It has been observed that when a CRT has gone through a repair process, the biasing of the internal dimming circuits may no longer be calibrated to the aircraft circuitry and in some cases when full brightness is selected an over bias condition occurs resulting in some loss of definition in the display. Another rather negative tendency is the “screen burn in effect,” where an image can be etched in the display if allowed to remain unchanged for extensive periods of time. This coupled with the effects of heat should always be considered when deciding how long a system should remain energized during a maintenance event.
CRTs can emit a small amount of X-ray radiation as a result of the electron beam’s bombardment of the phosphors. The amount of radiation escaping the front of the monitor is widely considered safe; however, they may contain toxic substances, such as cadmium within the phosphors.
At low refresh rates (below 50 Hz), the periodic scanning of the display may produce an irritating flicker that some people perceive more easily than others, especially when viewed with peripheral vision. A high refresh rate (above 72 Hz) reduces the effect. Computer displays and televisions with CRTs driven by digital electronics often use refresh rates of 100 Hz or more to largely eliminate any detectable flicker.
Liquid crystal display
The liquid crystal display (LCD) is the current panel of choice for many avionics manufacturers. LCDs are more energy efficient and offer safer handling and disposal than CRTs. Its low electrical power consumption enables use in many energy critical conditions such as backup instruments where emergency battery power may be required. It consists of an electronically modulated optical device made up of any number of pixels filled with liquid crystals and installed in front of a light source (backlight) or reflector to produce images in color or monochrome.Unlike the CRT there is no need for high voltages driving internal components resulting in lower risk of electrical shock to repair personnel. The earliest discovery leading to the development of LCD technology was the recognition of liquid crystals in the late 1800s.
In addition to low power consumption, LCDs are usually light weight and compact as compared to their predecessors. Depending on the method used for backlighting they provide little or no image flicker or geometric distortion. “Screen burn in” is also eliminated with this type of technology.
Viewing angle can cause variations in color and brightness and when left on for significant time periods without ample air circulation, partial screen overheat has been known to occur resulting in a discolored appearance of at least part of the displayed image. This is a temporary occurrence and will usually be self-resolving once cool down occurs.
Heat is only part of the dilemma. Cold will also impact proper operation. Many LCDs utilize an internal heating element which is activated any time power to the display occurs and ambient temperatures are below a certain threshold. In this situation, operation of the display may be inhibited until the internal components come up to a preset value which may take five minutes or more. This delayed startup has been misinterpreted as a failure and displays have been replaced simply because ample time was not allowed for warmup to occur.
In some cases replacement of lamps used for backlighting is an approved line maintenance function. Care should always be employed as damage can occur in the replacement process. Component or maintenance manuals should always be consulted for guidance prior to performing any actions.
Even though the use of electronic displays often provide pilots relief in the event of malfunction, the aircraft minimum equipment list (MEL) should always be consulted to determine which indications or indicators are considered critical for flight. AMT
Jim Sparks has been in aviation for 30 years and is a licensed A&P. He is the manager of aviation maintenance for a private company with a fleet including light single engine aircraft, helicopters, and several types of business jets. He can be reached at email@example.com.
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