Avionics Master Switches
Prior to the 1970s, very few aircraft had an avionics master switch for the simple reason that aircraft simply didn’t have many avionics to protect. Another reason aircraft from this era didn’t need an avionics master switch was because vacuum tube radios weren’t subject to damage from spikes in the charging system when the aircraft was started with the radios already turned on.
However, that all changed when tubes were replaced by silicon. If you start the aircraft’s engine with modern, solid-state radios turned on, there is a good chance you will damage the avionics. Even if it isn’t plain to see, the damage will have been done.
There are many types of avionics master switches in use today. Cessna, for example, first developed a relay that automatically removed power from the avionics bus whenever you cranked the engine or applied power to the aircraft ground power receptacle, to prevent avionics from engine-start transients. A problem presented by this configuration is that you won’t know if the circuit is working correctly or not until it is too late.
Another disadvantage is that you must use the aircraft battery to power the radios, rather than using external power. This means the aviation technician has a very limited amount of time to diagnose and address problems before the battery dies.
However, perhaps the biggest problem with the Cessna drop-out relays is this: if the battery voltage is low and you try to start the engine, the relay may fail to energize or only energize intermittently and allow voltage spikes to occur within your expensive avionics system. Many Beechcraft aircraft used an avionics master switch that works through a normally-closed avionics dropout relay, which is vulnerable to a similar problem.
State-of-the-art avionics account for up to one-third of the total value of your aircraft. To invest that kind of money in something and not have proper protection does not make economic or common sense. This is why avionics master switches are needed. Although the radios could be individually turned off prior to starting the aircraft, and therefore would not require an avionics master switch, there are many other components that do not have on and off switches.
These include the intercom, fuel computer, glidescope receiver, marker beacon, altitude encoder, HSI, flight detector, and more. The designers of these components assume they will be used in conjunction with an avionics master switch. Avionics subjected to voltage spikes will usually not show a problem immediately, but will nevertheless fail eventually.
In some aircraft, the avionics master switch is an ordinary switch located near the avionics bus. However, in some aircraft, where it is further away, a switch-breaker is needed. Further differences are found in other models where the switch powers a drop-out relay, rather than the avionics.
When installing an avionics master switch of the switch-breaker type, it is recommended that the device be rated approximately 25% greater than the maximum load the avionics could potentially draw. This will ensure protection in the event of a short. The purpose of using a switch-breaker as an avionics master switch is to protect the wiring going from the switch to the avionics bus, rather than to protect the avionics themselves.
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