AC 20-191

AC 20-191 Appendix A Term D efinition

• External Information the pilot derives from visual observation of real world Visual cues outside the cockpit.

Reference

• Extremely A probability of occurrence of 1 x 10-9 or less per hour of flight, or Improbable per event (e.g., takeoff, landing).

• Extremely A probability of occurrence between 1 x 10-9 and 1 x 10-7 per hour of Remote flight, or per event (e.g., takeoff, landing).

• Fail A system capable of completing the specified phases of an operation Operational following the failure of any single system component after passing a System point designated by the applicable safety analysis (e.g., Alert Height).

• Fail Passive A system that, in the event of a failure, causes no significant System deviation of aircraft flight path or attitude.

• Field of View As applied to a HUD, the angular extent of the display that can be seen from within the design eye box.

• Final The final bearing/radial/track of an instrument approach leading to a Approach runway, without regard to distance. For certain previously designed Course approach procedures that are not aligned with a runway, the FAC (FAC) bearing/radial/track of an instrument approach may lead to the extended runway centerline, rather than to alignment with the runway.

• Final The fix from which the final approach to an airport is executed. For Approach Fix standard procedures that do not involve multiple approach segments (FAF) intercepting the runway centerline near the runway, the FAF typically identifies the beginning of the straight-in final approach segment and the point where final segment descent may begin.

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AC 20-191 Appendix A Term D efinition

• Final The segment of an approach extending from the Glide Path Intercept Approach Waypoint (GPIWP) or Approach Intercept Waypoint (APIWP), Segment whichever occurs later, to the Glide Path Intercept Reference Point (GIRP).

• Flight The means available to the flight crew to maneuver the aircraft in a Guidance specific manner either manually or automatically. It may include a System number of components such as the autopilot, flight directors, relevant display and annunciation elements and it typically accepts inputs from the airborne navigation system.

• Flight Path The FPAP is a point usually at or near the stop end of a runway. The Alignment point is used in conjunction with the Runway Datum Point (RDP) and Point (FPAP) a vector normal to the World Geological Survey – 1984 (WGS-84) ellipsoid at the RDP to define the geodesic plane of a final approach and landing flight path. The FPAP typically may be the RDP for the reciprocal runway.

• Flight Path The Flight Path Control Point (FPCP) is a calculated point located Control Point directly above the Runway Datum Point. The FPCP is used to relate (FPCP) the vertical descent of the final approach flight path to the landing runway.

• Flight The accuracy with which the aircraft is controlled as measured by the Technical indicated aircraft position with respect to the indicated command or Error (FTE) desired position. Note: FTE does not include human performance conceptual errors, typically that may be of large magnitude (e.g., entry of an incorrect waypoint or waypoint position, selection of an incorrect procedure, selection of an incorrect NAVAID frequency, failure to select a proper flight guidance mode).

• Frequent Occurring more often than 1 in 1000 events or 1000 flight hours.

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AC 20-191 Appendix A Term D efinition

• GBAS A differential GNSS (e.g., GPS)-based landing system providing both Landing vertical and lateral position fixing capability. Note: Term may be System applied to any GNSS based differentially corrected landing system (GLS) providing lateral and vertical service for approach and landing equivalent to or better than that provided by a U.S. Type I ILS, or equivalent ILS specified by ICAO Annex 10.

• Glide Path A descent profile determined for vertical guidance during a final (GP) approach.

• Glide Path The glide path angle is an angle defined at the FPCP that establishes Angle (GPA) the descent gradient for the final approach flight path of an approach procedure. It is measured in the geodesic plane of the approach (defined by the RDP, FPAP, and WGS-84 ellipsoid’s center). The vertical and horizontal references for the GPA are a vector normal to the WGS-84 ellipsoid at the RDP and a plane perpendicular to that vector at the FPCP, respectively.

• Glide Path The point at which the extension of the final approach path intercepts Intercept the runway.

Reference Point (GIRP)

• Glide Path The point at which the established glide slope intercept altitude Intercept (MSL) meets the Final Approach Segment (FAS), on a standard day, Waypoint using a standard altimeter setting (1013.2 HPa or 29.92 in).

