Static shaft angle check failing with dual pMags

[LouNathanson]

Ever since I installed a second pMag, my EI Commander occasionally reports a shaft angle mismatch during its startup check of static (non-running) data. It only occurs when the prop is in a certain narrow range that is far from the actual firing points (which are in the vicinity of 0 and 180 degrees). The error is consistent at that particular crank angle range, and looks like this:


Shaft angle alert2016×1512 266 KB


Since the two pMags line up within a degree near the actual firing points, this doesn’t seem to affect operation, and my running timing deviation angle indication (TDA) is always about 2 degrees. In flight mag checks look good.

The right mag seems to be the main culprit, and this apparent distortion in the triggering magnet’s field seems to persist despite three IRANs performed by the Emag folks. Brad at Emag insists that this is nothing to worry about and that a comparison of static angle data is meaningless far away from 0 or 180 degrees.

However, the EI Commander folks are not comfortable moving the alarm point to any greater than 4 degrees of deviation because they say I’m the only one that has a static deviation this large. My inner pilot hates to take off with an error indication, but I’m kinda stuck between dueling vendors.

So, my question for the group is: Does anyone else that is running dual pMags with an EI Commander ever get this error?

 

[rv8ch]

Ever since I installed a second pMag, my EI Commander occasionally reports a shaft angle mismatch during its startup check of static (non-running) data. It only occurs when the prop is in a certain narrow range that is far from the actual firing points (which are in the vicinity of 0 and 180 degrees). The error is consistent at that particular crank angle range, and looks like this:


Shaft angle alert2016×1512 266 KB


Since the two pMags line up within a degree near the actual firing points, this doesn’t seem to affect operation, and my running timing deviation angle indication (TDA) is always about 2 degrees. In flight mag checks look good.

The right mag seems to be the main culprit, and this apparent distortion in the triggering magnet’s field seems to persist despite three IRANs performed by the Emag folks. Brad at Emag insists that this is nothing to worry about and that a comparison of static angle data is meaningless far away from 0 or 180 degrees.

However, the EI Commander folks are not comfortable moving the alarm point to any greater than 4 degrees of deviation because they say I’m the only one that has a static deviation this large. My inner pilot hates to take off with an error indication, but I’m kinda stuck between dueling vendors.

So, my question for the group is: Does anyone else that is running dual pMags with an EI Commander ever get this error?

I don't have this with my dual pmags, as one data point, but I have the original EI Commander. Do you have more information about how this is measured, and why it's ok if not near 0 or 180 degrees? Did you do the IRAN on both pmags, or only one? I can see some possible "slop" in the gears giving a slightly different angle when not running, but no idea what would be considered "normal". Very interesting!

 

[Toobuilder]

It only occurs when the prop is in a certain narrow range

Is this ”narrow range” also at some cardinal setting, say 90/270, by chance? And does the TDA increase and peak at the alarm point and then decrease again as it gets closer to firing?

 

[[email protected]]

Yes I get the same error when turning on the power, but never when the engine is running. I did a test and rotated the propeller to 14 different angles. The result is below. At most up to 7 deg diff. As far I understand it doesn't matter as long it's 0 at TDC. But I don't understand why EiCommander has this check during power up.

LEFT	RIGHT	Diff
1	1	0
174	176	−2
89	96	−7
38	44	−6
299	304	−5
260	267	−7
226	232	−6
191	193	−2
163	165	−2
114	120	−6
82	89	−7
37	41	−4
15	17	−2

 

[LouNathanson]

Is this ”narrow range” also at some cardinal setting, say 90/270, by chance? And does the TDA increase and peak at the alarm point and then decrease again as it gets closer to firing?

I didn't want to flood the post with data right off the bat, but I did perform the static check every three teeth all the way around. Yes, the problem does appear to be worst in the general neighborhood of 90 and 270, but not exactly. Note also that I did just have both mags overhauled just prior to collecting this most recent set of static data, so there was no shaft play at all in either one. As you can see from the plots, the right (older) mag is consistently worse than the left mag. In fact, the left mag was actually pretty good before this last IRAN. Now, it has a weird (and repeatable) spike right at 283 ATDC.

You can also see that the difference between left and right is less than two degrees anywhere near the normal firing ranges of 20-35 degrees BTDC. The in-flight display looks normal with a TDA around 2 degrees.

 

[Toobuilder]

Strange that the variation looks like a perfect eccentric, peaking on the far side of the firing point... that can’t be a coincidence, but for the life of me nothing jumps out as a cause.

 

[LouNathanson]

Strange that the variation looks like a perfect eccentric, peaking on the far side of the firing point... that can’t be a coincidence, but for the life of me nothing jumps out as a cause.

Yes, and nothing jumps out at Emag either. They even replaced the sense magnets this time I'm wondering if perhaps the shaft itself has a residual magnetism that may be superimposed on the sense magnet. I'm comfortable that there's nothing to worry about operationally, but it goes against my inner-pilot to routinely takeoff ignoring a persistent alarm.

 

[LouNathanson]

I was able to prevail on Bill and Ed at EI Commander to widen the set point for the static angle check on bootup to 6 degrees deviation between left and right. That should solve my problem, at least with my current pMags.