4-7 heading and not to track). Synthetic vision scene compression can result from FOR selections or display size limitations. Regardless, prominent topographical features should be easily identified and must be correlated with the actual external scene.
4.3.2.4 Identify dominant topographical features present in the SVS image in the outside view. Dominant topographical features present in the outside view should also be identifiable in the SVS image.
4.3.2.5 Terrain:
4.3.2.5.1 A potential terrain or obstacle conflict should be obvious to the pilot, and must not conflict with TAWS or HTAWS requirements. One mechanism for making such conflicts obvious on a primary display is an earth-based flight path vector. For guidance regarding alerts, use AC 23.1311-1C or AC 25.1322-1.
4.3.2.5.2 Displayed topographical features should not intersect guidance provided to the pilot to fly a published approach path. Terrain and 3-space position resolution should be accurate enough that topographical features should not intersect published approach paths.
4.3.2.5.3 Threatening terrain, close enough to generate a terrain warning alert, should appear above the artificial horizon zero pitch reference line if it is higher than the aircraft altitude.
4.3.2.6 SVS-based primary displays must be clear and unambiguous when recovery from unusual attitudes is required. An accurate, easy, quickglance interpretation of attitude should be possible for all unusual attitude situations and other “non-normal” maneuvers sufficient to permit the pilot to recognize the unusual attitude and initiate a recovery within one second.
We recommend you use chevrons, pointers, and/or permanent ground-sky horizon on all attitude indications to perform effective manual recovery from unusual attitudes. (For part 25 aircraft, refer to AC 25-11B. For part 23 aircraft, refer to Appendix H.) Always have some indication of both sky and ground visible on the PFD for use in initiating unusual attitude recovery. SVS displays used as an attitude indicator require an egocentric orientation.
4.3.2.7 The relationship of the display FOR to the actual field of view should be suitable for the pilot to smoothly transition from the head-down display to the head-up, out-the-window real features.
4.3.2.8 The terrain depiction should include large bodies of water. Differentiation between water and sky depictions should be clearly distinguishable.
4.3.2.9 The pilot’s view should not be depicted below the earth’s surface.
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4.3.2.10 The image should be updated at 15 Hz or better and function smoothly during all expected maneuvering reasonable for the class and type of aircraft.
4.3.2.11 The image latency for a PFD or HUD or equivalent display should be consistent with the image guidance in AC 25-11B or that necessary for expected maneuvering reasonable for the class and type of aircraft. For other displays in the flight deck, a larger lag can be acceptable subject to the intended function.
4.3.2.12 Undesirable display characteristics should be minimized (for example, jitter, jerky motion, and excessive delays).
4.3.2.13 Range:
4.3.2.13.1 The scene range should be the natural horizon for both egocentric and exocentric displays. For systems intended for use in approach, missed approach, take-off, and departure operations, the scene range should be whichever is less of the following: natural horizon, 40 nautical miles, or 10 minutes at maximum cruise speed.
4.3.2.13.2 The scene range from the eye position to the terrain horizon must not be misleading and must be appropriate to the intended function. The intent is to prevent misleading information due to a SVS range limitation to the horizon that could lead to crew confusion at a critical phase of flight. At some airports the missed approach can take the aircraft on a course towards mountains. Pilots need to see that the mountains will be a factor in their missed approach long before they get to the runway. Thus, it is not acceptable for an approach to appear as if it is over flat terrain until short final when the mountains in the distance finally start appearing on the PFD.
4.3.2.14 Although every possible scenario cannot be described, the crew should be able to perceive relative distances to prominent topographical features.
For example, the pilot should be able to identify an immediate terrain threat versus a distant terrain conflict.
4.3.2.15 All obstacles taller than 200 feet above ground level should be displayed.
These obstacles must be displayed in the scaled and aligned position relative to other features on the synthetic vision image.
Note: Obstacle display consideration should be given to reducing clutter when objects pose no threat, yet not suddenly appear at low altitudes during periods of task saturation.
