In the King Air 200 and B200 (perhaps other King Air models as well), the avionics electrical bus relays are normally closed. With the Avionics Master switch in the "OFF" position—which is electrically closed—when the Battery master switch is turned on, DC power is supplied through the closed Avionics Master switch to the avionics bus relay solenoids causing these relays to open and disconnect the avionics buses from DC electrical power. Subsequently selecting the Avionics Master switch to the "ON" position opens the switch which disconnects the DC power to the avionics bus relay solenoids, allowing the relays to default to the closed position thereby supplying DC power to the avionics buses. The checklist action item for the Avionics Master switch power failure is to pull the 5 amp Avionics Master circuit breaker, which electrically opens the avionics master switch circuit thereby restoring DC power to the avionics buses.

See the following circuit diagram: enter image description here (Source: own work)

This system works, and works well, by all appearances. However, the design does seem to be somewhat counter-intuitive.

My question is, what is the purpose of designing the avionics master switching system normally closed?

Despite being counter-intuitive, I can understand that the electrical loads required by the avionics demand the use of relays, and that the design allows for the relay solenoids to be powered for a minimal time. I think this is the most reasonable explanation for the design, but I am looking for documented confirmation. Is there some other design consideration that I have overlooked? Does anyone know of a documented reason for this design?

NOTE: While you are welcome to speculate with me on this issue in the comments or chat room, I am not looking speculative answers. I am looking for an answer coming from persons familiar with the King Air electrical system and preferably backed up with documentation, such as the maintenance manual, or other sources that I have overlooked.


2 Answers 2


Design elements like this generally have to do with failure modes and how you want the system to fail when (not if) it fails. In this case, if there was a momentary or worse, permanent, loss of DC power to the relay or it failed for another reason you would want them to stay closed (and continue in the "ON") position so you would not lose your avionics (provided the power supply was some how still operational to them). Unlike most other electric components (semiconductors) relays are actually electro-mechanical components and they do have moving parts with in them that can fail. In my experience (non avionics mostly) relays have a much lower cycle lifetime than their semiconductor friends.

If we dive even a bit deeper into them, a relay is effectively an electromagnet coil that actuates a small metal rod that generally moves a copper (or similar) strip to close a switch connect. Applying a steady state voltage to a coil to generate a magnetic field (and keep the relay in a desired position) also drives current through the coil. The coils, being very thin gauge wire, act like resistors and can generate a substantial amount of heat. Due to the very thin gauge of the wire this can lead to melting and shorting of the coil. The wire also tends to be resin coated which can heat and melt as well causing a short. In turn you are correct that they don't want to he help on for a long period of time.

This is the electrical equivalent of many mechanically similar ideas in aircraft design. For example some landing gear in planes is held up using positive pressure so should the pumps fail the landing gear may simply deploy or be able to be shaken down by releasing a lock, the idea being its better it can come down in the case of a system failure than be stuck up.

  • $\begingroup$ I don't buy the idea that a solenoid would generate damaging heat with prolonged use. The King Air uses a powered solenoid for the battery circuit, both generators, and the inverter in use. Most other aircraft do the same for the master switch. $\endgroup$
    – J W
    Aug 18, 2016 at 4:45
  • $\begingroup$ You might be right that potential failure states are the consideration here, but depending on which component failed that state could vary significantly. Here, the checklist action item for a failed avionics bus is to pull the avionics circuit breaker, which should again open the circuit to the relays and restore power to the buses. $\endgroup$
    – J W
    Aug 18, 2016 at 4:49
  • 1
    $\begingroup$ I have seen many relays fail from prolonged use for all kinds of reasons, shorts and melts included. The failure state of a component is for that component only and often assumes all else is working. Chain failure (and redundancy) is an entire aspect of these designs but is beyond the scope of this question. $\endgroup$
    – Dave
    Aug 18, 2016 at 4:52
  • $\begingroup$ Pulling the avionics circuit breaker will kill the avionics (as far as I know it will not cycle them unless you close the breaker again). The breaker is in line with the +V on the aircraft and is most likely pre relay. The relay control unit may have its own breaker. $\endgroup$
    – Dave
    Aug 18, 2016 at 4:53
  • 1
    $\begingroup$ Im pretty sure the why is because that is the desired safety failure state of the system. $\endgroup$
    – Dave
    Aug 18, 2016 at 13:29

My guess:

From a functional and electrical point of view the task could be done by a single, 30A, double-pole, manual switch taking the place of the relay contacts in the circuit. One chunky switch will probably be just as reliable as a dual-feed bus, 5A switch and two relays.

I reckon that Beechcraft might have chosen this configuration so they could (i) have a lighter, smaller 5A single-pole switch on the panel and (ii) avoid routing an additional four 30A wires to that panel. This choice becomes even more sensible if a highly-reliable dual-feed bus was already available.

Disclosure: I am an electronics engineer but have no aircraft engineering experience.

  • 1
    $\begingroup$ That's a good explanation for the use of relays instead of a central switch, but it misses the reason to use normally-close relays rather than usual normally-open ones. $\endgroup$
    – mins
    Aug 18, 2016 at 11:21

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