AC 20-167B CHAPTER 2. VISION SYSTEMS OVERVIEW
2.1 Enhanced Vision Systems (EVS).
2.1.1 An EVS is an electronic means to provide a display of the forward external scene topography using imaging sensors including forward looking infrared, millimeter wave (MMW) radiometry, MMW radar, and low-light-level image intensifying. EVS does not necessarily provide the additional flight information/symbology on a head-up display (HUD) or equivalent display that is required for EFVS operations under 14 CFR
91.176. While EVS share a similarity with EFVS technology, an EVS does not have to be integrated with a flight guidance system. An EVS may provide additional awareness about the external scene to the pilot, but without the additional capabilities provided by an EFVS, it is not eligible to conduct EFVS operations under § 91.176 (a) or (b).
2.1.2 The elements of an installed EVS are listed below, with the EVS diagram shown in Figure 1:
• EVS sensor system,
• Display processor,
• EVS display,
• Pilot controls/interface, and
• Mounting.
Note 1: While an EFVS contains all the elements of an EVS, for the purposes of this AC, any discussion of an EVS excludes an EFVS.
Note 2: For the purposes of this AC, any guidance that applies to a HUD also applies to displays equivalent to a HUD.
Note 3: A head-worn display (HWD) could potentially meet the requirement for a HUD-equivalent display. The FAA does not currently have any published installation criteria specifically addressing HWDs. Applicants should propose a means of compliance that demonstrates that the proposed HWD is equivalent to a HUD.
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AC 20-167B Figure 2-1. Enhanced Vision Diagram Note: Dashed lines and shading represent individual system boundaries for EVS and EFVS.
2.1.3 EVS sensors are currently categorized as passive or active sensors.
2.1.3.1 Passive Sensors.
Scene contrast detected by passive infrared sensors can be much different than that detected by natural pilot vision. On a dark night, thermal differences of objects, while not detectable by the naked eye, will be detected depending on the capabilities of an imaging infrared system.
However, contrasting colors in visual wavelengths distinguished by the naked eye may not be visible using an imaging infrared system. Sufficient thermal scene contrast allows shapes and patterns of certain visual references to be recognized in the infrared image by the pilot. However, depending on environmental conditions, they can also appear different to a pilot in the infrared image than they would with normal vision.
2.1.3.2 Active Sensors.
Scene contrast detected by active systems depends on several parameters:
whether the transmitter is centered on the aircraft velocity vector, display updates rates, latency, range resolution, sensitivity, dynamic range, and azimuth and elevation resolution. For an active infrared thermal imaging 2-2
AC 20-167B sensor, the infrared illuminator has the potential to illuminate more weather obscurations, which can compete with scene contrast and interpretation. One advantage of millimeter wave radar systems is their general immunity to weather obscurations.
2.1.4 Unlike the pilot's external view, the enhanced vision image can be a monochrome, twodimensional display. Some, but not all, of the depth cues found in the natural view are also found in the imagery. The quality of the enhanced vision image and the level of enhanced vision sensor performance depend on atmospheric and external visible and non-visible energy source conditions. Gain settings of the sensor, and brightness or contrast settings of the display, can significantly affect image quality. Certain system characteristics could create distracting and confusing display artifacts.
2.1.5 Unlike with EFVS, an aircraft with EVS does not necessarily provide the additional flight information or symbology required by § 91.176. Unlike an EFVS, an EVS may be displayed on a head down display or on a HUD or equivalent display. The EVS display may not be able to present the image and flight symbology in the same scale and alignment as the outside view. An EVS can provide a display of the external scene topography to the pilot but does not meet the regulatory requirements of § 91.176. An EVS cannot be used as a means to enhance flight visibility or to identify the required visual references in order to gain operational credit and descend below the decision altitude/decision height (DA/DH) or minimum descent altitude (MDA).
2.2 Enhanced Flight Vision Systems (EFVS).
2.2.1 An EFVS is defined in 14 CFR 1.1 as “an installed aircraft system which uses an electronic means to provide a display of the forward external scene topography (the natural or engineered features of a place or region especially in a way to show their relative positions and elevation) through the use of imaging sensors, including but not limited to forward-looking infrared, millimeter wave radiometry, millimeter wave radar, or low-light level image intensification.” An EFVS includes the display element, sensors, computers and power supplies, indications, and controls.
2.2.2 Under § 91.176(a)(1) and § 91.176(b)(1), the EFVS must present EFVS sensor imagery, aircraft flight information, and flight symbology on a head-up display, or an equivalent display, so that the imagery, information and symbology are clearly visible to the pilot flying in their normal position with the line of vision looking forward along the flight path.
