AC 20-167A

2-1 CHAPTER 2. VISION SYSTEMS OVERVIEW

2.1 Enhanced Vision Systems

2.1.1 An EVS is an electronic means to provide a display of the forward external scene topography through the use of imaging sensors, such as forward looking infrared (FLIR), millimeter wave (MMW) radiometry, MMW radar, and/or low-light-level image intensifying. EVS need not provide the additional flight information/symbology required in 14 CFR 91.176 on a Head Up Display (HUD) or equivalent display. While Enhanced Vision Systems share a similarity with EFVS technology, an EVS does not have to be integrated with a flight guidance system. Furthermore, while an EVS may provide additional situation awareness without the additional capabilities provided by an EFVS, it provides no credit towards operations conducted under 14 CFR 91.176 (a) and (b). 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.

 Mounting.

Visible External Scene Energy Atmosphere Sensor Sensor Conversion Sensor System Display Processor Pilot's Eye Enhanced Flight Vision System Aircraft State Data Head-up Display Processing Head-up Display Enhanced Vision System EVS Display Pilot Controls

Figure 1. Enhanced Vision Diagram (Dashed lines and shading represent individual system boundaries for EVS and EFVS)

2-2 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 Head Up Display (HUD) also applies to displays equivalent to HUD.

2.1.2 The categories of current EVS sensors are passive or active sensors.

2.1.2.1 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, not detectable by the naked eye, will be easily detected by many imaging infrared systems. 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 conditions, they can also appear different to a pilot in the infrared image than they would with normal vision.

2.1.2.2 Active Sensors: Scene contrast by active systems depends on several parameters such as: 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 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.

2.1.2.3 Unlike the pilot's external view, the enhanced vision image can be a monochrome, two-dimensional 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 the atmospheric and external visible and nonvisible 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.

2.2 Synthetic Vision Systems

2.2.1 Synthetic vision is a computer-generated image of the external scene topography relative to the aircraft that is derived from aircraft attitude, high-precision navigation solution, and database of terrain, obstacles, and relevant cultural features. A synthetic vision system is an electronic means to display a synthetic vision image of the external scene topography to the flight crew. Synthetic vision creates an image relative to terrain and airport within the limits of the navigation source capabilities (position, altitude, heading, track, and the database limitations). SVS provides situation

2-3 awareness but cannot be used in lieu of natural vision. The application of synthetic vision systems is through a primary flight display from the perspective of the flight deck (egocentric), or through a secondary flight display from the perspective correlating to outside the aircraft (exocentric, like a “bird’s eye” view of a moving map display). This AC addresses both perspectives. The elements of an installed SVS are listed below, with the SVS diagram shown in Figure 2.

 Display (includes interface and installation).

 System interface.

 Database of terrain and obstacles.

 Sources providing the aircraft altitude, attitude, and position.

Visible External Scene Energy Atmosphere Aircraft Altitude, Attitude, Position Database Synthetic Vision Display Processing Pilot's Eye Display

Figure 2. Synthetic Vision Diagram (Dashed lines represent individual system boundaries. External scene visible image is not affected by SVS.)

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2.2.2 Synthetic vision coordinate referenced databases are usually referenced to digital terrain elevation data (DTED). Digital terrain data can help produce a digital elevation model (DEM), a terrain database used to draw the terrain image on a synthetic vision display.

Note: For information purposes, there are three DTED levels with the following terrain spacing:

 DTED Level 0 uses 30 arc second spacing (nominally one kilometer).

 DTED Level 1 uses 3 arc second spacing (approximately 100 meters).

 DTED Level 2 uses one arc second spacing (approximately 30 meters).

2.2.3 With the exception of terrain and database processing (addressed in chapter 4, paragraph 4.3.7), synthetic vision characteristics may include:

2.2.3.1 Display factors, such as:

 Conformality.

 Refresh rate.

 Field of regard (FOR).

 Field of view (FOV).

2.2.3.2 Display placement factors, such as:

 Pilot’s primary FOV versus pilot’s secondary FOV. See figure A3-1 in AC 25-11B.

 Head down display (HDD) versus Head up display (HUD).

2.2.3.3 Terrain presentation factors, such as:

 Depiction (such as monochromatic, photo-realistic, elevation coloring).

 Gridding (presence versus absence, spacing).

 Color, shading, shadowing, and texturing. (This AC does not establish criteria for these items.)

 Resolution. DEM resolution and DTED level.

 Range indication (e.g., terrain based range marks, color scheme).

2.2.3.4 Terrain and cultural features, such as:

 Rivers, valleys, and mountains.

 Major landmarks, highways, and buildings within close proximity to airport.

 Hue and contrast between terrain and sky.

