PETITION FOR EXEMPTION 

Petitioner: Tim McAdams i • •J/\'iy' 

Manager, American Eurocopter Flight Simulation Center ' : " ; (• 

FAA Management Representative 

2701 Forum Drive ' - • ' -, il: 

Grand Prairie, TX 75052 

(972) 641-3796 Tim.McAdams@eurocopterusa.com 

-*^^*< ^ 

Code reference for which exemption is sought: 

14CFR Part 60, Appendix C, Table C2A Full Flight Simulator Objective Tests, requirement 

4.b.3 

Exemption 

American Eurocopter is seeking relief from the requirement that any geometric error 

between the Image Generator eye point and the pilot eye point must be 8° or less for the 

EC175 Level D FFS currently being designed. We request that a geometric error of 10 to 12 

degrees be allowed subject to justification and agreement with National Simulator Program 

Manager (NSPM) in accordance with helicopter characteristics and training needs, 

We request this exemption based on the following: 

In order to produce a realistic helicopter training experience, the pilot must be able to 

accurately hover in close proximity to objects and landing areas. To design a visual system 

that realistically replicates this requirement requires that the dome be as close as possible 

to the pilot. However, moving the dome closer increases the geometric error. American 

Eurocopter believes that the trade off of accepting a higher geometric error in order to 

provide an enhanced hover and landing experience greatly improves the helicopter training 

experience. There is no doubt that granting this exemption will result in a better trained 

pilot. Better training enhances safety and safer helicopter operations are a substantial 

benefit to the public as a whole. 

Summary for publishing in the Federal Register: 

American Eurocopter is seeking an exemption to 14 CFR Part 60, Appendix C, Table C2A, 

4.b.3, specifically, allowing a geometric error greater than 8 degrees. This exemption will 

allow an improved design that improves training for hovering and close proximity to objects 

operations. Improved training will significantly enhance safety. 

A technical description follows: 
EC175 Full Flight Simulator, Level-D 

EUROCOPTER 

INDRASistemasS.A. 

ll-April-2011 

Technical Note 

Visual Display System 

Parallax Error & Dome Screen Size. 
1 Introduction 

It is accepted that wide visual fields of view, both horizontal and vertical, with coverage for peripheral 

view, look down on side-door window and chin window areas, greatly contribute to simulator acceptance 

by pilots and training effectiveness. As for today technology status, the only feasible solution for FoV in 

excess of 210^ x 60^ is direct projection on a dome screen, which combined with adequate scene content 

and texture details also provide for a highly accurate perception of height above ground and ground speed 

magnitude and direction, as required for ground operations and low level hover and flight tasks. 

For multi-crew helicopters, a significant contribution to the Visual Display System error is the so called 

parallax error to distant objects because of different crewmember and Image Generator/Projection 

System eye point reference positions. The Visual Display System typically includes a switching capability to 

set the IG viewpoint to the pilot-flying eye point for critical tasks so the parallax error is effectively 

cancelled out (to the expense of a higher parallax error for the pilot-not-flying). For general tasks, or when 

both pilot and copilot are involved, a center view is selected (mid-point between design eye point 

positions) so the parallax error for both crew members simultaneously is minimized. 

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Figure 1 .-Dome Projection, Parallax Error. 

As a general rule, parallax error for distant objects might be reduced by increasing dome screen radius but 

large dome size, while increasing error for closer objects and degrading height perception on ground, has 

also severe effects on other areas of the simulator from a practical point of view. 

A structured decision process has been performed for the EC175 FFS-D Visual Display System design, 

leading to a direct projection dome screen because of relevance of ground operations and low level hover 

and flight tasks in the intended training. Current proposed design is presented, together with analysis and 

discussion on compliance of applicable requirements, training implications and impact on general 

simulator design and performance. 
This technical note has been compiled with the objective to obtain FAA feedback early in the simulator 

design process whether proposed solution might be acceptable for EC175 FFS Level D qualification. 

2 Proposed EC175 VDS Design 

Real image, front projection spherical dome. 

Internal screen radius: 3.25m (10.7ft). 

Physical screen size: Horizontal 240e (+1209) 

Vertical 909 (+302 up, -609 down) 

Target image coverage (center projection), optimized for EC175 cockpit FoV: 

Horizontal >2109(±1059) 

Vertical >809 (+259 up, -559 down) 

Pilot/Copilot Design Eye Point position: ±580mm (22.8in) right and left of centerllne. 

Offset 250mm (9.8in) aft of dome center. 

IG view point positions, selectable from Instructor Station: Pilot, 

Center, 

Copilot. 

See Figure 2 for proposed FoV coverage and cockpit structure masking from Center View (mid-point 

between pilot and copilot DEP positions) and Figure 3 from Pilot DEP, See Figure 4 and Figure 5 for dome 

screen main dimensions. 