An equivalent display must be some type of head-up presentation of the required information. A head-down display does not meet the regulatory requirement. For the EFVS monitor display criteria, see sections 4.4.2 and B.10 of this AC.
2.2.3 An EFVS is intended to be used to conduct an operation in which the enhanced vision image is used in lieu of natural vision to perform an approach or landing, determine the enhanced flight visibility, identify required visual references, or conduct a rollout. An EFVS requires a real-time imaging sensor that provides enhanced visibility in low 2-3
AC 20-167B visibility conditions and a level of safety suitable for the proposed operational procedure.
2.2.4 An EFVS with airworthiness approval will be designated as an EFVS approach system or EFVS landing system as described below. Under § 91.176, airworthiness approval being sought by the applicant may require additional basic equipage. Acceptable criteria for EFVS approach systems and EFVS landing systems are discussed in section 4.4 of this AC. EFVS approach systems and EFVS landing systems are briefly described in the following sections:
2.2.4.1 EFVS Approach System.
The installed EFVS has been demonstrated to meet an acceptable criteria, such as the criteria described in this AC, for an EFVS approach system to be used to conduct EFVS operations under § 91.176(b). The system design relies on sufficient visibility conditions to mitigate a loss of EFVS function in order to enable unaided flare and rollout. Operational guidance for use of an EFVS is published in AC 90-106B, Enhanced Flight Vision Systems.
2.2.4.2 EFVS Landing System.
The installed EFVS has been demonstrated to meet an acceptable criteria, such as the criteria described in this AC, for an EFVS landing system to be used for EFVS operations conducted under § 91.176(a). The system design relies on sufficient visibility conditions to mitigate loss of EFVS function in order to enable unaided flare and rollout. An EFVS that meets the certification criteria for an EFVS landing system also meets the certification criteria for an EFVS approach system. Operational guidance for the use of an EFVS is published in AC 90-106B.
2.2.5 Under § 91.176(a)(1) and § 91.176(b)(1), the basic elements of an installed EFVS are listed below. Guidance on specific system criteria that may be acceptable for an EFVS can be found in Chapter 4 of this AC.
2.2.5.1 The EFVS must have an electronic means to provide a display of the forward external scene topography (the applicable natural or engineered features of a place or region especially in a way to show their relative positions and elevation) through the use of imaging sensors including, but not limited to forward-looking infrared, millimeter wave radiometry, millimeter wave radar, and low-light level image intensification.
2.2.5.2 The EFVS must present sensor imagery, aircraft flight information, and flight symbology on a head-up display, or an equivalent display, so that the imagery, information, and symbology are clearly visible to the pilot flying in their normal position with the line of vision looking forward along the flight path. Aircraft flight information and flight symbology must consist of at least airspeed, vertical speed, aircraft attitude, heading, altitude, command guidance (as appropriate for the approach to be flown), 2-4
AC 20-167B path deviation indications, flight path vector, and flight path angle reference cue. Additionally, for aircraft other than rotorcraft, the EFVS must display a flare prompt or flare guidance when conducting § 91.176(a) operations.
2.2.5.3 The display should meet the following criteria:
2.2.5.3.1 The EFVS must present the displayed EFVS sensor imagery, attitude symbology, flight path vector, flight path angle reference cue (FPARC), command guidance as appropriate and other cues, which are referenced to the EFVS sensor imagery and external scene topography, so that they are aligned with and scaled to the external view.
Note: The European Union Aviation Safety Agency (EASA) imposes additional symbology requirements that are not required by the FAA for certification or for operational approvals. Applicants that will be seeking EASA approval should consult the latest EASA guidance material to ensure compliance with EASA regulations.
2.2.5.3.2 The EFVS must display the FPARC with a pitch scale. The FPARC must be selectable by the pilot to the desired descent angle for the approach and be sufficient to monitor the vertical flight path of the aircraft. The descent angle may also be automatically set to a value found in an onboard database.
2.2.5.3.3 The displayed sensor imagery, aircraft flight information, and flight symbology must not adversely obscure the pilot's outside view or field of view through the flightdeck window.
2.2.5.3.4 The EFVS includes the display element, sensors, computers/power supplies, indications, and controls. It may receive inputs from an airborne navigation system or flight guidance system.
2.2.5.3.5 The EFVS must provide display characteristics, dynamics, and cues suitable for manual control of the aircraft.
2.2.5.3.6 Installations designed to be eligible for EFVS landing system operations must also present a flare prompt or flare guidance and present height above ground level (such as that provided by a radio altimeter or equivalent device). Refer to section 4.4 of this AC for guidance on acceptable performance criteria.
