AC 120-28D Appendix 3 07/13/99 used to intercept and establish tracking of the final approach path), the transition from automatic to manual flight shall be evaluated during either the HUD demonstration or automatic flight control system demonstration, or both demonstration(s).
j. Any transition from automatic flight control to manual control using HUD guidance must not require exceptional piloting skill, alertness, strength or excessive workload.
k. If the HUD fails at any time during a go-around (GA), the pilot must be able to satisfactorily revert to use of head down displays or instruments. The transition must be completed without unacceptable flight path transients, or loss of climb performance that could adversely affect obstacle clearance.
l. During demonstration of any HUD intended for use in Category III operations (e.g., to monitor autoland), and particularly for any HUD intended for manual "pilot-in-the-loop" flight guidance for Category III approach and landing, both landing cases and go-around (GA) cases should be demonstrated where:
1) External visual reference is available at or below 50 ft. HAT to touchdown, and
2) External visual reference is not available at any time below 50 ft. HAT to touchdown, and, if applicable, is also not available for rollout, and
3) External visual references and HUD and instrument references disagree (e.g., localizer centering errors).
m. If rollout guidance is provided on the HUD, the HUD information must enable the pilot to safely control the airplane along the runway after touch down within the prescribed limits. Both normal tracking and any applicable non-normal capture or tracking conditions (e.g., recovery from displacements) should be assessed.
n. After touch down, loss of a Fail Passive rollout system for manual control with guidance, shall be annunciated with an appropriate visual alert and removal of the command guidance.
o. Rollout systems which display only lateral deviation as a cue for centerline tracking have generally not been shown to provide adequate information to adequately control the aircraft or recover from displacements. Consequently, such displays are typically considered to have excessive workload and require excessive pilot task compensation. Also, systems which display only situation information in lieu of command information have not been shown to be effective. If proposed, either type of such system would require successful proof of concept evaluation. [PoC]
8.4. Hybrid HUD/Autoland Systems [PoC]. Hybrid systems must be demonstrated to be acceptable to the FAA in a proof of concept evaluation during which specific airworthiness and operation criteria will be developed, and they must otherwise meet the requirements of 5.8 and this appendix.
8.4.1. Hybrid HUD/Autoland System Fail Operational Equivalency Concept. Combining an automatic landing system which meets the Fail Passive criteria of this appendix with a HUD which also meets that same criteria does not necessarily ensure that an acceptable Fail Operational system will result. These systems may be combined to establish a Fail Operational system for low visibility operations provided certain considerations are addressed:
AC 120-28D 07/13/99 Appendix 3
1) Each element of the system alone is shown to meet its respective requirements for a Fail Passive system.
2) The automatic landing system shall be the primary means of control, with the manual flight guidance system serving as a backup mode or reversionary mode.
3) Manual rollout flight guidance capability must be provided for hybrid systems which do not have automatic rollout capability. Such manual rollout capability must have been shown to have performance and reliability at least equivalent to that required of a Fail Passive automatic rollout system.
4) The transition between automatic mode of operation and manual mode of operator should not require extraordinary skill, training, or proficiency.
5) If the system requires a pilot to initiate manual control at or shortly after touch down, the transition from automatic control prior to touch down to manual control using the remaining element of the hybrid system (e.g., HUD) after touch down must be shown to be safe and reliable.
6) The capability of the pilot to use a hybrid system to safely accomplish the landing and rollout, following a failure of one of the hybrid system elements below alert height, must be demonstrated, even if operational procedures require the pilot to initiate a go-around.
7) Appropriate annunciations must be provided to the flightcrew to ensure a safe operation.
8) The combined elements of the system must be demonstrated to meet the required Fail Operational criteria necessary to support the operation (refer to Section 4 of the AC)
9) The overall system must also be shown to meet necessary accuracy, availability, and integrity criteria suitable for Fail Operational systems. Individual components must each be individually reliable
(e.g., a highly reliable automatic flight control system and an unreliable HUD would not be acceptable).
Hybrid System Go Around Capability.
Demonstrations are necessary for each element of the hybrid system for low altitude go-around (GA), in the altitude range between 50 ft. HAT and touchdown.
Hybrid system demonstrations must be conducted in the following conditions:
a. Without external visual reference,
b. With visual reference, and
c. With the presence of external visual reference that disagrees with instrument reference
(e.g., localizer centering errors).
