Exemption No. 10521 

UNITED STATES OF AMERICA 

DEPARTMENT OF TRANSPORTATION 

FEDERAL AVIATION ADMINISTRATION 

WASHINGTON, DC 20591 

 

In the matter of the petition of 

 

AMERICAN EUROCOPTER Regulatory Docket No. FAA-2011-0824

 

for an exemption from 14 CFR 

part 60 Appendix C, Table C2A 

Title 14, Code of 

Federal Regulations 

GRANT OF EXEMPTION 

 By letter dated August 4, 2011, Mr. Tim McAdams, Manager, American Eurocopter, 2701 

Forum Drive, Grand Prairie, TX 75052 petitioned the Federal Aviation Administration (FAA) 

on behalf of American Eurocopter Flight Simulation Center (AEFSC) for an exemption from 

part 60, Appendix C, Table C2A, 4.b.3 of Title 14, Code of Federal Regulations (14 CFR). 

The proposed exemption, if granted, would permit a geometric error of up to 12 degrees 

between the image generator (IG) eye point and the pilot eye point for the Eurocopter (EC) 

175 Level D full flight simulator (FFS). 

The petitioner requests relief from the following regulation:

Part 60, Continuous Field-of-View (FoV) tests as stated in Table C2A paragraph 4.b.3, 

require any geometric error between the image generator eyepoint and the pilot 

eyepoint to be 8° or less. 

The petitioner supports its request with the following information:

The petitioner states 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°. 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. The petitioner believes 

AFS-11-413-E 
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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. 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 

Technical Note 

Visual Display System (VDS) 

Parallax Error & Dome Screen Size. 

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’s technology status, 

the only feasible solution for FoV in excess of 210o x 60o 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 VDS error is the so called 

parallax error to distant objects because of different crewmember and Image 

Generator/Projection System eye point reference positions. The VDS 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 (the inability to detect the difference in the apparent position 

of an object viewed along two different lines of sight) for distant objects might be reduced by 

increasing dome screen radius or specifically the dome size. While this might reduce parallax 

error, there is a propensity to increase the error for closer objects and degrade height 

perception on the ground, which also has 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 VDS 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. The current proposed design is presented 

together with the 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. 

Proposed EC175 VDS Design 

Real image, front projection spherical dome. 

Internal screen radius: 3.25m (10.7ft). 

Physical screen size: Horizontal 240o (±120o) 

 Vertical 90o (+30o up, -60o down) 

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

 Horizontal >210o (±105o) 

 Vertical >80o (+25o up, -55o down) 

Pilot/Copilot Design Eye Point position: 

±580mm (22.8in) right and left of centerline. 

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

Design Eyepoint ((DEP) i.e. mid-point between pilot eyepoint (PEP) and copilot eyepoint 

positions (CPEP)) and Figure 3 from PEP. See Figure 4 and Figure 5 for dome screen main 

dimensions. 

Figure 2 ‐ EC175 Proposed VDS Design, FoV Coverage, Center or Design Eyepoint (DEP)

Figure 3 ‐ EC175 Proposed VDS Design, FoV Coverage, Pilot Eye Point (PEP)
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Figure 4  ‐ Dome Screen, Side View.

Figure 5  ‐ Dome Screen, Plan View.

Current Requirements and Future Trend 

In 14 CFR 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 eyepoint 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. 
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On 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 International Civil Air 

Organization (ICAO) document 9625, Manual of Criteria for the Qualification of Flight 

Simulation Training Devices. This is being done to demonstrate a common basis for 

convergence of national standards and to take 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 II 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. These tests evaluated 

height proximity perception and effect of parallax error on approaches to hover/landing in 

different device types (collimated/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 10o, with about 12o being acceptable provided a running landing was not intended, while 

15o and above were consistently interpreted as some problem on aircraft handling or poor 

pilot performance. 

In accordance to these results, initial wording of the draft doc 9625, Vol.-II, asks in the 

objective test section for a maximum parallax error of 10o, center image in dome screen 

projection and a limit of 20o for pilot-not-flying when pilot-flying viewpoint selected. 

Training Implications 

Strict compliance with the 14 CFR part 60 requirement would take the dome screen size to 

about 4.25m (14.0ft) radius. The proposed 3.25m (10.7ft) dome screen size was selected as a 

compromise value to keep the parallax error for both pilot and copilot within H-IWG 

recommendations for both practical low level flight conditions (close objects) and general 

flight (distant objects) with DEP selected. This would still provide adequate margin for 

IG/projection system misalignment or distortion. With a pilot-flying viewpoint (PEP, or 

CPEP) selected, parallax error for pilot-not-flying (about double than for DEP selection) is 

also 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.

Figure 7 ‐ Azimuth Parallax Error. Pilot View, Dome Screen 4.25m, Center Image.

Figure 8 ‐ Azimuth Parallax Error. Pilot View, Collimated System @ 40m, Center Image.
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Figure 9 ‐ Total Circular Error due to Parallax, Helicopter on Ground, Flat Terrain.  

Pilot View, Dome Screen 3.25m, Center Image.

Figure 10 ‐ Total Circular Error due to Parallax, Helicopter on Ground, Flat Terrain.

Pilot View, Dome Screen 4.25m, Center Image.

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

Pilot View, Collimated System @ 40m, Center Image.
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Error for close objects should increase if a larger dome size is used other than what is 

proposed 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 errors (root sum squared Azimuth and 

Elevation errors) that are due to parallax for a flat terrain viewed with an EC175 cockpit pilot 

DEP height of 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 -25o to -50o 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. 

