Subject: SYNTHETIC VISION AND PATHWAY 

DEPICTIONS ON THE PRIMARY FLIGHT 

DISPLAY

Date: 12/22/05 

Initiated By: ACE-100 

AC No: 23-26

Change: 

1. PURPOSE. 

a. This advisory circular (AC) sets forth an acceptable means, but not the only means, of 

showing compliance with Title 14 Code of Federal Regulations (14 CFR) part 23 for two new 

concepts in small airplanes. The two concepts are: (1) Synthetic Vision (SV), and (2) pathway 

depictions displaying the navigation course on the primary flight display. This AC addresses the 

two concepts in a head down display format only. 

b. This AC covers airplanes in the normal, utility, acrobatic, and commuter categories 

approved to fly under Instrument Flight Rules (IFR). 

c. Material in this AC is neither mandatory nor regulatory in nature and does not constitute 

a regulation. 

2. BACKGROUND. 

a. Description. SV is a technology that has the potential to reduce fatal Instrument 

Meteorological Conditions (IMC), night, and low-visibility accidents. These three areas of 

operation represent the majority of fatal accidents in general aviation. A computer generated-SV 

image is a pictorial scene viewed from pilots’ perspectives that is derived from: (1) aircraft state 

data (including heading, airspeed, and attitude), (2) a navigation position and direction, and (3) a 

database of terrain, obstacles, and relevant cultural features. Interest in this display is not new; it 

originated in the 1950's with the Joint Army-Navy Instrumentation Research (JANAIR) 

Program. Since then, many research programs have published findings supporting the benefits 

of both SV and display technologies. (See appendix 1.) 
AC 23-26 12/22/05 

b. Advanced General Aviation Transport Experiment (AGATE), Research 

Experience, and Safer Skies Recommendations. In 1994, the AGATE (comprised of National 

Aeronautics and Space Administration (NASA), FAA, and private industry) envisioned flight 

displays that artificially made all flying resemble daytime, clear weather conditions. The 

members of the AGATE group believed that by making all instrument flying displays as easy as 

flying on a clear day those displays would minimize the mistakes made while flying in IMC. 

This belief was supported by the General Aviation (GA) accident history. The AGATE program

solidified the FAA, NASA, and industry desire to make all flying as close to clear day conditions 

as possible, not only for increased GA utility but also for enhanced safety. 

(1) Perspective displays, first proposed in the early 1950’s as part of the JANAIR 

Program, required the maturation of electronic displays to come to fruition. The use of such SV 

displays enhances situation awareness and geographic awareness as demonstrated by a 

statistically significant body of subjective data, and by a finite set of objective measures. Most 

research results cite subjective pilot assessment of improved situation awareness. When 

combined with a pathway or tunnel display, improved tracking performance, with complex 

curved trajectories, has been reported. SV and pathway displays may improve performance and 

increase GA flight safety if there is an adequate demonstration of acceptable pilot workload and 

error detection. (NASA has reported equivalent or lower pilot workload levels when compared 

to conventional displays. This occurred in every simulation or flight experiment conducted so 

far in which comparative measurements of workload were obtained.)

(2) NASA’s goal in the AGATE program was to help the light airplane industry 

recover the health that it enjoyed in the 1970’s. Industry's goal in participating in AGATE was 

to get access to new technology that would help sell airplanes. The economic ramifications of 

this approach to GA flying are simple. To sell airplanes as useful transportation, they need to be 

nearly all-weather capable and offer safer operations than small airplanes offer today. The 

FAA's goal from AGATE participation was to get equipment that could also be retrofitted into 

the existing fleet, which would help reduce fatal accidents. 

(3) NASA has been working with high resolution, SV displays since the high-speed 

civil transport project occurred in the late 1980's through the 1990's. They have flown SV 

displays in their B-737 and are flying SV displays in their B-757 (for part 25) and in their C-206 

(for part 23). NASA SV GA studies published so far have used three-dimensional flight 

guidance; the guidance is three-dimensional in the sense the more traditional two-dimensional 

flight director pitch and roll commands are augmented by time-based predictive components. 

