Static RPM Test

Tom-D said:
You can start with far 43.15 Para (a) 1&2 plus (c) in its entirety.

If you can do that with a bad tach you sign the annual/100 hour off, I won't nor will any of the IAs I know, because IAs have a responsibility to the regs.
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Not taking sides with the question, but I want to know from Tom:

Does your opinion vary on this issue if we're talking fixed pitch vs CS prop?
I notice you mentioned 'the danger zone' and 'yellow arc'. FP systems don't have a yellow arc (that I'm aware of).
 
I'd still like to hear what Tom's normal practice is for verifying the tach as part of an annual.

As far as the yellow arc goes, in my limited experience a yellow arc on a tach is usually due to vibration at specific RPM ranges. I would guess it's possible to have one on a FP propeller mated to a specific engine, but I can't recall having seen one.
 
TMetzinger said:
I'd still like to hear what Tom's normal practice is for verifying the tach as part of an annual.

As far as the yellow arc goes, in my limited experience a yellow arc on a tach is usually due to vibration at specific RPM ranges. I would guess it's possible to have one on a FP propeller mated to a specific engine, but I can't recall having seen one.
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43.15 says that the static RPM and idle setting much be checked as Tom cited above (is that what you're looking for?). I find it interesting that my checklist has idle check but not static, but in the normal procedures it discusses a static run-up and with a hint that one should do it on a regular basis (presumably as a warning for engine trouble).
 
No, I get that the static and idle RPM get checked. But I assume that's with the mechanical tach. What I'm curious about is:

If the mechanical tach reads correctly for idle and static RPM - does the inspector still validate the tach against a known-good instrument like an optical tach? If not, how do you know the mechanical tach is accurate for the tests you've already completed?
 
TMetzinger said:
No, I get that the static and idle RPM get checked. But I assume that's with the mechanical tach. What I'm curious about is:

If the mechanical tach reads correctly for idle and static RPM - does the inspector still validate the tach against a known-good instrument like an optical tach? If not, how do you know the mechanical tach is accurate for the tests you've already completed?
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not usually. If it is showing what it should, I normally don't go any farther.
 
Skylane81E said:
Not really, while ill accept very minor error if you don't have a place to hang your hat that any error is ok then how do you know how much is allowed?
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Exactly what I'm asking -- what is "very minor error," and how do you justify whatever error you will accept? Also, I've yet to see a check of tach accuracy on any airplane manufacturer's annual checklist for a light single. I've had annuals on my planes done by a host of different IA's over the last 35 years, and I'm certain none of them ever checked it, although they all did static RPM checks using the installed tach as it was.
 
Jaybird180 said:
Does your opinion vary on this issue if we're talking fixed pitch vs CS prop?
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No, they both must meet the full power numbers, IAW the TCDS.

Jaybird180 said:
I notice you mentioned 'the danger zone' and 'yellow arc'. FP systems don't have a yellow arc (that I'm aware of).
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Many installations do not have a yellow arc, I mentioned them as an example of why you should have an accurate tach.
 
Ron Levy said:
Exactly what I'm asking -- what is "very minor error," and how do you justify whatever error you will accept? Also, I've yet to see a check of tach accuracy on any airplane manufacturer's annual checklist for a light single. I've had annuals on my planes done by a host of different IA's over the last 35 years, and I'm certain none of them ever checked it, although they all did static RPM checks using the installed tach as it was.
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Have you ever had a tach go bad?
 
Tom-D said:
You can start with far 43.15 Para (a) 1&2 plus (c) in its entirety.

If you can do that with a bad tach you sign the annual/100 hour off, I won't nor will any of the IAs I know, because IAs have a responsibility to the regs.
Click to expand...

Jaybird180 said:
Does your opinion vary on this issue if we're talking fixed pitch vs CS prop?
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Tom-D said:
No, they both must meet the full power numbers, IAW the TCDS.
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I was referring to your opinion as suggested above that 'an airplane with a bad tach is unairworthy'.
 