Figure 2.- EC175 Proposed VDS Design, FoV Coverage, Pitot/Copilot Center View. 
Figure 3.- EC175 Proposed VDS Design, FoV Coverage, Pilot Design Eye Point. 

2.81m 

1.63m 

<5.33ft) 

(5.33ft) 

Figure 4 .- Dome Screen, Side View. 
2 X 580mm 

(2 X 22.8in) 

3.25m 

(10.711) 

Figure 5 .- Dome Screen, Plan View. 

3 Current Requirements and Future Trend 

14CFR Part 60, Appendix C, Table C2A Full Flight Simulator Objective Tests, requirement 4.b calls for "Any 

geometric error between the Image Generator eye point and the pilot eye point must be 8° or less". 

Although not explicitly stated, it can be assumed this geometry error being dominated by the parallax 

error in the dome screen, plus any misalignment and distortion in the IG and projection system. 

By the other hand, the Royal Aeronautical Society, Flight Simulation Group is currently coordinating an 

international effort, with contribution from all main participants in the simulator training industry 

(regulator/qualifying authorities, helicopter and training operators, aircraft and simulator manufacturers) 

to propose an update of the ICAO document 9625, Manual of Criteria for the Qualification of Flight 

Simulation Training Devices, as a common basis for convergence of national standards and taking into 

account lessons learned from the huge amount of training time accrued to date, current technology status 

and expected evolution for the near future. Draft Volume I for Fixed Wing (Aeroplanes) has been already 

published for public review, with Volume ((for Rotary Wing (Helicopters) following now at final 

elaboration stage by the International Working Group for Helicopters (H-IWG). 

As a part of this effort, a series of systematic tests were conducted to evaluate height proximity 

perception and effect of parallax error on approaches to hover/landing in different device types 

(colllmated/dome projection), visual data base area resolution and scene content (low to high resolution, 

hi-res with plenty of familiar 3D objects) and IG shift induced visual errors. As for parallax error, 

experienced pilots/instructors posted no objection up to 109, with about 129 being acceptable provided a 
running landing was not intended, while 159 and above were consistently interpreted as some problem on 

aircraft handling or poor pilot performance. 

In accordance to these results, initial wordingof the draft doc 9625, Vol-ll, asks In the Objective test 

section for maximum parallax error of 109, center image in dome screen projection and a limit of 209 for 

pilot-not-flying when pilot-flying viewpoint selected. 

4 Training Implications 

Whilst strict compliance with 14CFR Part 60 requirement should take to dome screen size to about 4.25m 

(14.0ft) radius, following experience of both EC as training operator and Indra as simulator manufacturer, 

fully In agreement with H-IWG test results, the proposed 3.25m (10.7ft) dome screen size was selected as 

a compromise value to keep parallax error for both pilot and copilot within 14CFR Part 60 requirement for 

practical low level flight conditions (close objects, up to typical collimating distance) and limited to H-IWG 

recommendations in general flight (distant objects) when center Image viewpoint selected, still providing 

adequate margin for IG/projection system misalignment and distortion. When pilot-flying viewpoint 

selected, parallax error for pilot-not-flying (about double than for center view) is kept within the H-IWG 

recommendations. 

Figure 6 plots the estimated parallax error at the horizon line as a function of object distance up to 

practical infinity for the proposed 3.25m (10.7ft) dome screen size. See Figure 7 for comparison, showing 

parallax error in a 4.25m (14.0ft) dome screen radius and Figure 8 for a collimated system, where the error 

trends are Inverted: smaller errors for distant objects, increasing as the object Is getting closer. 

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Figure 6 .- Azimuth Parallax Error. Pilot View, Dome Screen 3.25m, Center Image. 
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Figure 7 .- Azimuth Parallax Error. Pilot View, Dome Screen 4.25m, Center Image. 

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Figure 8.- Azimuth Parallax Error. Pilot View, Collimated System €> 40m, Center Image. 

Figure 9.- Total Circular Error due to Parallax, Helicopter on Ground, Flat Terrain. 

Pilot View, Dome Screen 3.2Sm, Center Image. 
Figure 10 .- Total Circular Error due to Parallax, Helicopter on Ground, Flat Terrain. 

Pilot View, Dome Screen 4.25m, Center Image. 

-120 -103 -90 -73 -60 

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Figure 11.- Total Circular Error due to Parallax, Helicopter on Ground, Flat Terrain. 

Pilot View, Collimated System @ 40m, Center Image. 

Larger than proposed dome size should Increase error for close objects and, most of all, also degrade the 

perception of ground separation for final approach to hover, touchdown and ground handling, which is a 

common source of problems in collimated systems as the helicopter Is systematically miss-perceived to be 

high above ground level. 

