Reliability and Backup Equipment

Avionics equipment has to keep working in all conditions. An aircraft could be sitting on the ground on a hot day at more than 104°F (40°C), and a few min­utes later it could be flying through air as cold as -76°F (-60°C). Turbulent (rough) air and hard landings can shake up a plane considerably. Lightning can strike and shock a plane with millions of volts. Avionics have to work reliably through all of this.

If part of an avionics system does break down, however, it is designed to fail in a way that does not put an air­craft in danger. The most important sys­tems have at least one backup. If the main or primary system fails, the back­up takes over. There is often more than one backup, so if the first backup fails, yet another backup can take over. This is called “failsafe operation.”

The Space Shuttle has five flight computers. Four of the computers work together, and they constantly check each other. If one computer fails, the other three vote it out of the system and ignore it so that it cannot command the spacecraft to do anything dangerous. If a second computer fails, the other two can still land the Space Shuttle safely. If all four computers fail, the fifth com­puter takes over. If all five computers were programmed with the same soft­ware, they could all crash because of the same fault in their programming. The fifth computer, therefore, is programmed with different software.