Avionics Technology: Flight Displays

Enhancing situational awareness has long been a desirable circumstance in the world of aviation while providing the flight crew with necessary and wanted information has been a constant challenge for designers. Available space for equipment and instruments is almost always at a premium.

Minimum requirements are most often dictated depending on the basis for airworthiness. The majority of small single engine aircraft, at a minimum, will have an airspeed indicator, altimeter and a compass. Living in the information age causes many to strive for more data and given technological advancements in recent times provides availability of precision navigation along with enhancements to protect against controlled flight into terrain (CFIT) as well as in-flight collisions.

Throughout the first 70 years in aviation anytime new technology was to be made available for flight deck viewing, an evaluation had to be conducted to assess available real-estate to locate the new instrument. In some situations it became necessary to combine displays in one device.

A well-equipped vintage machine may have every square inch of instrument panel occupied by some type of gage or switching device and in the world of transport category aircraft, redundancy has always been an important facet requiring duplication. In an electro mechanical environment swapping inputs to flight deck instruments requires significant engineering and finesse to accommodate the wide array of analog gizmos and gadgets. 

Technological breakthroughs

The past 30 years has yielded significant technological breakthroughs in the realm of avionics. Electronic flight instrument systems started to appear and were fed digital data.  It was realized early on they would revolutionize the flight crew’s ability to increase awareness plus reduce wiring along with ancillary components.

The concept enables flight crews to not only view required information but to also choose what supplemental data can be highlighted. Information transfer capability can provide immediate relief in the event of a single display failure. When reversionary modes are used it is often possible to transfer information from a failed indicator to a second display where the operator can choose to observe multiple or composite indications.

Cathode ray tube

A cathode ray tube is a vacuum tube which consists of one or more electron guns, possibly internal electrostatic deflection plates, and a phosphor target. The entire front area of the tube is scanned repetitively and systematically in a fixed pattern called a “raster.” An image is produced by controlling the intensity of each of the three electron beams, one for each primary color (red, green, and blue) with a reference signal.

In all modern CRT displays as well as televisions, the beams are deflected using varying electric fields produced by coils and driven by electronic circuits within the case. The brightness, color, and persistence of the illumination depends upon the type of phosphor used on the CRT screen. Phosphors are available with persistence ranging from less than one microsecond to several seconds. For visual observation of brief transient events, a long persistence phosphor may be desirable. For events which are fast and repetitive, or high frequency, a short-persistence phosphor is generally preferable.

Color tubes use three different phosphors which emit red, green, and blue light and are packed together in stripes or clusters called "triads." Color CRTs have three electron guns, one for each primary color, arranged either in a straight line or in a triangular configuration (the guns are usually constructed as a single unit). A grille or mask absorbs the electrons that would otherwise hit the wrong phosphor. This type of display does have a tendency to degrade with age and over time will loose luminescence resulting in costly replacement. Display intensity can be controlled either by an external dimming circuit or by an ambient light sensor.

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 and 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. The phenomenon of “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 anytime 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 warm up to occur.

Replacement

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.

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 protected].