AC 20-191 aircr aft within the prescribed limits
• If applicable, promptly go-around, with minimum practical altitude loss
4.3.2.4 Autopilot performance must be compatible with either manual speed control, or, if applicable, autothrottle/autothrust speed control.
4.3.2.5 Mode definition and logic should be consistent with appropriate industry practice for mode identification and use (e.g., naming, mode arming, and engagement). Definition of new modes or features, not otherwise commonly used, should be consistent with their intended function, and the applicant should consider their potential for setting inappropriate or adverse precedent.
4.3.2.6 If the autopilot is used to control the flight path of the aircraft to intercept and establish the approach path, the pilot should be able to transition from automatic to manual flight at any time without undue effort, attention, or control forces, and with a minimum of disturbance of flight path.
4.3.2.7 A flight director system, or alternative form of guidance, if used, must be compatible with the autopilot.
4.3.2.8 The FAA recommends at least one autopilot and dual flight director systems with an independent display for each pilot. Dual guidance systems providing the same information to both pilots may be acceptable only if suitable comparison monitoring between the systems is available, and timely transfer to the standby system can be completed, and suitable annunciation to the flight crew is provided.
4.3.2.9 A fault must cause an autopilot advisory, caution, or warning, as necessary.
If a warning is necessary, the pilot must be able to detect the warning with a normal level of attention and alertness expected during an approach or goaround.
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AC 20-191
4.3.3 He ad Down Guidance . The following criteria applies to Head Down Guidance systems:
4.3.3.1 A flight director system, or alternative form of guidance, must be designed so that the probability of display of incorrect guidance commands is remote.
4.3.3.2 Wherever practical, a fault must cause guidance information to be immediately removed from view. If a warning is given instead, it must be such that the pilot will observe the warning while using the guidance information.
4.3.4 Head Up Guidance.
If the system includes a HUD, the following criteria are applicable to Head Up Guidance systems:
4.3.4.1 The workload associated with use of the HUD must comply with 14 CFR §§
23.2610, 25.1523, 27.1523, or 29.1523. Reference AC 23.1523, AC
25.1523-1 and AC 25-11, AC 27-1, or AC 29-2 as applicable for additional guidance.
4.3.4.2 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, AC 25.773 and AC 25-11, appendix F; or AC 27-1 or AC 29-2 for additional guidance.
4.3.4.3 If the autopilot is used to control the flight path of the aircraft to intercept and establish the approach path, the point during the approach at which the transition from automatic to manual flight takes place must be identified and used for the performance demonstration.
4.3.4.4 A flight director system, or alternative form of guidance, must be designed so that the probability of display of incorrect guidance commands is remote.
4.3.5 HUD use to monitor autoflight systems.
The HUD must be compatible with the autoflight system and permit the pilot to detect unsuitable autopilot performance.
4.3.6 Autothrottle/Autothrust.
4.3.6.1 If autothrottle/autothrust capability is installed, the applicant should identify any necessary modes, conditions, procedures, or constraints that apply to its use.
4.3.6.2 Use of the autothrottle/autothrust should not cause unacceptable performance of any autopilot modes intended for use. And any autopilot 4-14
AC 20-191 mode intended for use with autothrottle/autothrust should not cause unac ceptable autothrottle/autothrust performance.
4.3.6.3 The autothrottle/autothrust should expeditiously capture any commanded speed adjustments and maintain speed. The autothrottle/autothrust should not cause any hazardous conditions with normal use or in any probable failure modes, considering pilot intervention using normal piloting skills.
4.3.6.4 The autothrottle/autothrust should hold speed within -5 to +10 knots of the intended speed except for momentary gusts, in typical environmental conditions expected for use.
4.3.6.5 The autothrottle/autothrust should provide appropriate status, advisory, caution, and warning information for failures.
4.3.6.6 The autothrottle/autothrust should provide timely application of go-around thrust if a go-around mode is available.
4.3.6.7 The autothrottle/autothrust should not require undue flight crew attention or skill to recognize and respond to an engine failure during approach or goaround.
4.4 Approach System Evaluation.
The applicant should conduct an evaluation to verify that the pertinent systems as installed in the aircraft meet the airworthiness criteria of section 4.2 of this AC. The evaluation should include verification of approach system performance and a safety assessment for verification of the integrity and availability provisions. The applicant should demonstrate engine failure cases and other selected failure conditions identified by the safety assessment by simulator or flight test.