8.4.3. Hybrid System Transition From Automatic to Manual Flight.
A safe manual takeover of airplane control to complete the landing within the established touchdown footprint must be demonstrated. Use of appropriate takeover response time delays for the transition should be considered during the demonstration.
AC 120-28D Appendix 3 07/13/99 These demonstrations must be conducted in the following conditions:
a. Without external visual reference,
b. With visual reference, and
c. With the presence of external visual reference that disagrees with instrument reference
(e.g., localizer centering errors).
8.4.4. Hybrid System Pilot Not Flying (PNF). The pilot not flying (PNF) must have suitable information provided to accomplish appropriate assigned duties, to be an integral part of the crew, and to safely deal with immediate or subtle incapacitation of the Pilot Flying (PF) regardless of visual reference availability.
8.5. Satellite Based Landing Systems [PoC]. This appendix is intended to provide criteria, but not a comprehensive acceptable means of compliance for airworthiness approval of GNSS based low visibility landing systems (e.g., GLS). Airworthiness approval of a GLS requires an overall assessment of the integration of the airplane landing system components with other related non-airplane landing system elements (e.g., GLS differential transmitters, pseudolites, satellite constellation(s) characteristics, waypoint data sources and use, reference datum used) to ensure that the overall safety is acceptable.
The performance, integrity and availability of any ground station elements, any data links to the airplane, any satellite elements and any data base considerations, when combined with the performance, integrity and availability of the airplane system, should be at least equivalent to the overall performance, integrity and availability achieved when ILS is used to support Category III operations.
The following requirements apply to approach and landing systems using GNSS (e.g., GLS):
a. During the approach, the flightcrew must be advised if the GNSS service or landing system cannot support the required performance and integrity. This includes assessment of space vehicle (SV) degradation or failure, augmentation degradation or failure, including the effect of satellite vehicle geometry on the required performance, availability and integrity.
b. The GNSS system assessment should address failure mode detection coverage and adequacy of monitors and associated alarm times. GLS landing and rollout system performance, failure detection and annunciation should be consistent with any established ICAO Standards and Recommended Practices, FAA criteria, or other State criteria acceptable to FAA, unless otherwise approved by FAA.
c. The effect of airplane maneuvers on the reception of signals must be considered as necessary to maintain the required performance, availability and integrity. If applicable, loss and re-acquisition of signals should be considered. The effect of local terrain should also be considered.
8.5.1. Flight Path Definition. For Flight Path Definition considerations refer to Section 4.6 of this AC.
8.5.2. Aircraft Database. The required flight path may be uplinked to the airplane or may be stored in an aircraft database for recall and incorporation into the flight guidance and/or control system when required to conduct an approach, landing and rollout.
Corruption of the information contained in the data base used to define the reference flight path is considered Hazardous. Failures which result in unannunciated changes to the data base must be Extremely Remote.
AC 120-28D 07/13/99 Appendix 3 For a procedural specified flight path intended to support automatic landing or manual flight guidance below 100 ft HAT, the flightcrew should not be able to modify information in the data base which relates to the critical definition of that required flight path, for any segment(s) of the procedure considered flight critical.
8.5.3. Differential Augmentation. Differential augmentation uses a set of GNSS receivers at known locations to derive differential corrections for each of the satellite pseudo-ranges. This network of GNSS receivers typically also provides signal in space integrity monitoring. If such a ground based augmentation system is used to provide differential pseudo-range corrections, or other data to an airplane to support Category III operations, the overall integrity of that operation must be established.
The role of the ground based augmentation system in the landing system must be considered during the aircraft system certification process until such time as an acceptable national, or international standard, for the ground reference system is established.
8.5.4. Datalink. A data link may be used to provide data to the airplane to provide the accuracy necessary to support certain operations.
The integrity of the data link should be commensurate with the integrity required for the operation.
The role of the data link in the landing system must be addressed as part of the aircraft system certification process until such time as an acceptable national, or international standard, for the ground system is established.
8.6 Enhanced Vision Systems or Synthetic Vision Systems [PoC]. Enhanced Vision System are typically considered to be those systems using airplane based sensors to penetrate visibility restrictions, and provide the flightcrew with a corresponding enhanced forward view of the scene outside the airplane
(e.g., radar imagery presented in a perspective view, FLIR, LLTV). Synthetic Vision Systems (SVS) are typically those systems which create computer generated imagery or symbology representing how an outside forward vision scene would otherwise appear, or elements of that scene would appear, if a pilot could optically see through the visibility restriction or darkness.
