AC 20-167B
4.1.3.2 Operating limitations should be proposed to the FAA by the applicant. In accordance with §§ 23.2620, 25.1581, 27.1581, and 29.1581, operating limitations must be clearly defined and stated in the aircraft’s flight manual supplement in line with the intended function.
4.1.3.3 An EVS that does not meet the requirements in § 91.176 for EFVS operations cannot be used in lieu of natural vision to descend below DA/DH or MDA, regardless of whether it is on a HUD or on a head down display.
4.1.4 Guidelines for EVS/CVS.
The proposed system (EVS/CVS):
4.1.4.1 Should have a means to automatically or manually control display brightness.
4.1.4.2 Must not degrade presentation of essential flight information. 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/CVS image.
4.1.4.3 Must 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 if the vision system uses a head-up display or equivalent display. Refer to Society of Automotive Engineers (SAE) design standards for distortion prescribed in SAE Aerospace Standard (AS) 8055A. See section 4.1.5.2 of this chapter for additional HUD or equivalent display criteria.
4.1.4.4 Must not adversely affect any other installed aircraft system. See §§ 23.2500, 23.2505, 25.1301, 27.1301, and 29.1301 for function and installation. See §§ 23.2510, 25.1431, and 29.1431 for electrical equipment.
4.1.4.5 Must perform its intended function in each aircraft environment where system approval is desired. See §§ 23.2500, 23.2505, 25.1301, 27.1301, and 29.1301. For example, if the system is intended to perform in (or after exposure to) known icing conditions, provide a means to keep the EVS sensor window clear of ice accumulation.
4.1.4.6 Should display the mode of operation (EVS, or CVS) to the pilot/flightcrew. Consideration should be given to recording the EVS/CVS display status in a flight data recorder or some form of nonvolatile memory. The modes of the EFVS operation should be made available to the flight data recorder, in addition to all the information required by §§ 23.1459, 25.1459, 27.1459, and 29.1459 or by an operational rule that 4-2
AC 20-167B the applicant seeks to show compliance to during certification (e.g. 14 CFR 121.344).
4.1.4.7 Should not have undesirable display characteristics (for example, jitter, jerky motion, and excessive delays.) Refer to SAE design standards for jitter prescribed in SAE AS 8055A.
4.1.5 Display Implementation.
EVS/CVS can be incorporated into different display types installed in the flightdeck.
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.
4.1.5.1 Primary Flight Display (PFD).
Primary flight displays are flightdeck displays used to provide information needed to guide and control the aircraft and provide the aircraft altitude, attitude, and airspeed indications. EVS/CVS can be implemented on the primary head down display (HDD) in part 23 and 25 aircraft. Refer to AC 27-1B CHG 1-9, Certification of Normal Category Rotorcraft, and AC 29- 2C CHG 9, Certification of Transport Category Rotorcraft, for guidance on installation on rotorcraft. The following criteria apply to EVS/CVS implemented on a PFD:
4.1.5.1.1 The image, or loss thereof, should not adversely affect the PFD functionality.
4.1.5.1.2 Alignment of imagery presented on the PFD should be consistent with the real world, appropriate for the system’s intended function accounting for possible aircraft attitudes, turbulence, and wind effects, and not mislead the flightcrew. If two displays show EVS or CVS information, or a combination (EVS on one and CVS on another), the orientation of the displays should be the same.
4.1.5.1.3 All spatially referenced symbology must be scaled and aligned within each axis with the imagery so as not to present any misleading information to the pilot.
4.1.5.1.4 The field of regard (FOR) can be variable. However, in accordance with §§ 23.2600, 25.771, 27.771, and 29.771 and §§ 23.2500, 23.2505,
25.1309, 27.1309, and 29.1309, the FOR must be designed to ensure the displayed image is not distracting or misleading and does not adversely affect flightcrew performance and workload.
4.1.5.1.5 The display should not impede a clearly visible zero pitch reference line.
The zero pitch reference line should be distinct in visual appearance 4-3
AC 20-167B 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.
4.1.5.1.6 Under §§ 23.2600, 25.771, 27.771, and 29.771 and §§ 23.2600(a), 25.773,
27.773, and 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.
4.1.5.2 Head-Up Display or Equivalent Display.
4.1.5.2.1 A design with EVS/CVS imagery displayed on a HUD must account for the pilot compartment view, including validation that the display of EVS imagery does not conflict with the pilot compartment view.