4.4.1 Certification Plan.
An applicant should provide a certification plan(s) describing:
4.4.1.1 The means proposed to show compliance with the criteria of section 4.2 of this AC, with particular attention to methods that differ significantly from those described in this chapter.
4.4.1.2 How non-aircraft elements of the approach system relate to the aircraft system from a performance, integrity, and availability perspective (e.g., appropriate reference to ICAO Annex or U.S. Standard).
4.4.1.3 The assumptions on how the performance, integrity, and availability requirements of the non-aircraft elements of standard landing aids will be assured, if applicable.
4.4.1.4 The effect of the aircraft navigation reference point on the aircraft flight path and wheel-to-threshold crossing height.
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AC 20-191
4.4.1.5 The system concepts and operational philosophy to allow the FAA to determine whether criteria and guidance in excess of that contained in this chapter are necessary.
4.4.1.6 The applicant should reach an early agreement with the FAA on his/her proposed certification plan.
4.4.2 Performance Evaluation.
The applicant should demonstrate the airborne system performance meets provisions of this chapter using simulations and flight tests.
4.4.3 Airborne Performance System.
The airborne system performance demonstration should include at least the following conditions taking into account manual/coupled autopilot, autothrottle/autothrust configurations for CAT II approaches:
4.4.3.1 Wind Conditions:
• 20 kts - Head wind component
• 10 kts - Crosswind component
• 10 kts - Tailwind component Note: The FAA may accept lower limits if the applicant appropriately documents in the AFM and the certification plan.
4.4.3.2 The applicant must demonstrate performance by flight test, or analysis validated by flight test, using at least three different representative facilities for a minimum of 20 total approaches, with a representative range of environmental and system variables affecting overall performance. The applicant must achieve a minimum of 95% success in the number of attempted approaches.
4.4.3.3 The performance assessment must take into account at least the following variables with the variables being applied based upon their expected distribution:
• Configurations of the aircraft (e.g., flap settings),
• Center of gravity,
• Landing weight,
• Conditions of wind, turbulence, and wind shear,
• Characteristics of ground- and space-based systems and aids (i.e., ILS or GLS), and
• Any other parameter which may affect system performance (e.g., airport altitude, approach path slope, variations in approach speed).
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AC 20-191
4.4.3.4 The FAA will accept use of the Continuous Method and the Pass/Fail Method, both found in EASA AMC AWO 231, in lieu of the 95% probability of success tracking requirement described in section 4.2.6.2, and the minimum number of 20 approaches stated in section 4.4.3.2.
4.4.3.5 The FAA recommends wind model analysis to obtain approval of systems related to CAT II. If an applicant elects to use simulation with a wind model to support approval, the applicant should use the model specified in Appendix B.
4.4.3.6 For GLS, the applicant should assess fault free performance of GLS using simulations such as Monte-Carlo with the input variables distributed according to their performance characteristics and a model of the GLS guidance including fault free error characteristics defined by the GLS signal model as specified in 5.5.8 Appendix C.
Note: Applicants with approved airworthiness criteria for GAST D level of service meet all the airworthiness criteria of this chapter.
4.4.4 Safety Assessment.
In addition to any specific safety related criteria identified in this chapter, the applicant must conduct a safety assessment of the approach and landing system, considered separately and in conjunction with other systems, to show compliance with §§ 23.2500,
23.2510, 23.2605, 25.1309, 27.1309, or 29.1309. The applicant can use the guidance contained in AC 23.1309-1, AC 25.1309-1, AC 27-1, or AC 29-2, as applicable.
4.4.4.1 The applicant must consider the responses to failures of navigation facilities, taking into account ICAO and other pertinent state criteria for navigation facilities.
4.4.4.2 Documented safety analysis conclusions must include:
4.4.4.2.1 Pursuant to §§ 23.2500, 23.2510, 23.2605, 25.1309, 27.1309, or 29.1309, an FHA conducted in accordance with the guidance provided by AC 23.1309-1 , AC 25.1309-1, AC 27-1 or AC 29-2, respectively, as applicable; a summary of results from the fault tree analysis, demonstrated compliance, and probability determinations for significant functional hazards.