NASA’s three-dimensional presentations have shown reductions in pilot error and workload 

when compared to flying raw navigation data. The FAA's Civil Aero Medical Institute (CAMI) 

also has researched using SV and pathway flight guidance. Their results also show that this 

technology has the potential to meet the AGATE safety expectations. 

(4) In addition to the AGATE, NASA, and CAMI SV pathway research, “Safer Skies,” 

which was the FAA and industry accident reduction effort, produced recommendations for 

part 23 Controlled Flight Into Terrain (CFIT) and weather accidents calling for reduced pilot 

workload. These reports, available on the FAA's Internet site (www.faa.gov), recommend 

adopting technologies like SV. The reports cite that new technology with the potential to reduce 

pilots’ mental workload during high task operations, such as approach, should reduce the number 

of fatal accidents. 

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12/22/05 AC 23-26 

3. APPLICABILITY. 

a. This document is for airplane manufacturers, modifiers, part 23 airplane type certification 

engineers and their designees. The methods and procedures contained in this AC are available 

for use during all part 23 airplane certification activities. 

b. This material is neither mandatory nor regulatory in nature and does not constitute a 

regulation. It describes acceptable means, but not the only means, for showing compliance with

the applicable regulations. The FAA will consider other methods of demonstrating compliance 

that an applicant may elect to present. While these guidelines are not mandatory, they are 

derived from FAA and industry experience in determining compliance with the relevant 

regulations. If the FAA becomes aware of circumstances that convince us that following this AC

would not result in compliance with the applicable regulations, we will not be bound by the 

terms of this AC. The FAA may require additional substantiation or design changes as a basis 

for findings of compliance. 

c. This material does not change, create any additional, authorize change in, or allow 

deviations from regulatory requirements. Applicants should contact their Aircraft Certification 

Office (ACO) to determine the acceptability of any alternate means. 

4. DEFINITIONS. 

a. Synthetic Vision (SV). A computer-generated image of the external scene topography 

from the perspective of the flight deck that is derived from aircraft attitude (or state), highprecision navigation solution, and database of terrain, obstacles and relevant cultural features. 

b. Synthetic Vision System (SVS). An electronic means to display a synthetic vision 

image of the external scene topography to the flight crew. 

c. Pathway. A pathway display (often called a Highway In The Sky (HITS)) provides a 

picture of the selected or programmed navigation path to pilots using a perspective view of a 

path through the airspace. The three-dimensional pathway provides navigation position 

information to pilots. 

d. Flight Path Marker (FPM)/Velocity Vector. The FPM or velocity vector is a 

projection of the aircraft's path, that is, where the aircraft is going. This is sometimes called a 

velocity vector because that is what the symbol represents; however, since the symbol is 

typically an aircraft and not a vector, the symbol is more commonly called a flight path marker. 

If the flight path marker is designed to predict future position based on current aircraft state 

parameters, it may also be referred to as a flight path predictor. 

e. Digital Elevation Model (DEM). Terrain database data used to draw the terrain image 

on a synthetic vision display. 

f. Drawing Order. Drawing order is the sequential layering of symbology drawn on the 

Primary Flight Display (PFD) or other electronic display. 

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AC 23-26 12/22/05 

5. RELATED DOCUMENTS 

a. SAE ARP 4102/7, “Electronic Displays,” July 1, 1988. 

b. MIL-STD-1787C, “Aircraft Display Symbology,” January 5, 2001. 

c. RTCA DO-200A, “Standards For Processing Aeronautical Data,” September 28, 1998. 

d. RTCA DO-276, “User Requirements For Terrain and Obstacle Data,” March 5, 2002. 

e. TSO-C151b, “Terrain Awareness And Warning System,” December 17, 2002. 

f. Theunissen, E., “Integrated Design Of A Man-Machine Interface For 4D Navigation,” 

Delft: Delft University Press, 1997. 

g. Mulder, M., “Cybernetics of Tunnel-in-the-Sky Displays,” Delft: Delft University Press, 

