoff guidance system.

3.2.3.6.1 The applicant must evaluate the takeoff guidance system to ensure the performance is satisfactory for normal operations, aircraft failure cases (e.g., engine failure), and recovery from displacements from non -normal events. Guidance cues should be easy to follow and not increase workload significantly compared to the basic flight instruments . The applicant should not solicit any credit for performance improvements resulting from available visual cues .

3.2.3.6.2 The applicant must evaluate the takeoff guidance system displays to ensure that the pilot correctly interpre ts the display information to maintain the desired takeoff flight path. The applicant should evaluate the cockpit integration issues to ensure consistent operations and pilot response in all situations.

3.2.3.6.3 The system’s reference path is usually defined by the ILS localizer or other appro ved approach navigation aids (e.g., GBAS) that normally coincides with the runway centerline. As 3-2