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.
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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
AC 20-191 such, the applicant must d emonstrate that the takeoff guidance system accounts for any differences, if any, between the runway centerline and the reference lateral path. The applicant should demonstrate c ompliance by flight test, or by a combination of flight test and simulation. Flight testing must cover those factors affecting the behavior of the aircraft (e.g., wind conditions, ILS characteristics, weight, center of gravity).
3.2.3.6.4 The applicant should demonstrate that when the airplane is displaced from the runway centerline at any point during the takeoff or rejected takeoff, including engine failure cases, the takeoff guidance system will allow the “ pilot flying” (PF) to control the airplane smoothly back to the intended path in a controlled and predictable manner. Minor overshoot or oscillations around the centerline are permissible when not leading to unacceptable flight crew workload.
3.2.3.6.5 The applicant should use the perfor mance envelope and conditions in Figure 3-1 for evaluating the takeoff guidance system.
• Takeoff with all engines operating,
• Engine failure at or slightly after V1- continued takeoff*,
• Engine failure just prior to V1- rejected takeoff *; and
• Engine failure at a speed prior to VMCG – rejected takeoff (VFAILURE< VMCG)*.
*Wind and runway conditions consistent with basic aircraft takeoff performance demonstrations.
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AC 20-191 Figure 3-1. Performance Envelope for Evaluating Takeoff System Command Guidance V1 is takeoff decision speed VFAILURE – speed at which failure occurs VMCG is Ground Minimum Control Speed VLOF is Liftoff Speed Note 1: When showing compliance, the pilot may adjust the lateral flight path after liftoff to account for wind and drift effects.
Note 2: The 14m lateral region addresses tracking on the runway surface.
Note 3: The FAA expects the pilot to position the aircraft on the runway centerline.
Once on the runway, the takeoff guidance system should provide an indication to confirm proper operation.
Note 4: The lateral tracking criteria is referenced to the lateral offset of the aircraft’s centerline between the main landing gear (or aircraft center of gravity).
3.2.3.6.6 The system should have no characteristic making it difficult for a pilot to transition to a safe climb flight path, consistent 3-4
AC 20-191 with obstacle clearance for obj ects near the runway .
3.2.3.6.7 When demonstrating compliance, the applicant can take into consideration testing techniques such as normal pilot technique for responding to engine failure, application of rudder, maintaining safe bank angles, rotating to suitable pitch attitudes, and compensating for crosswind, as necessary .
3.2.3.6.8 When the rejected takeoff is the result of an engine failure , the actual performance should reflect the effects of a short-term increase in lateral deviation and then converge toward the centerline during the deceleration to a full stop.
3.2.3.6.9 During takeoff, the system must annunciate guidance signal loss to the flight crew.
3.2.3.6.10 The applicant should demonstrate t he vertical axis guidance of the takeoff system during normal operation provides an appropriate pitch attitude and climb speed when considering the following factors.
• A normal rate of rotation to the commanded pitch attitude, at VR-10 knots for all engines operating and VR-5 knots for an engine-out condition, does not result in a tail-strike.
• Acquire a pitch attitude resulting in the capture and tracking of the All-Engine Takeoff Climb Speed, V2 + X, where X is the All-Engine Speed Additive from the AFM (normally 10 knots or higher).
3.2.3.6.11 For engine-out operations, the applicant should demonstrate the takeoff guidance system provides commands to acquire a pitch attitude result ing in the capture and tracking of the following reference airspeeds.
• V2, for engine failure at or below V2. (The aircraft should attain this speed after liftoff.)
• Airspeed at engine failure, for failures between V2 and V2 + X.
• V2 + X, for failures at or above V2 + X. Alternatively, the applicant may use the airspeed at engine failure provided the system can achieve the minimum takeoff climb gradient at that speed.