4.3.3 Position Source. The horizontal position source used for the SVS display should at least meet the criteria for TAWS or HTAWS. The horizontal position source must not provide contradictory indications of horizontal terrain clearance. Additional guidance
4-9 for the horizontal position source can be necessary, depending on the intended functions of the SVS. TAWS guidance is contained in AC 25-23 and AC 23-18. HTAWS guidance is contained in AC 27-1B and 29-2C.
4.3.3.1 The synthetic vision display must not conflict with either the terrain warning or terrain awareness functions (for example, TAWS or HTAWS).
4.3.3.2 Any aircraft incorporating SVS from the egocentric perspective should also provide a TAWS or HTAWS or terrain warning system (refer to Appendix H for specifics on a terrain warning system). If terrain alerts and cautions are depicted on the SVS, they must be consistent across all displays in the cockpit when terrain threats are identified. In other words, terrain database sources should be consistent.
4.3.4 Altitude Source. The altitude source used for the SVS display should meet the criteria for TAWS or HTAWS installations, including the need to account for cold temperature errors which could place the aircraft closer to the surface than the altimeter indicates.
The altitude source should be consistent with that used for the onboard terrain awareness and alerting system on the aircraft and must not provide contradictory indications of vertical terrain clearance. Additional criteria for the altitude source may be necessary, depending on the intended functions of the SVS. TAWS guidance is contained in AC 25-23 and AC 23-18. HTAWS guidance is contained in AC 27-1B, AC 29-2C, and TSO-C194.
4.3.5 Attitude Source. The attitude source must not conflict with attitude information or integrity criteria provided by the primary flight display.
4.3.6 Heading/Track Source. The heading/track source must not conflict with heading/track information provided by the navigation display.
4.3.7 Terrain and Obstacle Database.
4.3.7.1 Terrain and obstacle database processing. The terrain and obstacle database, along with any other database used to create the SVS image should be compliant to RTCA/DO-200A, as applicable to the intended function. The minimum terrain database resolution and accuracy must be consistent with the intended function, and should comply with the resolution and accuracy listed in TSO-C151, Appendix A, section 6.3, or for helicopters, TSO-C194.
Position accuracy, symbology, and topographical information should be consistent with each other.
Runway elevations must be accurate at the approach ends and not just at the airport center.
4.3.7.2 Obstacle database. The obstacle database should include all available physical hazards greater than 200 feet above ground level, not just terrain.
The system should neither disregard nor corrupt obstacles available in the
4-10 database greater than 200 feet above ground level. Obstacles displayed should be those deemed hazardous to the flight (or phase of flight). The applicant should provide procedures in the Instructions for Continued Airworthiness to ensure the most current obstacle database is installed.
Note: It is difficult to apply one altitude value across a wide range of aircraft performance, from rotary wing to high performance fixed wing.
Generally, above 5,000 ft AGL, most obstacles are not tall enough to be hazardous. However, in the airport terminal area, even obstacles less than 1,000 ft AGL can be hazardous.
4.3.7.3 Navigation Database. The navigation database used by SVS for runway and airport information should be consistent with that used by other systems in the aircraft (for example, flight management systems). The applicant should provide procedures to document database currency in the Instructions for Continued Airworthiness.
4.3.7.4 Features. If flight path guidance features are provided by the SVS, they should complement (or correspond with) approved navigation system guidance.
4.3.7.5 Head-Up Displays. Providing complete guidance for synthetic vision systems is complicated by the many design variables available to manufacturers. Since synthetic vision displays can share many characteristics with a HUD, applicants should use HUD display guidance where appropriate.
4.3.7.6 Terrain display. If the intended function is “terrain awareness,” then “awareness” is consistent with the actual terrain. The display may not match the terrain perfectly, but the display should provide a reasonable representation of the terrain in a manner that does not misrepresent the threat posed by the terrain. The display, while possibly resolution or fieldof-view limited, may still adequately portray terrain awareness. The pilot should be aware of less than a 1:1 ratio of depiction. The pilot could be made aware of this ratio by training, the Airplane Flight Manual, symbology or other means. Display suitability must be matched against intended function.