4.1.5.2.2 The following tasks associated with the use of the pilot’s view should 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; and
• Accurate identification and utilization of visual references required for every task relevant to the phase of flight.
Note: Utilization of video imagery on the HUD or HUD equivalent display during ground operations should not be considered if sensor proximity to the taxiway surface causes distracting imagery.
4.1.5.2.3 Imagery on the HUD or HUD 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.
4.1.5.2.4 Specific design standards for resolution and line width, luminance and contrast ratio, chromaticity, and grayscale should be applied. Refer to SAE international design standards for HUD symbology, optical elements and video imagery, which are prescribed in SAE AS 8055A, Minimum Performance Standard for Airborne Head Up Display (HUD), dated July 2015. Recommended best practices can be found in: 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.
Note: The intellectual property in each of these documents is owned by SAE, (not the FAA), and is subject to change. In general, SAE has a 5- year review cycle for each of their documents.
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AC 20-167B
4.1.5.3 Secondary displays. EVS/CVS can be implemented with egocentric “inside aircraft” views or exocentric “outside aircraft” viewpoints on head down displays. This includes installed electronic flight bags (EFBs) or MFDs. The following criteria apply to EVS/CVS implemented on secondary displays:
4.1.5.3.1 EVS/CVS image, or loss thereof, must not adversely affect other approved display functionality (such as navigation displays). See §§ 23.2500,
23.2505, 25.1301, 27.1301, and 29.1301.
4.1.5.3.2 The secondary display should be such that it cannot be confused with, considered as, or used as a PFD.
4.1.5.3.3 If primary flight information is presented on a secondary display, it should be of sufficient quality that it does not cause someone to doubt the PFD data.
4.1.5.3.4 The orientation and perspective of the EVS/CVS view should be clear to the pilot. Generally, the orientation of the secondary display should be the same as the orientation of 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 The following are specific criteria for EVS installations:
4.2.2.1 The EVS depiction must be flightcrew de-selectable (if on the primary display, the pilot must be able to easily and quickly remove the sensor image.) For an EVS image displayed on a HUD or HUD 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 their hands from the primary flight and power controls, or their equivalent. See §§ 23.2500, 23.2505, and
25.1301, 27.1301, 29.1301, and §§ 23.2600(a), 25.773, 27.773 and 29.773.
4.2.2.2 The display mode (status of EVS), either through flightcrew de-selection or as a result of a failure, should be clearly indicated or obvious to the flightcrew. See §§ 23.2500, 23.2505, and 25.1301, 27.1301, 29.1301, and §§ 23.2600(a), 25.773, 27.773 and 29.773.
4.2.2.3 The display and sensor FOR should be sufficient for the intended functions.
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AC 20-167B
4.2.2.4 Specific design standards for resolution and line width, luminance and contrast ratio, chromaticity, and grayscale should be applied. Refer to SAE design standards for EVS symbology, optical elements and video imagery are also prescribed in SAE AS 8055A, SAE ARP 5288, and SAE ARP 5287.
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 airplanes certificated under part 25, all display characteristics listed in AC 25-11B should be applied. For airplanes certificated under part 23, all display characteristics listed in AC 23.1311-1C should be applied. For rotorcraft certificated under part 27 or 29, all display characteristics listed in AC 27-1B CHG 1-9 or AC 29-2C CHG 9 (respectively), should be applied. 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, fixed pattern noise and running water). Refer to SAE design standards for display characteristics prescribed in SAE AS 8055A.
4.2.2.6 Consider the following criteria for image characteristics for the EVS design:
4.2.2.6.1 On an 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 HUD equivalent display.
Minification up to a ratio of 3:1 has been shown to be acceptable.
4.2.2.6.2 The image data should be refreshed at 15 Hz or better.
Note: Refresh rates above 50 Hz for cathode ray tube (CRT) based HUDs may be required to prevent unacceptable levels of flicker. Refer to SAE AS 8055A.
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: An EVS display refresh rate 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 associated with that parameter.
4.2.2.7 For display on a HUD or HUD equivalent display, the system should provide a control of the EVS display contrast/brightness which is effective in dynamically changing background (ambient) lighting. The minimum system should not:
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AC 20-167B
• 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 an EVS is incorporated on the primary flight display, the EVS- based primary display should 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 the use of chevrons or pointers on all attitude indications to perform effective manual recovery from unusual attitudes. (For airplanes certificated under part 25, refer to AC 25-11B).
