ladesaparecida

Hello,

Rarely in aviation design do things get done by accident - they are usually done for a good reason. With this in mind, can anyone shed any light on why the ignition key switch (on a C172 SP at least) is laid out:

Right - Left - Both

Instead of going from Left - Right - Both as logic would suggest.

Thanks for your input.

Planespotta

When you turn the key to the "right" position, you are grounding out the left magneto; conversely, turn the key to the "left" position, and the engine is running on the left magneto with the right one grounded out through the P-lead.
Hope this helped

ladesaparecida

Thank you -

Explains what is going on with the magneto test - that you are effectively testing if the engine will run effectively on just one magneto - but the question remains -

why not have the ignition switch go in order from left to right instead of the:

Right, Left, Both, Start

Why not:

Left, Right, Both, Start

Thanks.

Planespotta

Ignore . . . .

rickair7777

I'm not sure there is a good reason. Maybe they just had to pick one. Maybe the engineer had never been in the military...a military guy would ALWAYS start with the left

There is no real operational difference between the right and left...they both fire all cylinders.

One mag powers the top spark plugs, the other the bottom plugs so I suppose that might make a difference?

Zayghami

I understand what your trying to say and now Im wondering why they do that....

Planespotta

A normal switch is designed so that it turns a device on through completing a circuit. Conventionally, this would be left to left and right to right.

However, for an aircraft ignition switch, we have an exception. We complete the circuit to turn the magneto off by grounding the P-lead. So "R" is on the left because that position actually completes the circuit for the left mag, turning it off. Same thing for the "L" position, which completes the circuit for the right mag. Switching these around would actually not work!

I know it may not seem logical . . . but neither does having FAs reported 3 times a day (or being abbreviated as "FA" instead of "AF")

Mitragorz

So when the key is on "R" the plane's running off the right mag.

Switching them would totally work! Switch the lables and switch the wiring, so that on "R" it's running on the right mag. Same exact thing, just worded differently. Instead of saying that the left mag is off, say that the right mag is on.

Instead of saying "R" (with the label on the left) completes the left mag's circuit, say that when on "R" (with the label on the right), the plane is RUNNING on the right mag.


Switch the labels and the wiring and it becomes logical. Move the current "R" wiring setup and label to the current "L" position and vice-versa.

250 or point 65

remember, you are also testing to make sure that you can effectively ground each magneto. if there is no drop in rpm, then you may have a dangerous situation if you try to rotate the prop after the flight.

Kilgore Trout

This caught my eye, don't mean to be a persnickety A+P, and I'm not going to vouch for all installations, but on all the horizontally opposed engines I've worked on this ain't true.

An example- an IO-520's Right mag runs the top right plugs, and bottom left plugs. Left mag runs top left and bottom right.

If for some reason you replace a wiring harness, the easiest way to avoid screwing this up is to search out a wiring diagram for your exact installation.

Good advice above in that you want to make sure there is a drop in rpm when doing mag checks, if you don't get one, could mean the magneto or magnetos are not grounded, and when turning a prop by hand to do stuff like hook up a tow bar, could lead to a mishap.

Another bit of unsolicited advice- never turn the master on with people or objects in or near the prop arc. A sticky starter solenoid can cause the prop to turn just as if you'd hit the starter intentionally. Not fun. Seen it happen. Nobody hurt but they got scared pretty bad.

Cheers,