4-1 CHAPTER 4. SYSTEM CRITERIA
4.1 EVS or SVS (or CVS when EVS and SVS combined) – General.
4.1.1 EVS/SVS/CVS Primary Displays. EVS, SVS, or CVS functionality can be superimposed on the electronic flight instrument system (EFIS), for example the primary flight display (PFD) as installed in the flight deck. In this configuration example, the EVS, SVS, or CVS image can be merged into the sky/ground shading of the attitude direction indicator. In addition to the traditional head down display PFD, this type of superimposed display could also be associated with a HUD or equivalent display system using EVS, SVS, or CVS capabilities. Maneuvering the aircraft during any phase of flight (taxi, approach, landing, rollout, etc.) shall not be predicated on EVS, SVS or CVS imagery alone.
4.1.2 EVS/SVS/CVS Secondary Displays. EVS, SVS, or CVS functionality can be selected on a multi-function display (MFD) or navigation display (ND) as one of many standalone type formats available in the flight deck. For example, an EVS, SVS, or CVS image can be one selection on the MFD, while an electrical synoptic could be another selection on the same MFD.
4.1.3 EVS, SVS, or CVS installed on side-mounted displays: While similar in concept to the MFD, side-mounted displays systems are, in general, limited due to the nature of the installation constraints of these devices. Side-mounted displays of EVS technologies can present unique certification challenges such as alignment or positioning concerns relating to the display installation. (See AC 120-76C, Guidelines for Certification, Airworthiness, and Operational Use of Electronic Flight Bag, for specific applications.)
4.1.4 The intended function for any installation of EVS/SVS/CVS must be clearly defined (appropriate 14 CFR §§ 23/25/27/29.1301), and the design and installation safety levels must be appropriate for the stated intended function. Although normally associated with use during flight, utilization of EVS display during ground operations should not be used if sensor proximity to the taxiway surface causes a distraction.
4.1.4.1 The intended function should be reasonable considering the actual functionality of the installation.
4.1.4.2 Operating limitations should be proposed to the FAA by the applicant.
Operating limitations must be clearly defined and stated in the aircraft’s flight manual supplement in line with the intended function.
4.1.4.3 An EVS must not be used for flight guidance or navigation. An EVS cannot be used in lieu of natural vision to descend below DA/DH or MDA under §§ 91.176, 121.651, 125.381, or 135.225 regardless of whether it is on a HUD, or on a head down display.
4.1.5 The proposed system (EVS, SVS, and/or CVS):
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4.1.5.1 Should have a means to automatically or manually control display brightness.
4.1.5.2 Must not degrade presentation of essential flight information. In other words, the pilot’s ability to see and use the required primary flight display information such as primary attitude, airspeed, altitude, and command bars/command guidance must not be hindered or compromised by the EVS/SVS/CVS image.
4.1.5.3 If the vision system uses a head-up display or the equivalent, then it may not distort the pilot compartment view, provide a distorted (i.e., nonconformal) view of the external scene, or interfere with the pilot’s ability to safely perform any maneuvers within the operating limitations of the aircraft, including taxiing, takeoff, approach, and landing. See paragraph
4.1.6.2 of this chapter.
4.1.5.4 Must not adversely affect any other installed aircraft system.
4.1.5.5 Perform its intended function in each aircraft environment where system approval is desired. For example, if the system is intended to perform in (or after exposure to) known icing conditions, a means can be required to keep the EVS sensor window clear of ice accumulation.
4.1.5.6 Should display the mode of operation (EVS, SVS, or CVS) to the pilot/crew. Consideration should be given to recording the EVS/SVS/CVS display status in a flight data recorder or some form of nonvolatile memory.
4.1.5.7 Should not have undesirable display characteristics (for example, jitter, jerky motion, and excessive delays).
4.1.6 Display Implementation. EVS/SVS/CVS can be incorporated into differing display types installed in the cockpit. AC 25-11B, Electronic Flight Displays and AC 23.1311- 1C, Installation of Electronic Display in Part 23 Airplanes, provide more information on electronic displays. The display is subject to all applicable primary flight information rules and guidance for the category of aircraft.