This appendix section is not intended to address acceptable means of compliance for airworthiness approval of either Enhanced Vision Systems or Synthetic Vision Systems. Criteria for approval of an enhanced vision system or synthetic vision systems must match the system's proposed intended use, and must follow and be based on successful completion of proof of concept testing acceptable to FAA.
Typically EVS or SVS systems would be expected to meet the same or equivalent performance accuracy, integrity, and availability criteria of other acceptable landing systems. Other limited uses, such as for assessing integrity alone (e.g., use as an independent landing monitor) may be assessed principally considering the proposed limited intended function. However, fidelity, alignment, penetration of weather, potential for misleading information, real time response, and any other relevant factor must be shown to be safe and appropriate for the intended application. If EVS or SVS information is to be presented on a headup-display (HUD), such EVS or SVS information must additionally meet any pertinent HUD provisions
(e.g., see Appendix 3, sections 8.3 and 8.4, as applicable). For a HUD presentation of EVS or SVS, a significant issue to be considered, even for no credit or limited credit, is the issue of potential blockage of cockpit window forward view (see provisions of Appendix 3, section 8.3 d.).
9. Airplane Flight Manual. Upon satisfactory completion of an airworthiness assessment and test program, the FAA-approved airplane flight manual or supplement, and any associated markings or placards, if appropriate, should be issued or amended to address the following:
AC 120-28D Appendix 3 07/13/99
1) Relevant conditions or constraints applicable to landing or landing and rollout system use regarding the airport or runway conditions (e.g., elevation, ambient temperature, runway slope).
2) The criteria used for the demonstration of the system, acceptable normal and non-normal procedures (including procedures for response to loss of guidance), the demonstrated configurations, and any constraints or limitations necessary for safe operation.
3) The type of navigation aids used as a basis for demonstration. 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., For ILS or MLS) based systems, the AFM should indicate that operation is predicated upon the use of an ILS (or MLS) facility with performance and integrity equivalent to, or better than, a United States Type II or Type III ILS, or equivalent ICAO Annex 10 Facility Performance Category III facility).
4) Applicable atmospheric conditions under which the system was demonstrated (e.g., demonstrated headwind, crosswind, tailwind) should be described as follows:
a) In the Limitations Section, the wind component values* used as a basis for statistical analysis, as supported by flight evaluation and validation, which may apply to use of the landing system, such as if credit for use is sought for low visibility operations, *Note: These are the wind values for which the applicable criteria of Appendix 3, (see paragraph 5) below), have been met.
b) In the Normal Operations Section, or equivalent section, maximum** wind component values experienced during the flight demonstration, described as "Demonstrated Wind Component(s)", **Note: These values are provided for information only.
c) For use of the landing system other than for low visibility credit (e.g., in wind or other conditions where system performance may not necessarily be supported by the statistical analysis), any necessary description of considerations, if other than the maximum demonstrated wind component values for the basic airplane*** apply.
***NOTE: FAA does not apply a "landing system" wind limitation unless unacceptable system characteristics dictate use of a limitation. This is consistent with specification of the demonstrated wind component value for the basic airplane, which is included in the AFM for information, and is not limiting.
5) For a landing or landing and rollout system meeting provisions of Appendix 3, 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 requirements of AC 120-28D Appendix 3 for <specify the pertinent Landing or Landing and Rollout capability Section(s) criteria met> when the following equipment is installed and operative:
<list pertinent equipment>" "This AFM provision does not constitute operational approval or credit for Category III use of this system."
AC 120-28D 07/13/99 Appendix 3
6) Airplane Flight Manual provisions should be consistent with the following:
a) The AFM may list the alert height demonstrated,
b) The AFM should not specify a DA, DH or RVR constraint, and
c) The AFM should not include visual segment specifications.
Examples of general AFM considerations, specific AFM provisions, and location of those provisions for applicable landing or landing and rollout systems are provided in Appendix 6.
AC 120-28D 07/13/99 Appendix 4 APPENDIX 4
WIND MODEL FOR APPROACH AND LANDING SIMULATION In carrying out the performance analysis, one of the following models of wind, turbulence and wind shear may be used:
Wind Model A 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 A4-1 (downwind) and Figure A4-2 (crosswind). Alternatively, the mean wind can be defined with magnitude and direction. The cumulative probability for the mean wind magnitude is provided in Figure A4-3, and the histogram of the mean wind direction is provided in Figure A4-4. The mean wind is measured at a reference altitude of 20 ft. AGL. The models of the wind shear and turbulence given in following sections assume this reference altitude of 20 ft. AGL is used.
