AC 20-191 procedures section(s)
• Flig ht Crew Operating Manual (FCOM) provisions, or equivalent
• Pilot qualification criteria (e.g., training requirements, Flight Standardization Board (FSB) provisions)
• Any other reference material necessary for an operator or flight crew to use the system safely.
3.4.3.2.3 Information to support preparation of any maintenance and preventive maintenance procedures necessary for safety, such as:
• Certification Maintenance Requirements (CMR),
• Periodic checks, or
• Other checks, as necessary (e.g., return to service).
3.4.3.2.4 Information applicable to lim itations, as necessary.
3.4.3.2.5 Information necessary for development of Non-normal Procedures.
3.5 AFM.
Pursuant to 14 CFR §§ 23.1581, 25.1581, 27.1581, or 29.1581, as applicable, an applicant must furnish AFM with each aircraft. The AFM should contain the following information:
3.5.1 The criteria and configurations used for the takeoff system demonstration.
3.5.2 Any constraints or limitations necessary for safe operation.
3.5.3 Acceptable normal and non-normal procedures.
3.5.4 The type of navigation facilities used as a basis for certification. This should not be taken as a limitation on the use of other facilities. The AFM may contain a statement regarding the type of facilities or condition s known to be unacceptable for use (e.g ., ILS localizer or GLS) .
3.5.5 Applicable atmospheric conditions under which the system was demonstrated (e.g., headwind, crosswind, tailwind).
3.5.6 All necessary performance, procedure, or configuration data to permit an operator to determine climb gradient and transition distances for safe obstacle clearance during a takeoff operation ; Note that this information need not be specifically included in the AFM if it is available to the operator using some other method acceptable to the 3-13
AC 20-191 operator and manufacturer (e.g., Flight Crew Operating Manual (FCOM), su pplementary performance information, separate AFM appendix).
3.5.7 For a takeoff system meeting the provisions of this chapter, the Normal Procedures, Normal Operations, or equivalent section, of the AFM should also contain the following statements:
• “The airborne system has been demonstrated to meet the airworthiness guidance of AC 20-191 (Chapter 3) for [specify the pertinent takeoff capability section(s) criteria met] when the following equipment is installed and operative:”
• “[list pertinent equipment]”
• “This AFM provision does not constitute operational approval for low visibility takeoff use of the system.”
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AC 20-191 CHAPTER 4. CATEGORY (CAT) II
4.1 Introduction.
4.1.1 This chapter contains airworthiness approval criteria for equipment used to conduct a CAT II weather minima approach using certain radio na vigation aids (navaids) such as ILS or GBAS. This chapter does not provide criteria for each potential combination of aircraft and non-aircraft elements.
4.1.2 This chapter addresses the approach phase of flight, beginning at the Final Approach Fix (FAF) and ending at the CAT II Decision Height ( DH).
4.1.3 ICAO Annex 10 includes standards for ILS, MLS, and GBAS intended to support CAT II operations. This chapter addresses approval of equipment for operations using either ILS class (II/D/2) or better performance or GBAS facility classification (D/G1/any/any 1). This chapter addresses ILS avionics with Technical Standard Order (TSO)-C34e, TSO-C36e authorization, and GBAS avionics with the latest TSO- C161b and TSO-C162b authorization.
Note 1: While the airworthiness criteria for the guidance system for the approach and landing system applies from the FAF to the DH, guidance should be available down to half the DH.
Note 2: GBAS approach service volume is defined in ICAO Annex 10 Amendment 91.
4.1.4 The standards for GBAS ( RTCA DO-253D Change 1 and ICAO Annex 10) define two general classes of GBAS service, Approach Services and the GBAS Positioning Service. The standards further diffe rentiate GBAS approach services into multiple types referred to as GBAS Approach Service Types (GAST). A GAST is defined as the matched set of airborne and ground performance and functional requirements intended to be used in concert to provide approach guidance wi th quantifiable performance.