4.4 CVS Specific Criteria.
4.4.1 CVS requires a real-time imaging sensor and display that provides demonstrated vision performance for its intended function. They also require a terrain and obstacle database and a precision navigation position for the synthetic portion of the display. The design assurance levels should be appropriate for the system’s intended function. The following criteria apply to CVS installations:
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4.4.1.1 CVS should meet the respective criteria of the EVS and SVS implementations.
4.4.1.2 The EVS and SVS depictions should be conformal with each other.
4.4.1.3 Fusion of EVS and SVS should require the images to be aligned within 5 milliradian (mrad) laterally and vertically at the boresight of the display.
Therefore, blended EVS and SVS images should not cause confusion to the flight crew. Significant image discrepancies between EVS and SVS due to failure conditions should be obvious to the crew.
Note: It is difficult to define every implementation in order to determine what is “significant.” However, the applicant is expected to reduce discrepancies to as near-zero as possible. The final determination will occur during flight test.
4.4.2 The CVS should meet the detailed performance criteria for EVS and SVS specified in paragraphs 4-2 and 4-3 of this chapter.
4.5 EFVS – General and Specific Criteria.
4.5.1 EFVS criteria would meet EVS criteria. However, there are additional EFVS general and specific criteria for operational credit, as follows:
4.5.1.1 Mitigate system failures more frequent than extremely improbable which produce effects the pilot (or the aircraft itself) cannot safely handle.
Design the aircraft systems so the entire fault probability is kept to an acceptable level, and complies with 14 CFR part §§ 23/25/27/29.1309 regulations.
4.5.1.2 The sensor image, combined with the required aircraft state and position reference symbology, is presented to the flight crew on the HUD, or an equivalent display, so that they are clearly visible to the pilot flying in his or her normal position and line of vision looking forward along the flight path. For HUD or equivalent display operations, the pilot flying views the EFVS sensor and symbolic information that is properly aligned and registered to enable a one-to-one (conformal) overlay with the actual external scene.
4.5.1.3 The HUD or equivalent display and displayed FOR should be sufficient for the EFVS information to be displayed conformally over the range of anticipated aircraft attitudes, aircraft configurations, and environmental (for example, wind) conditions. The aircraft state and position reference data is presented in the form of symbology overlaying the image presentation. The flight instrument data on the HUD or equivalent display must include:
Airspeed;
4-12 Vertical speed;
Aircraft attitude;
Heading;
Altitude;
Command guidance as appropriate for the approach to be flown;
Path deviation indications;
Flight path vector; and Flight path angle reference cue.
4.5.1.4 For an EFVS Landing System intended for landing (i.e., from DA/DH through touchdown and roll out at not less than1000ft RVR, the following specified aircraft flight information, in addition to that noted above in
4.5.1.3, must also be displayed:
Height above ground level such as that provided by a radio altimeter or other device capable of providing equivalent performance, and Flare prompt or flare guidance for achieving acceptable touchdown performance, as discussed in paragraph 6.2.
4.5.1.5 The minimum system should include a control of EFVS display contrast/brightness which is effective in dynamically changing background (ambient) lighting, prevents distractions to the pilot, prevents impairment of the pilot’s ability to detect and identify visual references, prevents masking of flight hazards, or otherwise degrades task performance or safety. If automatic control for image brightness is not provided, it should be shown that manual setting of image brightness meets the above criteria and does not cause excessive workload.
4.5.1.6 The EFVS display controls should be visible to, and within reach of, the pilot flying from any normal seated position. If each pilot station is equipped with EFVS, the EFVS display controls should be visible to, and within reach of, the respective pilot from any normal seated position. The position and movement of the controls should not lead to inadvertent operation. The EFVS controls, except those located on the pilot’s control wheel, should be adequately illuminated for all normal background lighting conditions, and should not create any objectionable reflections on the HUD or equivalent display or other flight instruments. There must be a means to allow the pilot using the display to immediately deactivate and reactivate the vision system imagery, on demand, without removing the pilot’s hands from the primary flight and power controls, or their equivalent. (Ref 14 CFR §§ 23.773(c)(3), 25.775(e)(3), 27.773(c)(3), and
29.773(c)(3)).
