Synthetic Vision (SV) displays replace the pilot’s traditional 2D primary flight display (PFD) with a 3D perspective view display. The 3D SV perspective display is rendered from an onboard terrain database, e.g., one derived from the Enhanced Ground Proximity Warning System (EGPWS) to create a virtual reality- like image. A comparison of a traditional 2D PFD with a 3D SV PFD is shown in Figure 1.
The first OEM large aircraft SV display was certified on the Honeywell Primus Epic® system by Gulfstream in January 2008. The natural perspective colorized 3D terrain offers an unmatched safety enhancement in that it always presents the pilot with a clear and natural view out the window even in degraded visibility conditions such as at night or during operations in inclement weather.
In addition to the 3D perspective terrain, obstacles, and runway environment, some certified SV displays include advanced Head-Up Display (HUD) symbology, like the flight path vector, to improve control precision and energy management. An SV display, with enhanced symbology, permits any typically trained pilot to fly a more tightly coupled approach easily and routinely, capable of achieving an enhanced level of performance. However, despite the improved all-weather terrain and energy awareness, and enhanced pilot performance provided by some of the new SV displays, certification of the SV systems to date has afforded no operational credit to incentivize operators outside the business and general aviation community to equip with the new technology.
NextGen and SESAR
Under the FAA’s NextGen and European SESAR programs, the aviation authorities in the US and Europe are redesigning their airspace regulations, usage and rules to increase accessibility to airports and increase landing frequencies under any weather conditions. However the ability to provide a consistent level of accessibility and safety is not possible with the conventional airborne avionics and limitations of some airport infrastructures.
For GPS Localizer Performance with Vertical guidance (LPV) operations, specifically wide area augmentation system (WAAS) guidance, monitoring is provided by an entire system for North America, and is limited to a time to alert (TTA) of false or misleading information to 6.2 seconds limiting the decision altitude (DA) of a CAT I standard operation to 200 ft. DA is the altitude at which a pilot must see outside visual references in order to continue the approach to landing. If the DA on an approach is lowered, and aircraft and crew are appropriately equipped, airport accessibility and landing frequency is increased.
There is a proposal for a new SV Guidance System (SVGS) that will enable aircraft to get lower than standard on the ILS and LPV approaches by solving the existing integrity and accuracy problems that are limiting operations. This SVGS system will change the performance of the airplane and it is believed that it will incentivize regional and air transport aircraft operators and OEMs to equip with SV technology.
SVGS — A New Generation Display and Guidance System
SVGS is a flight path vector (FPV) based SV PFD with additional pilot in the loop control display elements and system monitors that is intended to enable operations to lower than standard CAT I and LPV approach minimums at reduced infrastructure airfields (e.g., reduced lighting). SVGS is based upon previously certified Primus Epic® avionics architecture with a software update to an existing Primus Epic SmartView certified display.
Honeywell’s SVGS system is called SmartView Lower Minimums (SVLM). SVLM capabilities are based upon the design of the intuitive flight instrument elements that address previously complex training issues by pilot task reduction and increased pilot in the loop control in combination with a new airborne system monitoring for continuity of signal in space, CAT II level (or better) flight technical error, and new level of position assurance by nature of the flight display design elements.
HUD Symbology Concepts and Next Gen PFDs
The symbology design on SVLM intentionally adopts that of a HUD, and takes advantage of a high resolution, large format, color display. The SVLM display is a PFD that incorporates flight instrument, flight guidance and a terrain based attitude indicator using data approved in accordance with FAA DO-200A standards (Figure 2, left).
The HUD-like advanced flight instrument design symbology elements on the SVLM display include an expanded pitch ladder, which is conformal with the outside view; conformal FPV; and, conformal runway and airport symbols.
The FPV group symbology integrates many previously separate control and performance parameter elements that the pilot directly manipulates. Because the pilot is directly controlling path, the flying task becomes much easier. Because this is done on a much expanded pitch ladder scale, precision is improved. The FPV and the associated acceleration cue provide the crew with an intuitive way to fly the aircraft and greatly increase energy cues. They provide direct control of the aircraft's path through space and a direct indication of the effects of altitude and thrust on aircraft speed. The SVLM FPV group is registered with the (virtual) outside view.
The use of conformal symbology provides continuous knowledge of objects in the real world. The runway symbol cue is a good example; it allows continual knowledge of the runway location even in low visibility conditions. The transition from instrument flight to visual flight with a HUD or with SVLM requires context switching by the pilot, e.g., the pilot changes from the HUD symbology presented on the combiner glass, and must look beyond the symbology to visual cues in the natural world (note: sometimes it is a great challenge for the pilot to look through cluttered HUD formats to the far domain real world) or transitions from a head down SVLM PFD to a head out frame of reference. However, SVLM along with most HUDS, provide an aid in the form of conformal symbology that assist the pilots to prepare to visually acquire the runway when transitioning from the instrument to visual segment.