1999. 

h. Beringer, DB, “Development of Highway-in-the-Sky Displays for Flightpath Guidance: 

History, Performance Results, Guidelines,” Proceedings of the 43rd Annual Meeting of the 

Human Factors and Ergonomics Society, paper 3, 2000. 

i. Newman, RL and Mulder, M (2003), “Pathway Displays: A Literature Review,” 

Proceedings 22nd Digital Avionics Systems Conference, Indianapolis, October 2003, paper 9D6. 

j. Wilckens, V (1973), “Improvements in Pilot/Aircraft-Integration by Advanced Contact 

Analog Displays,” Proceedings of the 9th Annual Conference on Manual Control, pp. 175-192. 

k. Anderson, MW and Newman, RL (1994), “HUD Flight Testing: Lessons Learned,” 

presented at Southwest Section SETP Symposium, Stone Mountain, Georgia, May 1994. 

l. Schnell T., Kwon J., Merchant S., Etherington, T., Vogl, T., “Improved Flight Technical 

Performance in Flight Decks Equipped with Synthetic Vision Information System Displays,” 

International Journal of Aviation Psychology, 14(1), 2004. 

m. Schnell, T., Lemos, K., Keller, M., Yang, S., Synthetic Vision Systems, “Optimum

Display Characteristics,“ Final Report, NASA Langley Research Center, Aviation Safety 

Program, Hampton, VA, 2003. 

n. Schnell, T., Lemos K., “Terrain Sampling Density and Texture Requirements for 

Synthetic Vision Systems”, Final Report, Submitted to Rockwell Collins Advanced Technology 

Center, 400 Collins Rd. NE, Cedar Rapids, IA 52498, 2003. 

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6. CERTIFICATION 

a. Specific Features. This AC provides guidance for specific SV system features 

considered necessary for the safe operation of a SV equipped part 23 airplane. This AC 

separately addresses specific features considered necessary for pathway flight guidance. While 

the two technologies complement each other, they can be incorporated independently. The 

guidance in this AC is based on SV research and experience gained during the certification of

existing SV systems. New systems may need additional features for safe operation, and 

policy/guidance for those features will be issued on an individual basis. 

b. Heads-Up Displays. Providing complete guidance for SV systems is difficult because 

there are many design variables available to avionics manufacturers. Since SV displays have 

many characteristics in common with Head-Up Displays (HUD) when compared to traditional 

Head-Down Displays (HDD), applicants may use HUD display guidance where it makes sense. 

Specifically, symbols like the flight path marker may be based on HUD guidelines such as SAE 

ARP 4102/7; part 25 guidance on HUDs; and MIL-STD-1787C, the Department of Defense 

interface standard for aircraft display symbology. 

c. Intended Function. Applicants need to define clearly the intended function of their 

synthetic depiction and pathway (as applicable). This will help the FAA develop appropriate 

evaluation criteria. For example: 

(1) Will the terrain depiction be used only for terrain awareness or for maneuvering to 

avoid impacting terrain? 

(2) Are the elements of the pathway display intended to be used with or as a substitute 

for a Course Deviation Indicator (CDI)?

d. Appropriate Display. The applicant must show that the SV display is appropriate for 

this clearly stated intended function. For example, if the intended function is “terrain 

awareness,” the applicant must show that the “awareness” is consistent with the actual terrain. 

That is, the display may not match the terrain perfectly, but the display should provide a 

reasonable representation of the terrain in a manner that does not misrepresent the threat posed 

by the terrain. The display, while possibly resolution or field-of-view limited, may still 

adequately portray the terrain for terrain awareness. This effect is similar to a “compressed” 

rearview mirror in a car. The mirror compresses the traffic picture so “objects in this mirror are 

closer than they appear.” Though not “accurate,” this approach effectively shows traffic in the 

car’s blind spot. 

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AC 23-26 12/22/05 

7. SPECIFIC GUIDANCE FOR SYNTHETIC VISION. 

a. Synthetic Terrain/Vision Imagery. 

