approaches may require that pilots maintain altitudes for step-down fixes or the Minimum Descent Altitude (MDA) up to the missed approach. As databases are improved to include this vertical information, the pathway should not go below altitude minimums during the step-down portion of the approach. The final approach segment (i.e. from the final approach fix to the runway) for a nonprecision approach may require more than a 3-degree approach angle, either from the final approach fix or from an intermediate step-down fix, to ensure adequate obstacle clearance. Nonprecision approaches may also require a circle-to-land procedure. These types of approaches may not be suitable for a constant approach angle, as depicted by a pathway. The applicant should ensure that any approach for which a pathway is depicted can be safely flown using the pathway, and that the pathway provides adequate obstacle clearance for that approach.
On approaches for which a pathway is not suitable, the pathway should not be presented.
(c) Pathway guidance should be provided for the missed approach either on pilot selection of the missed approach, or on the aircraft passing the MAP. Passage of the MAP should consider only the lateral position of the aircraft, and not its altitude, i.e. descent or climb through the MDA should not affect waypoint sequencing. If pilots select the missed approach before reaching the MAP, pathway guidance should be provided first to the MAP, and then for the remainder of the missed approach procedure, unless the aircraft has deviated significantly from the approach procedure and it is not practical to proceed to the MAP. Once pilots select the missed approach, the vertical guidance provided by the pathway should be to an altitude not below the MDA. For example, if pilots initiate a climb, the pathway guidance should not indicate a descent to cross the MAP at the MDA. Conversely, if pilots descend below the MDA, or leveled at the MDA, the pathway should provide vertical guidance to an altitude no lower than the MDA.
(5) The intended function of the pathway needs to be clearly stated as addressed in section 5. The applicant and FAA need to be clear on which navigation sensor the pathway will use and how pilots are expected to use the pathway. As a minimum, the applicant should consider the following:
(a) Can the pathway have multiple navigation sources?
(b) If so, how do pilots know which navigation source is used for depicting the pathway?
(c) If the pathway is GPS driven, how will it be used for “ground-based” approaches?
(d) How will vertical minimums be addressed?
(6) For en route operations, the pathway must use barometric altitude for the vertical guidance because that is what is used in the airspace system. Barometric altitude is also used on approach unless the system uses navigation-system-derived vertical path guidance (Instrument Landing System (ILS) or GPS-based vertical navigation). Applicants should also consider providing pilots with the option of dimming or removing the pathway and reverting to traditional flight guidance if available.
b. Pathway Lateral and Vertical Limits. Applicants should show that the pathway boundary is appropriate for the stated intended function. If intended as a secondary source of navigation information, the boundary markings must be consistent/compatible with the primary 13
navigation sources (deviation indicators or flight directors). Research indicates that pilot training and experience strongly affect the usability of pathway boundaries. Therefore, applicants should show that their pathway boundaries allow for an acceptable level of usability performance.
Vertical limits for nonprecision and precision approaches are fixed at an MDA and Decision Altitude (DA). Pilots are still expected to comply with these limits by using the primary barometric altimeter. The AFM pathway description should clearly define the lateral and vertical limits of the pathway and how they are intended to be used.
c. Precision Approach Guidance. If an applicant chooses to provide precision approach guidance using a pathway depiction on an SV PFD, they should meet the applicable TSO performance requirements for the intended approaches.
(1) Applicants may consider using independent sensors (when available) to create the display symbology. One example of dual sensor implementation is to use GPS for the pathway and ILS raw data for the precision approach guidance.
(2) Applicants using the same navigation positioning sensor for different approach guidance displays (for example, the same sensor driving the ILS flight director and the Pathway) should demonstrate the clear and unambiguous annunciation of approach guidance failures in the pilots’ primary field of view.
d. Pathways and Terrain. Misapplying drawing order priorities could result in hazardously misleading information displayed on the PFD. For example, the pathway may pass behind or through terrain. The pathway is a compelling flight display and should never continue through terrain. Also, if the programmed flight plan or immediate flight path has a terrain conflict, the system should alert the pilots. Terrain conflicts should be obvious to pilots by the discontinuity of the pathway in or around terrain.
(1) Most research so far has been conducted during the approach phase of flight.
Applicants should consider issues associated with other phases of flight such as climb, en route, descent, and missed approach. In particular, nonground referenced climb trajectories may indicate misleading vertical information. Applicants should consider including departure and en route Air Traffic Control (ATC) scenarios for pathway operations.
(2) Applicants should also consider decluttering (removing) the pathway for unusual attitude recovery. Unusual attitude recovery will need to be examined during the pilots’ evaluation.
9. TEST AND EVALUATION METHODS AND CRITERIA FOR COMPLIANCE.
a. Pilot Evaluation. There are hundreds of variables that can distinguish one display system from another. Depending on the design implementation, an SV/pathway system might not provide a safety improvement. Because of the number of variables, a thorough FAA pilot evaluation will be necessary for the first implementation of any SV display system. Less FAA involvement may be possible on subsequent installations or system upgrades. Often, it is useful to gather subjective pilot assessments of the SV displays. Questionnaires used with flight evaluations and/or simulation are good tools to use for pilot assessment, but they need to be specific rather than merely solicit general impressions. Accepted and proven evaluation protocol, measures, and scales should be used where applicable to ensure the integrity of the 14
evaluation process. The questions should target specific information presented on the display, its intended function, and whether it is usable for flight tasks required for typical instrument and commercial ratings. Besides using FAA pilots (including Designated Engineering Representative (DER) pilots), the applicant should consider conducting assessments with representative end user/operational pilots. Applicants should coordinate plans for any pilot evaluation with the Small Airplane Directorate and their responsible ACO.
b. Evaluation Criteria. The integration of synthetic terrain/vision and/or pathway imagery on the background of a PFD will contain many features for which the FAA does not have standards or guidance. Therefore, besides the guidance in this AC, the display must be evaluated to ensure the following:
(1) The primary flight display meets its intended function; and,
(2) The additional and/or novel features of the synthetic terrain/vision and/or pathway imagery on the background of a PFD do not unacceptably compromise the functions and usability of the PFD. The existing level of safety must be maintained. For example, the integration of a terrain image cannot result in hazardously misleading information with existing tasks flown using the attitude indicator.