4.1.5 GBAS GAST C is the approved GBAS to support CAT I level of service. Applicants for CAT II airworthiness approvals typically seek approval through GBAS GAST D level of service using GBAS airborne equipment class ( GAEC) D. Applicants seeking CAT II airworthiness approval using GAST C and GAEC C avionics with TSO- C161b and TSO-C162b authorization need to show equivalence to performance achieved with ILS II/D/2 with additional criteria, limitations , and exceptions as 1 RTCA DO-253D Change 1, appendix I, contains a detailed discussion of approach service types, facility classification and airborne equipment classification for GBAS.
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AC 20-191 determined by the FAA .
4.2 Airworthiness Criteria.
4.2.1 Airborne Equipment.
Airborne equipment meeting the airworthiness criteria contained in this chapter for the CAT II landing system satisfy the operational requirements of CAT II ILS found in AC 120-118. The basic airworthiness criteria rely on meeting the specific airworthiness performance provisions and are independent of the specific implementation in the aircraft or the type of approach system selected for the approach.
4.2.2 Applicant’s Proposal.
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.
4.2.3 Intended Flight Path.
The applicant may establish the intended flight path in a number of ways, for example using a navigation aid (e.g., ILS, GBAS, etc.). Other methods may be acceptable if the applicant can show feasibility through other means (e.g., se nsing of the runway environment such as surface, lighting or markings or both with a vision enhancement system).
4.2.4 Onboard Navigation Systems.
Onboard navigation systems may have various sensor elements by which to determine the aircraft’s position. The sensor elements may include ILS, GBAS, or inertial information. Each of these sensor elements must perform within its appropriate limitations with regard to accuracy, integrity, and availability.
4.2.5 Indications.
The system must provide indications of the aircraft position with respect to the intended flight path. A list of accepted means of providing these indications to the pilot includes:
• Deviation displays with reference to navigation source (e.g., ILS receiver, GLS receiver, etc.)
• Onboard navigation system computations with corresponding displays of position and reference path
• Vision enhancement system, if applicable.
4.2.6 Approach System Accuracy Criteria.
The criteria for acceptable approach performance are based upon acquiring and tracking the required flight path to the appropriate approach procedure’s minimum altitude. The acquisition should be accomplished in a manner compatible with instrument procedure requirements and flight crew requirements for the type of approach being conducted.
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AC 20-191
4.2.6.1 Appr oach Guidance System.
The approach guidance system must not generate sustained oscillatory command information (e.g., flight director, HUD, etc.) or require unusual effort by the pilot to control the flight path.
4.2.6.2 Lateral and Vertical Tracking Performance.
The applicant must show the system will meet the following performance.
4.2.6.2.1 Lateral tracking performance f rom 1000 ft. Height Above Touchdown (HAT) to 300 ft. HAT will be stable without large deviations (i.e., within ±50 microamps deviation or equivalent) from the indicated course or path.
4.2.6.2.2 Lateral tracking performance from 300 ft. (HAT) down to 100 ft. HAT will be stable without large deviations (i.e., within ±25 microamp s deviation) from the indicated course, for 95% of the time.
4.2.6.2.3 Vertical tracking f rom 700 ft. HAT to 300 ft. HAT should be stable without large deviations (i.e., within ±75 microamps deviation or equivalent) from the indicated course or path.
4.2.6.2.4 Vertical tracking performance from 300 ft. HAT down to 100 ft. HAT will be stable without large deviations (i.e., within ±35 microamps deviation) from the indicated path or ±12 ft., whichever is greater, for 95% of the time. When applying this provision to path tracking in conjunction with CAT III, momentary excursions up to ±75 microamps during test demonstrations may be acceptable if the applicant shows satisfactory performance of the flight guidance system through touchdown and landing.
Note: The performance standards for ILS or GBAS GAST D signal in space contained in ICAO SARPs Annex 10, or an equivalent State standard, are acceptable standards for operations based on an ILS or GBAS, respectively.
