Impulse Couplings

captain_john · Post by **captain_john** » Tue May 09, 2006 4:47 am

The Cherokee Six and the Grummann Tiger both have operating procedures to start the engine on the left mag. After noise begins, switch to both.

I am told that this is to spare the life of the impulse coupling.

Q. What does the IC do and how could it be harmed by starting on both?

CJ

Mahlon · Post by **Mahlon** » Tue May 09, 2006 6:36 am

John,
The Impulse coupling is a device to retard the mag timing to TDC for starting purposes. It also makes the mag have a very hot spark at very low rotational speeds that the engine experiences during crank over with the starter. So it’s purpose is to provide a HOT, RETARDED spark for starting purposes.
The issue is that only one mag has an impulse coupling and it is the left mag. You don't want to try and start on a mag that doesn't have retarded timing for starting. Starting on a non retarded mag can cause engine kickback against the starter and this will often break the starter.
Most ignition switches will ground the right (non impulse mag) when in the start position. On engines that have separate mag switches and start switches the procedure is to turn the left mag on and leave the right mag off when engaging the starter button. That way you prevent the non impulse mag from firing at normal spark advance and possible the associated kickback issues.
Good Luck,
Mahlon
"The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at you own risk."

Post by **Spike** » Tue May 09, 2006 7:12 am

Thanx for the info, very short and very well explained. I love this place!

captain_john · Post by **captain_john** » Tue May 09, 2006 6:16 pm

Thanks Mahlon!

I knew that it wouldn't be a concern with electronically ignited ships but I did want to understand these planes I am flying right now a bit better than the average pilot.

It makes alot more sense now!

Thanks again!

CJ

Wicked Stick · Post by **Wicked Stick** » Tue May 09, 2006 6:59 pm

Some people install 2 impulse coupled mags so they can start on both all the time. They also reason that if one impulse mechinism goes bad, they can still start the airplane safely.

It's a good rule of thumb to inspect and overhaul if necessary a Mag every 500 hours. Sometimes they need work, sometimes not, but it's a good rule of thumb that my A&P taught me. He's taken them apart in front of me to show me what/how they work, and where they can wear out, and how to check them.

That's the beauty of participating in an owner assisted annual, you can really learn about your aircraft that way.

Of course sometimes it's hard to find an A&P who wants to take the time to educate.