(1) The synthetic terrain/vision image is intended to enhance the pilots’ awareness of

their spatial position relative to important features when flying in limited visibility. This is 

particularly useful during the critical phases of flight such as instrument approaches. These 

features include terrain and runways. The terrain image is based on the use of data from a DEM 

that is stored within the SV system. Additional features such as obstacles, runways, and 

landmarks may be integrated into the display to enhance situation awareness of cultural features

posing a significant hazard to aircraft or are of other significance to aerial navigation. As 

cultural features are ephemeral, care must be taken in evaluating the accuracy of the obstacle 

database against the actual obstacles. The time frame that the obstacle database is updated is a 

key component to accuracy. While terrain changes slowly, cultural features may change quickly. 

For example, a transmission tower may be installed or removed, or change height overnight. 

This is typically documented on maps and charts that are updated frequently. Since synthetic 

vision displays may be more compelling to pilots than a map or chart, it is important that the 

displayed obstacles accurately portray the actual surroundings. 

(2) Currently, integrating synthetic terrain/vision as a background image on a PFD is a 

novel feature that may have the potential to not only degrade PFD readability and interpretability 

but may also provide misleading terrain and orientation cues. These potential safety issues must 

be assessed under both normal and abnormal conditions. In addition, the applicant’s system

must be shown to provide a level of safety at least equal to the conventional flight instruments it 

replaces. One method to accomplish this is to provide a means to remove the SV display and 

revert to traditional blue over brown, but this is not required. 

b. Terrain Alerting. Any airplane equipment incorporating an SV system should also 

provide some type of terrain warning for pilots. The terrain warning feature should be 

incorporated on the Multifunction Display (MFD) or separate display unless the applicant can 

demonstrate that the feature is effective on the PFD. SV systems on the PFD should provide 

adequate altitude and distance cues if used for terrain warning. 

(1) Applicants may use TSO-C151b, Class A, B, or C standards; or applicants may 

develop their own terrain warning system. Applicants who want to develop their own terrain 

warning system should include: 

(a) A one-minute caution and 30-second warning if the airplane’s current flight 

path will collide with terrain or an obstacle. 

(b) Aural call-out for both the caution and the warning (CAUTION – TERRAIN, 

TERRAIN; WARNING – TERRAIN, TERRAIN). 

(c) Terrain impact region highlighted on the moving map. 

(d) A safety margin or buffer of at least 100 feet for cumulative errors in both the 

GPS altitude and terrain database. 

(e) A terrain database/DEM developed using the criteria in RTCA DO-200A, 

“Standards For Processing Aeronautical Data,” September 28, 1998. 

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(f) The SV display must not provide any information that is in conflict with or

incompatible with either the terrain warning or terrain awareness functions of the Terrain 

Awareness and Warning Systems (TAWS). 

(2) SV may be so compelling that pilots may try to use it beyond the intended function. 

Current SV systems may not offer the depth/distance cueing necessary to be used for terrain 

avoidance. Another way of saying this is that error margins may still be too large to use any SV 

system alone. The current systems must be used with other flight and navigation information. If 

other systems are not included to augment the SV system for terrain awareness, the applicant 

must show that the error margins due to field of view size, depth perception, resolution of the 

database used, resolution of the display, update rate of the display and any other factors are small 

enough that SV alone is adequate for terrain awareness in all flight conditions expected. Display 

size constraints may result in a “compressed” display that has the potential to cause misleading 

altitude and range estimation. In addition, cumulative errors from GPS, terrain databases, and 

barometric altimetry systems may contribute to misleading distance and height cues. Adequate 

mitigation should be provided to avoid the effects of such hazards. Such mitigation may be 

incorporated in the design or may include training requirements or procedures to use other 

navigation sources. However, it is unwise to depend solely on Aircraft Flight Manual (AFM) 

limitations. 

c. Airplane Reference Relative to Terrain. If terrain is displayed on the same

instrument/display as the primary attitude indicator, pilots should clearly be able to distinguish 

between terrain above and below the airplane. In other words, terrain above the aircraft's altitude 

should appear above the zero pitch line, and conversely, terrain below the aircraft's altitude 

should appear below the zero pitch line. Displays using a flight-path-marker should be designed 

so that when the flight-path-marker appears above the terrain, the airplane will clear the terrain 

and vice versa. 

d. Heading Integrity. Protection against hazardously misleading information resulting 

from erroneous display of heading must be provided. The use of independent sources, one 

source for the SV and one source for the flight guidance, may be considered, but is not the only 

method of compliance. 

e. Zero Pitch Line. 