4.4.4.2.2 Information regarding "alleviating flightcrew actions" that were considered in the safety analysis. This information should list appropriate alleviating actions, if any, and should be consistent with the validation conducted during testing. If alleviating actions are identified, the applicant should describe those actions in a form suitable to aid in developing, as applicable:
• Pertinent provisions of the AFM procedures section(s) 4-17
AC 20-191
• Fl i ght Crew Operating Manual (FCOM) provisions, or equivalent
• Pilot qualification criteria (e.g., training requirements, FSB provisions)
• Any other reference material necessary for an operator or flight crew to safely use the system
4.4.4.2.3 Information to support preparation of any necessary maintenance and preventive maintenance such as:
• Certification maintenance requirements (CMR)
• Periodic checks
• Other checks, as necessary (e.g., return to service)
4.4.4.2.4 Information applicable to lim itations, as necessary.
4.4.4.2.5 Identification of applicable syste ms, modes, or equipment necessary for use of the landing syste m. This information may be used, to aid in development of flight planning , dispatch criteria, or procedures or checklists for pilot selection of modes or assessment of system status, prior to initiation of approach or during approach.
4.4.4.2.6 Information necessary for development of non-normal procedures.
4.4.5 Continued Airworthiness.
4.4.5.1 Pursuant to 14 CFR §§ 23.1529, 25.1529, 27.1529, or 29.1529, an applicant must prepare an Instructions for Continued Airworthiness (ICA) for use by operators. The applicant must submit to the FAA a program showing how the applicant distributes changes made to the ICAs by the applicant or by the manufacturers of the ILS or GBAS avionics installed in the aircraft.
ICAs established during the approval process form the basis for an operator’s maintenance program.
4.4.5.2 Pursuant to 14 CFR §§ 21.3, the applicant must report any failure, malfunction, or defect whose occurrences could impact the safety of the flight crew or the structural integrity of the aircraft itself.
4.5 AFM.
Pursuant to 14 CFR §§ 23.2620, 25.1581, 27.1581, or 29.1581, as applicable, an applicant must furnish an AFM with each aircraft. The AFM should contain the following information:
4.5.1 Relevant conditions or constraints applicable to the approach and landing system use 4-18
AC 20-191 regardi ng the airport or runway conditions. For example:
• Runway elevation
• Ambient temperature
• Approach path slope
• Runway slope
• Ground profile under the approach path
4.5.2 The criteria and configurations used for the demonstration of the system .
4.5.3 Any constraints or limitations necessary for safe operation.
4.5.4 Acceptable normal and non-normal procedures.
4.5.5 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 known to be unacceptable for use (e.g., ILS or GLS). The AFM should indicate the operation is predicated upon the use of an ILS (or GLS) facility with performance and integrity equivalent to, or exceeding, a United States Type II or Type III ILS, GBAS GAST D, or equivalent ICAO Annex 10 Facility Performance CAT II facility.
4.5.6 Applicable atmospheric conditions under which the system was demonstrated (e.g., headwind, crosswind, tailwind).
4.5.7 Any necessary performance, procedure, or configuration data to permit an operator to determine climb gradient and transition distances for safe obstacle clearance during a missed approach, balked landing, or rejected landing. T his information need not be specifically included in the AFM if it is available to the operator using some other method acceptable to the operator and manufacturer (e.g., FCOM, supplementary performance information, separate AFM appendix).
4.5.8 For an approach and landing system meeting the provisions of this chapter, the Normal Procedures, Normal Operations, or equivalent section s, of the AFM should also contain the following statements:
“The airborne system has been demonstrated to meet the airworthiness provisions of AC 20-191 (Chapter 4) for [specify the pertinent approach 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 CAT II use of the system.”
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AC 20-191 CHAPTER 5. CATEGORY (CAT) III
5.1 Introduction.
5.1.1 This chapter contains airworthiness approval criteria for equipment used to conduct an approach, landing, and rollout in CAT III weather minima using an ILS or GBAS.
Landing and rollout may combine various combinations of aircraft sensors and system architecture with various combinations of ground and space -based elements. This AC does not provide criteria for each potential combination of airborne and non- airborne elements.
5.1.2 This chapter addresses the approach , landing and rollout phase of flight, beginning at the FAF, through the CAT III DH, ending at the landing , and rollout phase.
5.1.3 ICAO Annex 10 i ncludes standards for ILS and GBAS systems intended to support CAT III operations. This chapter addresses operations using either ILS or GBAS for approaches, using ILS avionics produced in accordance with TSO-C34e, TSO-C36e authorizations, and GBAS avionics with TSO-C161b and TSO-C162b authorizations.
5.1.4 The standards for GBAS (RTCA DO- 253D Change 1 and ICAO Annex 10) define two general classes of GBAS service s, Approach Service , and the GBAS Positioning Service. The standards further differentiate GAST. A GAST is defined as the matched set of airborne and ground performance and functional requirements intended to be used in concert in order to provide approach guidance with quantifiable performance.