4.1.6.1 Primary Flight Display (PFD). Primary displays are cockpit displays used to provide information needed to guide and control the aircraft and provide the aircraft altitude, attitude, and airspeed indications.
EVS/SVS/CVS can be implemented on the primary Head Down Display (HDD) in 14 CFR part 23 and 25 aircraft. Refer to AC 27-1B and AC 29- 2C for guidance on installation on rotorcraft. The following criteria apply to EVS/SVS/CVS implemented on a PFD.
4.1.6.1.1 The image, or loss thereof, must not adversely affect the PFD functionality.
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4.1.6.1.2 Align the displayed image with the aircraft’s inertial axis, physical axis or as appropriate for the intended function. The alignment can be variable and/or “phase of flight” dependent. The relationship between the image and the aircraft’s heading angle, pitch angle, roll angle, and track angle should generally align with the trajectory of the aircraft, should be recognizable by the flight crew, and not be misleading. For example, if two displays show EVS, SVS, or CVS information, or a combination (EVS on one and SVS on another), the orientation of the displays should be the same. Alignment of imagery presented on the PFD should be consistent with the real world and appropriate for the system’s intended function accounting for possible aircraft attitudes, turbulence, and wind effects.
4.1.6.1.3 Scale and align all spatially referenced symbology within each axis with the imagery so as not to present any misleading information to the pilot.
4.1.6.1.4 The Field of Regard (FOR) can be variable, but must be designed to ensure the displayed image is not distracting or misleading and does not adversely affect crew performance and workload. (Reference 14 CFR §§ 23/25/27/29.771 and §§ 23/25/27/29.1309.)
4.1.6.1.5 The display should not impede a clearly visible zero pitch reference line, distinct in visual appearance relative to any possible terrain, obstacle, or cultural feature display appearance.
Note: Including a flight path vector or velocity vector to show the pilot the aircraft’s trajectory relative to displayed terrain is recommended.
4.1.6.1.6 Per 14 CFR §§ 23/25/27/29.771 and §§ 23/25/27/29.773, pilot tasks must not be degraded by the displayed imagery. Depending on the intended function of the display information, the imagery must not provide the pilot with misleading information regarding detection, accurate identification, and avoidance of terrain, obstacles, and other flight hazards.
4.1.6.2 Head-Up Display or Equivalent. EVS/SVS/CVS can be implemented on a HUD, HDD or equivalent. The following criteria apply to EVS/SVS/CVS implemented on a HUD or equivalent display:
4.1.6.2.1 A design with EVS/SVS/CVS imagery displayed on a HUD must account for the pilot compartment view requirements found in 14 CFR §§ 23.773,
25.773, 27.773, and 29.773, including validation that the display of EVS imagery does not conflict with the pilot compartment view. 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 EVS:
Detection, accurate identification and maneuvering, as necessary, to avoid traffic, terrain, obstacles, and other hazards of flight.
4-4 Accurate identification and utilization of visual references required for every task relevant to the phase of flight.
Note: Although normally associated with use during flight, utilization of video imagery on the HUD or equivalent display during ground operations should not be considered if sensor proximity to the taxiway surface causes a distraction.
4.1.6.2.2 Imagery on the HUD or equivalent display must be conformal with the real world and appropriate for the system’s intended function accounting for possible aircraft attitudes, turbulence, and wind effects.
4.1.6.2.3 Society of Automotive Engineers (SAE) International design standards for HUD symbology, optical elements and video imagery are also prescribed with SAE Aerospace Standard (AS) 8055, Minimum Performance Standard for Airborne Head Up Display (HUD), dated March 1999, SAE Aerospace Recommended Practice (ARP) 5288, Transport Category Airplane Head Up Display (HUD) Systems, dated May 2001, and SAE ARP 5287, Optical Measurement Procedures for Airborne Head Up Display (HUD), dated March 1999. Apply specific design standards for resolution and line width, luminance and contrast ratio, chromaticity, and grayscale.
4.1.6.3 Secondary displays. EVS/SVS/CVS can be implemented with ego-centric “inside aircraft” views or exocentric “outside aircraft” viewpoints on head down displays. This includes installed EFBs or MFD. The following criteria apply to EVS/SVS/CVS implemented on secondary displays:
4.1.6.3.1 The orientation and perspective of the EVS/SVS/CVS view should be clear to the pilot.
4.1.6.3.2 If PFD information is displayed, it should meet PFD integrity requirements and availability criteria.
4.1.6.3.3 EVS/SVS/CVS image, or loss thereof, must not adversely affect other approved display functionality (such as navigation displays).