TMetzinger said:
No, I get that the static and idle RPM get checked. But I assume that's with the mechanical tach. What I'm curious about is:

If the mechanical tach reads correctly for idle and static RPM - does the inspector still validate the tach against a known-good instrument like an optical tach? If not, how do you know the mechanical tach is accurate for the tests you've already completed?
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Seems prudent to me to make the initial assumption that it is correct, considering checking the tach accuracy is part of troubleshooting a different scenario (failure to meet stated numbers).

Unfortunately, as I type this I don't think it passes the sniff test. Pitot Static instruments get tested for accuracy and they don't use the onboard instruments to check the oboard instruments.
 
Probably more fuel for the fire, but I just re-read an FAA SAIB (NE-08-21) about prop rpm restrictions and placards. The SAIB acknowledges that there are many inaccurate tachs out there, and suggests you have the tach checked for accuracy, but does not provide an acceptable range of tach error.
 
Tom-D said:
not usually. If it is showing what it should, I normally don't go any farther.
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OK. That's what I figured.

It does raise the question though how one would know the tach is actually correct. Let's say the tach on a rental airplane actually reads 100 RPM low. A student squawks the low RPM at idle but has no issue getting it to reach redline during normal operations. A&P adjusts the idle using the mechanical tach without checking it for accuracy. (Let me know if this is something that the maintenance manual or the A&P body of knowledge would make unlikely).

Over multiple years of inspection, the engine is actually running 100 RPM faster than the mechanical tach reads and has been overreved by 100 RPM in cruise for hundreds of hours.

Now, is this scenario impossible or unlikely?
 
TMetzinger said:
OK. That's what I figured.

It does raise the question though how one would know the tach is actually correct. Let's say the tach on a rental airplane actually reads 100 RPM low. A student squawks the low RPM at idle but has no issue getting it to reach redline during normal operations. A&P adjusts the idle using the mechanical tach without checking it for accuracy. (Let me know if this is something that the maintenance manual or the A&P body of knowledge would make unlikely).

Over multiple years of inspection, the engine is actually running 100 RPM faster than the mechanical tach reads and has been overreved by 100 RPM in cruise for hundreds of hours.

Now, is this scenario impossible or unlikely?
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Probably quite common. It's just that if you did overrev by 100 RPM I doubt it's going to have any serious effect, that's like 4 percent. The engineers that built these things way back when were way smarter than I am. They knew the potential error of a tach and how it'd drift over time. They built the engine to handle such error.
 
TMetzinger said:
OK. That's what I figured.

It does raise the question though how one would know the tach is actually correct. Let's say the tach on a rental airplane actually reads 100 RPM low. A student squawks the low RPM at idle but has no issue getting it to reach redline during normal operations. A&P adjusts the idle using the mechanical tach without checking it for accuracy. (Let me know if this is something that the maintenance manual or the A&P body of knowledge would make unlikely).

Over multiple years of inspection, the engine is actually running 100 RPM faster than the mechanical tach reads and has been overreved by 100 RPM in cruise for hundreds of hours.

Now, is this scenario impossible or unlikely?
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If it was properly run after the annual, and it was actually over the red line by 100 RPM the A&P-IA should have had it corrected. because the owner may have the prop pitched too low, who would know until it was verified by a electronic tach?

And if it is actually the tach, you have a known discrepancy, and 91.405 does not give much latitude to not get it fixed.
 
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Tom-D said:
If it was properly run after the annual, and it was actually over the red line by 100 RPM the A&P-IA should have had it corrected. because the owner may have the prop pitched too low, who would know until it was verified by a electronic tach?
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Wait a second. You said several posts ago that you didn't verify the tach if it made numbers (which it would in this case since the static RPM range is generally more than 100 RPM). So I'm assuming in this case that you wouldn't know the tach was off by 100 RPM.
 
Jaybird180 said:
Unfortunately, as I type this I don't think it passes the sniff test. Pitot Static instruments get tested for accuracy and they don't use the onboard instruments to check the oboard instruments.
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There is no specific requirement to check the pitot system or airspeed indicator accuracy. Contrary to popular misconception, 91.411 does not look at either one.
 