2-5  Man-made obstacles (e.g., radio towers, bridges, tall buildings) and airport depiction.

 Airport depiction, including markings (e.g., extended runway centerline and numbers, taxiway markings, approach lighting system).

 Range marks or indications.

2.2.3.5 Functions, such as:

 Alerting (terrain, obstacles, traffic, and airspace).

 Declutter.

 Mode reversion.

 Image control (FOV, brightness, contrast, registration).

2.2.3.6 Integration of guidance symbology (if incorporated), such as:

 Guidance cues (flight path vector, flight path angle reference cue, velocity vector, command guidance cues, deviation indicators, and trend indicators).

 Aircraft reference relative to terrain (indication of relationship between zero pitch line and terrain).

 Pathway markings (“breadcrumbs,” rectangles) and boundaries, to indicate past or intended future aircraft track.

 Navigation information (course lines, fixes, icons, special-use airspace).

 Unusual attitude recovery.

 Crosswind indication (and image display based on cross-track).

2.3 Combined Vision Systems

2.3.1 The CVS concept involves a combination of SVS and 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 For combined vision systems applicants are expected to meet applicable performance criteria for each incorporated system.

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2.4 Enhanced Flight Vision Systems

2.4.1 An EFVS, as defined in 14 CFR § 1.1, is “an installed aircraft system which uses an electronic means to provide a display of the forward external scene topography (the natural or manmade 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.” 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 his or her normal position with the line of vision looking forward along the flight path (ref 14 CFR § 91.176(a)(1)(i)(B)).

During an instrument approach, the enhanced vision image is intended to enhance the pilot's ability to detect and identify the visual references required by the regulations.

2.4.2 An EFVS requires a real-time imaging sensor providing demonstrated vision performance in low visibility conditions and a level of safety suitable for the proposed operational procedure. This allows the required visual references to become visible in the image before they are visible naturally out-the-window. Depending on atmospheric conditions and the strength of energy emitted and/or reflected from the scene, the pilot can use the imagery produced by an EFVS to see these visual references on the display better than the pilot can see them through the window without enhanced vision.

Currently approved enhanced flight vision systems use passive infrared sensor technology. However, an active or passive sensor, as described in chapter 2, paragraph

2.1.2, could be used provided it meets the performance criteria in this AC.

2.4.3 Depending on the intended operational function of the EFVS, airworthiness approval will be designated as an EFVS Approach System or EFVS Landing System as described below. Depending on the airworthiness approval being sought by the applicant, additional basic equipage may be required by § 91.176. Those additional requirements are discussed in paragraph 4.5 of this AC.

2.4.3.1 EFVS Approach System – The installed EFVS has been demonstrated to meet the criteria for AC 20-167A for an EFVS Approach System to be used for instrument approach operations from DA/DH or MDA to 100 TDZE while all system components are working, but may have failure modes that result in the loss of EFVS capability. Above 100 feet above the TDZE, the pilot is expected to conduct a go-around if the EFVS fails.

Descent below 100 feet above the TDZE through touchdown and rollout is conducted using natural vision, so any failure of the EFVS should not prevent the pilot from completing the approach and landing.

2.4.3.2 EFVS Landing System – The installed EFVS has been demonstrated to meet the criteria for AC 20-167A for EFVS Landing System to be used for instrument approach and landing operations that rely on sufficient visibility conditions to enable unaided rollout and to mitigate for loss of

2-7 EFVS function. An EFVS that meets the certification criteria for EFVS Landing System also meets the certification criteria for an EFVS Approach System.

2.4.4 The basic elements of an installed EFVS are required by 14 CFR § 91.176(a)(1)(i), and are listed below, with the EFVS diagram in Figure 1. Specific system criteria for EFVS systems can be found in Chapter 4 of this AC.

2.4.4.1 The EFVS must have an electronic means to provide a display of the forward external scene topography (the applicable natural or manmade 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;

2.4.4.2 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 his or her 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, height above ground level such as that provided by a radio altimeter or other device capable of providing equivalent performance, command guidance as appropriate for the approach to be flown, path deviation indications, flight path vector, and flight path angle reference cue. Additionally, for aircraft other than rotorcraft, the EFVS must display flare prompt or flare guidance. The display should include:

2.4.4.2.1 The EFVS must present the displayed EFVS sensor imagery, attitude symbology, flight path vector, and flight path angle reference cue (FPARC), 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;

2.4.4.2.2 The EFVS must display the flight path angle reference cue (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.

2.4.4.2.3 The displayed sensor imagery, aircraft flight information, and flight symbology do not adversely obscure the pilot's outside view or field of view through the cockpit window.