Applicants for CAT III airworthiness seek approval through GBAS GAST -D level of service.
5.2 Airworthiness Criteria.
5.2.1 The airworthiness criteria contained in this chapter for the CAT II I landing and rollout system satisfy the operational requirements described in AC 120-118. The basic airworthiness criteria are independent of the specific implementation in the aircraft or the type of approach landing system and rollout selected for the approach.
5.2.2 Performance provisions for touchdown performance, landing sink rates , and attitudes, etc. (see section 5.4.2.4, below) are the same for landing systems with automatic flight control, and systems for manual flight control with command information as guidance.
5.2.3 The applicant’s type certification proposal should include criterion such as the system’s intended function, accuracy, reliability, and fail- safe features. The proposal should also address the operational concepts contained in AC 1 20-118.
5.2.4 The low visibility landing system is intended to guide the aircraft to a touchdown in the prescribed touchdown zone, with an appropriate sink rate and attitude without exceeding prescribed load limits of the aircraft. The rollout system is intended to guide the aircraft to converge on and track the runway centerline, from the point of 5-1
AC 20-191 touchdown t o a safe taxi speed.
5.2.5 The overall assurance of performance and safety of an operation can only be assessed when all elements of the system are considered.
5.2.6 The low-visibility landing system must be shown to be satisfactory with and without the use of any outside visual references, except that outside visual references will not be considered when assessing lateral tracking performance. The airworthiness evaluation will also determine whether the c ombination of guidance and outside visual references would unacceptably degrade task performance or require exceptional workload and pilot compensation . The evaluation should include normal operations and non-normal operations with system and aircraft failure conditions.
5.2.7 General Airworthiness Criteria .
5.2.7.1 The applicant establishes the intended flight path. The path may be established in a number of ways, including the use of navigation aids (e.g., ILS or GLS). Other methods may be acceptable if the applicant can show feasibility through other means, for example:
• Sensing of the runway environment (e.g., surface, lighting or markings or both) with a vision enhancement system, or
• By a vision enhancement system.
5.2.7.2 On-board navigation systems may have various sensor elements by which to determine aircraft position. The sensor elements may include ILS, Inertial information, GLS, or other GNSS elements. Each of these sensor elements should comply with their performance provisions within appropriate limitations.
5.2.7.3 The system should provide an indication of the aircraft position with respect to the intended lateral path using:
• Deviation displays with reference to a navigation source (e.g., ILS receiver, GLS receiver, etc.), or
• On-board navigation system computations with corresponding displays of position and reference path.
5.2.8 Accuracy.
5.2.8.1 Acquisition Performance.
The acquisition should be accomplished in a manner compatible with instrument procedure requirements and flight crew requirements for the type of approach being conducted. The approach guidance system must not generate sustained oscillating command information (e.g., flight director, HUD, etc.) or require unusual effort by the pilot to control the flight path.
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AC 20-191
5.2.8.2 La teral and Vertical Tracking Performance.
The applicant must show the system will meet the following performance parameters:
5.2.8.2.1 Stable lateral tracking performance from 1000 ft. Height Above Touchdown (HAT) to 300 ft. HAT should be stable without large deviations (i.e., within ±50 microamps deviation or equivalent) from the indicated course or path.
5.2.8.2.2 Stable lateral tracking performance from 300 ft. HAT to touchdown without large deviations (i.e., within ±25 microamps deviation) from the indicated course, for 95% of the time.
5.2.8.2.3 Stable vertical tracking from 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.
5.2.8.2.4 Stable vertical tracking performance from 300 ft. HAT to until commencing the flare maneuver (e.g., 100 ft. ) without large deviations (i.e., within ±35 microamps deviation) from the indicated path or ±12 ft., whichever is greater, for 95% of the time. 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.
5.2.8.2.5 The FAA will accept use of the Continuous Method and the Pass/Fail Method, both found in EASA AMC AWO 231, in lieu of the 95% probability of success tracking requirement described in sections 5.2.8.2.1, 5.2.8.2.2, 5.2.8.2.3 and
5.2.8.2.4 using a minimum of 20 approaches .
5.2.9 Approach and Landing System Performance .
5.2.9.1 Stable Approach.
The applicant should conduct a stable approach (i.e., “normal maneuvering” without excessive attitudes, sink rates, path deviations, or speed deviations) to the point where a smooth transition is made to the landing.
