Rotary switches are a staple in aviation instrument panels, and I would bet every pilot has used one. The mag switch—or ignition switch—is a perfect example of a multi-pole key-style rotary switch. Key switches like this offer a sense of familiarity for pilots. They remind us of keyed ignitions in cars but with the added benefit of doing mag checks via the LEFT-RIGHT-BOTH arrangement. Let’s compare the switch configurations for a car and plane in a table format to clarify things. Now, the (parenthesis) just means that position is momentary or that it springs back.
Car Ignition vs. Magneto Switch
An important thing to note here is that both of these switches control three circuits independently; you could call them three-pole switches. The circuits must be independent, separate poles in all positions; otherwise, they would be tied together.
What if I told you that you can control a dozen or more circuits with a single switch and have more switch positions than you will know what to do with?
Commercial rotary switches will get you all the same functionality of a Cessna key switch, except you will lose the momentary start position (and likely the key, but hear me out).
In my case, I wanted a switch design that would allow the EFII system on my Lycoming to behave similarly to a traditional magneto key switch. The main challenge is that there are more circuits to control and more switch positions than normal. Here is my attempt at a solution.
In this design, the switch controls four circuits plus the master ON/OFF function. This gives the ability to check the health of the fuel system, injectors, and ignition simultaneously during run-up. If I need to isolate a circuit further for diagnostics, the pull breakers are available. There is a separate momentary push-button for starting the engine and a built-in “ALARM” position that switches all engine functions to the redundant backup. In my specific case, the “master” switch function is already tightly integrated to the dual battery electrical system, so linking it to this switch just adds convenience. Designing a system like this seems daunting at first, but just go line by line and define how each circuit should behave.
Finished Installation
Now the entire start-up/run-up sequence is handled with just these switches.
Pros:
- Panel layout was simplified from five toggle switches to a single rotary switch.
- The user interface is familiar, and performing run-up checks can be done with two clicks.
- Added backup “ALARM” position simplifies emergency procedures and is very fast.
Cons:
- Combining five functions into one switch reduces inherent redundancy.
- Wiring the switch requires careful soldering and jumper wires.
- No longer have a physical key required for starting unless an additional “kill switch” is added.
Reflection
Redundancy and single point of failure is going to be the gut reaction response here. I get it. Consider if I have five individual toggle switches instead of the rotary switch. If any one of those switches failed, the engine will not start, so I’m not sure much has changed with this design. I will miss the physical key for safety and theft reasons, but locking the doors should do the job (and locking the hangar).
Other Use Cases
Now, don’t let the magneto switch legacy trap you, as a rotary switch could be used for many other things to clean up a panel—like lights, for example. With LEDs and their low-current draw, it becomes feasible to use a rotary switch rated for 3 or 5 amps to do some interesting things with that array of LEDs on your plane that has the local news shouting “Aliens!” every time you go for a night flight. Here is some food for thought.
Considerations for Buying a Rotary Switch
- Current Rating: The current rating must be greater than the circuits you are trying to control; 1 amp is good for control signals and relays, but lights will require a higher rating.
- Positions: The number of switch positions must be equal to or greater than the number of positions you require (usually 12 positions maximum). A stop pin can always be added to the panel to limit the switch travel.
- Poles and Decks: The number of poles and/or decks of the switch must be more than the number of circuits you wish to control. Poles and decks function in a similar way. A single-deck, two-pole switch can control two circuits. A two-deck, one-pole switch can control two circuits. Therefore, a two-pole, two-deck switch can control four circuits.
- Shorting vs. Non-shorting: Switches function differently. If you have a circuit that absolutely cannot momentarily go to OFF between switch positions (like my master bus circuit), you need a “shorting” switch, also known as make-before-break. For the lights example above, a “non-shorting” switch would be fine and only cause the lights to drop out for a few milliseconds between positions.
- Availability: Grayhill brand switches have the most available documentation and are most widely available on the resale market like eBay. Note that unless the switch is in its original packaging, the actual detailed part number won’t be marked on the switch, and you’ll have to test it with a multimeter to verify.
- Knobs: Don’t forget to select a switch knob! The possibilities are endless, and knobs for a quarter-inch shaft seem to be the most common.
Remember, this is experimental aviation at its finest. Trust but verify, or better yet, don’t trust at all and test it out before committing. Be aware when people start shouting about redundancy; pause and take a moment to dive into the actual failure modes of your system. Every switch and circuit breaker that is inline with the power to the component is a single point of failure. We are generally OK with this because we check these functions in every preflight and run-up by default. If your master switch doesn’t work, I doubt you’ll make it to the hold short line. If your fuel injector or fuel pump switch doesn’t work … well, you get the point. Now get back to designing that panel!
Rotax Switch
Since drafting this article, I have discovered the company RS Flight Systems has created a brilliant rotary switch with a key that does almost exactly as my design, specifically for Rotax engines. It is a seven-pole, seven-position, really clean design that includes a momentary (START) position. If I had a Rotax engine, I would look no further than this unit.
In this case: One switch failure does it all!
For the Plasma CD Ignitions we produce at Lightspeed Engineering, I highly recommend using independent toggle switches, one for each ignition. These rotary switches can fail and I see no good reason to have a single failure point for both ignitions (or both EFI circuits).
As i noted in the article, redundancy is often the immediate reaction when considering a design like this, and that is a fair reaction. Redundancy is a complex systems engineering challenge and deserves its own article (or series of articles). There are many considerations beyond just the number of parallel components, like the specific failure modes and whether those failure modes are detectable during pre-flight. In the case of mechanical switches—whether rotary or toggle—a failure is typically obvious during start-up or run-up, and the aircraft would be grounded until corrected.