(1) The zero pitch line should be constructed so it has a high contrast against most 

possible backgrounds. This is not normally an issue with traditional Attitude Direction 

Indicators (ADIs) because the zero pitch line on these indicators is the boundary between two 

different colored areas. Many ADIs include a contrasting line between the sky and ground. It is 

possible to use a single-color line in terrain-depicting displays where the sky and ground 

representations are of known uniform colors. However, displays expected to portray a 

realistically colored terrain representation or a color enhanced terrain depiction having multiple, 

generic hues may require a zero pitch line having an outline that will contrast against all the 

hues. 

(2) Changes in the shape and edges of background terrain images may provide false 

cues of aircraft attitude to pilots, just as the real world can do in VFR or IFR-on-top conditions. 

Pilots may mistakenly use the terrain image to interpret a level horizon line. For example, a 

sloping terrain on limited field-of-view display may lead pilots to use the flight controls to 

change aircraft attitude based on a false sensation of attitude. Dynamically changing terrain 

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AC 23-26 12/22/05 

background shapes and edges may also create the false sensation of changing attitude. Existing 

guidance recommends a solid, bold zero pitch line extending across the entire display,

representing the aircraft's attitude relative to the horizon. However, this line should not be so 

bold that it hides important display features. 

f. Moving Map Display that Corresponds to and Complements SV PFD Display. 

(1) SV depictions on the PFD have the potential to provide pilots with enhanced terrain 

and landmark awareness during nonprecision and precision approaches. However, the display 

may not provide depth perception and may not provide a field-of-view for pilots to know what is 

to the left and right of the display view area. When viewing the terrain on a limited field-of-view 

display, pilots may mistakenly infer the location of the aircraft relative to the terrain. 

(2) There should be a second, complementary plan view display. The second display 

should indicate the PFD field-of-view and depict at least the same terrain, obstacles, and features 

that appear on the PFDs SV display. This display should be the navigation display, but it could 

also be an MFD or a third, separate display. Ideally, the terrain alerting system should be part of 

this display (unless incorporated into the PFD) so that hazardous terrain or obstacles are 

highlighted. 

g. Terrain Color and Depiction. Terrain coloring and shading techniques that are 

effective in conveying terrain information to pilots should be considered. These techniques must 

make the separation between sky and ground obvious. Currently blue over brown is the 

conventional method; however multiple colors, used on the SV display to portray effectively 

terrain height, contour, contrast, and hydrological features will be evaluated and considered. 

Constant color (brown) terrain has been used effectively and can be enhanced with texturing and 

shading. 

(1) Experience has shown that large bodies of water (i.e. oceans, major rivers, large 

lakes) should be displayed. This might be done effectively using a blue over dark blue or blue 

over black concept. Applicants might consider applying different techniques such as texturing to 

distinguish between ground/water and sky. 

(2) Also, NASA and University of Iowa research showed that some particular terrain 

portrayal coloring techniques are more effective than constant color terrain displays. Applying 

color bands to depict the height of the terrain relative to the airplane may enhance elevation cues. 

Shading, texturing, and shadowing techniques have also proven effective in realistic terrain 

portrayal. For shadowing, the light source (sun angle) positioning must be carefully controlled to 

avoid obscuring important terrain features by shadows. Applicants should consider symbology 

contrast, sunlight readability, and brightness at night when using lighter colors for terrain. 