(GPIWP)

• Glideslope Part of the ILS that projects a radio beam upward at an angle of (GS) approximately three degrees from the approach end of an instrument runway. The glideslope (GS) provides vertical guidance to aircraft on the final approach course for the aircraft to follow when making an ILS approach along the Localizer (LOC) path.

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AC 20-191 Appendix A Term D efinition

• Global A world-wide position, velocity, and time determination system that Navigation uses one or more satellite constellations.

Satellite System (GNSS)

• Global The NAVSTAR Global Positioning System operated by the United Positioning States Department of Defense (DOD). It is a satellite-based radio System navigation system composed of space, control, and user segments.

(GPS) The space segment is composed of satellites. The control segment is composed of monitor stations, ground antennas, and a master control station. The user segment consists of antennas and receiverprocessors that derive time and compute a position and velocity from the data transmitted from the satellites.

• Go-Around A transition from an approach to a stabilized climb.

• Guidance Information used during manual control or monitoring of automatic control of the aircraft that is of sufficient quality to be used by itself for the intended purpose.

• Head Up A display with the following characteristics:

Display • A head-up presentation not requiring the transition of visual(HUD) attention from head down to head up,System

• displays imagery and or/symbology conformal (as defined in SAE AS 8055) with the pilot’s external view,

• permits simultaneous view of the imagery (if presented), aircraft flight symbology and the external view, and

• display characteristics and dynamics are suitable for manual control of the aircraft.

• Hybrid A combination of two, or more, systems of dissimilar design used to System perform a particular operation.

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AC 20-191 Appendix A Term D efinition

• ICAO Type An instrument approach operation to a DH below 75m (250 ft) Type B Instrument B instrument approach operations are further categorized as Category Approach I, II, and III defined above.

• Improbable -9 A probability of occurrence greater than 1 x 10 but less than or -5 equal to 1 x 10 per hour of flig ht, or per event (e.g., takeoff, landing).

• Independent A system that is not adversely influenced by the operation, Systems computation, or failure of some other identical, related, or separate system (e.g., two separate ILS receivers).

• Infrequent Occurring less often than 1 in 1000 events or 1000 flight hours.

• Instantaneous The angular extent of a HUD display that can be seen from either eye Field of View from a fixed position of the head.

• Integrity A measure of the acceptability of a system, or system element, to contribute to the required safety of an operation.

• Landing For the purpose of this AC, landing begins at 100 ft., the DH or the AH and ends at the first contact of the wheels for fixed wing aircraft with the runway.

• Landing For the purpose of this AC, rollout starts from the first contact of the rollout wheels with the runway and finishes when the aircraft has slowed to a safe taxi speed (in the order of 30 knots).

• Missed The flight path followed by an aircraft after discontinuation of an Approach approach procedure and initiation of a go-around. Typically, a “missed approach” follows a published missed approach segment of an IAP, or follows radar vectors to a MAP, return to landing, or diversion to an alternate.

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AC 20-191 Appendix A Term De finition

• Monitored A HUD that has internal or external capability to reliably detect HUD erroneous sensor inputs or guidance outputs, to assure that a pilot does not receive incorrect or misleading guidance, failure, or status information.

• Non-Normal Conditions or configurations of the aircraft that would not normally Conditions be experienced during routine flight operations usually due to failures

(e.g., engine failure, electrical failure, autothrottle/autothrust failure, etc.).

• NOTAM Notice to Airmen - A notice distributed by means of telecommunication containing information concerning the establishment, condition, or change in any aeronautical facility, service, procedure or hazard, the timely knowledge of which is essential to personnel concerned with flight operations. (ICAO - IS&RP Annex 6).

• Performance A measure of the accuracy with which an aircraft, a system, or an element of a system operates compared against specified parameters.

Performance demonstration(s) typically include the component of FTE.

• Rare Normal A condition which must be expected to normally occur, but does so Conditions only very infrequently (e.g., unusually strong winds, significant wind gradients, significant turbulence, significant in-flight icing, significant mountain wave activity).