4.3 CVS Specific Criteria.
4.3.1 CVS requires a real-time imaging sensor and display that provides demonstrated vision performance for its intended function. It also requires 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:
4.3.1.1 CVS should meet the respective criteria of the EVS and SVS implementations. Installation guidance for SVS displays can be found in AC 20-185A.
4.3.1.2 The EVS and SVS depictions should be conformal with each other.
4.3.1.3 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 flightcrew. Significant image discrepancies between EVS and SVS due to failure conditions should be obvious to the flightcrew.
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.
4.3.2 The CVS should meet the detailed performance criteria for EVS specified in sections
4.2 and 4.3 of this chapter and SVS performance criteria listed in AC 20-185A.
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AC 20-167B
4.4 EFVS – General and Specific Criteria.
4.4.1 If the intended function of the EFVS is to meet § 91.176, the EVS criteria above as well as the following EFVS general and specific criteria, are applicable to the EFVS:
4.4.1.1 Under §§ 23.2500, 23.2505, 25.1309, 27.1309, and 29.1309, the design assurance level of the aircraft systems must be commensurate with the identified failure probabilities.
4.4.1.2 The sensor image, combined with the required aircraft state and position reference symbology, must be presented to the flightcrew on the HUD, or an equivalent display, so that they are clearly visible to the pilot flying in their normal position and line of vision looking forward along the flight path. For HUD or HUD 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. See SAE AS8055A criteria on conformality.
4.4.1.3 The HUD or HUD equivalent display and displayed FOR must 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 must be presented in the form of symbology overlaying the image presentation. Section 91.176(a)(1)(B) requires the following flight instrument data on the HUD, or HUD equivalent display:
• Airspeed;
• 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.
Note: EASA has additional symbology requirements that are not required by the FAA for EFVS certification or for operational approvals.
Applicants that will be seeking EASA approvals should consult the latest EASA guidance material to ensure compliance with EASA regulations.
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AC 20-167B
4.4.1.4 In addition to criteria in 4.4.1.3, the following specified aircraft flight information required for operation by § 91.176(a)(1)(i)(B) must also be displayed for an EFVS landing system:
• Height above ground level such as that provided by a radio altimeter or other device capable of providing equivalent performance; or
• For aircraft other than rotorcraft, flare prompt or flare guidance for achieving acceptable touchdown performance, as discussed in section
6.2.
4.4.1.5 Flare Cue.
Under § 91.176(a)(1)(i)(B), an EFVS landing system for aircraft other than rotorcraft must have a flare cue because it is intended to enable landing in low visibility. The flare cue, whether a flare prompt or flare guidance, should demonstrate that it meets the landing criteria found in section 6.2. For some aircraft, flare guidance may be necessary because a flare prompt is not sufficient.
4.4.1.6 Flare Prompt.
The flare prompt indicates to the pilot when to initiate the flare but does not provide direction/guidance on how to execute the flare maneuver. The flare prompt should be presented at an appropriate time and with sufficient saliency so that the flare maneuver will be neither too early nor too late and within the touchdown zone as described in section 6.2 of this AC. The appearance and dynamic behavior of the flare prompt should be distinguishable from command guidance.
4.4.1.7 Flare Guidance.
Flare guidance provides explicit command guidance to the pilot for when to initiate the flare and provides directional cues on how to execute the flare maneuver. The flare guidance directs the flare maneuver so that the flare would be neither too early nor too late, and within the touchdown zone as described in section 6.2 of this AC.
4.4.1.8 The minimum system should include a control of EFVS display contrast/ brightness, for manually controlled EFVS systems, and should include a control for automatic EFVS systems that 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, and prevents other negative impacts on 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.
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AC 20-167B
4.4.1.9 Under §§ 23.2600(a), 25.773(e)(3), 27.773(c)(3), and § 29.773(c)(3), the EFVS display controls must be visible to and within reach of the pilot flying, from any normal seated position with the seat belt and shoulder harness (if applicable) fastened. If each pilot station is equipped with EFVS, the EFVS display controls must be visible to, and within reach of each pilot from any normal seated position. The position and movement of the controls must not lead to an inadvertent operation. The EFVS controls, except those located on the pilot’s flight controls, must be adequately illuminated for all normal background lighting conditions, and must not create any objectionable reflections on the HUD or HUD 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 their hands from the primary flight and power controls, or their equivalent.