(3) The traditional flight symbology, such as the pitch ladder, and the new symbology 

used on the SV displays (like towers and traffic) should always be viewable without being 

washed out against the terrain background. Applicants need to consider the layering priority of 

symbology. For example, the pitch ladder should always appear on top of obstacles and terrain 

features. 

h. Minimums Audio Callout Capability. Applicants are encouraged to incorporate either 

a pilot selectable or automatic altitude alert with audio callout to remind pilots they are 

approaching minimums. Pilots may “see” the runway environment on their SV display and 

continue below minimums inadvertently because they were so intent on following the approach 

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12/22/05 AC 23-26 

guidance. This scenario is similar to pilots fixating on a flight director and descending below 

minimums. Alerting pilots that they are nearing minimums reduces the opportunity for this 

situation to occur. 

i. Cultural Features. With a three-dimensional display, pilots should be able to determine 

their relative position to important landmarks like the runway, terrain, and obstacles (such as 

towers). These features can easily be cross-referenced on a published approach chart. Since the 

FAA's goal is to reduce accidents (see Safer Skies recommendations, available at www.faa.gov), 

applicants should consider adding features that increase pilots’ situation awareness and reduce 

pilots’ mental workload. As cultural features and obstacles may change, only those features that 

have sufficient duration and importance, or that pose a significant hazard to aircraft, should be 

considered. The published Notice to Airmen (NOTAM) and transmitted Automatic Terminal 

Information Service (ATIS) remain as the primary means of disseminating significant, temporary 

obstacle information. 

(1) Synthetic Vision systems are intended as an aid to situational awareness and are not 

intended as substitutes for approach charts. But once an applicant decides to incorporate threedimensional terrain, they should consider displaying other features and information necessary to 

reproduce a clear, daytime picture that also correlates directly to approach charts. Information 

displayed on a synthetic vision system that might be helpful to pilots could include; terrain, 

obstacles, towers, and runway orientation. These depictions are not mandatory. When used, 

however, they should be evaluated for accuracy, with particular emphasis on the database and its 

update cycle. 

(2) Display of small lakes, rivers, roads, train tracks, and so on, may be considered on 

the PFD. While these features are common on MFDs/moving maps, display of secondary 

features should not inhibit the visibility of the primary features (terrain features, obstacles and 

hydrological features). The risk is that the display could become too cluttered and it would need 

to be evaluated by the FAA. 

(a) Obstacles. Besides including synthetically drawn terrain on the PFD, 

applicants should consider including charted obstacles (existing government databases include 

all obstacles greater than 200 feet in height). Once an applicant commits to presenting a threedimensional depiction of the terrain on the PFD, pilots may be compelled to trust the display in 

low visibility conditions such as haze. In many areas like the Midwestern US, the terrain will 

look like a flat background. The terrain depiction increases pilots’ comfort level that their flight 

altitude is safe; however, towers and tall buildings can present a significant safety threat. Pilots 

using an SV PFD may be enticed to rely on the terrain information, forgetting about tall 

obstacles such as towers. SV displays should include all hazards (readily available in databases) 

that could be catastrophic to an airplane, not just terrain. 

NOTE: The limitations of the on-board database supporting the SV display 

should be clearly identified and there should be analysis of the potential for 

Hazardously Misleading Information (HMI) resulting from pilots misinterpreting 

the SV display because of these limitations. For example, if the database is 

comprised solely of Digital Terrain Elevation Data (DTED) and no other features 

(such as towers, buildings, and vegetation), then the fidelity of the data should be 

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AC 23-26 12/22/05 

identified. Any operational limitations that result from this analysis should be 

reflected in pilots’ procedures for operating the system and found in the Aircraft 

Flight Manual Supplement (AFMS) and the pilots’ operating manuals. 