See Figure 9 to Figure 11 for estimated total circular error {root sum squared Azimuth and Elevation 

errors) due to parallax for a flat terrain, being EC17S cockpit pilot DEP height about 2m (6.6ft) when the 

helicopter on ground. The proposed 3.25m (10.7ft) dome screen radius provides minimal error and 

excellent screen-eye distance match in the range -25^ to -50^ for ground view through cockpit side and 

chin windows (ref Figure 3). For the 4.25m (14.0ft) dome this optimal view range is shifted forward, giving 

somewhat degraded height perception above terrain immediate around the helicopter, while the 

collimated system, in accordance to general experience, provides quite poor performance for height 

perception in the close ground areas. 
5 Other Considerations 

5.1 Motion system 

Larger dome size means increased height of DEP above motion platform to keep the same down view 

coverage, and so total dome height and weight of cockpit, dome and upper projectors support structures. 

Higher center of gravity position, increased moving load mass and corresponding (quadratic) growing 

moments of Inertia should Impose on the Motion System payload handling requirements close or above 

limiting performances for available commercial equipment. 

Dome Radius 

Height above Motion Platform 

DEP 

Total Dome 

CoG 

3.25m 

2.8m (9.2ft) 

4.4m (14.6ft) 

>1.25m (4.1ft) 

4.25m 

3.7m (12.1ft) 

5.8m (19.0ft) 

>2.25m (7.4ft) 

Because a coordinated longitudinal/lateral displacement Is required for Pitch/Roll displacement to be 

perceived as a pure rotation by the trainee, with no parasitic linear accelerations, there is an inherent 

coupling among Pitch/Roll and X/Y axes. So, higher DEP height also reduces the effective workspace 

available for motion cueing. 

5.2 Building/Facilities 

Larger dome size increases the Motion envelope, taking the obstruction free space required to highly unpractical values. Higher drawbridge access complicates building design and emergency escape system. 

Dome Radius 

Required Free Envelope 

Longitudinal 

Lateral 

Vertical 

Drawbridge Access @ Settled 

3.25m 

13.7m (44.9ft) 

13.2m (43.3ft) 

11.0m (36.1ft) 

>4.0m (13.1ft) 

4.25m 

15.7m (51.2ft) 

15.2m {49.9ft) 

12.5m {41.0ft) 

>5.0m {16.5ft) 

5.3 Eye Strain and Ocuiar Fatigue 

When focusing at a distance closer than Infinity, the human ocular system reacts through the so called 

accommodation-vergence reflex loop. In a direct projection dome screen the Image Is formed at a 

constant distance, about the dome screen radius, so accommodation (change of eye lens focal length) and 

vergence (inward rotation of the eyes to keep the projection of the Image in the center of the retina) no 

longer contribute to depth perception {which is provided in the simulator by other different monocular 

mechanisms, mostly related to scene content and detail) but pose a risk for eye strain and ocular fatigue 

because of the involved eye muscles being permanently under stress, the closer the objects, the greater 

the strain on the muscles. 
There is not much literature on dome screen vision, but there is extensive ergonomics research available 

on computer workstations and data visualization displays, which can be considered essentially similar with 

regard to near vision fatigue. In accordance to published data, eye strain and ocular fatigue develop for a 

sustained visualization task If the viewing distance is less than the resting point of accommodation (RPA, 

default accommodation distance, the distance at which the eyes focus in total darkness, when there is 

nothing to focus on) and the restlngpoint of vergence (RPV, also known as dark vergence, the distance at 

which the eyes are set to converge when there is no object to converge on), with the stress on the 

vergence system being the dominant factor for vision strain while shifts in the resting point of 

accommodation are more related to reduction in visual acuity or keenness when switching to view a 

distant target. 

RPA averages about 0.75m (30in) increasing to about 1.0m {40in) with age. RPV averages about 1.15m 

{45in) for straight view, coming to 0.90m (35in) with a 30-degree downward gaze angle. So, dome screen 

radiuses are in both cases well beyond the viewing distance required not to develop near vision fatigue, 

with the larger dome providing no clear advantage. 

6 Conclusions 

There Is no one single Visual Display System solution being optimal from all points of view at the same 

time, so it shall always be a trade-off compromise among regulatory compliance, training aspects and 

practical simulator design considerations. 

Proposed VDS design provides an excellent coverage of EC175 cockpit fields of view and pretty good 

height perception for ground operations and low level hovers and flight tasks. While in strict sense, 3.25m 

(10.7ft) dome radius parallax error for distant objects In center view is over the current gs requirement by 

14CFR Part 60, it is still kept within practical acceptable values, and in any case, view point switching 

allows for pilot-flying error cancellation for critical flight tasks.