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4.5.1.7 The approach path situation references and command guidance should be based on the navaid for the straight-in instrument approach procedure in use.
4.5.1.8 Under 14 CFR § 91.176, upon reaching the decision altitude/decision height (DA/DH) or minimum descent altitude/minimum descent height (MDA/MDH), the required visual references presented in Table 1 must be distinctly visible and identifiable to the pilot. Sensor performance criteria can be quantified in terms of the range of the enhanced flight visibility compared to the flight visibility when low visibility conditions exist. The visual references of the runway environment must be seen by the sensor at operationally relevant distances.
Table 1. EFVS Visibility and Visual Reference Requirements From the authorized DA/DH to 100 feet height above touchdown zone elevation of the runway of intended landing, the approach light system (if installed) or both the runway threshold and the touchdown zone are distinctly visible and identifiable to the pilot using an EFVS.
(A) The pilot must identify the runway threshold using at least one of the following visual references -
(1) The beginning of the runway landing surface;
(2) The threshold lights; or
(3) The runway end identifier lights.
(B) The pilot must identify the touchdown zone using at least one of the following visual references -
(1) The runway touchdown zone landing surface;
(2) The touchdown zone lights;
(3) The touchdown zone markings; or
(4) The runway lights At 100 feet height above touchdown zone elevation of the runway of intended landing and below that altitude, the enhanced flight visibility using EFVS is sufficient for one of the following visual references to be distinctly visible and identifiable to the pilot. For EFVS Approach System operations the listed features must be identified without reliance on the EFVS Approach System. For EFVS Landing and Rollout Operations the listed features may be identified using an EFVS Landing System.
(A) The runway threshold;
(B) The lights or markings of the threshold (C) The runway touchdown zone landing surface; or (D) The lights or markings of the touchdown zone.
4.5.1.9 The minimum detection EFVS range (figure 3, below) can be derived by using an assumed minimum distance of the aircraft at the nominal
4-14 Category I (200 ft) decision altitude before which the EFVS should image the runway threshold. On a 3 degree glide slope, the horizontal distance from the aircraft to the runway threshold is approximately 2816 feet (3816 feet from the precision touchdown zone markers). Use this range as a minimum performance value. These values do not take into account pilot decision time or actual atmospheric conditions, the use of non-precision approaches which can require greater distances, or the use of approaches with higher DAs, DHs, or MDAs.
Figure 3. Minimum Detection Range
4.5.1.10 The EFVS imagery displayed on the HUD or equivalent display must account for the pilot compartment view requirements found in 14 CFR §§
25.773, 23.773, 27.773, and 29.773, including validation that the display of imagery does not conflict with the pilot compartment view. The display of EFVS sensor imagery must be on a system that compensates for the interference caused by the provided imagery. Additionally, the system must provide an undistorted and conformal view of the external scene, a means to deactivate the display and does not restrict the pilot from performing specific maneuvers. The following tasks associated with the use of the pilot’s view must not be degraded below the level of safety that existed without the video imagery:
Detection, accurate identification and maneuvering, as necessary, to avoid traffic, terrain, obstacles, and other hazards of flight.
Accurate identification and utilization of visual references required for every task relevant to the phase of flight.
Note: Although, the EFVS image requirements relate primarily to the approach and landing phases of flight, the EFVS image when viewed head up during ground operations should not create unacceptable distractions due to sensor proximity to the taxiway surface.