4.2.7 Approach System Integrity Criteria .
Pursuant to §§ 23.2500, 23.2510, 23.2605, 25.1309, 27.1309, or 29.1309, the onboard components of the landing system, when considered separately and in relation to other associated onboard systems, must consider all specific safety related criteria identified in this AC, or as identified in accordance with the operating rules. The applicant can use the following criteria as advisory material for the application of §§ 23.2500, 23.2510,
23.2605, 25.1309, 27.1309, or 29.1309 to Approach and Landing Systems:
4.2.7.1 ILS.
The applicant must establish the aircraft system response to loss of ILS 4-3
AC 20-191 guidance signals (localizer (LOC) or glideslope (GS)). The applicant must also establish the aircraft system response during a switchover from an active LOC or GS transmitter to an optional backup transmitter.
4.2.7.2 GLS.
The applicant must establish the aircraft system response to loss of GLS guidance signals.
4.2.7.2.1 When using GBAS, the required flight path is uplin ked to the aircraft after the approach has been selected. This path will be used for flight guidance or the autopilot or both when required to conduct an approach, landing, and rollout.
4.2.7.2.2 Corruption of the information contained in the received VHF Data Broadcast (VDB) to define the reference path is hazardous. Failure resulting in undetected changes to the approach path definitions data must be extremely remote.
4.2.7.2.3 The flight crew must not be able to modify information that relates to the critical definition of required flight path.
4.2.8 Approach System Continuity Criteria .
The applicant should verify the continuity of navigation during normal aircraft maneuvering while in approach. For GLS, this includes verifying VDB signal reception at all angles relative to the airframe, as the GBAS facility could be located anywhere with respect to the approach path.
4.2.9 Approach System Availability Criteria .
While on approach below 500 ft., the demonstrated probability of a successful landing should be at least 95% (i.e., no more than 5% of the approaches result in a go-around due to the combination of failures in the landing system and the incidence of unsatisfactory performance).
4.2.10 Go-around.
4.2.10.1 Go-Around Safety.
An aircraft conducting a CAT II instrument approach must be capable of safely executing a go-around from any point in an approach prior to touchdown with the aircraft in a normal configuration or specified nonnormal configurations (e.g., engine out for multiengine aircraft).
4.2.10.1.1 A go-around should not require unusual pilot skill, alertness, or strength.
4.2.10.1.2 The system should have information indicating the availability of a safe go-around flight path.
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AC 20-191
4.2.11 Information, A nnunciation, and Alerting .
This section identifies the provisions for information, annunciations, and alerting.
4.2.11.1 Compliance.
The design of the Information, Annunciation, and Alerting must comply with §§ 23.2605, 25.1322, 27.1322, or 29.1322 as applicable. Reference AC
23.1311-1, AC 25.1322, AC 27-1, or AC 29-2 for additional guidance.
4.2.11.2 Information guidance.
This section identifies criteria for basic situational and guidance information.
4.2.11.2.1 The system must present the mode and system malfunction indications in a manner consistent with the human f actors cockpit design philosophy. The indications must be grouped in a logical and consistent manner and be visible under all expected normal lighting conditions.
4.2.11.2.2 For manual control of approach flight path, the appropriate flight display(s), whether head down (HDD) or HUD, must provide sufficient information, without excessive reference to other cockpit displays, to enable a suitably trained pilot to :
• Maintain the approach path,
• Make the alignment with the runway, and if applicable, safely flare and roll out, or
• Go-around.
4.2.11.2.3 The information provided along with any additional information necessary to the design of the system must be sufficient to allow the pilots to monitor the progress and safety of the approach operation .
4.2.11.2.4 The system must have flight performance monitoring to include at least the following:
• Unambiguous identification of the intended path for the approach, and, if applicable, flare and roll-out, (e.g., ILS approach identifier/frequency, and selected navigation source, GBAS Runway Path Identifier (RPI), approach service type, etc.).
• Indication of the position of the aircraft with respect to the intended path (e.g., raw data localizer and glide path, deviation from FAS or equivalent).
4.2.11.3 Annunciation.
The system should provide a positive, continuous, and unambiguous 4-5
AC 20-191 indication for the modes in operation, as well as those armed for eng agement. In addition, where mode engagement is automatic (e.g., LOC and glide path acquisition), the system must provide a clear indication when the mode has been armed by either crew action, or automatically by the system itself (e.g., a pre-land test annunciating LAND 3).