Note: Generally, the orientation of the secondary display should be the same as the primary display to decrease the possibility of pilot confusion.
4.2 EVS Specific Criteria.
4.2.1 An EVS requires a real-time imaging sensor and display, providing improved vision performance. These systems do not qualify for additional operational credit over and above what is already approved, and are installed on a non-interference basis.
4.2.2 EVS installations must meet the following criteria:
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4.2.2.1 The EVS depiction must be crew de-selectable (if on the primary display, the pilot should be able to easily and quickly declutter the EVS or remove sensor image). For an EVS image displayed on a HUD or equivalent display, the system must provide 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.
4.2.2.2 The display mode (status of EVS), either through crew de-selection or as a result of a failure, should be clearly indicated or obvious to the crew.
4.2.2.3 The display and sensor FOR should be sufficient for the intended functions.
4.2.2.4 SAE design standards for EVS symbology, optical elements and video imagery are also prescribed in SAE AS 8055, SAE ARP 5288, and SAE ARP 5287. Apply the specific design standards for resolution and line width, luminance and contrast ratio, chromaticity, and grayscale.
4.2.2.5 Consider the following criteria for display characteristics for the EVS design, regardless of the display type:
4.2.2.5.1 For part 25 airplanes, apply all display characteristics listed in AC 25-11B.
For part 23 airplanes, apply all display characteristics listed in AC
23.1311-1C. For other aircraft without corresponding criteria, the guidance in AC 25-11B or AC 23.1311-1C serves as recommended guidance.
4.2.2.5.2 The display should not have undesirable display characteristics (such as blooming, “burlap,” and running water).
4.2.2.6 Image characteristics.
4.2.2.6.1 On a HDD, the relationship of the display FOR to the actual field of view should be suitable for the pilot to smoothly transition from the HDD to the out-the-window real world view or HUD or equivalent display. The recommended maximum display minification is 3:1.
4.2.2.6.2 The image data should be refreshed at 15 Hz or better.
4.2.2.6.3 The image latency should be less than 100 milliseconds where the latency is measured from the image source time of applicability to the display of the image.
Note: EVS display refresh rate, when used in direct aircraft or power plant manual control tasks (such as attitude, engine parameters, etc.), equal to, or greater than 15 Hz has been found to be acceptable. Any lag introduced by the display system should be consistent with the aircraft control task
4-6 associated with that parameter. In particular, display system lag (including the sensor) for attitude which does not exceed a first order equivalent time constant of 100 milliseconds for aircraft with conventional control system response is generally acceptable.
4.2.2.7 For display on a HUD or equivalent display, the minimum system should provide a control of the EVS display contrast/brightness which is effective in dynamically changing background (ambient) lighting. In addition, the minimum system must not:
distract the pilot, impair the pilot’s ability to detect and identify visual references, mask flight hazards, or degrade task performance or safety.
4.2.2.8 When EVS is incorporated on the primary display, the EVS-based primary display must be clear and unambiguous when recovery from unusual attitudes is required. An accurate, easy, quick-glance 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 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.)
4.3 SVS Specific Criteria.
4.3.1 Synthetic Vision Systems require a terrain and obstacle database, a precision navigation position, display, and height, attitude, and heading/track inputs. The design and installation safety levels should be appropriate for the system’s intended function. AC 25-11B and AC 23.1311-1C provide additional general display guidance.
4.3.2 Image characteristics:
4.3.2.1 Failure or de-selection of the synthetic vision depiction should be obvious to the crew.
4.3.2.2 The display can depict the scene from the pilot’s view looking through the front window (egocentric) or from outside the aircraft (exocentric).
However, if implemented on a primary display, then the display should depict the scene from the pilot’s perspective looking through the front window.
4.3.2.3 The Field of Regard (FOR) should be appropriate for the system’s intended function and account for possible aircraft attitudes, turbulence, and wind effects (for example, to orient the line of sight according to
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)).