Checkride; RPM drop between magnetos

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chrixxer

Filing Flight Plan
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Chrixxer
I'm a few days away from my check ride, and took the plane I'm going to be in out for solo practice this morning. It's a PA-28-140. The panel was at some point replaced, and the new tach is digital (and eye level for the right seat). I've flown this plane before, though it had the "classic Piper" layout (analog tach by your knees) at the time.

Today I noticed the RPM drop when switching mags was just barely in spec, per the checklist - neither dropped anything close to 175 rpm (max allowed), but the left mag dropped about 90, and the right mag dropped about 48. Max difference between the two is 50 rpm, again, per the checklist.

I checked with the FBO mechanic and his response was "yeah, it's always done that, it's no bid deal." (The plane flies about 1000 hours a year, and goes up 2-3x/day.) I asked the FBO owner and he stated, "yeah, it's usually about 50-70 rpm difference, it always has been, but you only notice it now on the digital tach, the analog tach bounced around too much to ever see the difference."

I checked the POH, and it simply says: "Differential drop should not exceed 50 RPM while the total drop on either magneto should not exceed 175 RPM." (Emphasis added.) I.e., it's not mandatory ("may not"), just recommended.

I have no issues personally flying this plane exhibiting that behavior, because I know it and I trust it and the flight school / FBO. (Heck, I've personally logged about half my training hours in it, including 15 solo hours.) But will it be an issue with a DPE?
 
My first defense to the DPE is that it IS within limits. Second would be this:
chrixxer said:
I checked the POH, and it simply says: "Differential drop should not exceed 50 RPM while the total drop on either magneto should not exceed 175 RPM." (Emphasis added.) I.e., it's not mandatory ("may not"), just recommended.
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Is the drop more than 175? No. Is the difference between the two greater than 50? No. So....you're good.
 
It was within the 50 rpm spec (barely) this morning, but I was a little unnerved by the fact that a 50-70 rpm drop (out of spec) is "normal" for this plane. I'm going to go with what the POH says and if the DPE has an issue, I suppose we can switch planes (this shop has multiple -140s and I've flown them all)...

Thanks all!
 
SkyHog said:
Change the checklist.
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Treating the DPE like he's an idiot is a nice way to get him to bust you. Read the last special emphasis area in the PTS.

That drop is quite small, and I'd be concerned that one mag is weak, fouling, mistimed or misgapped. I typically see drops of at least 80 RPM.

"It's always done that" translates as "it's over my head" or "the owner won't pay for it." In other words, they are gambling with YOUR butt.

It may be "in spec" (barely), but it's trying to tell you something is wrong. Especially EVERY TIME it goes out of spec.
 
Someone made an equipment change. It has revealed that there's a difference between the mags that is barely within spec. That means there has always been a latent issue but nobody knew it.

Your call as PIC to fly or not fly.
 
MAKG1 said:
Treating the DPE like he's an idiot is a nice way to get him to bust you. Read the last special emphasis area in the PTS.

That drop is quite small, and I'd be concerned that one mag is weak, fouling, mistimed or misgapped. I typically see drops of at least 80 RPM.

"It's always done that" translates as "it's over my head" or "the owner won't pay for it." In other words, they are gambling with YOUR butt.

It may be "in spec" (barely), but it's trying to tell you something is wrong. Especially EVERY TIME it goes out of spec.
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Why is changing the checklist to match the POH a trick? Nothing specifies what your checklist should say...
 
SkyHog said:
Why is changing the checklist to match the POH a trick? Nothing specifies what your checklist should say...
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The checklist already matches the POH. The argument about that was to use weasel words like "should" to mean "I can ignore this."
 
MAKG1 said:
The checklist already matches the POH. The argument about that was to use weasel words like "should" to mean "I can ignore this."
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It's not a weasel word (but maybe that's just me as a lawyer talking), it has a specific meaning. In my world (I'm a Tau Beta Pi member and worked on rockets, prior to my career as a federal litigator), "should" means "should, but it's not a huge issue if it doesn't" and "must" or "shall" means "must, and it's impermissible to continue if it doesn't."