3.2.3.6.12 The applicant should demonstrate t he workload associated with the use of the takeoff system compl ies with the Handling Quality Rating System (HQRS) criteria in AC 25-7, Flight Test Guide for Certification of Transport Category Airplanes, 3-5
AC 20-191 as amended, or equivalent cr iteria for the type of aircraft. The takeoff guidance system should provide required tracking performance with “ satisfactory” workload and pilot compensation, under foreseeable normal conditions. The takeoff guidance system may provide reduced (i.e., HQRS Rating Adequate) path tracking performance (e.g., up to 14m lateral deviation on the ground, ref . Figure 3-1) with adequate workload (i.e., heightened pilot effort and attention) during takeoffs with engine failures.
3.2.4 Integrity.
The applicant must demonstrate the takeoff guidance system provides command information as lateral guidance, which will enable maintaining the aircraft on the runway during the takeoff roll through acceleration to liftoff or, during a deceleration to a stop during a rejected takeoff.
3.2.4.1 The applicant must evaluate the design of the onboard components of the low-visibility system, separately and in relation to other systems, pursuant to 14 CFR Parts 23, 25, 27, and 29; and sections 23.2500, 23.2510, 23.2605,
25.1309, 27.1309, and 29.1309, respectively, as applicable. The following criteria is provided for the application of§ 25.1309 to the takeoff guidance system:
3.2.4.1.1 The takeoff guidance system design should not possess characteristics, in normal operation or when failed, that would degrade takeoff safety or lead to a hazardous condition .
3.2.4.1.2 The probability of the takeoff guidance system generating misleading information that could lead to an unsafe condition must be improbable when the flight crew is alerted to the condition by suitable fault annunciation or by information from other independent sources available within the pilot's primary field of view. For airworthiness, the applicant must demonstrate the effectiveness of the fault annunciation or information from other independent sources . Any single failure of the aircraft that could impact the takeoff path (e.g., engine failure, single electrical generator or bus failure, single Inertial Reference Unit (IRU) failure) must not cause loss of guidance information or give incorrect guidance information .
3.2.4.1.3 The probability of the takeoff guidance system generating misleading information that woul d be unsafe to follow, must be extremely improbable , if:
• No means are available for the takeoff system to detect and annunciate the failure, and
• No information is provided to the pilot to immediately 3-6
AC 20-191 detect the malfunction and take corrective action.
3.2.4.1.4 I LS. The applicant must establish the aircraft system response to loss of ILS localizer transmitter. The applicant must also establish the aircraft system response during a switchover from an active localizer or to an optional backup transmitter.
3.2.4.1.5 GLS. The applicant must establish the aircraft system response to loss of GLS guidance signals .
3.2.4.2 In the event of a probable failure (e.g., engine failure, electrical source failure), if the pilot follows the takeoff display and disregards external visual references, if applicable, the aircraft performance must meet the criteria illustrated in Figure 3-1.
In showing system compliance, the applicant may not factor the proportion of takeoffs made in low visibility into the probabilities of performance or failure effects.
3.2.5 Information, Annunciation, and Alerting.
This section identifies information, annunciations, and alerting criteria for the takeoff guidance system. The design of the controls, indicators, and alerts must minimize crew errors that could cause a hazard. The takeoff guidance system must present the mode and system malfunction indications in a manner compatible with the procedures and assigned tasks of the flight crew. The takeoff guidance system must group the indications in a logical and consistent manner and ensure they are visible under all expected normal lighting conditions.
3.2.5.1 Compliance.The design of the Information, Annunciation, and Alerting HUD must comply with 14 CFR §§ 23.2605, 25.1322, 27.1322, or 29.1322 as applicable. Reference AC 23.1311-1, AC 25.1322-1, AC 27-1, or AC 29- 2 for additional guidance.
3.2.5.2 Annunciation.The applicant must demonstrate prior to initiating takeoff and during takeoff, the takeoff guidance system provides positive indications of the following information to the flight crew:
• System status,
• Modes of engagement and operation, and
• Guidance source.
If pitch or speed or both guidance is also provided, the takeoff guidance system must detect and promptly announce to the pilot failures that would result in rotation at an unsafe speed, or an unsafe pitch rate or pitch angle.