4.2.11.4 Alerting.
Alerting is intended to address the need for warning, caution, and advisory information for the flight crew.
4.2.11.4.1 Warnings.
ACs 23-8, 23-17, 23.1309-1, 23.1311-1, 25.1309-1, 25-1322-1,
27.7, and 29-2 provides an acceptable means for the system to provide information to alert the flight crew to unsafe system operating conditions and to enable the flight crew to take appropriate corrective action. The system must provide a warning indication for any condition requiring the flight crew to take immediate corrective action. The system design should account for flight crew alerting cues, corrective action required, and the capability of detecting faults.
4.2.11.4.2 Cautions.
The system must provide a caution for any condition requiring immediate flight crew awareness and timely subsequent action.
The system must provide a means to advise the flight crew of failed aircraft system elements affecting the decision to continue or discontinue the approach.
4.2.11.4.3 Advisories.
The system must provide a means to inform the flight crew when the aircraft has reached the Alert Height (AH) or DH, as applicable, requiring pilot awareness in a timely manner. It should not generate advisories after takeoff commences.
4.2.11.4.4 Excessive-Deviation Alerting .
The FAA does not require excessive-deviation alerting but will approve systems that meet appropriate criteria (e.g., EASA CS- AWO.B.CATII.115).
4.2.11.4.5 Excessive-Deviation Alerting Method of Compliance .
Meeting EASA CS-AWO.B.CATII.115 criteria are an acceptable means of compliance for airworthiness approval.
4.2.11.4.6 System Status.
The system should provide a means for the operator and flight crew to determine prior to departure and the flight crew to 4-6
AC 20-191 determine af ter departure, the capability of the aircraft elements to accomplish the intended approach.
4.2.11.4.7 System Identification.
The system should identify system status indications by names that are different than operational authorization categories (e.g., do not use names such as “CAT I,” “CAT II,” “CAT III”).
4.2.12 Multiple Landing Systems and Multi -Mode Receivers (MMR) for CAT II.
Multiple landing systems typically consist of one or more MMRs that are capable of providing navigation information for ILS, GLS, or other combinations of landing sensor systems. The MMR characteristics should be consistent with applicable related ARINC characteristics for MMRs.
4.2.12.1 Approach Procedure.
From the pilot’s perspective, the approach procedure should be the same regardless of the navigation source being used.
4.2.12.2 MMR.
The MMR must provide a means to confirm that the intended approach aid(s) was correctly selected (e.g., the current ILS audio identification, GLS Reference Path Data Selector (RPDS)).
4.2.12.3 Indications.
The MMR must indicate the loss of deviation data on the display. It is acceptable to have a single failure indication for each axis common to all navigation sources.
4.2.12.4 Annunciations.
The following criteria apply to annunciations when using a multi-mode approach system:
4.2.12.4.1 The MMR must positively indicate t he navigation source
(e.g., ILS, GLS, FMS) selected for the approach in the primary field of view at each pilot station .
4.2.12.4.2 The MMR must indicate the data designating the approach
(e.g., ILS frequency, GLS channel number) in a position readily accessible and visible to each pilot.
4.2.12.4.3 The MMR should use a common set of mode ARM and ACTIVE indications (e.g., LOC and GS) for ILS and GLS operations. ARM mode indicates the MMR is ready and waiting to capture the signal while ACTIVE mode indicates the MMR is currently operating in that specific navigation mode.
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AC 20-191
4.2.12.4.4 Th e MMR must provide a means for the flight crew to determine a failure of the non -selected navigation receiver function, in addition to the selected navigation receiver function. When considering equipment failures, the failure indications must not mislead through incorrect association with the navigation source. For example, it would not be acceptable for the annunciation “ILS FAIL” to be displayed when the selected navigation source is GLS, and the failure actually affects the GLS receiver .
4.2.12.4.5 Outside the approach phase, the MMR should have the capability to indicate a failure of each element of a multi - mode approach and landing system as an advisory to the flight crew without pilot action.