2.2.6 Enhanced flight vision systems.
Enhanced flight vision systems display both symbology and imagery.
2.2.6.1 Symbology luminates a small fraction of the total display area of the HUD, leaving much of that area free of symbology and imagery that could interfere with the pilot's view out the window through the display.
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AC 20-167B
2.2.6.2 The EVS image is in the center of the pilot's regulated “pilot compartment view.” In accordance with §§ 23.2600, 25.773(a), 27.773(a), and
29.773(a), the pilot compartment view must be clear, undistorted, and free of glare and reflection that could interfere with the normal duties of the minimum flightcrew. A video image can be more difficult for the pilot to see through than symbols displayed on the HUD. Unlike symbology, the video image illuminates, to some degree, most of the total display area of the HUD with much greater potential interference with the pilot compartment view. The outside scene must be visible, both through and around the display when the EVS image is displayed.
2.2.7 Unlike the pilot's external view, the enhanced flight vision image is historically a monochrome, two-dimensional display. Some, but not all the depth cues found in the natural view are also found in the imagery. The quality of the enhanced flight vision image and the level of enhanced flight vision sensor performance could depend significantly on the atmospheric and external light source conditions. Gain settings of the sensor and brightness or contrast settings of the HUD can significantly affect image quality. Certain system characteristics could create distracting and confusing display artifacts. Finally, this is a sensor-based system that is intended to provide a conformal perspective. Refer to figure 1 for a diagram of the system.
2.2.8 For the primary display, the regulations also make provision for an equivalent display.
Section 91.176(a)(1)(i)(B) states that EFVS sensor imagery, aircraft flight information, and flight symbology must be presented “on a head-up display, or an equivalent display, so that the imagery, information and symbology are clearly visible to the pilot flying in their normal position with the line of vision looking forward along the flight path.” To meet part 91 operational requirements, an equivalent display must be some type of head-up presentation of the required information. A head-down display does not meet the operational regulatory requirement. For guidance on monitoring display criteria, see section 4.4.2 of this AC.
2.3 Combined Vision Systems (CVS).
2.3.1 The CVS concept involves a combination of SVS and either EVS or EFVS. Some examples of a CVS include database-driven synthetic vision images combined with real-time sensor images superimposed and correlated on the same display. This includes selective blending of the two technologies based on the intended function of the vision system for which approval is sought. For example, on an approach, most of the arrival would utilize the SVS picture. As the aircraft nears the runway, the picture gradually and smoothly transitions from synthetic to enhanced vision, either for SVS picture validation or displaying the runway environment.
2.3.2 CVS applicants are expected to meet applicable performance criteria for each incorporated system. EVS and EFVS related guidance is located in this document. SVS related guidance can be found in AC 20-185A.
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AC 20-167B CHAPTER 3. A IRWORTHINESS PACKAGE CONTENTS
3.1 Airworthiness Package.
For the purpose of this AC, the applicant should include the following items in the airworthiness package it submits for approval.
• Intended function for the proposed system (See section 3.2 of this AC.)
• General operation for the proposed system (See chapter 4 of this AC.)
• Performance criteria and evaluation plan for the proposed system (See chapter 6 of this AC, and sample test considerations in the appendices.).
• Installation considerations for the proposed system (See chapter 5 of this AC.)
3.2 Intended Function.
3.2.1 EVS or CVS.
Define the intended functions of the EVS or CVS. Include what features will be displayed and the criticality of pilot decision-making using the display features under §§ 23.2500, 23.2505, 25.1301, 27.1301, and 29.1301, define additional intended functions (for example, terrain alerting), as appropriate to the aircraft type. It is not acceptable to identify the function of an EVS or CVS display as providing situational awareness only. If the intended function of the CVS is to be used for operational credit, then the CVS must also meet the requirements of an EFVS.
3.2.2 EFVS.
Clearly define the intended function of the EFVS (EFVS approach system or EFVS landing system), including whether its use to assess enhanced flight visibility meets or exceeds the required flight visibility of the instrument approach being flown and to visually acquire the visual references required to operate below the DA/DH or MDA as specified in § 91.176(a) and (b). The purpose of an EFVS is to enable flight in the visual segment of an approach when natural flight visibility is not adequate to meet the flight visibility requirements of the instrument approach being flown. In accordance with the operational requirements in § 91.176, an EFVS must demonstrate its ability to provide sufficient enhanced flight visibility and to clearly identify the required visual references at DA/DH or MDA necessary to conduct an EFVS operation.
