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What Happened to My Map?

By Tim Shaver
Reprinted with permission from FAA Safety Briefing

While an inoperative landing light or flap failure can certainly add anxiety to your flight, a primary flight display (PFD) or multi-function display (MFD) screen that suddenly goes blank can be even more distressing. While these systems are much more reliable than the mechanical instruments they replace, as more aircraft migrate to glass-cockpit integration (some condensing nearly the entire instrument panel into a single display), pilots and mechanics will need to maintain a keen awareness of how to recognize and troubleshoot avionics component failures.

Here is the good news. Today’s display systems are designed to gracefully degrade when a failure occurs. If one display fails, the remaining display will revert to a mode that combines critical information from both displays on a single screen. This helps prevent the system from displaying hazardous or misleading information to the pilot. However, for mechanics to successfully troubleshoot these failures it is critical that they understand the system design as well as the role the individual components have in the overall system. For example, an air-data computer malfunction could result in the display’s failure to present airspeed, altitude, vertical speed, outside air temperature, and true airspeed.

What Happened to My Map?

In some systems, a loss of information will be communicated to the pilot through a large red “X” over the inoperative indicator. In more advanced systems, a central maintenance system can provide detailed failure information that pinpoints the problem to a specific component or, in some cases, to an individual wiring problem. A component built-in test, known as “bite” capability, has become a highly reliable troubleshooting tool. However, don’t let these advances fool you into thinking that avionics troubleshooting is now as simple as pushing a button. Knowledge of system operation and integration is essential, especially when there is a system problem and no associated component faults are found.

Because today’s display systems are designed to eliminate hazardous or misleading information, some display problems may occur not as a result of a component failure, but because a system input is unavailable or invalid. For example, some systems may use valid GPS data as a critical input for the display of attitude, moving map, own-ship position, and heading. If the GPS input is not valid, the system will not display this information. Many situations external to the aircraft could cause the GPS signal to be invalid. These could range from planned GPS outages (typically covered by a NOTAM) to electromagnetic interference or sunspot activity.

Avionics display systems have evolved to provide more detailed information than ever, and much of it is crucial if we are to realize the improvements envisioned in the Next Generation Air Transportation System. Today’s avionics mechanics have access to excellent troubleshooting tools that are integrated into the aircraft’s avionics suite. This can provide quick and accurate problem identification. However futuristic and easy these systems are when they function properly, it is understanding the overall system operation, architecture, and integration that is the key to properly address advanced avionics problems.

Tim Shaver is Avionics Maintenance Branch Manager in the FAA’s Aircraft Maintenance Division. He is a commercial pilot and holds A&P certificate.