A Crash Course on Simulator Visual System Evaluation

The Crucial Role of Visual Systems in Flight Simulation

When it comes to modern flight simulation, visual systems are far more than just a supplementary feature; they form an absolutely essential component. Early flight simulators didn't have visual systems, but as the technology matured, these systems have become more and more important to the immersive experience that a flight simulator needs to provide. These systems are responsible for providing the critical "out-the-window" views that pilots rely on to orient themselves, perceive their surroundings, and make informed decisions. They are a primary means through which the simulator communicates the aircraft's attitude, position, and movement in relation to the external environment; alongside the aircraft's instruments of course.

The importance of visual systems extends across all phases of flight training, testing, and checking. From the initial orientation and familiarisation with the airport environment to the precise maneuvers required for landing and taxiing, visual cues are indispensable. They allow pilots to practice essential skills, such as maintaining situational awareness, judging distances and heights, and responding to dynamic changes in weather and terrain.

Whether training for routine procedures or handling complex emergencies, the visual system provides the necessary context and realism to make the simulation both effective and relevant. For both aeroplanes and helicopters, these systems are not a luxury, but a fundamental requirement for creating a credible and valuable training experience.

General Requirements and Qualification Levels

The qualification of visual systems within Flight Simulation Training Devices (FSTDs) is governed by specific regulations and standards set forth by aviation authorities. These regulations ensure that visual systems meet the necessary criteria for their intended use in pilot training and evaluation.

EASA and FAA Standards

Both the European Union Aviation Safety Agency (EASA) and the Federal Aviation Administration (FAA) establish standards for visual systems in FSTDs.

• EASA CS-FSTD(A) (Certification Specification for Flight Simulation Training Devices (Aeroplanes)) mandates that the visual system in an aeroplane FSTD must adhere to all standards applicable to the qualification level requested for that FSTD. For Full Flight Simulators (FFSs), these qualification levels range from A to D, with Level D representing the highest fidelity. For Flight Training Devices (FTDs), the levels are 1 and 2.

• Similarly, FAA Part 60 outlines visual system requirements for different levels of airplane FFS (A, B, C, D) and FTDs. These requirements are designed to ensure that the visual system provides appropriate cues and realism for the specific training tasks conducted in the simulator.

• EASA CS-FSTD(H) also specifies visual system standards for helicopter FSTDs, with qualification levels ranging from A to D for FFS and 1 to 3 for FTD.

• FAA Part 60 includes Appendix C, which details requirements for helicopter FFS (Levels B, C, D), and Appendix D, which covers requirements for helicopter FTDs (Levels 4, 5, 6, 7).

These regulations provide a structured framework for evaluating and qualifying visual systems, ensuring consistency and adherence to established standards.

Visual Systems on Lower-Level Devices

It's important to note that even when visual systems are added to lower-level training devices that don't specifically require them or attract specific training credits (such as FTDs, Flight Navigation Procedures Trainers (FNPTs), and Basic Instrument Training Devices (BITDs)), they are still subject to assessment. This assessment aims to confirm that the visual system does not negatively affect the overall qualification of the FSTD.

Furthermore, if a visual system on an FTD is intended to be used for maneuvering by visual reference (e.g., visual approaches and landings), it should, at a minimum, comply with the visual system requirements for a Level A FFS. This ensures that the visual system provides the necessary fidelity and cues to support these critical flight tasks.

Field of View (FOV) Requirements

Field of View (FOV) is a critical parameter for visual systems, as it determines the extent of the visual scene presented to the pilot. Adequate FOV is essential for maintaining situational awareness, perceiving the aircraft's relationship with its surroundings, and accurately judging distances and speeds. Both EASA and the FAA specify minimum FOV requirements for various FSTD types and qualification levels.

Aeroplane FFS

Both EASA and the FAA define minimum continuous FOV requirements for aeroplane Full Flight Simulators (FFS).

• For lower qualification levels (A, B), FAA Part 60 specifies a minimum continuous FOV of 45° horizontal and 30° vertical for each pilot. This ensures a basic level of peripheral vision for these training scenarios.

• For higher qualification levels (C, D), FAA Part 60 mandates a significantly wider FOV, requiring a minimum continuous FOV of at least 176° horizontal and 36° vertical. This expanded FOV provides a more immersive and realistic representation of the outside world, particularly important for advanced training and certification.

