Zero RPM drop on mag check after 500-hour IRAN

[toto]

Just had Bendix dual-mag out for IRAN at Kelly. Mags reinstalled, engine runs smooth as glass. Mag check is clean, no stumbling on L or R. Switching to "Off" kills engine immediately. 

So .. my concern is that there is no discernible drop during mag check on either side. 

After my first run-up, I scrubbed the flight and sent it back to the shop. They found no problems, and said that the RPM drop is minimal but present (they estimated 10rpm drop per side when checked at 2000rpm). 

Went back out, same mag check at run-up. Engine perfectly smooth on L, R, or Both. Engine dies as expected when switched "Off."

Flew the plane on a post-mx flight within gliding distance of the field for an hour. Everything is perfect. 

Should I be concerned? I've never had a mag show zero RPM drop on a mag check at run-up. I have flown brand new mags, newly overhauled mags, newly IRANed mags, and all basically do the same thing. This zero-drop check is very unusual. But the engine is running great, and I don't want to go digging around looking for a problem if there isn't one. 

TIA for advice

 

[GeeBee]

Do you have an engine monitor? What are the EGTs doing when you go to single mag?

[PT20J]

Timing could be too advanced. It’s easy to check and I’d have it done before flying as advanced timing can lead to detonation.

Here’s Mike’s article on mags https://resources.savvyaviation.com/wp-content/uploads/articles_eaa/EAA_2010-12_the-mag-check.pdf

Skip

[toto]

14 minutes ago, GeeBee said:

Do you have an engine monitor? What are the EGTs doing when you go to single mag?

Only the single-port analog one with factory gauge. It’s pretty useless during the mag check. 

[GeeBee]

It is better than nothing, do you see a rise?

 

[toto]

31 minutes ago, GeeBee said:

It is better than nothing, do you see a rise?

 

Yes, definitely see a small rise during the mag check. When I switch between “L” and “R” it looks the same, but a definite increase from “Both.”

[GeeBee]

Does it rise from "Both" to "Right" then back down when switched back to "Both"? Ditto for the left?

[toto]

2 hours ago, GeeBee said:

Does it rise from "Both" to "Right" then back down when switched back to "Both"? Ditto for the left?

Well, unfortunately I don’t think I waited long enough on “Both” to tell. I have a video of it, but basically the temp ticks up when I go from “Both” to “Right” but then it’s not back on “Both” for very long before I go to “Left.”

I’ll look again and see if I can see movement between the two. 

[toto]

1 hour ago, toto said:

Well, unfortunately I don’t think I waited long enough on “Both” to tell. I have a video of it, but basically the temp ticks up when I go from “Both” to “Right” but then it’s not back on “Both” for very long before I go to “Left.”

I’ll look again and see if I can see movement between the two. 

I looked at the video carefully, and I really can’t see much on the EGT. The temperature definitely increases the first time I switch to one mag, but doesn’t noticeably decrease when back on Both. 

[toto]

3 hours ago, PT20J said:

Timing could be too advanced. It’s easy to check and I’d have it done before flying as advanced timing can lead to detonation.

Here’s Mike’s article on mags https://resources.savvyaviation.com/wp-content/uploads/articles_eaa/EAA_2010-12_the-mag-check.pdf

Skip

I’ll see if they can check the timing. When I talked to the shop yesterday, they made the point that the internal timing is set by the overhaul shop and not by them - but I’m sure that they can re-check the engine timing. 

[PT20J]

1 minute ago, toto said:

I’ll see if they can check the timing. When I talked to the shop yesterday, they made the point that the internal timing is set by the overhaul shop and not by them - but I’m sure that they can re-check the engine timing. 

That’s correct - the internal timing (E-gap) should have been set correctly by the mag shop. The mag to engine timing should match the engine data plate — 25 deg for a dual mag IO360.

[toto]

12 minutes ago, PT20J said:

That’s correct - the internal timing (E-gap) should have been set correctly by the mag shop. The mag to engine timing should match the engine data plate — 25 deg for a dual mag IO360.

Great, thank you. I’ll ask them. 

I sent you a PM with the mag check video. 

[Wildhorsetrail]

Sounds like the switch is not grounding out the mag as appropriate in L or R but is in both.

Sent from my SM-A326U1 using Tapatalk

[toto]

A bit more info, for what it’s worth..

Went back to the shop today, did a run up with the mechanic sitting in the plane next to me. The mag drop at 2000rpm is consistently zero (arguably it's between zero and ten rpm, but hard to tell for sure). EGT does increase visibly on either L or R, and drops back on Both. Hard to tell exactly how much with my analog gauge, but it’s clear and repeatable.

So here’s where it gets interesting (maybe). The mechanic suggested trying the mag check at a lower RPM. Sure enough, if you run the engine up to 1300rpm, it shows a solid drop of 30-40rpm on each side, and returns to 1300 on Both. 

So if I do the mag check by the book, I see no rpm drop. But if I do it at 1300rpm, it looks basically normal.

Does this tell me anything one way or the other?

 

[cferr59]

What happens if you move the switch to off? Also, have you tried doing the mag check lean?

[toto]

9 minutes ago, cferr59 said:

What happens if you move the switch to off? Also, have you tried doing the mag check lean?

The engine dies immediately when switched to “Off.” I do this every time I shut down. 

I didn’t try a lean mag check. I did try leaner than full rich, and it was no different.

[cferr59]

7 minutes ago, toto said:

The engine dies immediately when switched to “Off.” I do this every time I shut down. 

I didn’t try a lean mag check. I did try leaner than full rich, and it was no different.

So it seems like the p-leads are ok. Is there any change in smoothness during the mag check?

[Ragsf15e]

Interesting that your poh calls for 2000rpm?  My mag check (IO-360A1A) is 1700rpm.  
Did the shop double check the timing?

[toto]

59 minutes ago, cferr59 said:

So it seems like the p-leads are ok. Is there any change in smoothness during the mag check?

Not noticeably. The engine is running smoothly throughout.

[toto]

35 minutes ago, Ragsf15e said:

Interesting that your poh calls for 2000rpm?  My mag check (IO-360A1A) is 1700rpm.  
Did the shop double check the timing?

Yeah, my POH says 1900-2000. 

They have not re-done the timing yet. The mechanic was very confident that it was correct. 

[Will.iam]

I agree with PT20J the timing being advanced can show little drop in mag check.  Another tale tale sign is that your cylinder head temps will be hotter than normal with that advanced setting at takeoff power setting.

[Ragsf15e]

1 hour ago, toto said:

Yeah, my POH says 1900-2000. 

They have not re-done the timing yet. The mechanic was very confident that it was correct. 

I’m no mechanic, so take this for the free advice it is… have them confirm the timing.  My right mag was advanced recently and it would barely drop.  Maybe 20 rpm.  At the annual my mechanic found the timing out of whack by like 6 degrees, fixed that and now it’s normal.

[Yetti]

My mags don't show much drop if at a lean setting.  I do mag checks at 1700rpm

[MikeOH]

Another data point: I do mag checks at 1700, but not full rich (IO-360A1A) and typically see 25-40 rpm drop.

[A64Pilot]

11 hours ago, Ragsf15e said:

Interesting that your poh calls for 2000rpm?  My mag check (IO-360A1A) is 1700rpm.  
Did the shop double check the timing?

We have a yellow arc under 2000, 1500 to 2000 if memory is correct

Edited March 10, 2022 by A64Pilot