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Mirror, mirror on the... FSTD



Introduction

The majority of Full Flight Simulators (FFSs), and some fixed-based devices, currently in service use stretched mylar mirrors as part of their collimated displays. During the last year, we at SIM OPS have been requested by several customers for help with maintaining these systems and have replaced the mirrors on a number of them. So we thought we should look at the systems, not just the mirrors, in more detail.


Brief systems overview

Before looking at the problems that can arise it might be useful to briefly look at what a typical display system comprises and why. When you are sitting in an FSTD fitted with a collimated display you are actually looking at an image reflected on a mirror. The projectors creating the image are above your head facing forwards. The image is then focused onto a screen above the crew’s heads in one of two ways;


Back projection - the image is projected into the back of the screen, in this case called a Back Projection Screen (BPS). The screen is made of perspex with a special coating that transmits the image to the front of the screen (think about a pinhole camera with grease-proof paper from your school days). This image is reflected by the main mirror which is directly in front of the crew.


Front projection - the projectors are pointed at “fold mirrors”, one per channel, mounted above the main mirror. These reflect the image onto the front of the screen mounted above the crew's heads, this case called a Front Projection Screen (FPS). This image is then again reflected by the main mirror.


Both these systems have their advocates and their detractors along with their advantages and disadvantages.


The main mirror itself is a fibreglass structure with a wide field of view in azimuth (original systems were 150 degrees but now 220 degrees is often achieved) and up to 45 degrees vertically with a radius of either 10 or 11 feet (over 3 metres). Over this is stretched a mylar mirror which is sucked into shape by a vacuum pump and associated controller to ensure the correct amount of suction and “draw” for the required curvature.


The reason for this somewhat complex light path is to collimate the light entering the crew's eyes, which means to make the light (nearly) parallel. That is to focus the eyes at near infinity and ensure the two crew get (very nearly) the same Out of The Window (OTW) view, particularly important when landing.


So what goes wrong?

First thing to make clear is while these can experience problems in service they are by most measures very reliable.


Mirror problems; We have seen two distinct categories of problems with mylar mirrors, catastrophic failures (mylar ripping) and gradual degradations.


Gradual degradation. Over time it is quite common for dust to accumulate on the surface of the mylar (and fold mirrors on FPS systems) whilst this sounds very bad it is not necessarily something to get too excited about. As long as the dusting is light, evenly distributed across the surface and the qualification light levels are still being achieved there is no need to panic. The human eye is a wonderful thing and tends to look through the dust and focus on the image displayed.


The biggest problem we have seen in this situation is when someone notices the dust and in an attempt to establish how much dust there is touches the mirror. This does two things, the covering is no longer even and if in the crew’s field of view provides a distraction that draws the eye, it also results in corrosion on the surface that has been touched due to acids on the skin. Another problem we have seen with dust is when a dust-affected mirror is deflated for transport to a new location during an FSTD re-location. While the mirror is deflated the mirror ripples and with the vibrations during the move the dust settles in these ripples; when the mirror is subsequently refitted and sucked into shape the dust is no longer evenly spread and looks like “tiger stripes”


So what can be done about dust? It is possible to “wash” mirrors. This is a highly specialised procedure, not without risk, and the results can by no means be guaranteed. It normally involves spraying de-ionised water on the surface and dabbing off with lint-free absorbent materials. However SIM OPS can propose an alternate “dry cleaning” alternative.


A final note on dust, some of the TDMs offer systems that create a positive pressure in the mirror cell aimed at eliminating the ingress of dust. Our experience is that, whilst these are helpful, they are no panacea.


Another gradual degradation is where the mirror “stretches” over time, leading to it no longer being able to achieve the required shape and distorted images. Whilst there is a tendency for a new mirror to stretch a little the main cause we have seen over time is a badly set or faulty controller over-drawing the mirror causing it to stretch. SIM OPS can help you check the mirror for distortion and if it still meets the original specification/qualification criteria, often called shooting the mirror. Unfortunately though if the mirror has stretched the only answer is to replace the mylar.


Catastrophic failures, the most common being that the mirror rips. Fortunately, this is extremely rare and we are yet to be convinced that any mirror has ever “popped” randomly in service.


The best defence against accidental mirror damage in service is discipline, making sure that anyone who enters the mirror cell knows to be careful and the dangers (to not have tools sticking out of their back pockets for example) and is briefed of the potential to get disoriented when working next to the mirror. Ensure to keep the door to the mirror cell locked!


Apart from damage caused by people inside the mirror cell, we have also known mirrors to rip due to items being dropped from above during maintenance on the projector platform above the flight deck. Murphy’s law comes into play here, any tool, nut, bolt or washer, etc dropped will impact the mirror. Similarly should the roof of your simulator bay leak it will do so immediately above an FSTD mirror causing water contamination.


We have seen on some older systems mirrors fail due to the adhesive holding the mylar in place degrading and letting the mirror slip out of shape, as with the case where the mirror stretches out of shape this does need the mylar to be replaced.


Projectors

We have come a long way with projector technology since the days of CRT three colour tubes through LCoS to the current batch of purpose-built projectors with incredible lamp lives. But with all the solutions we know there will come a day when light levels just cannot be maintained at qualification levels and refurbishing is no longer economic or, in the case of obsolescence, not possible. The good news is that projectors can be fairly easily replaced and certainly SIM OPS can achieve this for you.



How can SIM OPS Help?

As mentioned in the blog above SIM OPS has an extensive capability to assist. We have specialists that can clean mirrors and have a new “dry” cleaning capability. For Mylar Replacement SIM OPS retains a team who can quickly deploy should it become necessary to replace the mirror skin (mylar) and maintain a supply of skins ready to ship where needed. When the time comes to replace the projectors we can both provide independent advice on the current projectors on the market and replace them. Finally, if you are considering replacing your Image Generator (IG) talk to us.




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