(b) Runways. Displaying airport runways is a desirable feature, especially while 

conducting an instrument or visual approach. The runway anchors the approach. Another reason 

to display runways is to minimize the effects of false alerts from TAWS if the TAWS database 

does not include smaller airports but the SV system does. TAWS alerting envelopes are a 

function of distance from the airport (per TSO C151), and all predictive TAWS alerts must be 

inhibited at some time on final approach to avoid nuisance alerts. Therefore, many TAWS

AFMS have requirements that the TAWS be inhibited when flying into airports that are not in 

the TAWS airport database. For most existing TAWS, it is not readily obvious whether the 

destination airport is in the database. As TAWS equipment is installed in smaller aircraft flying 

into more remote locations with shorter runways, the problems associated with airports not in the 

TAWS database will only increase. Portraying airport runways on the PFD and/or MFD would 

be one way to mitigate this problem. 

j. DEM Resolution. The DEM resolution is one factor that determines how well the SV

terrain depiction will match the terrain environment. NASA experiments have shown that a 

terrain resolution of 30-arc-seconds “rounds off” the terrain peaks and fills in valleys. This 

makes the terrain appear less hazardous for the peaks than it is and potentially reduces some

safety benefit. Conversely, for the valleys, the terrain appears higher and is therefore 

conservative. The same set of NASA experiments pointed out that even though pilots preferred 

terrain created using higher resolutions (one and three-arc-seconds), a SV display using a 30-arcsecond database could provide more situation awareness (and, therefore, safety) than the 

conventional instrument panel. Therefore, 30-arc-second resolution should be considered the 

minimum safety standard for SV displays. Depending on the intended function of the terrain, 

applicants should strive to match the DEM resolution specified in Technical Standard Orders 

(TSOs) for TAWS with areas of varying terrain and near airports. More importantly, applicants 

should clearly define the resolution and measuring units of the DEM used by their SV system in 

the AFMS and the pilots’ handbooks. 

(1) The guidance in this AC is based on the terrain display being approved for 

awareness and not for navigation or terrain avoidance. If an applicant wants more credit for the 

terrain display and has the terrain database integrity to support it, the DEM resolution will need 

to be higher than is acceptable for awareness only. 

NOTE 1: TSO-C151B, Appendix 1, paragraph 6.3, recommends a resolution of 

15-arc seconds for TAWS. Also, RTCA DO-276 specifies a high resolution of 

three-arc-seconds in the enroute environment and one-arc-second in terminal 

areas. While these resolutions are not required for SV systems, they are 

recommended targets for resolution. 

NOTE 2: The DEM resolution needed on an SV display depends on the intended 

function of that display. Applicants need to consider how they are going to use 

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the terrain database information for their SV display, and this information should 

be given to the FAA at the beginning of an SV certification program. Common 

elevation references are average elevation, maximum elevation, and sometimes, 

the elevation of the geometric center of the area. As post-spacing increases, the 

difference between the DEM value and the actual elevation of a point within a cell 

may differ significantly. The elevations used for an SV display should be 

conservative; use the highest elevation for a given cell. This concept is identical

to the current sectional charts labeling the highest elevation in the given 

quadrangle (square sector) of latitude and longitude. 

(2) Applicants should provide an acceptable means to indicate the currency status of a 

database. TAWS databases approved under TSO C151b should also be adequate for synthetic 

vision. 

k. Terrain Database Integrity. Besides resolution, a high integrity terrain database is 

critical to safe implementation of SV. Database integrity should be developed and maintained in 

a manner similar to current databases for TAWS or FMS. The level of integrity of the database 

should be commensurate with the intended function, which determines failure classifications. 

DO-200A and DO-276 offer applicants procedures to address database integrity. Applicants 

should provide the FAA with thorough explanations on how they develop and maintain their 

database integrity. 

l. Display Update Rates. 

(1) The primary features of the PFD need to update fast enough to provide a smooth 

depiction of motion for all reasonable flight maneuvers appropriate for the class of airplane. The 

loss of terrain update (for example "frozen screen," and failure to update) should be shown to be 

improbable. Besides failure annunciation, the terrain display should be replaced with the 

traditional blue-over-brown display when SV related errors are detected. 