Original signed by Kim Smith Manager, Small Airplane Directorate Aircraft Certification Service
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Appendix 1
APPENDIX 1 -BACKGROUND FOR SYNTHETIC VISION AND PATHWAY
DISPLAYS-RESEARCH OVERVIEW PROVIDED BY NASA (REPRINTED) The two display concepts applied to the Primary Flight Display (PFD) that are the subject of this Advisory Circular, Synthetic Vision (SV) and Pathway or Highway in the Sky Displays, have both been under investigation within the flight display research community for more than three decades. Prior to the more recent research in SV, these investigations usually have addressed the technologies separately. With the advent of more contemporary SV concepts, SV and Pathway or Highway in the Sky displays have become more closely coupled, as will be discussed.
The earliest flight display work in both technologies was limited graphically to connected straight line segments by the rendering capabilities available then as the state of the art (i.e., stroke generators). Because Pathway Displays attempted to represent the intended flight path of the airplane connecting geospatial waypoints, and because of the two-dimensional nature of Instrument Landing Systems (ILS), which generated rectangular boundaries, the earliest Pathway Displays were quite amenable to stroke presentations. The natural inclination to include a runway representation at the end of the final approach segment of the Pathway Display led to its initial coupling with Synthetic Vision. In addition to a runway representation, some primitive attempts were also made to represent first, the ground plane, and eventually terrain. As computer graphics technology has matured, pathway (and terrain) presentations have improved dramatically, although the basic concept of presenting the desired vertical and lateral path ahead of the airplane, viewed from the pilots’ positions, in a three dimensional perspective scene has clearly been maintained. Within the flight display research community, while terminology may vary between Pathway, Highway, or even Tunnel Displays, and some concepts may employ different flight guidance strategies (including the total lack of flight-director-like guidance) and different pathway elements, common confusion over the definition of this type of flight display has never arisen.
However, even within the flight display research community, the term Synthetic Vision has had different interpretations through the years, which can still lead to some confusion. However, the community viewpoint has finally converged to an acceptance of the interpretation of Synthetic Vision as a pictorial scene viewed from pilots’ perspectives, which is rendered by a graphics computer based on a geospatial database of terrain, obstacles, and perhaps cultural features.
Rudimentary Synthetic Vision Displays of the airport environment contained only a perspective runway outline and a horizon line, augmented perhaps with alphanumeric flight information when character generators became available (the terminology for these displays at that time was contact analog, rather than Synthetic Vision, displays). However, with the advent of raster graphics engines, filled polygons allowed for the presentation of more realistic, although somewhat cartoonish, airport scenes (which were commonly termed pictorial displays) and surrounding terrain.
Somewhat concurrent with the initial use of raster graphics in flight display research was a low visibility landing program conducted by the FAA which was titled the Synthetic Vision Flight Demonstration Program. Actually, using today's terminology, the program investigated Enhanced Vision Systems (EVS), which attempt to improve visual acquisition by enhancing significant components of the real world scene outside the aircraft. Enhanced vision systems typically use imaging sensors to penetrate weather phenomena such as darkness, fog, haze, rain, A1-1
Appendix 1
APPENDIX 1 -BACKGROUND FOR SYNTHETIC VISION AND PATHWAY
DISPLAYS-RESEARCH OVERVIEW PROVIDED BY NASA (REPRINTED) (CONTINUED)
and/or snow, and the resulting enhanced scene, a sensor image, is presented on a Head-Up Display (HUD), through which the outside real world may also be visible.
Synthetic Vision acquired its present definition somewhat concurrent with the emergence of texturing capabilities, which allowed raster graphics engines to render more highly realistic scenes (in some concepts, aerial and/or satellite photography are used to provide photo-realistic scenes). Synthetic Vision Displays today are characterized by their ability to represent, in an intuitive manner, the visual information and cues that a flight crew would have in daylight -- Visual Meteorological Conditions (VMC). The view of the outside world is provided by melding computer generated airport scenes from on-board databases and flight display symbologies, in some cases with either information derived from a weather penetrating sensor
(e.g., information from runway edge detection or object detection algorithms) or with actual imagery from such a sensor. The visual information and cues are depicted based on precise positioning information relative to the onboard terrain database, and may include traffic information from surveillance sources (such as Traffic Alert and Collision Avoidance System (TCAS), Airport Surface Detection Equipment (ASDE), etc.) and other hazard information (such as wind shear).
A bibliography was created to provide insight into the technical foundation of this Advisory Circular regarding Synthetic Vision and Pathway in the Sky Displays. While quite extensive, it is not intended to be exhaustive. In terms of the more recent papers, the bibliography is perhaps weighted heavily with those papers generated by either NASA researchers working within, or industry and university researchers sponsored by NASA programs in Large Screen Pictorial Displays, High Speed Research External Visibility Systems, and Synthetic Vision. The entire bibliography is intended to offer some of the more influential sources of highly relevant research-based information regarding the development and certification of Synthetic Vision and Pathway in the Sky Displays. NOTE: this bibliography is extensive, about 27 pages, and is therefore available online at the following address:
For a Word Document:
http://www.faa.gov/certificatio n/aircraft/aceNASABibliography.doc For a PDF Document http://www.faa.gov/certificatio n/aircraft/aceNASABibliography.pdf
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