December 8, 2016

Mag Switches and Turn Coordinators


Mag Switches and Turn Coordinators


This month I have chosen to address two additional questions sent in by one of our readers from Golden, B.C. The first question deals with the seemingly peculiar arrangement of the mag-switch positions and the second deals with the evolution of the turn coordinator.

Both issues deal with the same basic premise of “why did they do that?” As an aside, I must confess that I often ask the same question when I see an unintuitive design. Perhaps my background in engineering psychology primes me to approach issues in this way. Anyway, let’s dig some dirt and see what we can learn about mag-switches and turn coordinators!

The venerable key type dual-mag switch has graced our cockpits for decades, but even after all these years the labeling of the switch positions still eludes many pilots. Mag switches of the type in question are labeled (left to right) “OFF R L BOTH START”.

If the issue isn’t obvious, let me spell it out; the (R)ight and (L)eft positions are seemingly on the wrong side of each other. Or are they? I suppose that depends on your perspective. Let’s consider the design from the perspective of the engineer who no doubt came up with this device for operating the magnetos. The purpose of selecting one magneto is to check their operation independently. But selecting one actually allows the pilot to check two things simultaneously: 1) that one mag shuts off (grounds out), and 2) the other mag still operates and does so within the allowed RPM drop.

In order to check the magnetos independently, one magneto is grounded while the remaining magneto runs. By design when you select the (R)ight position the left magneto is grounded, leaving the right magneto circuit open. Likewise, selecting the (L)eft position grounds out the right magneto, leaving the left magneto circuit open. In the BOTH position, both the right and left magneto circuits are open. So, relative to the magneto that is being checked for proper grounding, the R L labeling refers to the side that is still running. I suppose someone labeled it this way to remind the pilot which magneto they were monitoring when checking them independently.

So, in reality, the labeling of the conventional dual-mag switch does make sense (sort of) if we consider the full function of the switch: grounding check, and RPM drop check. Could they have labeled the switches L R instead of R L? Perhaps, but we still wind up with the question “are we grounding that side or running that side?” I’m not sure that one labeling convention is better than the other. Maybe a placard to indicate exactly what the switch does is in order!

Interestingly, many newer installations use a single magneto unit that contains both magnetos, so right and left labeling becomes slightly moot (actually, the two magnetos are still separate but contained in a common housing). In addition, the individual mags run each cylinder, not just one side of the engine. Otherwise, you’d have a pretty rough running engine, if at all!

Turn coordinators: Why is it that turn coordinators (TCs) seem to have replaced the turn & slip indicator in recent years? Answer: Actually, TCs have been replacing some turn & slip indicator installations since the early 70’s but you can still buy the old ball & needle type indicator (brand new) from several instrument companies. It’s just that the TC has become the preferred instrument as a result of its increased capabilities over that of its predecessors.

Once called the turn & bank indicator (for reasons yet unclear since it didn’t indicate bank!) the turn & slip indicator was the conventional rate of turn and slip/skid indicator until the early 1970’s. About this time someone decided that the single axis autopilot devices for light aircraft (wing levelers, etc.) needed a better source of yaw rate than the conventional turn & slip indicator.

Since bank is inherently tied (or coupled) to yaw, it takes little imagination to guess what the engineers came up with. By taking the horizontal gyro of the turn & slip indicator and canting it about 30 degrees the new instrument could sense not only yaw but bank. This gives the autopilot a measure of impending yaw more quickly than the actual yaw.

For example, you have to be in the process of yawing to have a yaw rate to measure and react to, but the existence of a roll rate can precede the existence of a yaw. Since the roll will lead to a yaw more times than not, this gives the autopilot advance notice and helps keep corrections small.

The turn coordinator was also designed with better dampening that helps the autopilot better react to established rates rather than fleeting changes in rate of yaw or roll as a result of turbulence.

Interpreting the TC requires a little extra care than with the older turn & slip indicator. While the turn & slip indicator sensed only yaw (e.g. heading changing) the TC will indirectly indicate a rate of roll while the angle of bank is changing and then settle to whatever yaw rate is occurring once the angle of bank is constant.

Some pilots think this confuses the need for control input (rudder vs. ailerons), but I find it easier to fly the TC in partial panel situations. However, I’m not aware of any research that makes a case for either instrument while partial panel. Whether or not you like the TC, if you don’t have an autopilot that relies on it, you might have the option to replace it with the older style turn & slip indicator. You can buy them new and if there’s a model approved for your aircraft it can usually be easily retrofitted.

I’ve heard pilots complain about their TC, saying that it’s sloppy in bumpy weather and partial panel is almost impossible. I’m real skeptical when I hear these sorts of complaints about the TC. Although it may be the pilot preference issue cropping up again, I’ll bet money that the real cause of this sort of complaint is a bad TC.

I say this because I’ve experienced this problem first hand and the fix was installing a new TC. Perhaps the dampening mechanism goes bad! In any case, if you’ve never flown with a new TC, and therefore have no comparison, any judgments about what it should be doing are relative.

One thing that didn’t change with the redesign of the turn & slip indicator was the inclinometer (ball). The ball is separate from the rest of the instrument and still provides the same slip/skid info in a TC as it did for the earlier instruments. Luckily, no one has come up with a reason to change that part yet, although I’ve noticed that it’s a little smaller in newer instrument.

So, intuitive design failures not withstanding, there are good reasons for both the way the mag switch is labeled and for the evolution to the TC. I can’t say that this is always the case, but a little systems knowledge goes a long way to clearing up these sorts of mysteries. Challenge yourself to learn why things are the way they are!

That’s all the dirt digging we have for this month. If you find this sort of article interesting, send me your question(s) to and I’ll be happy to consider them for upcoming months.

This month’s Pilot Primer is written by Donald Anders Talleur, an Assistant Chief Flight Instructor at the University of Illinois, Institute of Aviation. He holds a joint appointment with the Professional Pilot Division and Human Factors Division. He has been flying since 1984 and in addition to flight instructing since 1990, has worked on numerous research contracts for the FAA, Air Force, Navy, NASA, and Army. He has authored or co-authored over 180 aviation related papers and articles and has an M.S. degree in Engineering Psychology, specializing in Aviation Human Factors.