(2) Update rates vary for different sources of information. This variation may be 

acceptable, depending on the intended function and the implementation of the SV display. For 

example, the update rate on an SV function integrated with the ADI may need to be substantially 

higher than an SV function integrated with a lateral nav display. The applicant should show by 

simulation or flight test that the display update rate is appropriate. For integration into primary 

flight or navigation displays, the SV display should be updated at a rate equivalent to that of the 

rest of the display. This will prevent disagreement between the primary display functions and 

the SV background. 

m. Aircraft Flight Manual Supplement (AFMS). The AFMS should contain limitations 

for pilots on use of the applicant’s system. These limitations should be explained in detail. 

Warnings, cautions, and notes should also address the proper use and potential misuse of the 

display for terrain awareness and avoidance. 

NOTE: One possible reference for describing the terrain data are the data 

elements given in RTCA DO-276 (Section 3, Table 3-1). In addition, DO-276 

provides other requirements for terrain data content and quality that could be 

included in the description if applicable. 

n. Unusual Attitude Recovery. Historically, the FAA has required all but essential flight 

information to be removed from the PFD in unusual attitudes. This “decluttering” was done to 

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AC 23-26 12/22/05 

aid pilots in recovering the airplane. Thus, the FAA approached the first SV systems similarly 

by asking applicants to remove the synthetic depiction and revert to the traditional “blue-overbrown” display during unusual attitudes. 

(1) Recently, the FAA’s center for physiological research, CAMI, conducted 

experiments to determine the effect the synthetic depiction had on unusual attitude recovery. 

One recommendation from that research was that the synthetic depiction remains on the PFD for 

unusual attitude recovery. There was little performance difference between recoveries with and 

without the synthetic depiction. But there is a possibility that pilots may be temporarily confused 

by the significant change to their primary attitude display. Applicants should consider leaving 

the synthetic vision depiction on the PFD for unusual attitude recovery 

(2) Some indication of both sky and ground should always be visible on the PFD for 

use in initiating unusual attitude recovery. Pilots should be able to initiate a recovery toward the 

correct horizon and altitude within one second of recognition (reference MIL-STD-1787C, 

Appendix E, Figure 91). 

8. SPECIFIC GUIDANCE FOR PATHWAY DISPLAYS. 

a. Pathway. Pathway depictions researched have typically, but not always, resulted in 

better pilot performance or lower pilot workload, or both. Research and experience highlighted 

the following recommendations: 

(1) The pathway and flight path marker symbology should be damped enough so it is 

easy to fly. An undamped flight path marker may cause pilots to “chase” the symbol. Damping 

relates directly to pilots’ workload, task saturation, and distraction. 

(2) The pathway should be easy to reacquire when it is not visible on the PFD. FAA 

ACO pilots may evaluate the pathway implementation at their discretion. As the synthetic vision 

systems mature and become more universal, FAA oversight will be adjusted as needed. 

(3) The pathway depiction must not interfere with the primary benefits of the synthetic 

vision system; which are increased situation and terrain awareness. Applicants, therefore, should 

not make the pathway so compelling that pilots are reluctant to leave the flight path or become

fixated on flying through hoops or boxes. 

(4) Applicants should be aware that a poorly implemented pathway can increase pilot 

workload and would not be approvable. The following are examples of issues that applicants 

should consider because, if implemented poorly, they could be misleading to less experienced 

pilots. These examples of important characteristics include: 

(a) The glide path depicted by the display should reflect the conditions that exist 

between any Missed Approach Point (MAP) waypoint and the runway (or should not be 

presented, except by pilot selection). This may require various angles to clear obstacles, not 

always 3.0 degrees; and, 

(b) During the final approach segment, the pathway may follow a navigation 

database with coded vertical navigation path data on lateral navigation-only procedures. This 

provides pilots with a presentation that descends along this lateral navigation path. Some

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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 

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AC 23-26 12/22/05 

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 

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12/22/05 AC 23-26 

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 

15 (and 16) 

12/22/05 AC 23-26 

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, 

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AC 23-26 12/22/05 

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/certification/aircraft/aceNASABibliography.doc 

For a PDF Document 

 http://www.faa.gov/certification/aircraft/aceNASABibliography.pdf 

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