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Tom-D said:
If it was properly run after the annual, and it was actually over the red line by 100 RPM the A&P-IA should have had it corrected. because the owner may have the prop pitched too low, who would know until it was verified by a electronic tach?

And if it is actually the tach, you have a known discrepancy, and 91.405 does not give much latitude to not get it fixed.
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Looking up the FAR, I found this article:
http://www.aopa.org/asf/hotspot/articles/pc9903.html
where in says
As if the trauma of the incident and the resultant damage were not enough, the FAA threw the book at the owner, charging him with violation of FAR 91.405(b), requiring a logbook entry for the airplane's return to service; FAR 91.407(a), requiring logbook entries for the maintenance performed; FAR 91.7(a), operating an unairworthy aircraft (inoperative heater, disconnected gear door linkage rods); and FAR 91.13(a), being careless or reckless. Despite an appeal to the National Transportation Safety Board, the owner-pilot ultimately suffered a 45-day suspension of his commercial pilot certificate and the resultant black mark on his FAA record.
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Mind you, this started with simply a broken heater and ended with a pilot certificate action. I would not have made the leap of logic that the heater would be cited as a cause for the 91.79(a) violation, but according to the article it is. Using that logic, what else can an owner-operator be violated for?
 
TMetzinger said:
Wait a second. You said several posts ago that you didn't verify the tach if it made numbers (which it would in this case since the static RPM range is generally more than 100 RPM). So I'm assuming in this case that you wouldn't know the tach was off by 100 RPM.
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each TCDS will have a high RPM allowable, and a low RPM allowable, If it is with in those allowable limits, why would I go any farther? we have no discrepancy.

When the tach shows us what we need to see it is airworthy, when we see a RPM out side the allowable we need to know why because we have a discrepancy to repair.

I do not use the electronic tach to prove the tach wrong until I have reason to do so.
 
Jaybird180 said:
Looking up the FAR, I found this article:
http://www.aopa.org/asf/hotspot/articles/pc9903.html
where in says


Mind you, this started with simply a broken heater and ended with a pilot certificate action. I would not have made the leap of logic that the heater would be cited as a cause for the 91.79(a) violation, but according to the article it is. Using that logic, what else can an owner-operator be violated for?
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You name it.
 
Tom-D said:
each TCDS will have a high RPM allowable, and a low RPM allowable, If it is with in those allowable limits, why would I go any farther? we have no discrepancy.

When the tach shows us what we need to see it is airworthy, when we see a RPM out side the allowable we need to know why because we have a discrepancy to repair.
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So if you see the tach read at the bottom of the TCDS allowable RPM range, you'd be perfectly happy without checking to see if it's reading high? Or, as is more likely, the tach is reading low when it indicates at the top of the allowable range?

I do not use the electronic tach to prove the tach wrong until I have reason to do so.
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Then contrary to what you said earlier, you don't check to see if the tach is accurate before releasing the plane for flight, and it's entirely possible that due to tach inaccuracy, you may be releasing the aircraft in a legally unairworthy as well as possibly unsafe condition.

:nono: You do know that the allowable RPM ranges are what the engine is actually turning, not what the tach says, right?
 
TMetzinger said:
If the mechanical tach reads correctly for idle and static RPM - does the inspector still validate the tach against a known-good instrument like an optical tach? If not, how do you know the mechanical tach is accurate for the tests you've already completed?
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A few years ago I had the "job" of checking people's tachs against an optical tach as part of an inspection for an air race. The mechanical tachs were all over the place in comparison with the optical tach. The digital tachs were basically right on.
 
Everskyward said:
A few years ago I had the "job" of checking people's tachs against an optical tach as part of an inspection for an air race. The mechanical tachs were all over the place in comparison with the optical tach. The digital tachs were basically right on.
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That's what I'd expect.