3.2.5.3 Alerting.
The takeoff guidance system must alert the flight crew whenever a failure or 3-7
AC 20-191 condition prevents it f rom meeting the performance specified in sections
3.2.3 and 3.2.4. The removal of guidance alone is not adequate annunciation. Alerts must be timely, unambiguous, readily evident to each flight crew member, and compatible with the alerting philosophy of the airplane. Annunciations must be located to ensure rapid recognition and must not distract the pilot making the takeoff or significantly degrade the forward view.
3.2.5.3.1 Annunciations.During takeoff, whenever the takeoff guidance system does not provide valid guidance appropriate for the takeoff operation, it must clearly annunciate the failure to the flight crew, and the invalid guidance must be removed. The location of the annunciation must be positioned to ensure rapid recognition without distracting the pilot or significantly degrading the forward view.
3.2.5.3.2 Warnings.The takeoff guidance system must provide warnings for conditions requiring immediate pilot awareness and action
(e.g., failure of the guidance system).
3.2.5.3.3 Cautions.The takeoff guidance system must provide cautions for conditions requiring immediate pilot awareness and possible subsequent pilot action. These alerts need not generate a Master Caution light that would be contrary to the takeoff alert inhibit philosophy. Caution annunciations should not cause flight crew distraction during takeoff roll.
3.2.5.3.4 Advisories.The takeoff guidance system must provide advisories for conditions requiring pilot awareness in a timely manner. It should not generate advisories after takeoff commences.
3.2.5.3.5 System Status.
The takeoff guidance system should provide a system status
(e.g., status of built-in test equipment (BITE)/self-test).
3.2.6 Availability.
The probability of loss of takeoff guidance system should be remote.
3.3 Airborne Systems for Takeoff.
This section covers the criteria for a HUD that is integrated with ILS localizer or GBAS signals. In all cases, this AC assumes the sensors used for approach and landing will guide the aircraft during takeoff.
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AC 20-191
3.3.1 HUD for Ta keoff Guidance.
The following criteria apply if an applicant employs a HUD to provide takeoff guidance:
3.3.1.1 The workload associated with use of the HUD must comply with 14 CFR §§
23.2610, 25.1523, 27.1523, or 29.1523 as applicable. Reference AC 23.1523 and AC 23.1311-1; AC 25.1523-1 and AC 25-11, AC 27-1; or AC 29-2 as applicable for additional guidance.
3.3.1.2 The HUD installation and display presentation must meet 14 CFR §§
23.2600(a), 25.773, 27.773, or 29.773 as applicable. Reference AC 23.1311- 1; or AC 25.773 and AC 25-11, appendix F; or AC 27-1; or AC 29-2 for additional guidance.
3.3.1.3 For the entire takeoff path and for all normal and non-normal conditions, except loss of the HUD itself, the HUD takeoff system must provide acceptable guidance and flight information to enable the PF to complete the takeoff or abort the takeoff. HUD takeoff system use should not require excessive workload, exceptional skill, or excessive reference to other cockpit displays.
3.3.2 GBAS.
An applicant may use the guidance in this chapter to obtain airworthiness approval of a GBAS receiver for low visibility takeoff guidance. In addition, the applicant must evaluate the effect of loss and signal reacquisition during takeoffs .
3.4 Takeoff Guidance Evaluation.
An applicant should submit performance data for components used for takeoff guidance.
3.4.1 Certification Plan .
The applicant should provide a certification plan that includes a description of the aircraft systems, the certification basis, the certification methods, and compliance documentation. The certification plan should describe how any non-aircraft elements relate to the operation of aircraft systems from a performance, integrity, and availability perspective.
3.4.1.1 For ILS-based system elements, the applicant can predicate satisfaction of these performance provisions upon compliance with the ICAO Standards and Recommended Practices (SARPs), equivalent State standard, or by reference to an acceptable standard for performance of any navigation service.
3.4.1.2 For a Global Navigation Satellite System (GNSS) based system such as GBAS, the applicant can predicate satisfaction of these performance 3-9