Impulse coupling problem?

[Skip Miller]

Help with this problem, please. The club has a Piper Arrow that is hard to start cold. Many of our pilots flood it while trying to start the plane, and then must go through the flooded start procedure. Yesterday I walked onto the tarmac at this point - just as the pilot was about to try to start it again. One cylinder was firing regularly, but without enough "kick" to light off the other cylinders. Old Crusty, present as usual, suggested letting off the key immediately after a "kick" and sure enough the engine stumbled into life, obviously rich. Once smoothed out and running, the plane flies perfectly, and passes mag checks both on the ground and at full power in the air. Letting the key return to "both" engages the other mag and it was able to start successfully.

I know that with the key in the crank position, one mag is grounded and the one with the impulse coupling is operating.

Is there a failure mode whereby one or more of the plugs will not fire when the impulse coupling is operating, but all work normally when rpm builds up and the impulse coupling is not operating? Or is this really a pilot technique issue. I haven't flown that plane so I can't give a first hand report.

Thanks in advance.

-Skip

 

[Tom-D]

It's the mag switch not the mag or the impulse coupling.

The switch is not opening the "P" lead circuit on the start position. but it wiil when the switch is released. then the engine starts

 

[Skip Miller]

It's the mag switch not the mag or the impulse coupling.

The switch is not opening the "P" lead circuit on the start position. but it wiil when the switch is released. then the engine starts

Please tell me a bit more. As I understand it:

Two mags: the one without the impulse coupling should have the P lead grounded during start, open on "both".

The one with the impulse coupling should be open for "start" and "both" key positions.

So if, as you suggest, the P lead on the impulse coupling mag is grounded during start, then there should be no spark activity from either mag... but clearly one cylinder is firing. That's the clue I was chasing down. Why just one cylinder? What could cause that?

-Skip

 

[Hiperbiper]

You might check to see the right mag position on the switch is indeed grounding the RIGHT mag...

A Lance here had the same problem you describe; turned out "nobody" (you know; the person who does all the stupid stuff... ) had the P leads switched.

When in the start position the left mag (the one with the impulse coupling) was being grounded. The right mag was unable to start the engine reliably.
Letting the key go would un-ground the left mag and KABOOM! fire in the Hole(s)!

It might not be your problem but it's cheap and easy to check...

Chris

 

[Tom-D]

Please tell me a bit more. As I understand it:

Two mags: the one without the impulse coupling should have the P lead grounded during start, open on "both".

The one with the impulse coupling should be open for "start" and "both" key positions.

So if, as you suggest, the P lead on the impulse coupling mag is grounded during start, then there should be no spark activity from either mag... but clearly one cylinder is firing. That's the clue I was chasing down. Why just one cylinder? What could cause that?

-Skip

In the start position the "P" lead on the mag with the impulse coupling is open. the mag without the impulse coupling is grounded.

In the Both position both are open.

If the engine kicks back on the starter, the mag without the impulse coupling is fireing at advanced position. (P lead is open during start)

When (as you said in your first post) the engine starts when you reliece the starter, the switch has the impulse coupled mag grounded during start. or a very bad ground allowing the mag to fire randomly as the "P" lead opens and closes. but opens the "P" lead as you reliece the switch. and the engine starts.

This is a very common problem with Piper key start Ign switches.

 

[etsisk]

Reading the subject line, I thought this was about something else ENTIRELY.


at least, I HOPED it was!

 

[Skip Miller]

This is a very common problem with Piper key start Ign switches.

Thanks, Tom. I'll squawk it accordingly.

-Skip

 

[Henning]

Help with this problem, please. The club has a Piper Arrow that is hard to start cold. Many of our pilots flood it while trying to start the plane, and then must go through the flooded start procedure. Yesterday I walked onto the tarmac at this point - just as the pilot was about to try to start it again. One cylinder was firing regularly, but without enough "kick" to light off the other cylinders. Old Crusty, present as usual, suggested letting off the key immediately after a "kick" and sure enough the engine stumbled into life, obviously rich. Once smoothed out and running, the plane flies perfectly, and passes mag checks both on the ground and at full power in the air. Letting the key return to "both" engages the other mag and it was able to start successfully.

