My new pponk has Slick dual impulse mags - and I’m replacing my old L-R-Both-Start keyed switch with a push button and two mag toggle switches. Is there any disadvantage of starting the engine with both mags on when you have dual coupled impulse mags?

Both mags having impulse couplings should be an advantage for starting. Both plugs in the cylinders will fire. imop

flyer wrote:Both mags having impulse couplings should be an advantage for starting. Both plugs in the cylinders will fire. imop

I agree.

Flyer or David,

Pawnees had two mag switches and button start. Why did the mechanic not want us to start with both on when only left had impulse? Can't remember how many times I took off on left mag. Most of the planes had impulse on both mags so just flipped both switches to start.

Jim

I may have figured it out. Mr. Bencke at Cambridge, Nebraska had the three nicest Pawnees I ever flew. He was FAA mechanic of the year in the 50s. He loaded one while I flew and they started really well. Does the non impulse mag turn easier? The 12 volt battery is back by the tailwheel and most Pawnees just barely start.

contactflying wrote:Flyer or David,

Pawnees had two mag switches and button start. Why did the mechanic not want us to start with both on when only left had impulse? Can't remember how many times I took off on left mag. Most of the planes had impulse on both mags so just flipped both switches to start.

Jim

If only one is impulsed, the non impulsed can fire before TDC as its advanced. This can cause kickback. That's the main reason.

Thanks David. Mr Bencke was a serious guy and I did what he asked. I found every field with a platte map and worried a bit. He didn't say a thing until the end of the season. He said, "You missed one field. I sent you to the wrong field."

corefile wrote:.... I’m replacing my old L-R-Both-Start keyed switch with a push button and two mag toggle switches. .....

I prefer a key type mag switch, but definitely also much prefer a separate start button.
It's nice to be able to turn the engine over with the mags off.

As far as key switch vs toggles for mags,
the key switch helps keep the honest people honest.
I also think the toggles might make it easier to accidently leave the mags on--
the act of physically removing the key from the switch seems more positive.
I remove the key when the mags are turned off,
and hang it on the altimeter knob, put it up on the glareshield, or down on the floor.

Well said, hotrod180. I occasionally got into a Pawnee with hot mags. Often I had been the pilot who left them on. Only left the key on both once in 10,000 hours in Cessnas.

contactflying wrote:Well said, hotrod180. I occasionally got into a Pawnee with hot mags. Often I had been the pilot who left them on. Only left the key on both once in 10,000 hours in Cessnas.

Chalk up another "advantage" for the Rotax engines, which are shut down by turning both mag switches to OFF, rather than pulling the mixture (no "idle cut-off" position for the throttle). One would have to turn the mag switches back ON after shutting down to get to an "unsafe" condition with the mags.

That's a really "good thing", since one of the oddities of the Rotax 912-series is that it uses a dry-sump lubrication system, and some of the oil tends to slowly seep back down into the engine sump. You have to turn the prop (sometimes as many as 10-12 blades) to pump all the oil back into the oil tank to a) properly check the oil level, and to ensure you don't blow a seal on startup. Having the mag switches OFF is imperative in that situation!

My plane uses a "key start" switch that is also the master switch for the battery... The second time I started the plane, I somehow glossed over the "mag switches ON" item on the checklist... Let's just say that even the "easy-starting" Rotax 912 doesn't start so "easy" when the mags are off... Bet I won't skip that checklist item again!

I agree, hotrod180. My mechanic taught me to leave the mags off until the engine is turning over with the pushbutton. My designated examiner on my PP check ride taught me to put the keys on the glareshield where anybody outside the cockpit (near the prop?) can see them.

Ross