OFF-R-L-BOTH switch

[brien23]

For those of you who have a mag switch (OFF-R-L-BOTH) that do not have the start position and only one impulse drive and the check list says start on L mag bad things can happen on starting on BOTH. Should you not be able to start on the L mag and not advance the throttle but start on the R mag and normal advance the throttle you have the mag's to mag switch wires reversed.

 

[Clip4]

For those of you who have a mag switch (OFF-R-L-BOTH) that do not have the start position and only one impulse drive and the check list says start on L mag bad things can happen on starting on BOTH. Should you not be able to start on the L mag and not advance the throttle but start on the R mag and normal advance the throttle you have the mag's to mag switch wires reversed.

The throttle position has no effect. You have two mags both set to the same spark advance prior to top dead center. One mag has an impulse coupling to provide a different advance for starting. Start on which ever mag has the impulse coupling, or better yet have the second mag upgraded with an impulse coupling an startvwith both on.

 

[brien23]

The throttle position has no effect. You have two mags both set to the same spark advance prior to top dead center. One mag has an impulse coupling to provide a different advance for starting. Start on which ever mag has the impulse coupling, or better yet have the second mag upgraded with an impulse coupling an startvwith both on.

If you try to start on the mag without impulse coupling you will not be able to advance the throttle for more rpm. Upgrade to two impulse coupling's kind of expensive and dumb instead of following the check list.

 

[hindsight2020]

...or better yet go with the siamese mag that has an impulse coupling between the shared housing and the drive, so when the coupling spring and/or plastic peripherals fail you terminally lose all ignition inflight yai!

 

[Dan Thomas]

If you try to start on the mag without impulse coupling you will not be able to advance the throttle for more rpm. Upgrade to two impulse coupling's kind of expensive and dumb instead of following the check list.

The impulse coupling kicks out at around 400 RPM. The throttle has no relation to RPM during starting. The only reason the mag switch cuts out the non-impulse mag is because that mag can generate an advanced spark as the engine begins to fire and can cause a kickback before adequate RPM is reached.

 

[Clip4]

...or better yet go with the siamese mag that has an impulse coupling between the shared housing and the drive, so when the coupling spring and/or plastic peripherals fail you terminally lose all ignition inflight yai!

Service and parts on a Bendix Dual mag can be a bit of a challenge and Lycoming is more than willing to let you convert to a non D engine at overhaul.

 

[Tom-D]

The impulse coupling kicks out at around 400 RPM. The throttle has no relation to RPM during starting. The only reason the mag switch cuts out the non-impulse mag is because that mag can generate an advanced spark as the engine begins to fire and can cause a kickback before adequate RPM is reached.

If it is turning fast enough to generate a spark, it is turning too fast to kick back.

 

[Tom-D]

...or better yet go with the siamese mag that has an impulse coupling between the shared housing and the drive, so when the coupling spring and/or plastic peripherals fail you terminally lose all ignition inflight yai!

That is not an option on the majority of Lycoming products that are found in the Pipers

 

[Dan Thomas]

If it is turning fast enough to generate a spark, it is turning too fast to kick back.

I've had my A65 kick back during hand-propping if the non-impulse mag wasn't shut off. It can happen.

 

[brien23]

Service and parts on a Bendix Dual mag can be a bit of a challenge and Lycoming is more than willing to let you convert to a non D engine at overhaul.

The impulse coupling kicks out at around 400 RPM. The throttle has no relation to RPM during starting. The only reason the mag switch cuts out the non-impulse mag is because that mag can generate an advanced spark as the engine begins to fire and can cause a kickback before adequate RPM is reached.

If your going to try to start the engine on the Right and only have a impulse on the left you won't get to 400 rpm it will only chug along less and not be able to go any faster. Should you want to try it on a start type switch (OFF-R-L-BOTH-START) ground out the left P lead on the left mag that has a impulse coupling and disconnect the right P lead from the mag that does not have a impulse coupling and try to start it.

 

[Tom-D]

I've had my A65 kick back during hand-propping if the non-impulse mag wasn't shut off. It can happen.

A-65 should have two impulse couplings.

When hand propping you need them properly equipped and timed properly.
My J-3 came from Lockhaven new, with a A65 with two.