• Redundant The presence of more than one independent means for accomplishing a given function or flight operation. Each means need not necessarily be identical.

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AC 20-191 Appendix A Term D efinition

• Rejected A discontinued landing attempt. A rejected landing is typically Landing initiated at low altitude, but prior to touchdown. If from or following an instrument approach, a rejected landing typically is considered to be initiated below DA(H) or MDA(H). A rejected landing may be initiated in either Visual Meteorological Conditions (VMC) or Instrument Meteorological Conditions (IMC). A rejected landing typically leads to or results in a go-around and if following an instrument approach, a “Missed Approach.” If related to consideration of aircraft configuration(s) or performance, it is sometimes referred to as a “Balked Landing.” The term “rejected landing” is used to be consistent with regulatory references such as found in 14 CFR 121, appendix E, and policy references as in FAA Order 8400.10.

• Remote A probability of occurrence greater than 1 x 10-7 but less than or equal to 1 x 10-5 per hour of flight, or per event (e.g., takeoff, landing).

• Required That section of the visual aids or of the approach area that should Visual have been in view for sufficient time for the pilot to make an Reference assessment of the aircraft’s position and rate of change of position, in relation to the desired flight path. In Category III operations with a DH, the required visual reference is that specified for the particular procedure and operations (ICAO - IS&RP Annex 6 - DH definition - Note 2).

• Runway The RDP is used in conjunction with the FPAP and a vector normal Datum Point to the WGS-84 ellipsoid at the RDP. The RDP defines the geodesic (RDP) plane of a final approach flight path to the runway for touchdown and rollout. It is a point at the designated lateral center of the landing runway defined by latitude, longitude, and ellipsoidal height. The RDP is typically a surveyed reference point used to connect the approach flight path with the runway. The RDP may or may not The RDP may or may not necessarily be coincident with the designated runway necessarily be coincident with the designated runway threshold.

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AC 20-191 Appendix A Term D efinition

• Runway That segment of an approach from the glidepath intercept reference Segment point (GIRP) to Flight Path Alignment Point (FPAP).

(RWS)

• Safe An approach that does not penetrate the Obstacle Limitation Surfaces Approach for Precision Approach Category II or III as defined by ICAO SARPs Annex 14 Section 4.2.15 and Table 4-1.

• Safe Landing Landing in the touchdown landing box which includes longitudinally between 200 ft and 3000 ft. or the end of the touchdown zone, and laterally from with the outbound landing gear within 70 ft. from the runway centerline.

• Safe Taxi The speed at which the pilot can safely leave the runway or bring the Speed aircraft to a safe stop.

• Standard In the context of this section of this AC, is a navigation service Landing Aid provided by a State that meets internationally accepted performance (SLA) standards (e.g., ICAO Standards and Recommended Practices (SARP’s) or equivalent State standards).

• Synthetic Information provided to the flight crew by instrumentation or Reference electronic displays. May be either command or situation information.

• Synthetic A guidance system which integrates a synthetic vision system scene Vision depiction with additional functionality that includes a flight path Guidance vector and flight path angle reference cue, and display elements that System include runway of intended landing to support operations below (SVGS) standard precision approach operating minima.

• Takeoff Takeoff guidance system includes position sensors, displays, Guidance indicators and controls as a minimum. The takeoff guidance system System provides directional command guidance to the pilot during a takeoff, or takeoff and rejected takeoff phase of flight.

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AC 20-191 Appendix A Term D efinition

• Threshold The height of the straight line extension of the glide path above the Crossing runway at the threshold.

Height (TCH)

• Threshold The theoretical height above the runway threshold at which the Crossing aircraft’s glideslope antenna would be if the aircraft maintained the Height (TCH) trajectory established by the mean ILS glideslope or MLS glidepath.

(Pilots Handbook of Aeronautical Knowledge)

• Touchdown The first 3000 ft. of usable runway for landing, unless otherwise Zone specified by the FAA, or other applicable ICAO or State authority

(e.g., for Short Take Off and Landing (STOL) aircraft, or in accordance with a Special Federal Aviation Regulation (SFAR)).