Note 1: The EFVS is not intended to replace the technologies or procedures already used to safely fly the aircraft down to the DA/DH or MDA.
Note 2: The purpose of an EFVS is to enhance flight visibility for the pilot in a majority of cases/weather and ambient lighting conditions.
Note 3: Past test data obtained by the applicant for an approved EFVS which demonstrated compliance may be considered for inclusion in this airworthiness package if shown to be relevant to the new application using existing practices for establishing similarity of equipment installations from one aircraft to another. For example, if the 3-1
AC 20-167B certification project intends to certify a previously approved EFVS approach system as an EFVS landing system, the applicant will need to collect new data to demonstrate the unique aspects of the EFVS landing system and may re-use data collected for the approved EFVS approach system to demonstrate aspects common to both EFVS approach systems and EFVS landing systems. In order to re-use previously collected data the applicant will need to show that the previously collected data is still applicable to the current certification application.
Note 4: Where appropriate, use of technical standard order (TSO) criteria and performance standards can be beneficial to the FAA and the applicant.
3.3 Certification of a pre- 14 CFR 91.176 EFVS as an EFVS Landing System.
On December 13, 2016, the FAA published Amendment 91-345 (81 FR 90172) to update then existing §§ 91.175(l) and (m), the prior regulation addressing EFVS use during the segment of an approach that extended from DA/DH or MDA down to 100 feet above the touchdown zone elevation (TDZE). The updated rule expanded the use of EFVS to landings under specified conditions. Section 91.176 was added to cover EFVS use during both landing and approach. Section 91.176(a) addresses EFVS use during landing. The rules for approach that were previously contained in § 91.175(l) and (m) were re-designated as § 91.176(b). EFVS systems that were eligible for operation under §§ 91.175(l) and 91.175(m) are now eligible for § 91.176(b) operations as EFVS approach systems. Systems approved prior to the effective date of § 91.176 (March 13, 2017) must meet the updated applicable airworthiness standards from the rule to be eligible for § 91.176(a) operations as EFVS landing systems. The EFVS landing system application should include the following:
• How the EFVS meets all the requirements of § 91.176(a)(1).
• How the EFVS meets the applicable airworthiness standards for EFVS landing systems as described in AC 20-167B.
Note: Past test data obtained by the applicant for an approved EFVS which demonstrated compliance may be considered for inclusion in this airworthiness package if shown to be relevant to the new application using existing practices for establishing similarity of equipment installations from one aircraft to another.
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AC 20-167B CHAPTER 4. SYSTEM CRITERIA
4.1 EVS (or CVS when EVS and SVS combined) – General Criteria.
Unless otherwise specified, the EVS/CVS mandatory criteria in this chapter are in accordance with §§ 23.2600(a), 25.773, 27.773 and 29.773. Applicants showing compliance to § 23.2600 should use the criteria found in § 23.773(c), Amendment 23- 63, or ASTM F3117M-24a (or later as stated in the FAA’s part 23.), or propose their own means of compliance.
4.1.1 EVS/CVS Primary Displays.
EVS or CVS functionality can be superimposed on the electronic flight instrument system, such as the primary flight display (PFD) as installed in the flightdeck. In this configuration example, the EVS 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 HUD equivalent display system using EVS or CVS capabilities. Maneuvering the aircraft during any phase of flight (taxi, approach, landing, rollout, etc.) should not be predicated on EVS or CVS imagery alone.
4.1.2 EVS/CVS Secondary Displays.
EVS or CVS functionality can be displayed on secondary electronic displays or display windows but is not used for PFD applications. Examples of secondary displays include multi-function displays (MFD), navigation displays, and exocentric displays.
4.1.2.1 The secondary vision system image or loss thereof should not adversely affect other approved secondary display functionality (e.g., navigation display).
4.1.2.2 The secondary display should be such that it cannot be confused with, considered as, or used as a PFD.
4.1.2.3 If primary flight information is presented on a secondary display, the information should be of sufficient quality that it does not cause someone to doubt the PFD data.
4.1.3 Intended Function.
Under §§ 23.2500, 23.2505, 25.1301, 27.1301, and 29.1301, the intended function of any installation of EVS/CVS must be clearly defined as appropriate to the aircraft in which the system is being installed. The design and installation safety levels must be appropriate for the stated intended function. Although normally associated with use during flight, utilization of an EVS display during ground operations should not be used if sensor proximity to the taxiway surface causes a distraction.
4.1.3.1 The intended function should be reasonable considering the actual functionality of the installation.
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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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