Again, I have no issue flying this aircraft (indeed, it's my favorite PA28-140, the one I first soloed in, the one that always starts on the first attempt and has never — other than the deviation between the mags at run-up — given me any reason to doubt its mechanical (or other) integrity). I trust the FBO/school. I know the owner personally and have seen first-hand his dedication to his planes and students. I don't think he'd take unnecessary risks, and based on the service I've seen him have performed on his fleet, I don't think "owner doesn't want to pay" is in any way a factor.

And the authors of the POH evidently know how to distinguish between should, must, etc.:

"The carburetor air filter must be cleaned at least once every fifty hours. Under extremely adverse conditions of operation it may be necessary to clean the filter daily. Extra filters are inexpensive and a spare should be kept on hand and used as a rapid replacement."
 
My recommendation is if it really concerns you, and could cause an area of distraction or undue stress and anticipation just make sure your checkride is scheduled in one of the other planes! Takes all the worry out of the way then.
 
chrixxer said:
It's not a weasel word (but maybe that's just me as a lawyer talking), it has a specific meaning. In my world (I'm a Tau Beta Pi member and worked on rockets, prior to my career as a federal litigator), "should" means "should, but it's not a huge issue if it doesn't" and "must" or "shall" means "must, and it's impermissible to continue if it doesn't."

Again, I have no issue flying this aircraft (indeed, it's my favorite PA28-140, the one I first soloed in, the one that always starts on the first attempt and has never — other than the deviation between the mags at run-up — given me any reason to doubt its mechanical (or other) integrity). I trust the FBO/school. I know the owner personally and have seen first-hand his dedication to his planes and students. I don't think he'd take unnecessary risks, and based on the service I've seen him have performed on his fleet, I don't think "owner doesn't want to pay" is in any way a factor.

And the authors of the POH evidently know how to distinguish between should, must, etc.:

"The carburetor air filter must be cleaned at least once every fifty hours. Under extremely adverse conditions of operation it may be necessary to clean the filter daily. Extra filters are inexpensive and a spare should be kept on hand and used as a rapid replacement."
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It is indeed a weasel word in a safety context.

It means you can't be violated for it. Yes, that's the lawyer in you if you consider that as identical to good decision making.

Yup, I've worked on rockets, too. "Must" is not allowed. It's "shall" (required) vs. "should" (discretion) or "will" (best effort). Too bad NASA doesn't run Piper. It's not relevant. We're not writing an engineering requirements document.

The airplane is trying to say something is wrong. It is foolish to ignore that.
 
Mr Stating the Obviois, but you try to burn off the plugs?
 
Consider whether when the plane was built with the precision of the tach in that day and age, whether a few tens of RPM was able to be discerned by the needle and gauge markings. Was 50 rpm the distinction between drop and no drop? I don't know. I'm just asking and here to learn.

Sorry, didn't answer your question...I'll ask another: would DPE accept the above reasoning, due to the logic used? Or blind adherence to a 1-10 rpm delta?

Yesterday was supposed to be my solo. Did my 3 take offs and landings with CFI, then got signed off, then went to do solo. At run-up, got more than 200 rpm drop on R mag...spit, miss, rough running. Definately a no-go. If I had gotten no-drop (i.e., something indiscernable by tach & ear), would've also been a no-go...for me as PIC. Flying '82 C-152, mech tach. (Soloed today tho!)
 
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455 Bravo Uniform said:
Yesterday was supposed to be my solo. Did my 3 take offs and landings with CFI, then got signed off, then went to do solo. At run-up, got more than 200 rpm drop on R mag...spit, miss, rough running. Definately a no-go. If I had gotten no-drop (i.e., something indiscernable by tach & ear), would've also been a no-go...for me as PIC. Flying '82 C-152, mech tach. (Soloed today tho!)
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The 152 has the O-235 in it, an engine famous for fouling sparkplugs. Tell the mechanic to put REM37BY plugs in it--or preferably Tempest UREM37BY. They have extended electrodes that are very fouling-resistant and I put them in every engine that can take them.
 
Note that a change in behavior is more significant than the overall mag drop (or difference). It's a fallacy that if 175 is the max drop allowed that smaller drops somehow indicate a healthier engine. In fact, it the absurd case it indicates either malfuctioning ignition switch or p-leads or mistimed mags.
 
"Should" be no more than 50 and 48 "should" be considered to be less than 50 by any DPE you get. If anything, having the digital tach "should" leave no room for argument. There's virtually no possibility of you and your DPE disagreeing on what the actual number was because it's right there in front of his face.
 
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