4.5.2 EFVS General System Criteria.
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4.5.2.1 For EFVS implemented on a HUD, the image must be compatible with the FOV and head motion box of a HUD designed against SAE ARP 5288 (transport category head-up display (HUD) systems). When used in a given phase of flight, the HUD and EFVS FOR must provide a conformal image with the visual scene over the range of aircraft attitudes and wind conditions. A display that provides the pilot monitoring with EFVS sensor imagery is not required for EFVS Approach System operations. However, for EFVS Landing System operations, when a minimum flightcrew of more than one pilot is required for the conduct of the operation, the aircraft must be equipped with an operable display that provides the pilot monitoring with EFVS sensor imagery. Any display used to monitor the approach must be located in the cockpit in accordance with 14 CFR §§ 23/25/27/29.1321 so that any pilot seated at the controls can monitor the airplane’s flight path and instruments with minimum head and eye movement. Aircraft that pre-date 14 CFR §§ 23/25/27/29.1321 must meet the arrangement and visibility requirements in those sections for the installation of pilot monitoring displays. Any symbology displayed must not adversely obscure the sensor imagery of the runway environment.
4.5.2.2 EFVS display criteria must meet the airworthiness certification requirements in 14 CFR parts 21, 23, 25, 27, and 29 (as applicable), see Appendix A. Some of these requirements could be specific to EFVS and could be in addition to all other requirements applicable to the HUD and the basic avionics installation.
4.5.2.3 The current FAA guidelines for head-up displays apply with respect to EFVS. These criteria include well-established military as well as civil aviation standards for HUDs as defined in MIL-STD-1787C, Aircraft Display Symbology, and AC 25-11B. SAE design standards for HUD symbology, optical elements, and video imagery are also prescribed within SAE AS 8055, SAE ARP 5288 and SAE ARP 5287. Apply specific design standards for image size, resolution and line width, luminance and contrast ratio, chromaticity, and grayscale.
4.5.2.4 The EFVS image, when superimposed on the HUD symbology and when used in combination with other aircraft systems, must meet the criteria below. The EFVS image and installation must:
4.5.2.4.1 Be suitable for and successfully perform its intended function.
4.5.2.4.2 Allow the accurate identification and utilization of visual references, using both EFVS and natural vision as appropriate.
4.5.2.4.3 Have acceptable display characteristics to accomplish the intended function.
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4.5.2.4.4 There must be a means to allow the pilot using the display to immediately deactivate and reactivate the vision system imagery, on demand, without removing the pilot’s hands from the primary flight and power controls, or their equivalent. (Ref 14 CFR §§ 23.773(c)(3), 25.775(e)(3), 27.773(c)(3), and 29.773(c)(3)).
4.5.2.4.5 Not degrade the presentation of essential flight information on the HUD or equivalent display.
4.5.2.4.6 Not be misleading and should not cause confusion or any significant increase in pilot workload.
4.5.2.4.7 Be aligned with and scaled to the external scene, and should include the effect of near distance parallax.
4.5.2.4.8 Not cause unacceptable interference with the safe and effective use of the pilot compartment view.
4.5.2.4.9 Not misleadingly alter the color perception of the external scene so as to preclude the pilot’s performance of any required tasks by causing confusion or any significant increase in its workload, e.g. discerning PAPI lights accurately on approach.
4.5.2.4.10 Allow the pilot to recognize significantly misaligned or non-conformal conditions to the external scene that may preclude the pilot’s performance of any required maneuvers.
4.5.2.5 Additionally, the EFVS installation and image should have an effective control of EFVS display brightness without causing excessive pilot workload and not cause adverse physiological effects such as fatigue or eyestrain.
4.5.2.6 A HUD modified to display EFVS must continue to meet the conditions of the original approval and be adequate for the intended function, in all phases of flight in which the EFVS is used. An accurate, easy, quickglance interpretation of attitude should be possible for all unusual attitude situations and other “non-normal” maneuvers to permit the pilot to recognize the unusual attitude and initiate recovery within one second.
We recommend you use chevrons, pointers, and/or permanent ground-sky horizon on all attitude indications to perform effective manual recovery from unusual attitudes. Refer to AC 25-11B for guidance on electronic flight deck displays.
4.5.2.7 14 CFR § 91.176 requires that the EFVS display a flight path vector (FPV) and flight path angle reference cue (FPARC) on the HUD or equivalent display. The following criteria apply to these symbols:
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4.5.2.7.1 The position and motion of the FPV should correspond to the aircraft’s earth referenced flight path vector.
