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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AC 20-167B
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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AC 20-167B
4.4.2.4 The EFVS installation and the EFVS image, when superimposed on the HUD symbology must:
4.4.2.4.1 Under § § 23.1301 25.1301, 27.1301 29.1301 and 23.1309, 25.1309,
27.1309 and 29.1309, be suitable for and successfully perform its intended function.
4.4.2.4.2 Allow the accurate identification and utilization of visual references, using both EFVS and natural vision, as appropriate.
4.4.2.4.3 Under § § 23.1301 25.1301, 27.1301 29.1301 and 23.1309, 25.1309,
27.1309 and 29.1309, have acceptable display characteristics to accomplish the intended function.
4.4.2.4.4 Have 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.
4.4.2.4.5 Not degrade presentation of essential flight information on the HUD. The pilot's ability to see and use the required PFD information such as primary attitude, airspeed, altitude, and command bars must not be hindered or compromised by the EFVS image on the HUD.
4.4.2.4.6 Under § § 23.2600, 25.771(a), 27.771(a), 29.771(a) and 25.1302(b), not be misleading, cause confusion, or significantly increase pilot workload.
4.4.2.4.7 Be aligned with and scaled to the external scene.
4.4.2.4.8 Not cause unacceptable interference with the safe and effective use of the pilot compartment view.
4.4.2.4.9 Under § § 23.2600, 25.771(a), 27.771(a), 29.771(a) and 25.1302(b), not alter external scene colors in a way that causes confusion, significantly increases workload and/or prevents the pilot from performing any required tasks (e.g., discerning precision approach path indicators).
4.4.2.4.10 Allow the pilot to recognize significantly misaligned or non-conformal conditions to the external scene that may preclude the pilot’s performance of any required maneuvers.
4.4.2.5 The EFVS installation and image should have an effective control of EFVS display brightness without causing excessive pilot workload and not cause adverse physiological effects such as fatigue or eyestrain.
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AC 20-167B
4.4.2.6 A HUD modified to display EFVS must continue to meet the conditions of the original approval and be adequate for the intended function, in all phases of flight in which the EFVS is used. An accurate, easy, quickglance interpretation of attitude should be possible for all unusual attitude situations and other “non-normal” maneuvers to permit the pilot to recognize the unusual attitude and initiate recovery within one second.
The use of chevrons, pointers, and/or permanent ground-sky horizon on all attitude indications to perform effective manual recovery from unusual attitudes has been shown to be an acceptable means to aid in recovery from unusual attitudes. Refer to AC 23.1311-1C, AC 25-11B, AC 27-1B CHG 1-9 and AC 29-2C CHG 9 for guidance on electronic flightdeck displays.
4.4.2.7 Under § 91.176 the EFVS must display a flight path vector (FPV) and flight path angle reference cue (FPARC) on the HUD or HUD equivalent display. The following criteria apply to these symbols:
4.4.2.7.1 The position and motion of the FPV should correspond to the aircraft’s earth-referenced flight path vector. The display should provide for a FPV depiction when at the limit of the FOR.
4.4.2.7.2 The dynamic response of the FPV should not exhibit undue lag or overshoot due to pilot control inputs. If flight path vector quickening is used, adequate testing should demonstrate that the quickening employed actually improves performance and does not introduce any undesirable dynamics to the display. Refer to AC 25-11B, paragraphs A.7 and F.5.4.5;
4.4.2.7.3 The display of the FPARC should be suitable for monitoring the vertical flight path of the aircraft.
4.4.2.7.4 The system must provide the pilot a means to select the desired descent angle that is represented by the FPARC and may also provide a means for the descent angle to be selected automatically from a database.
4.4.2.8 The display of attitude symbology, FPV, FPARC, and other visual elements which are earth-referenced must be aligned with and scaled (i.e., conformal) to the external view.
4.4.2.9 The EFVS display of imagery, flight information and flight symbology should be a suitable visual reference for the pilot during the manual performance of any maneuvers within the operating limitations of the aircraft, including taxiing, takeoff, approach, landing and rollout.
4.4.3 EFVS Detailed System Criteria.
The performance of EFVS imaging systems does not solely depend upon system design, but also depends upon the target scene characteristics such as the runway, light structures, electromagnetic radiation, and atmospheric conditions.
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AC 20-167B
4.4.3.1 Minimum System Performance.
Integration of the major components includes the installed sensor, its interconnections with the sensor display processor, the display device, pilot interface, and aircraft mechanical interface, which can include the radome for the sensor.
