Ignition RPM Drop Behavior

[FlyingMonkey]

I don't think this is any cause for concern, but I'm curious if anybody else has similar experiences with their 912 engines...

 

I have noticed for a while now (maybe as long as I've had the airplane) that the RPM drop behavior is a little different between the two ignitions.  They both drop about the same amount (on my analog tach it's a little hard to say precisely, but less than 200rpm and probably closer to 100-250rpm).  

 

When switched to only ignition #1, the drop is a slow decrease, takes about a second to "ease" down to the new RPM.  In contrast, ignition #2 drops much more quickly, almost immediately dropping to the new RPM in probably half the time it takes #1 to do so.  There also might be a *slight* amount of additional roughness on #2, but it could well be my imagination.  That's really the only difference, and I can increase RPM smoothly on either ignition.

 

Do others have a similar experience, or is my #2 ignition worth looking into?  I suspect that the difference is in which plugs (top v bottom) are firing in which cylinder and how the combustion process happens, or some other minor difference.  As I said, both drop a similar RPM and the engine runs fine on either ignition.  Plus are gapped to 0.026".

[gbigs]

You are reading too much into RPM drop on the mags.  The only thing to care about in the mag check is detecting a dead mag or one about to die.

[FlyingMonkey]

You are reading too much into RPM drop on the mags.  The only thing to care about in the mag check is detecting a dead mag or one about to die.

 

If you get extraordinary roughness it can indicate a plug problem, excessively rich mixture, or other issues as well.  The mag check is such a great tool because it detects a LOT of things, not just a mag failure.  

[Anticept]

I wouldn't worry. As long as it settles in the right place.

 

Try checking at 4k if you're doing 3500. Roger said 4k is better to check at, it helps the mag check spread grow and identify problems sooner.

[FastEddieB]

You are reading too much into RPM drop on the mags.  The only thing to care about in the mag check is detecting a dead mag or one about to die.

That is, of course, patently and incontrovertibly false*.

 

Manufacturers specify both a maximum permissible drop, and a maximum difference between the two mags. Drops larger than that specified, or larger than the maximum difference allowed, are certainly a cause for concern.

 

 

 

 

*I would have hoped that the significance of the size of the drops and the maximum difference would be explained to every student pilot very early on, and that every licensed pilot would know this.

[FlyingMonkey]

I wouldn't worry. As long as it settles in the right place.

 

Try checking at 4k if you're doing 3500. Roger said 4k is better to check at, it helps the mag check spread grow and identify problems sooner.

 

I have done run ups everywhere from 3000-4000rpm, the behavior is consistent.  

 

I'm with you, it's not worrying, just interesting.

[Anticept]

I have done run ups everywhere from 3000-4000rpm, the behavior is consistent.

 

I'm with you, it's not worrying, just interesting.

Front cylinders get slightly more fuel. Might have something to do with top vs bottom plugs firing. Try doing it in reverse order.

[Anticept]

You are reading too much into RPM drop on the mags. The only thing to care about in the mag check is detecting a dead mag or one about to die.

I know you are trying to help, but what you said is outright dangerous.

[gbigs]

I know you are trying to help, but what you said is outright dangerous.

 

What i said is SOP.  It is the proper process in the CT and it's how it's done in the higher performance Cirrus SR22T (TSIO550K).    The ignition check (mags) is a differential check UP TO A MAX delta.  The entire check is to detect a failed or failing mag.   BEFORE TAKEOFF, not in the air.

 

Here is the CT POH ignition check -notice it is UP TO 300 drop on one mag, and UP TO 120 rpm between mags.  Anything less is of no consequence.  Notice I provide information without patronizing or insulting anyone.

magCT.bmp

[Tom Baker]

What i said is SOP.  It is the proper process in the CT and it's how it's done in the higher performance Cirrus SR22T (TSIO550K).    The ignition check (mags) is a differential check UP TO A MAX delta.  The entire check is to detect a failed or failing mag.   BEFORE TAKEOFF, not in the air.

 

Here is the CT POH ignition check -notice it is UP TO 300 drop on one mag, and UP TO 120 rpm between mags.  Anything less is of no consequence.  Notice I provide information without patronizing or insulting anyone.

A mag drop of zero is of great consequence.

[Anticept]

If you have a mag drop of zero, then your engine will either die when you switch to the other mag, or it is not grounding the mag you are testing and is a very dangerous condition to ground and maintenance crews. There WILL be a mag drop; it's physically impossible with current engine design not to have some kind of drop in a properly functioning system.

 

They don't write every single thing in the aircraft flight manuals because they would become maintenance manuals!

 

Anyways, airframe manufacturers are not engine manufacturers. They really, really should include the engine operating manuals with the AFMs, or at least publish a revised version with any airframe manufacturer modifications.