• EASA CS-FSTD(A), while emphasizing the importance of FOV, does not specify exact numerical FOV requirements. Instead, it focuses on ensuring that the visual system, including its FOV, meets the requirements of validation and subjective tests conducted by qualified pilots.

Helicopter FFS

FOV requirements also vary for helicopter FFS, reflecting the unique operational demands of rotary-wing aircraft.

• FAA Part 60 Appendix C specifies the minimum FOV for helicopter FFS. For Levels B and C, the minimum continuous FOV is 75° horizontal and 30° vertical. For the highest qualification level, Level D, the minimum continuous FOV is 176° horizontal and 56° vertical. The wider vertical FOV in Level D reflects the importance of vertical cues in helicopter operations.

• EASA CS-FSTD(H) acknowledges the importance of FOV but emphasises that the choice of display system and the resulting FOV should fully consider the FSTD's intended use. The decision should strike a balance between the specific training and testing objectives and the technical feasibility of achieving a particular FOV.

Aeroplane FTDs

For aeroplane Flight Training Devices (FTDs) seeking additional training credits based on the capabilities of their visual system, the visual system must meet the FOV standards of at least a Level A FFS.

• FAA Part 60 provides specific minimum FOV requirements for lower-level airplane FTDs when used for pilot flying tasks. These requirements include a minimum of 18° vertical and 24° horizontal FOV.

Helicopter FTDs

Similarly, for helicopter FTDs that aim to receive visual credits, FAA Part 60 Appendix D requires that the visual system meets the visual standards of at least a Level A FFS.

Additional FOV

It is important to note that the specified FOV requirements represent the minimum standards. FSTD sponsors may choose to incorporate additional FOV beyond these minimums, provided that the minimum FOV requirements are consistently maintained. This allows for enhanced realism and training capabilities in specific scenarios.

Key Technical Characteristics of Visual Systems

Beyond the fundamental requirements for Field of View, several key technical characteristics define the quality and fidelity of a visual system in a flight simulator. These characteristics contribute significantly to the realism of the simulation and its effectiveness for pilot training.

Collimation

Collimation is a technique used in visual display systems to 'project the image at infinity', providing the viewer with more realistic cues for judging distances.

• For aeroplane FFS, collimation requirements vary. Some qualification levels may not mandate collimation, focusing instead on ensuring the visual system meets the overall validation and subjective test standards. However, FAA Part 60 generally requires a continuous collimated FOV for airplane FFS, recognizing its benefits for distance perception.

• EASA CS-FSTD(H) also discusses the basic principles of collimated displays in the context of helicopter simulators. Collimated displays are noted to provide more realistic cues for distant objects by aligning light rays so that the viewer's eyes focus as if looking at an object at a great distance.

Optical Discontinuities and Artifacts

A fundamental requirement for all visual systems, whether in aeroplane or helicopter simulators, is the absence of optical discontinuities and artifacts.

• The visual system must be designed and calibrated to avoid any distortions, abrupt changes in the image, or other visual anomalies that could create unrealistic or misleading cues for the pilot. These discontinuities and artefacts can detract from the realism of the simulation and potentially hinder the training process.

Textural Cues

Visual textural cues play a crucial role in providing pilots with information about their motion and orientation, especially during critical phases of flight like takeoff and landing.

• For both aeroplane and helicopter FFS, the visual system must provide sufficient textural cues to allow pilots to accurately assess their sink rate and depth perception during takeoff and landing. These cues help pilots judge their height above the ground and control their descent rate.

• In Level A aeroplane FFS, the visual system should provide sufficient visual cueing to support a pilot's ability to make changes in the approach path using runway perspective. This requires accurate representation of runway markings, surface details, and the overall perspective of the runway as the aircraft approaches.

• Similarly, helicopter FFS Level A requires sufficient visual cueing to support changes in the approach path using the Final Approach and Take-Off (FATO) area perspective. This is essential for helicopter operations, which often involve approaches to confined or unprepared landing areas.

Horizon and Attitude Correlation

A critical aspect of any visual system is the accurate correlation between the displayed horizon and the simulated attitude indicator.

• In both aeroplane and helicopter FFS, the visual system must ensure that the displayed horizon line and the aircraft's attitude (pitch and roll) as shown on the simulator's attitude indicator are in precise agreement. Any discrepancy between these cues can lead to disorientation and negatively impact the pilot's control of the aircraft.