4.4.1.10 The approach path situation references and command guidance should be based on the specific navigation source for the straight-in instrument approach procedure in use.
4.4.1.11 Under § 91.176, upon reaching the DA/DH or minimum descent altitude (MDA), 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 (See appendix C, “Determining Quantified Visual Advantage”). The visual references of the runway environment must be seen by the sensor at or beyond the distance required in the visual segment of the instrument approach being flown.
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AC 20-167B Table 1-1. Title 14 CFR 91.176 EFVS Visibility and Visual Reference Requirements.
From the authorized DA/DH to 100 feet height above TDZE 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.
1. Under § 91.176, the pilot must identify the runway threshold using at least one of the following visual references:
a. The beginning of the runway landing surface; or
b. The threshold lights; or
c. The runway end identifier lights.
2. Under § 91.176, the pilot must identify the touchdown zone using at least one of the following visual references:
d. The runway touchdown zone landing surface; or
e. The touchdown zone lights; or
f. The touchdown zone markings; or
g. 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 following features must be identified without reliance on the EFVS approach system. For EFVS landing and rollout operations the following listed features may be identified using an EFVS landing system:
1. The runway threshold; or
2. The lights or markings of the threshold; or
3. The runway touchdown zone landing surface; or
4. The lights or markings of the touchdown zone.
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4.4.1.12 The minimum detection EFVS range (figure 4-1) can be derived by using an assumed minimum distance of the aircraft at the nominal Category I (200 ft.) DA/DH before which the EFVS should image the runway threshold. On a 3 degree glideslope, the horizontal distance from the aircraft to the runway threshold is approximately 2,816 feet (3,816 feet from the precision aim point markings). In all cases, in accordance with § 91.176(a)(3), the EFVS must provide sufficient enhanced visibility at the DA/DH or MDA equal to the prescribed visibility of the instrument approach being conducted. These values do not take into account pilot decision time or actual atmospheric conditions. However, the use of nonprecision approaches can require greater distances, or initiating approaches with higher DAs, DHs, or MDAs.
Figure 4-1. Minimum Detection Range
4.4.1.13 Under §§ 23.2600(a), 25.773, 27.773, and 29.773, the EFVS imagery displayed on the HUD or HUD equivalent display must account for the pilot compartment view requirements, 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, and a means to deactivate the display that 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; and 4-12
AC 20-167B
• Accurate identification and utilization of visual references required for every task relevant to the phase of flight.
Note: Utilization of video imagery on the HUD or HUD equivalent display during ground operations should not be considered if sensor proximity to the taxiway surface causes distracting imagery.
4.4.2 EFVS Display Criteria.
Unless otherwise specified, §§ 23.2600(a), 25.773, 27.773 and 29.773 serve as the regulatory bases for the EFVS mandatory display criteria in this section.
4.4.2.1 For EFVS implemented on a HUD, the image should be compatible with the field of view (FOV) and head motion box of a HUD designed against SAE ARP 5288 (transport category head-up display (HUD) systems), or an equivalent standard. 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, in accordance with § 91.176(a)(1)(ii) 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 a display that provides the pilot monitoring with EFVS sensor imagery. Under §§ 23.2500, 25.1321,
27.1321, and 29.1321, all displays used to monitor the approach must be located in the flightdeck so that either pilot seated at the controls can monitor the airplane’s flight path and instruments with minimum head and eye movement. Any symbology displayed should not adversely obscure the sensor imagery of the runway environment.
4.4.2.2 Under 14 CFR parts 21, 23, 25, 27, and 29, the EFVS display must meet applicable airworthiness certification requirements (See appendix A).
Some of th ese requirements could be specific to EFVS and could be in addition to all other requirements applicable to the HUD and the basic avionics installation.
4.4.2.3 Current FAA guidelines for HUDs apply with respect to EFVS. These criteria include well-established military and civil aviation standards for HUDs as defined in MIL-STD-1787D, Aircraft Display Symbology, and AC 25-11B. Specific design standards for image size, resolution and line width, luminance and contrast ratio, chromaticity, and grayscale should be applied. SAE design standards for HUD symbology, optical elements, and video imagery are also prescribed within SAE AS 8055. Recommended practices are included in SAE ARP 5288 and SAE ARP 5287.
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