5.2.9.1.1 If the landing system is designed to perform an alignment function prior to touchdown, to correct for crosswind effects, the system should operate in a manner consistent with the pilot's manual technique for crosswind landings . Typically, the pilot's manual technique includes the wing low , side slip procedure. Non-availability of the alignment mode, or failure of the alignment mode to perform its intended function must 5-3
AC 20-191 be easily detectable, or be suitably annunciated, so that the flig ht crew can take appropriate action.
5.2.9.1.2 The "landing flare to touchdown" maneuver should reduce the aircraft sink rate to a value not exceeding the sink rate for landing as listed in the AF M.
5.2.9.2 Speed Control Performance.
Airspeed must be controllable to within +/- five knots of the approach speed*, except for momentary gusts, up to the point where the throttles are retarded to idle for landing. For operations flown with manual control of approach speed, the flight crew must be able to control speed to within +/- five knots of the approach speed.
*This criterion is not specific to low visibility systems but must be met by low visibility systems.
5.2.9.3 Landing System Performance.
All types of low visibility landing systems, including automatic flight control, guidance for manual control, and hybrid systems, must meet the performance specified in section 5.2.9.4. The performance values may vary where justified by the characteristics of the aircraft. Additional performance provisions apply for GLS systems. See sections 5.2.9.5 through 5.2.9.7.
5.2.9.4 Nominal Performance (ILS, GLS).
The nominal performance provisions establish compatibility of the aircraft system performance with basic navigation system accuracy in fault-free conditions and nominal environmental conditions. Nominal conditions include both normal and rare normal conditions. The variables affecting performance that an applicant must include in the assessment are discussed in section 5.2.9.9. The nominal performance criteria and probabilities are as follows:
5.2.9.4.1 Probability of long itudinal touchdown earlier than a point on the runway 200 ft. (60m) from the threshold must not exceed
10-6.
5.2.9.4.2 Probability of longitudinal touchdown beyond 2700 ft.(823m) from threshold must not exceed 10-6.
5.2.9.4.3 Probability of lateral touchdown with the outboard landing gear more than 70 ft. (21.3 m) from runway centerline must not exceed 10-6 (assuming a 150 ft runway width) .
5.2.9.4.4 Probability of s ink rate at the structural limit load must not exceed 10-6. An acceptable means of establishing that the structural limit load is within its specified limit is to show separately and independently that:
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AC 20-191
• The limit load that results from a sink rate at touchdown not greater than 10 fps or the limit rate of descent used for certification under 14 CFR Part 25 subpart C (see § 25.473), whichever is greater. For part 23, 27, and 29 aircraft, the limit load conditions specified in §§ 23.2220,
27.473, or 29.473 apply, respectively.
• The lateral side load does not exceed the limit value determined for the lateral drift landing condition defined in § 25.479(d)(2). For Part 23, 27, and 29 aircraft, the level landing conditions specified in §§ 23.2220, 27.479, or
29.479 apply, respectively.
5.2.9.4.5 Probability of bank angle resulting in hazard must not exceed
10-7. For example, a hazard to a fixed wing aircraft means a bank angle resulting in any part of the wing including the tip, high lift device, or engine nacelle touching the ground. For rotorcraft the hazard definition will be based on the rotorcraft’s unique capabilities. Reference AC 27 -1 or AC 29- 2 for additional information on helicopters.
5.2.9.5 GLS.
The applicant must assess the effect of nominal GLS errors on landing system performance via simulation using the GLS Noise Model provided in
5.5.8 Appendix C or an approved equivalent model), including the fault-free elements, as described in sections C.1.2 through C.1.6.
5.2.9.5.1 When using GBAS, the required flight path is uplinked to the aircraft after the approach is selected. This path is used for flight guidance or the autopilot or both when required to conduct an approach, landing, and rollout.
5.2.9.5.2 Corruption of the information contained in the received VDB to define the reference path is considered a hazardous condition. Failure resulting in undetected changes to the approach path definitions data must be extremely remote.
5.2.9.5.3 The flight crew must not be able to modify information that relates to the critical definition of required flight path.
5.2.9.6 Compatibility with Rare Undetected Non-Aircraft System Error Conditions (e.g., satellite faults, ionospheric anomalies).
For any value of GLS navigation system error resulting from faults, in conjunction with nominal conditions:
• The probability of longitudinal touchdown earlier than a point on the runway 200 ft. (60 m) from the threshold must not exceed 10-5.
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