Occulting

Occulting refers to the system's ability to obscure visibility, simulating conditions like fog, rain, or other visibility-reducing phenomena.

• Both aeroplane and helicopter FFS must have the capability to simulate a range of visibility conditions. A minimum of ten distinct levels of occulting must be available and demonstrable, allowing instructors to create a variety of challenging weather scenarios for training.

Scene Content

The level of detail and realism in the visual scene is crucial for creating an immersive and effective training environment.

• For aeroplane FFS, the visual scene should be comparable in detail to what a pilot would see in the real world. This includes a significant number of visible textured surfaces, such as terrain, buildings, and runway details, as well as accurate representation of lights. The system must provide sufficient surfaces with appropriate textural cues to support visual approaches, landings, and taxiing during daylight conditions.

• Similar requirements exist for helicopter FFS, with specific requirements for the number of polygons and light points depending on the qualification level and whether it is day or night.

• In both aeroplane and helicopter simulators, the visual scene should correlate accurately with any integrated aircraft systems, such as terrain awareness and warning systems (TAWS), traffic collision avoidance systems (TCAS), and head-up guidance systems (HGS). This ensures that the visual cues presented to the pilot are consistent with the information provided by the aircraft's instruments and systems.

Colour, Brightness, and Contrast Ratio

Accurate representation of colour, brightness, and contrast is essential for visual realism and pilot perception.

• Daylight visual systems should provide full-colour presentations that accurately reflect the colours of the real-world environment. These systems must also meet specific brightness and contrast ratio requirements, which vary depending on the qualification level of the simulator.

• Twilight (dusk/dawn) and night visuals have their own specific requirements. Twilight scenes require accurate representation of the changing light conditions, while night visuals must accurately portray light sources and ambient illumination levels.

• FAA Part 60 also includes specific contrast ratio requirements to ensure adequate image clarity and distinguishability.

• To ensure ongoing accuracy, the visual system should allow for quick and easy confirmation of colour, Runway Visual Range (RVR), focus, and intensity settings.

Surface Resolution

Surface resolution refers to the level of detail that the visual system can display.

• Surface (Vernier) resolution should be demonstrated using a test pattern of objects that subtend a small visual angle. This ensures that the system can accurately portray fine details and edges, which are important for visual acuity and distance judgment.

Light Points

Accurate representation of light points is crucial, especially for night and low-visibility operations.

• The system should accurately portray the environment in relation to the FSTD's attitude. This means that light sources should move and change perspective correctly as the simulator changes its orientation.

• Realistic visual effects for the simulator's ownship (own aircraft) external lights are also needed, including landing lights, navigation lights, and strobe lights.

• Light points must be displayed without distracting jitter, smearing, or streaking, which can be disorienting and unrealistic.

• The system should be capable of focus effects that simulate the perspective growth of light points as the aircraft approaches them. This is important for accurately portraying the visual cues during approaches and landings.

Focus Effects

In addition to light point focus effects, the visual system should be capable of simulating other environmental effects.

• Systems should be able to provide focus effects that simulate rain, accurately portraying how rain affects visibility and distorts the visual scene.

These technical characteristics, when properly implemented and evaluated, contribute to a high-fidelity visual system that provides pilots with a realistic and effective training environment.

Visual Ground Segment (VGS) Testing

A crucial aspect of evaluating visual systems, particularly in aeroplane simulators, is the Visual Ground Segment (VGS) test. This test focuses on the accuracy of the visual scene during the critical approach and landing phase.

Aeroplane VGS Testing

The Visual Ground Segment test for aeroplanes assesses several key items that directly impact the pilot's perception and decision-making during an Instrument Landing System (ILS) approach. These include:

• RVR (Runway Visual Range) modelling accuracy: The simulator must accurately portray the simulated visibility conditions, including the Runway Visual Range, which is a critical factor in determining whether a landing is permitted.

• Glideslope/localiser modelling accuracy: The visual system must accurately represent the glideslope and localiser guidance cues, which provide the pilot with vertical and lateral guidance during the approach.

The VGS test is particularly important at decision height (DH), the altitude at which the pilot must decide whether to continue the approach or execute a go-around. Accurate visual cues at this point are essential for a safe and effective transition to visual flight.

