This invention relates to aircraft flight instrument display units having a filter glass positioned in front of the display screen and more particularly to a protective cover for the filter glass. 
In modern aircraft, more and more of the flight instrument systems rely on relatively large video-type cathode ray tube (CRT) instrumentation display screens as compared to electromechanical flight instrument displays.  A filter glass having optical-quality coatings applied thereon is positioned in front of the display.  The coatings are applied primarily to reduce the intensity of reflections inherent in the aircraft cockpit environment without reducing the light transmission from the display.  The optical-quality coating can, for example, be the type specified by the Aerospace Material Specification Number AMS2521A. The specification is available from the Society of Automotive Engineers, 400 Commonwealth Drive, Warrendale, PA. 
The coating, along with the filter glass, is susceptible to scratching, chipping, or cracking, due to inadvertent bumping or abrasion during processing of the display unit on the production line, in testing or during various servicing procedures in the aircraft cockpit.  Excessive damage due to bumping or abrasion requires the replacement of the filter glass and sometimes the entire display unit, at great cost in both time and materials. 
Presently, to avoid scratching and chipping during production, testing, and servicing, a protective self-adhesive tape is applied to the filter glass.  However, the protective tape does not prevent inadvertent cracking of the filter glass itself. 
While the use of the protective tape is effective in preventing scratching and chipping, eventually, the tape must be removed and the filter glass must be carefully cleaned of adhesive without damaging the optical-quality coating.  During the course of a production cycle, various tests are performed on the display unit prior to final acceptance which requires that the protective tape be installed and removed several times, thereby increasing the chances of a mishap.  The cleaning process is tedious and labor intensive and may take approximately thirty minutes, and; even with careful cleaning, there are times when the filter glass coating is damaged in the process. 
Another problem associated with the use of the protective tape is the generation of static electricity, especially during removal of the tape.  As is well known, static electricity can be damaging to the electronic components used in CRT display units which can require the replacement of the entire display unit for servicing. 
In order to overcome the foregoing problems, the present invention provides a protective cover for a flight instrument filter glass.  The protective cover is rigid, easily installed and removed, and prevents scratching, chipping, or cracking of the filter glass during production, testing, or servicing of the flight instrument display unit. 
One embodiment of the invention contemplates a protective cover for a filter glass for a flight instrument display unit mounted in a cockpit instrument panel having a display screen mounted in an opening of the display unit.  The display unit includes a chassis and a frame-like bezel mounted on the chassis in front of the filter glass.  The bezel masks the peripheral interface between the filter glass and the display unit opening.  The bezel includes a front surface adjacent to the front of the filter glass and a rear edge spaced from the front surface adjacent to the instrument panel.  The protective cover comprises a faceplate portion having a sidewall and a lip spaced from the faceplate and extending from the sidewall in a direction parallel to the plane of the faceplate.  The space between the lip and faceplate being approximately equal to the space between the front surface of the bezel and rear edge of the bezel for retaining the faceplate in spaced relationship from the front of the filter glass. 
Further advantages and details of the invention can be had from the following description and claims taken together with the accompanying drawing.