4.2.12.4.6 During an approach, a failure of the active element of a multi-mode approach and landing system must be accompanied by a warning, caution, or advisory.
4.2.13 Multi-Mode Receivers This section provides guidance for MMRs used for CAT II for retrofit certifications, and for certification of ILS installations with either new or modified receivers.
4.2.13.1 Retrofit Considerations.
4.2.13.1.1 Typical receiver configurations for retrofit applications include:
• An ILS receiver from a new supplier
• A modified ILS receiver from the same supplier (e.g., for purposes of providing improved Frequency Modulation (FM) Immunity)
• A re-packaged receiver from the same supplier (e.g., the ILS partition in an MMR, or the transition from ARINC 700 to 900 series equipment)
• A stand-alone GLS receiver
• A GLS partition in an MMR
4.2.13.1.2 General certification considerations include:
• Certification Process. An “impact assessment” should address all new receiver functionality considering:
o Differences between the current certification basis and that which is requested, if applicable.
o The functionality being added 4-8
AC 20-191 o Credit that can be taken for the existing approval
• Equi pment Approval. The applicant must follow 14 CFR Part 21, subpart O, to obtain a TSO authorization where appropriate, including software qualification and receiver environmental qualification to the appropriate levels.
• Aircraft installation approval should consider the following:
o Impact on aircraft system safety assessments o Equipment approval (e.g., antenna positions, range, polar diagrams, coverage, compatibility between receiver and antenna) o Electromagnetic Interference (EMI)/Electromagnetic Compatibility (EMC) testing o Functional integration aspects of the receiver with respect to other systems, controls, warnings, displays o Electrical loading o Flight data recorder requirements o Suitable AFM provisions o Certification means of compliance for the receiver installation (e.g., specification of ground or flight testing or both, as necessary)
4.2.13.2 Re-certification.
Re-certification of an ILS function following the Introduction of a new or modified ILS navigation receiver installation. The certification program should consider the differences between the new configuration and the pre-existing ILS receiver system. The applicant may use an impact assessment to establish a certification basis. The assessment will typically address the following:
4.2.13.2.1 New or Modified ILS Impact Assessment.
An impact assessment should consider the following aspects of the new or modified ILS receiver, or receiver function, for equivalence with the existing ILS receiver configuration:
• Hardware design
• Software design
• Signal processing and functional performance
• Failure analysis
• Receiver function, installation, and integration (e.g., with controls, indicators, and warnings) 4-9
AC 20-191 The impact assessment should also identify any additional con siderations such as:
• Future functionality provisions that have no impact on system operation
• Shared resources to support future functionality
• Based upon the assumption that the applicant can show the ILS receiver, or receiver function, to be equivalent to the current ILS configuration, the applicant may propose that the new installation be treated as a new ILS receiver (replacing the existing receiver) for installation on a given aircraft type
4.2.13.2.2 New or Modified ILS Failure Analysis .The failure characteristics of the new or modified installation should be shown to be equivalent to systems using ILS data. This ensures the failure characteristics are compatible and do not invalidate any original or previous safety assessments.
4.2.13.2.3 New or Modified ILS Autoland or HUD Guidance Landing Function Flight Testing (if necessary) .
For systems using a new ILS, or combined MMR receiver, complete a minimum of eight (8) approaches terminating in a successful (automatic or HUD) landing, and rollout (if applicable) using the flight control/guidance system, including a minimum of two (2) ILS facilities. Approaches should include captures from both sides of the final approach course, at angles and distances representative of typical Instrument Approach Procedures (IAPs), and, if applicable, from below and above the GS.
4.2.13.2.4 Approach and Landing Performance.
The applicant should show the approach and landing performance (flight path deviation, touchdown data, etc.) is equivalent to that achieved in the original ILS certification.
The FAA may require recorded flight test data to support equivalency demonstration.
4.2.13.2.5 New or Modified ILS Documentation.
The applicant should provide the following documentation for certification:
• An impact assessment including effects on system safety assessments.
• A flight test report, if applicable.
• Revisions to the flight manual where appropriate.
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