Helicopter VGS Considerations

While the ILS approach is specific to aeroplanes, similar considerations for altitude and RVR apply to helicopters to assess the accuracy of the visual scene during approaches to landing areas. The simulator must accurately portray the visual environment during helicopter approaches, providing pilots with the necessary cues for safe and precise manoeuvring.

Data Sources for VGS Calculations

Regardless of the aircraft type, the Qualification Test Guide (QTG) should clearly indicate the data sources used for the VGS calculations. This ensures transparency and allows evaluators to verify the accuracy and validity of the simulated visual scene.

Visual System Response Time and Transport Delay

A crucial aspect of visual system evaluation is assessing its responsiveness to pilot control inputs. This involves two key metrics: visual response time and transport delay.

Visual Response Time

Visual response time refers to the interval between an abrupt control input from the pilot and the moment the visual system completes the display of the first video field containing the new information resulting from that input.

• A means of accurately recording this response time is a mandatory requirement. This measurement helps ensure that the visual system reacts quickly and accurately to the pilot's actions, providing timely feedback and contributing to a realistic flight experience.

Transport Delay

Transport delay encompasses the total processing time within the entire FSTD system, measured from the pilot's initial control input to the corresponding visual system response.

• The total transport delay must remain within specified tolerances. Excessive delay can negatively impact the pilot's ability to control the simulated aircraft, leading to unrealistic handling and potential training deficiencies.

• Testing methods for transport delay are carefully defined. These methods take into account the type of controller system used in the simulator (whether it's a replica of the real aircraft's controller or a software-emulated version) and how to accurately account for any delays that may be inherent in the simulated aircraft's systems.

• The transport delay test should also consider the different visual system modes, such as daylight, twilight, and night. The delay characteristics may vary slightly between these modes, and the testing must ensure that the delay remains within acceptable limits for each.

• For helicopter FSTDs, there's a specific consideration related to the interaction between visual and motion systems. While the visual change in response to a control input may begin before the motion system initiates its response, the motion system's acceleration must occur before the visual system completes the display of the first video scan containing the new information. This ensures a coordinated and realistic presentation of cues.

• The overall latency of the FSTD, which represents the relationship between the pilot's controls, the visual system, the instruments, and the motion system (if fitted), can be effectively established through either transport delay tests or visual response time tests. These tests provide a comprehensive assessment of the simulator's responsiveness and the coordination of its various components.

Functions and Subjective Tests

While objective measurements and technical specifications are crucial, the evaluation of a simulator's visual system also involves assessing its functionality and its overall effectiveness from a pilot's perspective. This is where functions and subjective tests play a vital role.

• Accurate replication of the functions of the simulated aeroplane or helicopter systems is essential. This includes verifying the correct correlation between the visual system and other simulator components. For example, if the simulator is equipped with a weather radar display, the visual depiction of weather should accurately reflect the information presented on the radar.

• Qualified and experienced pilots conduct subjective assessments of the simulator's visual system. These evaluations focus on how well the visual system supports the pilot's ability to perform various flying tasks. This includes assessing the realism and accuracy of visual cues during manoeuvres such as visual approaches and landings, where pilot judgment and perception are paramount.

  By combining objective measurements with these functional and subjective evaluations, a comprehensive assessment of the visual system's quality and suitability for training is achieved.

Visual Databases (Airport/Landing Area Models)

For Full Flight Simulators (FFSs), a key element of the visual system is the provision of realistic and detailed visual databases representing airports or landing areas. These models are crucial for training pilots in a variety of operational scenarios.

Requirement for Airport/Landing Area Models

FFSs must be equipped with visual systems that can present an out-the-window view of these airport or landing area models. This allows pilots to practice approaches, landings, taxiing, and other ground operations in a simulated environment that closely resembles the real world.

Minimum Model Content

Specific minimum content requirements for these airport/landing area models are defined, and these requirements vary depending on the qualification level of the FFS. Higher qualification levels generally demand more detailed and accurate models. These details can include:

• Runway markings and lighting

• Taxiway layouts

• Terminal buildings and other airport infrastructure

• Terrain features surrounding the airport

Representativeness and Acceptability

The airport or landing area models used in the simulator should be representative of real-world, operational airports. However, in some cases, fictional airports may be used for specific training purposes. In all cases, the models must be acceptable to the relevant regulatory authority. This ensures that the models meet the standards for accuracy and realism necessary for effective training.