SVLM enhances existing SV features and ads features derived from HUD symbology (but now with color!). SVLM is a flight path centered and FPV based PFD flight instrument that provides a geographically correct terrain depiction attitude indicator with position assurance. In addition, it provides supplementary flight guidance during the final phase of the instrument approach segment via runway approach indicator. In addition to the runway approach indicator, a geometric Precision Approach Path Indicator (G-PAPI), flight path referenced vertical deviation and conformal lateral deviation with crab symbol have been added to the SV display (Figure 3). SVLM symbology also includes approach deviation arrowheads to alert the pilot during excessive lateral or vertical deviation (not shown in Figure 3). These new features and symbology are added to the PFD for the purpose of enhanced topographic and energy awareness and to obtain operational credit for SmartView to fly to lower than standard CAT I minima.
New Approach Monitors
In addition to the symbology enhancements previously described and reduced SBAS lateral and vertical integrity limits, the heart of the SVLM system resides with the new monitors. The new monitors had to be developed and added to the no operational credit version of the SmartView PFD to afford the integrity required for a lower than standard minimums operation and to meet time to alert requirements that are not currently supported by today’s CAT I or LPV approaches. The SVLM concept contains five new monitors which are designed to alert the flight crew when airplane position solutions and database values are out of a predefined tolerance. The monitors are introduced as a result of a functional hazard assessment (FHA) that considered effects of functional failures when moving from a “no operational credit” SV Primus Epic certification to a lower than standard CAT I ILS and LPV200 approach. The five new monitors of SVLM are:
- Runway Data Integrity
- Delta Position
- Virtual Inner Marker
- Flight Technical Error
The runway data integrity monitor compares two independent sources of runway information to enhance integrity of the displayed runway.
The delta position monitor is a realtime position monitor that becomes active when the aircraft is stabilized on approach, about 1000 ft above the runway elevation, and it continues through the instrument segment to the approach minimum. The design of this new monitor is based on CAT II time-to-alert requirements. It continuously compares the approach deviations from the instrument approach being flown with a “reference approach”. The reference approach is an idealized approach based on the approach geometry and navigation database values and its position is updated by an inertial reference system coasting algorithm.
The altitude monitor function protects against GPS altitude errors and associated LPV vertical deviations along with mis-set altimeters (pilot error or rapidly changing barometric pressure). It looks at the tracking of three independent altitude sources: GPS altitude, corrected barometric altitude, and radio altitude plus the terrain database.
The virtual inner marker utilizes a concept of an artificial inner marker as a decision altitude monitor that enables the use of a barometric decision altitude (DA) for approaches with minimums below standard CAT 1 where radio altitude decision height (DH) is normally used.
Flight technical error monitor will alert the crew when an excessive lateral or vertical deviation is present. This monitor activates when the system has determined that the current position on ILS or LPV approach is offset such that a safe landing cannot be made without exceptional piloting skill and without full visual references available.
Flight technical, system performance, and human factors evaluations of SVLM were conducted with FAA and EASA in 2013. Eighteen participant pilots from FAA, EASA and OEMs flew the SVLM functional system prototype. Flight tests were conducted using Honeywell’s Falcon 900EX EASy II test aircraft for system performance development and verification. Simulator testing to evaluate low visibility transition to landing verification was conducted in Boeing’s M-Cab 777 simulator.
The test data demonstrated pilot performance well within CAT II approach criteria and reduced pilot workload that enables either a manually flown or monitored approach to lower than standard minimums with a level of system alerting and monitoring that meets and exceeds CAT II level of safety. In addition, the flight tests and simulator evaluations demonstrated that operations with an SVLM guidance PFD (head down display) facilitated a successful transition from instrument to visual segment and landing at both 150ft DA with 1400ft RVR and 100ft DA with 1000ft RVR. The flight tests demonstrated that the SVLM system provides independent position fixing and new monitoring systems to the required level of integrity, time to alert, and performance for a lower than standard CAT I operation with minimal crew training.
Next Steps Towards SVGS Approval
RTCA SC213 federal advisory committee is in the final stages of completing minimum aviation performance standards for an SVGS system. Upon completion, it is predicted that FAA will in turn update airworthiness and certification guidance and issue updated approach plates to permit lower than standard minimums on approved approaches with approved SVGS equipment and crew. It is hoped that the approval process and publication of approach plates will, in turn, provide the desired incentive of operators and specifically the airline industry to equip with the natural view 3D SV PFDs.
This article was written by Thea Feyereisen, Engineer Fellow, Advanced Technology, Honeywell Aerospace (Phoenix, AZ). For more information, Click Here .