Wind Shear The wind shear component is that portion which affects the air mass moving along the ground
(i.e., ground friction). The magnitude of the shear is defined by the following expression:
VV h wref = +/Ge7/Gf70 20407 01501520.l n ..
where Vwref is the mean wind speed measured at h ft. and V20 is the mean wind speed at 20 ft. AGL.
Turbulence The turbulence spectra are of the Von Karman form.
Vertical Component of Turbulence.
The vertical component of turbulence has a spectrum of the form defined by the following equation:
() () () w w LL L w w w Φ Ω Ω Ω = +/Ge7/Gf7 +/Ge6/Ge8/Ge7 /Gf6/Gf7 2 2 2 116 1 2 67 1339 211 3 3 9 σ π ..
.
where:
wΦ = spectral density in (ft./sec)2 σw = root mean square (rms) turbulence magnitude in ft/sec = 0 1061 20. (ft / sec)V where V20 is expressed in knots Lw = scale length = h (for h < 1000 ft.)
Ω = spatial frequency in radians/ft. = ω /VT ω = temporal frequency in radians/sec, and VT = airplane speed in ft./sec.
AC 120-28D Appendix 4 07/13/99 Horizontal Component of Turbulence.
The horizontal component of turbulence consists of a longitudinal component (in the direction of the mean wind) and lateral component. The longitudinal and lateral components have spectra of the form defined by the following equations:
Longitudinal Component:
() ()() ΦΩ Ω u u u u L L = + 2 2 1 1399 σ π .
56 Lateral Component:
() ()() ()() v v v v v LL L Φ Ω Ω Ω = + + 2 2 2 11 6 1 2 67 1 339 21 1 3 3 9 σ π ..
.
where the RMS Turbulence Scales are defined as below σ w Vk t s .= 01061 20 .( )
a. When hf t≥ 1 000,. σσσ uvw ==
b. When hf t< 1 000,.
σσσ uvw h== +/Gea/Gea/Gea /Gfa/Gfa/Gfa10177 0 000823 04 ..
.
c. When h ≤ 0 σσσ uvw == /Gea/Gfa 10177 04 .
.
and where the Turbulence Scales are defined as below
a. When hf t≥ 10 0 0,. LLL uvw == = 1 000,
b. When hf t< 1 000,. Lh w = () LLh huv == +/Gea/Gfa 10177 0 000823..
1.2
c. When hf t≤ 0 LLL uvw == = 0
AC 120-28D 07/13/99 Appendix 4
0.01 0.1 1 10 20 30 40 50 60 70 80 90 99 99.9 99.99
0.010.111020304050607080909999.999.99 30 20 10 0 -30 -20 -10 -0 Probability of Exceedance, % Probability of not Exceeding, %
HEADWIND-TAILWIND DESCRIPTION TAILWIND SPEED .KT (WIND FROM 180 DEG) HEADWIND SPEED .KT (WIND FROM 0 DEG) Figure A4-1
0.01 0.1 1 10 20 30 40 50 60 70 80 90 99 99.9 99.99
0.010.111020304050607080909999.999.99 30 20 10 0 -30 -20 -10 -0 Probability of Exceedance, %
CROSSWIND DESCRIPTION -5 -15 -25 5 15 25 Probability of Exceedance, % CROSSWIND SPEED .KT COUNTERCLOCKWISE FROM 0 DEG CROSSWIND SPEED .KT CLOCKWISE FROM O DEG Figure A4-2
AC 120-28D Appendix 4 07/13/99 0 5 10 15 20 25 30 35 40 Wind S peed V w kt 100 80 60 40 20 10 8 6 4 2
1.0
0.8
0.6
0.4
0.2
0.1
ANNUAL PERCENT PROBABILITY OF MEAN WIND SPEED-
EQUALING OR EXCEEDING GIVEN VALUES Figure A4-3
PERCENT PROBABILITY OF EXCEEDANCE
AC 120-28D 07/13/99 Appendix 4 Wind Direction His togram Wind Direction Relative to Runway Heading P e r c e n t 0 2 4 6 8 10 12 180190200210220230240250260270280290300310320330340350 0 10 20 30 40 50 60 70 80 90 100110120130140150160170180
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Wind fr om the L eft Wind fr om the R ight Figure A4-4