Model Development

The development of these detailed airport and landing area models relies on various source materials to ensure accuracy. These materials may include:

• Airport pictures

• Construction drawings

• Maps

These resources help to create a visually accurate and geographically correct representation of the airport environment.

Considerations for Older Visual Systems

It is acknowledged that older visual systems may have limitations in terms of the level of detail they can display. Regulators take these limitations into account when evaluating the suitability of such systems, balancing them against the overall training objectives.

Circling Approach Requirements

For training in circling approaches (where the pilot approaches one runway but lands on another), specific requirements apply. The visual database must include detailed models of both:

• The initial approach runway

• The intended landing runway

This allows pilots to practice the visual manoeuvres involved in circling approaches, which require accurate perception of both runways and the surrounding terrain.

Enhanced Visual Systems (EVS)

Enhanced Vision Systems (EVS) are advanced technologies that provide pilots with a greatly improved view of the external environment, particularly in low-visibility conditions. These systems use sensors, such as infrared cameras, to "see" through fog, smoke, and darkness.

EASA Guidance on EVS Qualification

EASA CS-FSTD(A) AMC5 provides guidance on the qualification of Full Flight Simulators (FFSs) equipped with EVS. This guidance is particularly relevant for training and checking requirements that are detailed in Joint Operations Evaluation Board (JOEB) or EASA Operations Evaluation Board (OEB) reports. These reports specify how EVS can be used in flight operations and, consequently, how it should be represented in a simulator.

Testing for EVS

The evaluation of EVS within a simulator involves specific tests to ensure its accurate representation. Some key tests include:

• Transport delay: Similar to the general visual system, the transport delay of the EVS display must be carefully measured and verified to be within acceptable limits. This is crucial to ensure that the EVS display provides timely and accurate information to the pilot.

• Thermal crossover demonstrations: EVS often relies on thermal imaging, which detects differences in heat signatures. The simulator must accurately demonstrate the phenomenon of "thermal crossover," where temperature differences between objects become minimal, potentially reducing the effectiveness of the EVS.

By rigorously testing these and other relevant parameters, regulators can ensure that EVS is accurately represented in the simulator, providing pilots with effective training for its use in real-world operations.

Differences Between Aeroplane and Helicopter Visual Systems

While the fundamental requirements for visual systems in flight simulators share many similarities, there are also key differences that reflect the distinct operational characteristics of aeroplanes and helicopters.

Aircraft-Type-Specific Visual Cues

It's important to recognise that some visual cues are specifically tailored to the aircraft type being simulated.

• Helicopter FSTDs: These simulators place a greater emphasis on visual cues that are essential for hover and low-speed manoeuvring. This includes accurate representation of ground proximity, vertical movement, and the visual cues needed for precise control in confined spaces.

• Aeroplane FSTDs: In contrast, these simulators often focus more on cues related to runway perspective during approaches. This includes accurate depiction of runway markings, approach lighting systems, and the visual cues that help pilots judge their position and alignment with the runway.

Airport Models vs. Landing Area Models

The content of the visual databases also differs to reflect the operational environments of each aircraft type.

• Airport models: Aeroplane simulators typically require detailed models of airports, including runways, taxiways, and terminal buildings.

• Helicopter landing area models: Helicopter simulators may utilise models of a wider variety of landing areas, including:

  • Confined areas

  • Offshore platforms

  • Rooftops

These differences in visual cueing and database content ensure that the simulator provides a realistic and relevant training environment for the specific aircraft type.

Ensuring Visual Realism in Flight Simulation

The evaluation of visual systems in flight simulators is a complex and detailed process. The standards set by authorities like EASA and the FAA provide a comprehensive framework to ensure that these systems meet the demanding requirements of pilot training and evaluation.

This framework encompasses a wide range of factors, from fundamental requirements like field of view and the absence of optical distortions to more nuanced aspects like the accurate representation of textural cues, light points, and environmental effects. The goal is to create a visual environment that is not only realistic but also provides the specific cues that pilots rely on during various flight operations.

Furthermore, the criteria and procedures are tailored to the unique characteristics of both aeroplanes and helicopters, recognising the different visual demands of each aircraft type.

By adhering to these rigorous standards, the aviation industry can ensure that flight simulators provide pilots with a high-fidelity visual experience, ultimately contributing to safer and more effective flight training.