4.5.2.7.2 The dynamic response of FPV should follow pilot control inputs. Refer to SAE ARP 5589 for dynamic response guidance.
4.5.2.7.3 The display of the FPARC should be suitable for monitoring the vertical flight path of the aircraft.
4.5.2.7.4 The system must provide the pilot a means to select the desired descent angle that is represented by the FPARC, and may also provide a means for the descent angle to be selected automatically from a database.
4.5.2.8 The display of attitude symbology, FPV, FPARC, and other visual elements which are earth-referenced, must be aligned with and scaled (i.e., conformal) to the external view.
4.5.2.9 The EFVS display of imagery, flight information and flight symbology should be a suitable visual reference for the pilot during the manual performance of any maneuvers within the operating limitations of the aircraft, including taxiing, takeoff, approach, landing and rollout.
4.5.3 EFVS Detailed System Criteria.
4.5.3.1 The performance of EFVS imaging systems does not solely depend upon system design, but also depends upon the target scene characteristics such as the runway, light structures, electromagnetic radiation, and atmospheric conditions.
4.5.3.2 Since the purpose of the EFVS sensor is to provide a visual advantage over the pilot’s out-the-window view, the design should include a general performance analysis. This analysis includes calculated performance, which indicates the viability of the system to meet the proposed intended function, specifically including the calculated performance of the sensor operation within the range of the environment proposed.
4.5.3.3 Likewise, since the purpose of the EFVS sensor is to provide a visual advantage over the pilot’s out-the-window view, the general performance analysis should include the calculated transmission of electromagnetic energy in the visible spectrum and other relevant frequencies. The analysis should portray length of transmission over a path with generalized extinction coefficients at a given wavelength.
Note: Examples of acceptable sensor models are MODTRAN and LOWTRAN which can be used to estimate the performance of infrared systems. Other models (FASCODE) for radar systems may be used for these types of sensors and provide a basic measure of signal attenuation
4-18 helpful in assessing performance and viability for the functions required under 14 CFR § 91.176.
4.5.3.4 Minimum system performance. Integration of the major components includes the installed sensor, its interconnections with the sensor display processor, the display device, pilot interface, and aircraft mechanical interface, which can include the radome for the sensor.
4.5.3.4.1 Latency. EFVS latency should be no greater than 100 milliseconds (msec). A longer lag time can be found satisfactory, provided it is demonstrated not to be misleading or confusing to the pilot. Latency should not be discernible to the pilot and should not affect control performance or increase pilot workload. EFVS latency causes, at best, undesirable oscillatory image motion in response to pilot control inputs or turbulence. At worst, EFVS latency may cause pilot-induced oscillations if the pilot attempts to use the EFVS for active control during precision tracking tasks or maneuvers in the absence of other visual cues.
4.5.3.4.2 EFVS FOR. The minimum fixed FOR should be 20 degrees horizontal and 15 degrees vertical. In applications where the FOR is centered on the flight path vector the minimum vertical FOR should be 5 degrees (± 2.5 degrees) and 20 degrees horizontal.
4.5.3.4.2.1 The minimum EFVS FOR should not only consider the HUD FOV (i.e., how large of an area displayed), but also the area over which this area subtends (i.e., what is shown on the conformal display). The FOR portrayed on the HUD is established by three primary determinants:
HUD and EFVS sensor Field-Of-View;
Orientation of the HUD with respect to the aircraft frame of reference (for example, boresight and proximity to pilot’s eye); and Orientation (for example, attitude) of the aircraft.
4.5.3.4.2.2 SAE ARP 5288 states, “The design of the HUD installation should provide adequate display fields-of-view in order for the HUD to function correctly in all anticipated flight attitudes, aircraft configurations, or environmental conditions such as crosswinds for which it is approved.
Limitations should be clearly specified in the AFM if the HUD cannot be used throughout the full aircraft flight envelope.” A quantitative EFVS FOR was established as minimum design criteria to be qualitatively checked during certification flight test for sufficiency in meeting its intended function. The EFVS FOR requirement resulted after consideration of the minimum field of regard criteria