4.4.3.1.1 Latency.
EFVS latency should be no greater than 100 milliseconds (msec). A longer lag time can be found satisfactory, provided it is demonstrated not to be misleading or confusing to the pilot. Latency should not be discernible to the pilot and should not affect control performance or increase pilot workload. At best, EFVS latency causes undesirable oscillatory image motion in response to pilot control inputs or turbulence. At worst, EFVS latency may cause pilot-induced oscillations if the pilot attempts to use the EFVS for active control during precision tracking tasks or maneuvers in the absence of other visual cues.
4.4.3.1.2 EFVS FOR.
The minimum fixed FOR should be 20 degrees horizontal and 15 degrees vertical. In applications where the FOR is centered on the flight path vector, the minimum FOR should be 5 degrees (± 2.5 degrees) vertical and 20 degrees horizontal.
1. The minimum EFVS FOR should not only consider the HUD FOV
(i.e., how large of an area displayed), but also the area over which this area subtends (i.e., what is shown on the conformal display). The FOR portrayed on the HUD is established by three primary determinants:
• HUD and EFVS sensor field-of-view;
• Orientation of the HUD with respect to the aircraft frame of reference (for example, boresight and proximity to pilot’s eye); and
• Orientation (for example, attitude) of the aircraft.
2. The EFVS image should be compatible with the field-of-view and head motion box of a HUD designed against SAE ARP 5288, or equivalent standard. The HUD and EFVS FOR should provide a conformal image with the visual scene over the range of aircraft attitudes and wind conditions for each mode of operation. Limitations should be clearly specified in the aircraft flight manual (AFM) if the HUD cannot be used throughout the full aircraft flight envelope.
3. A variable FOR is permissible, assuming a slewable sensor (i.e., variable field-of-regard), centered on the flight path vector, with a minimum +/ 2.5 degrees about the flight path vector to allow for momentary flight path perturbations and to allow sufficient fore/aft view of the required visual references.
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4.4.3.1.3 Off-axis rejection.
A source in object space greater than 1 degree outside the FOV should not result in any perceptible point or edge-like image within the field of view.
The EFVS should preclude off-axis information from folding into the primary FOR imagery and creating the potential for misleading or distracting imagery.
4.4.3.1.4 Jitter.
When viewed from the HUD eye reference point, the displayed EFVS image jitter amplitude should be less than 0.6 mrad when installed on the aircraft. Jitter for this use is defined in SAE ARP 5288. This implies that the EFVS and HUD cannot exhibit jitter greater than that of the HUD itself.
4.4.3.1.5 Flicker.
Flicker is brightness variations at a frequency above 0.25 Hz per SAE ARP 5288. The minimum standard for flicker should meet the criteria of SAE ARP 5288, or equivalent. Flicker can cause mild fatigue and reduced flightcrew efficiency. In accordance with §§ 23.2600(a), 25.773, 27.773 and 29.773, the EFVS and HUD cannot exhibit flicker greater than that of the HUD itself.
4.4.3.1.6 Image artifacts.
The EFVS must not exhibit any objectionable noise, local disturbances, or an artifact that prevents the system from meeting its intended function.
The EFVS design should minimize display characteristics or artifacts (for example, internal system noise, fixed pattern noise, or running water droplets) which obscure the desired image of the scene, impair the pilot's ability to detect and identify visual references, mask flight hazards, distract the pilot, or otherwise degrade task performance or safety. See §§ 23.2600(a), 25.773, 27.773 and 29.773.
4.4.3.1.7 Image conformality.
The accuracy of the integrated EFVS and HUD image should not result in a greater than 5 mrad display error at the center of the display at a range of 2000 ft (100 ft altitude on a 3 degree glideslope). Under SAE ARP 5288, the total HUD system display accuracy error, as measured from the HUD eye reference point, should be less than 5 mrad at the HUD boresight, with increasing error allowable toward the outer edges of the HUD. Errors away from the boresight should be as defined in SAE ARP 5288, or an equivalent standard. The primary EFVS error components include the installation misalignment of the EFVS sensor from aircraft/HUD boresight and sensor parallax. A range parameter is used in the EFVS conformability requirement to account for the error component associated with parallax. There is no boresight error allowed for the EFVS sensor, since it is assumed any error can be electronically compensated during 4-17
AC 20-167B installation. With EFVS operations, the aircraft is essentially flown irrespective of the EFVS/HUD dynamic error, to the MDA or DA/DH.