• Wet Runway A runway surface that is neither dry nor contaminated but has visible dampness, moisture or water less than 1/8 inch in depth or both. Wet is considered to be any condition “not clear and dry” on any part of the useable area of the runway (useable area does not include edges, sides, melting of ice or snow banks at edges or sides, area beyond the advertised plowed and sanded surface, overruns, etc.).

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AC 20-191 Appendix B APPENDIX B. WIND MODEL FOR APPROACH AND LANDING SIMULATION B.1 Wind Models.

This appendix describes three types of wind models, namely wind model A, B and C. In carrying out the performance analysis, the applicant may use one of the models of wind, turbulence and wind shear. Wind model analysis is required to obtain approval of systems related to CAT III, and the FAA recommends wind model analysis to obtain approval of systems related to Category II. GAST D also must implement a wind model analysis.

B.2 Wind Model A.

B.2.1 Mean Wind.

The mean wind is the steady state wind measured at landing. This mean wind is composed of a downwind component (headwind and tailwind) and a crosswind component. The cumulative probability distributions for these components are provided in Figure B-1 (downwind) and Figure B-2 (crosswind). Alternatively, the mean wind can be defined with magnitude and direction. The cumulative probability for the mean wind magnitude is provided in Figure B-3, and the histogram of the mean wind direction is provided in Figure B-4. The mean wind is measured at a reference altitude of 20 ft. AGL. The models of the wind shear and turbulence given in the following sections assume this reference altitude of 20 ft. AGL is used.

B.2.2 Wind Shear.

The wind shear component is that portion that affects the air mass moving along the ground (i.e., ground friction). The magnitude of the shear is defined by the following expression:

 h+ 0.15V = 0.20407 V ln wref 20  0.15  where Vwref is the mean wind speed measured at h ft. and V 20 is the mean wind speed (ft/sec) at 20 ft. AGL.

B.2.3 Turbulence.

The turbulence spectra are of the Von Karman form.

B.2.3.1. Vertical Component of Turbulence.

The vertical component of turbulence has a spectrum of the form defined by the following equation:

B-1

AC 20-191 Appendix B L σ2 ( 2w1+ 2.67 1.339L Ω) Ω w w ) wΦ ( =  ( ) 112 62π1+ 1.339L Ω  w  where:

= spectral density in (ft./sec)2 Φ w σw = root mean square (rms) turbulence magnitude in ft/sec = 0.1061V20(ft/sec).

Where V 20 i s expressed in ft/sec.

Lw = scale length = h (for h < 1000 ft) Ω = spatial frequency in radians/ft = ω/VT ω = temporal frequency in radians/sec, and VT = aircraft speed in ft./sec.

B.2.3.2. Horizontal Component of Turbulence:

The horizontal component of turbulence consists of a longitudinal component (in the direction of the mean wind) and lateral component (see Figure B-5). The longitudinal and lateral components have spectra of the form defined by the following equations:

B.2.3.2.1 Longitudinal Component:

( ) L σ2Φu Ω = ( u u π 1+(1.399ΩLu) 2 ) 56 B.2.3.2.2 Lateral Component:

L 2v σv (1+ 2.67(1.339L Ω)2 ) Φv(Ω) v = 2 ( +( 11 π 1 1 .339L 2 ) 6 vΩ) B-2

AC 20-191 Appendix B B.2.3.3. Turbulence Intensities and Scale Lengths.

The RMS turbulence intensities are defined as below σw = 0.1061V 20 (ft/sec)

• When h≥1,000ft. σ = σ = σ u v w

• When h<1,000ft.

0.4  σ =σ =σ  u v w  0.177+ 0.000823h 

• When h≤0ft.

 1 

0.4 σ =σ =σ u v w  0.177 The turbulence scales are defined as below

• When h≥1,000ft. L = L = L = 1,000 u v w

• When h<1,000ft L = h w  1 

1.2 L = L = h u v ( 0.177+ 0.000823h)

• When h≤0ft. L = L = L = 0 u v w B-3