2.4.4.3 The EFVS includes the display element, sensors, computers and power supplies, indications, and controls. It may receive inputs from an airborne navigation system or flight guidance system; and

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2.4.4.4 The display characteristics, dynamics, and cues that are suitable for manual control of the aircraft.

2.4.4.5 Depending on the EFVS intended function, installations designed to be eligible for EFVS Landing System operations must also have a flare cue and radio altitude. Refer to the performance criteria in paragraph 4.5 of this AC.

2.4.5 Enhanced flight vision displays both symbology and imagery.

2.4.5.1 Symbology illuminates a small fraction of the total display area of the HUD, leaving much of that area free of reflected light that could interfere with the pilot's view out the window through the display.

2.4.5.2 The EVS image is in the center of the pilot's regulated "pilot compartment view." It should be free of interference, distortion, and glare that would adversely affect the pilot's normal performance and workload. A video image can be more difficult for the pilot to see through than symbols also 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. It is sufficient for the pilot to see around the video image, but the outside scene must be visible through and around it.

2.4.6 Unlike the pilot's external view, the enhanced flight vision image is a monochrome, two-dimensional 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 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.

2.4.7 Although the European Aviation Safety Agency (EASA) uses the term “EVS” as equivalent to the FAA “EFVS,” do not confuse an EFVS with an Enhanced Vision System (EVS).

2.4.7.1 Unlike with EFVS, an aircraft with EVS does not necessarily provide the additional flight information or symbology required by 14 CFR § 91.176.

An EVS is not required to use a HUD, and 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 situation awareness to the pilot, but does not meet the regulatory requirements of 14 CFR § 91.176. An EVS cannot be used as a means to determine enhanced flight visibility or to identify the required visual references in order to gain operational credit and descend below the DA/DH or MDA.

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2.4.7.2 For the primary display, the regulations also make provision for an equivalent display. Specifically, 14 CFR § 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 his or her normal position with the line of vision looking forward along the flight path.” An equivalent display must be some type of headup presentation of the required information. A head-down display does not meet the regulatory requirement. For the monitoring display, see paragraph 4.5.2.1 of this AC.

3-1 CHAPTER 3. AIRWORTHINESS PACKAGE CONTENTS

3.1 Airworthiness Package The applicant is responsible for the following contents in the airworthiness package (for the purpose of this AC). This AC specifically addresses SVS with attitude displayed on the primary flight display (PFD), EVS/SVS/CVS with a head down display other than a PFD (such as a navigation display or multifunction display), EVS image displayed on a head up display, and an EFVS.

3.1.1 Intended function: Information on intended function can be found in section 3.2.

3.1.2 General operation: Information on the general operations and specific performance criteria can be found in Chapter 4.

3.1.3 Performance criteria and evaluation plan for the respective system. Performance evaluation is addressed in Chapter 6 with sample test considerations found in the appendices of this document, and

3.1.4 Installation considerations: Information regarding installation considerations can be found in Chapter 5 of this document.

3.2 Intended Function.

3.2.1 EVS, SVS, or CVS: Define the intended functions of your EVS, SVS or CVS. Include what features you will display and the criticality of pilot decision-making using the display features. Define additional intended functions (for example, terrain alerting) according to AC 25-11B and 14 CFR §§ 23/25/27/29.1301.

3.2.2 EFVS: Clearly define the intended function of your EFVS (EFVS Approach System or EFVS Landing System). This must include its use to assess enhanced flight visibility and to visually acquire the visual references required to operate below the DA/DH or MDA as specified in 14 CFR §§ 91.176 (a) and (b), 121.651, 125.381, and 135.225, and the criticality of pilot decision-making based on what is visible using the EFVS display.

The purpose of the EFVS is to provide a visual advantage over the pilot’s out-thewindow view using natural vision. In low visibility conditions, the “enhanced flight visibility” should exceed the “flight visibility” and the required visual references should become visible to the pilot at a longer distance with an EFVS than they would out-thewindow using natural vision. Visual advantage using an EFVS must be demonstrated before descending below DA/DH or MDA because this is the point in an instrument approach procedure where the operating rules permit an EFVS to be used in lieu of natural vision for operational benefit.

Note 1: The EFVS is not intended to replace the technologies or procedures already used to safely fly the aircraft down to the MDA/MDH or DA/DH.

3-2 Note 2: While the goal of EFVS is to exceed the natural flight visibility in the majority of cases/weather conditions, there may be meteorological conditions where the EFVS does not provide a significant advantage.

Note 3: Past test data for existing EFVS sensor technology which demonstrated compliance should be considered for inclusion in this airworthiness package.

Note 4: Where appropriate, use of TSO criteria and performance can be beneficial to the FAA and the applicant.

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):