From this point to 100 ft. height above touchdown zone elevation, the EFVS conformality error introduces error in the pilot’s ability to track along the extended centerline/vertical glide path as the pilot flies the flight path vector and glidepath reference line toward the EFVS image of the runway.
4.4.3.1.8 Sensor/sensor processor.
1. Dynamic range.
The minimum required dynamic range for passive EFVS should be 48 db.
For active EFVS, side lobes should be 23 db below the main beam, and 40 db dynamic range plus sensitivity time control.
2. Sensor image calibration.
Visible image calibrations and other built-in tests that cannot be achieved within a total latency of 100 milliseconds should occur only on either pilot command or be coordinated by aircraft data to only occur in non-critical phases of flight. If other than normal imagery is displayed during the nonuniformity correction (NUC) or other built-in tests, the image must be removed from the pilot’s display in order to comply with §§ 23.2600(a),
25.773, 27.773 and 29.773. This prohibits excessive times to complete maintenance or calibration functions which would remove or degrade the EFVS imagery during critical phases of flight, unless the pilot commands the action (with full knowledge of effect, based on training and experience). Abnormal imagery should be removed from the display to eliminate the potential for any misleading information.
3. Sensor resolution.
As a minimum, EFVS resolution performance must adequately resolve (for pilot identification) the runway threshold and the touchdown zone to enable the intended function. See §§ 23.2500, 23.2505, and 25.1301,
27.1301, and 29.1301. For example, an EFVS should resolve a 60 ft wide runway from 200 ft above touchdown zone elevation with a typical 3- degree glideslope. The sensor resolution was established by providing this resolution at a minimum range, allowing the pilot to continue the descent below DA/DH or MDA. (These values do not take into account pilot decision time or actual atmospheric conditions, or the use of non-precision approaches that may require greater distances.) A 60 ft.-wide runway was chosen as the International Civil Aviation Organization (ICAO) minimum runway width to support an instrument approach procedure.
4. Passive sensor optical d istortion.
Optical distortion should be 5 percent or less across the minimal FOR as defined in section 4.4.3.1.2 and no greater than 8 percent outside the minimal FOR.
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5. Sensor sensitivity.
In this context, the EFVS sensor sensitivity should be at least a noise equivalent temperature difference (NETD) of 50 degrees mK tested at an appropriate ambient temperature for passive EFVS systems, or -20dB sm/sm (square meter/square meter) surface at Rmax from 200 ft height above touchdown zone elevation with a typical 3-degree glideslope for active EFVS systems. Passive sensors for different visible or short-wave infrared sources can require very sensitive detectors, as specified by low noise equivalent powers.
6. Failure messages.
EFVS malfunctions detected by the system, and which can adversely affect the normal operation of the EFVS, should be annunciated. As a minimum, specific in-flight failure message(s) for sensor failure and frozen image must be displayed to the flightcrew. See §§ 23.2500,
23.2505, 25.1309, 27.1309, 29.1309 and AC 25-11B, appendix F.
7. Blooming.
The sensor should incorporate features to minimize blooming, which can create an unusable or objectionable image. Objectionable blooming is defined as the condition that obscures the required visual cues specified in Table 1. Blooming to the extent the required visual references are no longer distinctly visible and identifiable is unacceptable.
8. Image persistence.
The image persistence time constant should be less than 100 milliseconds.
However, burn-in or longer image persistence caused by high energy sources (for example, the sun saturating the infrared sensor elements), should be removed from the image to meet the provisions of section
4.4.3.1.6 by a secondary on-demand process (for example, the NUC process).
9. Dead pixels.
Dead pixels or sensor elements replaced by a “bad pixel” replacement algorithm should be limited to 1 percent average of the total display area, with no cluster greater than 0.02 percent within the minimum FOR. A small number of disparate dead pixel elements can be effectively replaced by image processing, but the algorithms will eventually degrade the image quality and accuracy due to the sheer number and closely-spaced location of the elements.
10. Parallax.
The effects of parallax caused by lateral, vertical, and longitudinal offset of the sensor from the pilots’ design eye points must not impede the EFVS from performing its intended function, as evaluated during flight test. See §§ 23.2500, 23.2505, 25.1301, 27.1301, and 29.1301. Parallax should not degrade landing performance parameters (e.g., flare height, sink rate, 4-19
AC 20-167B touchdown location, and groundspeed during landing, exit and taxi) between EFVS operations and visual operations in the same aircraft.
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