 

[Tom-D]

If your going to try to start the engine on the Right and only have a impulse on the left you won't get to 400 rpm it will only chug along less and not be able to go any faster. Should you want to try it on a start type switch ground out the left P lead on the left mag that has a impulse coupling and disconnect the right P lead from the mag that does not have a impulse coupling and try to start it.

Why wouldn't it accelerate on either mag? There is no reason the engine shouldn't run normally on either.

 

[Dan Thomas]

A-65 should have two impulse couplings.

When hand propping you need them properly equipped and timed properly.
My J-3 came from Lockhaven new, with a A65 with two.

Mine was in a homebuilt and had only one until I finally put another impulse mag on it. Until then I simply left the non-impulse mag off until it was running.

Many of the new Skytec starters have a kickback protections system in the starter drive so that kickbacks, which can happen if a non-impulse mag fires at 25 or 30 degrees or whatever BTDC, don't rip the gears out of the starter.

 

[Dan Thomas]

Why wouldn't it accelerate on either mag? There is no reason the engine shouldn't run normally on either.

Not sure where he's coming from on any of this.

 

[GRG55]

Mine was in a homebuilt and had only one until I finally put another impulse mag on it. Until then I simply left the non-impulse mag off until it was running.

Many of the new Skytec starters have a kickback protections system in the starter drive so that kickbacks, which can happen if a non-impulse mag fires at 25 or 30 degrees or whatever BTDC, don't rip the gears out of the starter.

My Aztec has a "shower of sparks" with two sets of points, instead of an impulse coupling, in the LH "starting" mag. When the starter rocker switch is depressed it grounds/disables the main points in both the LH and RH magnetos - to prevent the potential for kick-back, as you have noted Dan.

 

[3393RP]

Problems starting the Aztec? You need a couple of these.

 

[IK04]

My C-85 equipped airplane has two separate SPST switches for the mags and starts on either one... They likely both have impulse couplings...

 

[GRG55]

Problems starting the Aztec? You need a couple of these.

You have my plane mixed up with the one in your avatar.

 

[brien23]

Not sure where he's coming from on any of this.

I started this due to several people in our club starting the planes in Both not Left as the check list required. Starting it in the BOTH could cause kickback not every time but it can happen and did causing starter damage. I bring this to the attention of this forum to save someone who might be starting their plane in BOTH the damage it could cause.

 

[Tom-D]

I started this due to several people in our club starting the planes in Both not Left as the check list required. Starting it in the BOTH could cause kickback not every time but it can happen and did causing starter damage. I bring this to the attention of this forum to save someone who might be starting their plane in BOTH the damage it could cause.

The CFI guidance is missing ??????

 

[GRG55]

I could not follow the original post. If only one impulse, start on that mag only and switch to both after the engine starts.

I had the same difficulty with the original post.

I started this due to several people in our club starting the planes in Both not Left as the check list required. Starting it in the BOTH could cause kickback not every time but it can happen and did causing starter damage. I bring this to the attention of this forum to save someone who might be starting their plane in BOTH the damage it could cause.

This is a clearer explanation of what you were trying to convey than your opening post.

 

[Dan Thomas]

Making the conscious decision to start on one mag is necessary only if the airplane doesn't have a START position on the mag switch. If it has the non-start switch (or two toggle switches) and a separate start button, you might need to start on the impulse mag only and switch to both after start. If the engine has two impulse mags there's no need to do that. A mag switch with the start position will cut out the non-impulse mag in the start position.

The impulse coupling does two thing: it retards the spark to near TDC to avoid kickback, and it winds up a spring to snap the mag's rotor forward quickly to generate plenty of electricity for a hot spark.

The Bendix dual mag that Lycoming used on some engines has one drive gear and therefore one impulse coupling. If the coupling fails, and they indeed can, the entire ignition system is in trouble. There was an issue some years ago with the impulse spring corroding and breaking, which allowed the dual mag's timing to fall to near TDC instead of the 25 or 30 degrees before top dead center, and the engine's power would fall to practically nothing. In an airplane with two separate mags the mag that didn't fail would take you home; the dual mag is entirely done when the drive quits.

Continental bought the Bendix magneto line and since their engines didn't use that dual mag, they stopped building it.