Other Considerations 

Motion System: 

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

down view coverage. As a consequence, the total dome height and weight, including 

projectors and dome support structures, increases the requirements or demands on the motion 

system. The higher center of gravity position, increased moving load mass and corresponding 

(quadratic) growing moments of inertia imposed on the motion system payload handling will 

drive the requirements close to or above limiting performances for available commercial 

equipment. 

Dome Radius 3.25m 4.25m 

Height above Motion 

Platform 

DEP 2.8m (9.2ft) 3.7m (12.1ft) 

Total Dome 4.4m (14.6ft) 5.8m (19.0ft) 

CoG >1.25m (4.1ft) >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. 

 
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Building/Facilities: 

Larger dome size increases the motion envelope, taking the obstruction free space required to 

highly un-practical values. Higher drawbridge access complicates building design and 

emergency escape system. 

Dome Radius 3.25m 4.25m 

Required Free Envelope 

Longitudinal 13.7m (44.9ft) 15.7m (51.2ft) 

Lateral 13.2m (43.3ft) 15.2m (49.9ft) 

Vertical 11.0m (36.1ft) 12.5m (41.0ft) 

Drawbridge Access @ Settled >4.0m (13.1ft) >5.0m (16.5ft) 

Eye Strain and Ocular 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). However, it does 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 resting point 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. 
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Conclusions 

There is no single VDS solution that is optimal from all points of view at the same time. As a 

result, there will always be a trade-off to achieve regulatory compliance, while meeting 

training aspects and practical simulator design considerations. 

The proposed VDS design provides an excellent coverage of EC175 cockpit FoV 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 8o requirement by 14 CFR 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. 

The petitioner believes 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. The petitioner states a grant of exemption would result in a better trained pilot. 

Better training enhances safety and safe helicopter operations are a substantial benefit to the 

public as a whole. 

A summary of the petition was published in the Federal Register on October 4, 2011 

(76 FR 61473). No comments were received. 

The FAA's analysis is as follows:

The FAA has fully considered the petitioner’s request and supporting information. The FAA 

finds that a grant of exemption would be in the public interest and would provide a level of 

safety equivalent to that provided by the current regulations. 

The FAA finds that there is no single visual display system (VDS) solution that is optimal, for 

purposes of helicopter simulator visual systems, from all points of view at the same time. As 

a result, there will be a trade-off to achieve regulatory compliance and training capabilities, 

while meeting practical simulator design considerations. In a strict sense, the computed 

geometric error for distant objects in center view may exceed the current 8° requirement by 

14 CFR part 60 for typical VDS dome sizes of wider bodied helicopter simulators (such as the 

Eurocopter EC175). It is recognized, however, that the geometric error may still be kept 

within subjectively acceptable values through optimizing the design eye point position and 

view point switching that allows for pilot-flying error cancellation for critical flight tasks. 
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As the petitioner has remarked, the FAA is aware of and has participated in the ongoing work 

of the Royal Aeronautical Society in proposing updates to the ICAO 9625 standards for 

helicopter simulators. While the ICAO document has not yet been completed, the FAA 

agrees with the conclusion that minor deviations from the 8° parallax error tolerance will have 

minor effects that are limited to specific training tasks which may be susceptible to parallax 

error (such as running landings). These effects can easily be mitigated through eye point 

switching and/or training task restrictions on flight simulation training device (FSTD) use as 

determined through subjective assessment. 

Furthermore, the FAA also agrees with the petitioner’s argument that height perception of 

objects in close proximity is a common complaint of FSTD users with collimated VDS’s 

(required for FSTDs qualified prior to part 60 becoming effective) and could also be 

problematic on a direct view VDS with a large dome radius. Since much of a helicopter 

pilot’s eye is focused on objects in close proximity during critical training maneuvers (such as 

hover and landing), improvements in height perception cues are likely to improve the overall 

training experience and are worth the relatively minor tradeoff in geometric error. 

The FAA agrees with the assertion that allowing a geometric error greater than 8° will allow 

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

operations. Improved training can significantly enhance safety. 

The FAA’s Decision 

In consideration of the foregoing, I find that a grant of exemption is in the public interest. 

Therefore, pursuant to the authority contained in 49 U.S.C. §§ 40113 and 44701, delegated to 

me by the Administrator, American Eurocopter Flight Simulation Center is granted an 

exemption from 14 CFR part 60, Continuous Field-of-View tests as stated in Table C2A 

paragraphs 4.b.3 to the extent necessary to allow (AEFSC) to manufacture and obtain FAA 

qualification of its Level D FSTD. This grant of exemption is further subject to the following 

conditions and limitations: 

Conditions and Limitations 

1. A geometric error of no greater than 12° is allowed subject to evaluation by a 

National Simulator Program (NSP) Aviation Safety Inspector by conducting ground 

and flight maneuvers to validate the geometric/parallax error does not negatively 

affect qualified training, testing, or checking tasks. 
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2. If the objective/subjective testing is successful, the NSP Inspector will make a 

recommendation to the National Simulator Program Manager (NSPM) for final 

approval.

3. Any training maneuver found to not be subjectively acceptable due to 

geometric/parallax error will be task restricted on the FSTD’s Statement of 

Qualification as determined appropriate by the NSPM.

This exemption terminates on May 31, 2014, unless sooner superseded or rescinded. 

Issued in Washington, D.C., on April 30, 2012. . 

/s/ 

John S. Duncan 

Deputy Director, Flight Standards 

 Service