I know that with the key in the crank position, one mag is grounded and the one with the impulse coupling is operating.

Is there a failure mode whereby one or more of the plugs will not fire when the impulse coupling is operating, but all work normally when rpm builds up and the impulse coupling is not operating? Or is this really a pilot technique issue. I haven't flown that plane so I can't give a first hand report.

Thanks in advance.

-Skip

Actually, not uncommon, and probably not a mag/impulse couple problem. First, swap the p-leads and see if it works better. If no, the switch has gone bad.

 

[John Reed]

Just make sure that the mags, especially the right one, are timed exactly to the engine. I also recommend that a 500 hour inspection of the magnetos be accomplished if it's been over 500 hours since the last inspection.

 

[Tom-D]

Just make sure that the mags, especially the right one, are timed exactly to the engine. I also recommend that a 500 hour inspection of the magnetos be accomplished if it's been over 500 hours since the last inspection.

I have seen several people state that mags should be inspected at 500 hours. what is the criteria of a 500 hours mag inspection? where do you get this inspection from?

 

[Witmo]

I have seen several people state that mags should be inspected at 500 hours. what is the criteria of a 500 hours mag inspection? where do you get this inspection from?

The mag manufacturers publish service bulletins with inspection criteria. Basically the internal components are checked and the magneto is internally timed (E-Gap set). The mags are then installed and timed to the engine.

 

[Tom-D]

The mag manufacturers publish service bulletins with inspection criteria. Basically the internal components are checked and the magneto is internally timed (E-Gap set). The mags are then installed and timed to the engine.

What you stated requires a complete disassembly of the magneto, Timing "E" gap and setting the lead or lag requires special tools and equipment not found outside of a repair facility. Repair facilites will not do this unless they are doing a complete overhaul of the mag. IOW what has been suggested in this thread is, we time life the mag at 500 hours, because the shops in the field will not service mags to this extent, they just remove and replace the whole unit.

 

[Witmo]

What you stated requires a complete disassembly of the magneto, Timing "E" gap and setting the lead or lag requires special tools and equipment not found outside of a repair facility. Repair facilites will not do this unless they are doing a complete overhaul of the mag. IOW what has been suggested in this thread is, we time life the mag at 500 hours, because the shops in the field will not service mags to this extent, they just remove and replace the whole unit.

Yes, there are some special tools required but they are not extremely expensive and any powerplant mechanic should be able to completely disassemble and reassemble a magneto, set the the E gap, install and time the magneto to the engine. If they can't they must have been sleeping during that training. The overhaul just replaces many of the parts with new.

(edit) Perhaps what I've said is too general. Some mags are more difficult to work on than others and may require a repair station with specialized facilities. The Bendix mags on my 150 are easy to work on.

 

[Tom-D]

Yes, there are some special tools required but they are not extremely expensive and any powerplant mechanic should be able to completely disassemble and reassemble a magneto, set the the E gap, install and time the magneto to the engine. If they can't they must have been sleeping during that training. The overhaul just replaces many of the parts with new.

(edit) Perhaps what I've said is too general. Some mags are more difficult to work on than others and may require a repair station with specialized facilities. The Bendix mags on my 150 are easy to work on.

My point is there is no such thing as a 500 hour inspection on mags. Most A&Ps can do the work but won't due to the liability. or because they do not have an overhaul manual for the mag. Its all about FAR 43.2 and the insurance company.

And by the way,, I won't even do my own mags, I send them out to Al at Scavage Magneto, in Hayward Ca.

Because the sum of the parts and time is more than most overhauled mags cost.

 

[Witmo]

My point is there is no such thing as a 500 hour inspection on mags. Most A&Ps can do the work but won't due to the liability. or because they do not have an overhaul manual for the mag. Its all about FAR 43.2 and the insurance company.

And by the way,, I won't even do my own mags, I send them out to Al at Scavage Magneto, in Hayward Ca.

Because the sum of the parts and time is more than most overhauled mags cost.

I can see not having the overhaul manual as a problem but "liability"? What's so special about a magneto that there'd be more liability for a mechanic than any other part of an airplane or maintenance procedure? I know plenty of mechanic's who do as you do and send them off to be overhauled but that's an individual choice based on profit margin and not due to their being overly complex or a magnet for liability claims.