Full flight simulators require constant supervision and regular maintenance to ensure uninterrupted operation. Visual system is one of the systems that require more frequent maintenance works due to its changeable components that get worn out through time. SimHelp engineer briefly presents how CRT was replaced in one of the BAA Training simulators.
But first …why CRT?
These projector types are usually chosen as an option for simulators because of some significant advantages. For example, it has a long service life. CRTs maintain good brightness to 10,000-15,000 hours. However, it depends on the intensity of the projected light, parameters and, of course, how often the device is turned on and off.
It has also a very fast response time (especially when compared to LCD-based projectors), resulting in minimal (if any) motion blur when showing a video with fast motion which is essential in order to ensure as realistic training experience as possible.
CRT projectors installed in the visual system of the full flight simulator has three separate CRTs and lenses which generate colour images. Overall picture on the screen is obtained by red, green and blue portions of the incoming video signals being processed and then sent to particular CRTs where those images are focused by their lenses.
To notice that something is wrong with a cathode ray tube is rather easy, if one of the colors is seen as more intensive than the others it means that one of the CRTs is functioning not properly. Even though the overall quality of the projected view is sufficient to identify even the smallest details it is still regarded as a serious defect by supervising authorities. EASA CS-FSTD (A) will not allow to certify the simulator if the maximum image brightness is less than 6ft-lamberts (20 cd/m2).
The main steps
First of all, engineers have to reduce the intensity of the desired tube in the system and stop the image generation progamme so that the new CRT will not get damaged during the replacement procedure. After reducing the intensity and stopping the image generation, it is time to turn the projector off, cut the power off and then wait for it to cool down. In the meantime, engineers have to prepare for the tube replacement.
After 10-20 minutes, engineers start removing the projector cover.
Later they remove the doors and the tube itself.
The tube needs to be handled with the utmost care as it can get easily damaged.
Later, after installing the new tube into the projector the steps are the same but in a reverse order. Engineers need to close the doors, assemble the cover, turn the power on and, finally, to calibrate the new projector tube so that it meets all the requirements.
Even though the replacement does not sound like a difficult task, it needs a lot of preparation and has to be carried out by specialists who have extensive knowledge in simulator maintenance, electronics and image generation systems.