Why start on Left Mag only?

jd21476

jd21476

Line Up and Wait
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jd21476
So Ill admit that when I bought my Piper Comanche a few months back I did not read the POH cover to cover. I finally got around to doing so this week and one interesting thing is that it says to start the engine on the LEFT mag and then go to both. Why? I was starting on BOTH prior to this.
 
Timing on the left mag is retarded for starting, starting on both may result in kickback as the right mag will fire too early.
 
That explains the kickback that I got once or twice. So I will start on the left from now on
 
Not even sure how this possible. It goes L-R START. Not sure how it could be in left mag only and then in start? Makes no sense.


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Tommar98 said:
Not even sure how this possible. It goes L-R START. Not sure how it could be in left mag only and then in start? Makes no sense.
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there are jumpers on the back of the switch... those allow you to configure which mags are active in the start position.
 
Tommar98 said:
Not even sure how this possible. It goes L-R START. Not sure how it could be in left mag only and then in start? Makes no sense.


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Click to expand...
I think what you are missing is that on some planes the starter is not part of the mag switch.
 
jd21476 said:
So Ill admit that when I bought my Piper Comanche a few months back I did not read the POH cover to cover. I finally got around to doing so this week and one interesting thing is that it says to start the engine on the LEFT mag and then go to both. Why? I was starting on BOTH prior to this.
Click to expand...
Seems like this detail would be mentioned on the checklist for starting the engine.
 
Tommar98 said:
Not even sure how this possible. It goes L-R START. Not sure how it could be in left mag only and then in start? Makes no sense.
Click to expand...

A lot of older airplanes have a starter switch separate from the magneto switches.

As @Stewartb mentioned above, most start switches ground one magneto (usually the right one) while the starter is engaged.

In some airplanes the starting magneto (usually the left one) will have two sets of points instead of an impulse coupling. One set of points is retarded timing and used for starting. The other set are the main points, which will also be grounded by the starter switch whenever it is engaged.
 
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Tommar98 said:
Not even sure how this possible. It goes L-R START. Not sure how it could be in left mag only and then in start? Makes no sense.
Click to expand...

Nope. OFF-R-L-BOTH
Electrical_Ignition-Switches_ACS-KEYED-IGNITION-SWITCHES.jpg
 
My Navion in the day had one like the above. The start was a second toggle (I believe this is how they came from the factory). That old key switch was pretty shot and when I replaced it, I got one that had the start position.

That worn switch was one of the reasons I'll always treat mags as hot. You could test this switch all you want to see if the mags grounded out, but after you removed the key, it would unground one. The mechanic found this the hard way when working on the mags and having the thing discharge into her hand.


My buddy's Waco has a button on the floor like an old car and some of the older Cessna have a Pull knob that is the start (both a switch and a thing to move the starter in position).
 
GRG55 said:
A lot of older airplanes have a starter switch separate from the magneto switches.
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Mine does, but the owner's manual says to start on both. Impulse couplings on both sides.
 
flyingron said:
My Navion in the day had one like the above. The start was a second toggle (I believe this is how they came from the factory)...
Click to expand...

That's how my 1961 Cherokee 160 was as well; it had a black momentary contact push button switch for the starter.

My 1979 Aztec has 4 individual toggle switches for the magnetos and a separate two-way rocker with both momentary contact switches in one assembly for the starters.
 
Stewartb said:
And for you newbies who are scratching your heads? Most of us with conventional L/R/Both/start key switches have the switch wired to ground one mag in the start position so the key switch does the mag selection for you.
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I was, indeed, scratching my head thinking the same thing. 'cause in the airplanes I have flown (not many), you have to go past R/L/Both to get to start... I was thinking - how the hell do you go from L to start without passing both. This explained it and all I had to do was keep scrolling down!
 
Deelee said:
I was, indeed, scratching my head thinking the same thing. 'cause in the airplanes I have flown (not many), you have to go past R/L/Both to get to start... I was thinking - how the hell do you go from L to start without passing both. This explained it and all I had to do was keep scrolling down!
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It's a Piper thing,, be certain to check the ignition switch AD
 
That explains it! I want to be a piper guy.... love the Arrow... but for now I’m a Cessna guy.
 
Tommar98 said:
Not even sure how this possible. It goes L-R START. Not sure how it could be in left mag only and then in start? Makes no sense.


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Click to expand...
Mine is RIGHT-LEFT-BOTH then you push a button to start
 
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