It seems to me that Tom has said two things:

If the tach isn't accurate, the plane isn't airworthy.
He doesn't check the tach accuracy during an annual inspection UNLESS it appears to be erroneous during engine idle and static runup checks.

To me this is contradictory, if we are going to be super-strict about it.

On the other hand, I have no problem believing that the collective wisdom on the subject is "if the tach looks good during the engine checks, it's airworthy and we don't check it further". This indicates to me that whatever the acceptable RPM range for the engine checks is, that's the acceptable range for tach error, because that's the range the inspector is letting go out the door without further checks.
 
TMetzinger said:
That's what I'd expect.

It seems to me that Tom has said two things:

If the tach isn't accurate, the plane isn't airworthy.
He doesn't check the tach accuracy during an annual inspection UNLESS it appears to be erroneous during engine idle and static runup checks.

To me this is contradictory, if we are going to be super-strict about it.

On the other hand, I have no problem believing that the collective wisdom on the subject is "if the tach looks good during the engine checks, it's airworthy and we don't check it further". This indicates to me that whatever the acceptable RPM range for the engine checks is, that's the acceptable range for tach error, because that's the range the inspector is letting go out the door without further checks.
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What I have said is, when you know the tach is not accurate it isn't airworthy and you must correct the discrepancy.
Any Tach that shows the RPM is with in the limits of the TCDS will be close enough for what we use Tachs for.

That's why Ron has not seen any one declare an aircraft unairworthy as a result of a tach being wrong. the inspectors see what they need to see and sign off the aircraft as airworthy.
As he has pointed out, there is no criteria to judge a tach bad or good. so when you go hunting for a tach discrepancies there is nothing to make the decision bad or good.

BUT when we see the RPM is not with in limits of the TCDS we do have a reason to check the tack, and decide what is wrong, Prop, engine or tach.
 
All of this argument points out the need for a standard in the FARs, same as the annual tach calibration we have in Canada. However, another standard is another regulation, and another regulation costs everyone money--for the calibration check, for a new tach (because so many of the those old tachs are way off, in my experience), for the installation of the new tach and another calibration check. I can see a $600 bill for the owners of a lot of old SE airplanes, then another $25 or so annually to check it if the mechanic is decent about it.

The tach drive cable should be replaced then, too. It's often a contributor to wobbly needle indications. And they do break.

Canadian limits are 4% of the RPM in the middle of the cruise range, i.e. 100 RPM at 2500. The magnets in the mag-drag-cup type tachs (which covers most of the tachs out there in singles) get weak and they start to underread. Very seldom will they overread unless they have a tiny pilot shaft between the needle shaft and cup shaft, and the lubricant gets dried out in there and will drag the needle higher.

I have an ancient flyweight-driven tach in my Jodel that doesn't weaken with age, though the lube did get sticky and made it erratic. I opened it up and cleaned it and off we went. Gotta love homebuilts. It's adjustable, too, if it ever needs it, and it will read accurately no matter the direction of cable rotation. No backward scale needed for the Continental A-series engines.

Dan
 
Dan Thomas said:
All of this argument points out the need for a standard in the FARs, same as the annual tach calibration we have in Canada. However, another standard is another regulation, and another regulation costs everyone money--for the calibration check, for a new tach (because so many of the those old tachs are way off, in my experience), for the installation of the new tach and another calibration check. I can see a $600 bill for the owners of a lot of old SE airplanes, then another $25 or so annually to check it if the mechanic is decent about it.

The tach drive cable should be replaced then, too. It's often a contributor to wobbly needle indications. And they do break.

Canadian limits are 4% of the RPM in the middle of the cruise range, i.e. 100 RPM at 2500. The magnets in the mag-drag-cup type tachs (which covers most of the tachs out there in singles) get weak and they start to underread. Very seldom will they overread unless they have a tiny pilot shaft between the needle shaft and cup shaft, and the lubricant gets dried out in there and will drag the needle higher.

I have an ancient flyweight-driven tach in my Jodel that doesn't weaken with age, though the lube did get sticky and made it erratic. I opened it up and cleaned it and off we went. Gotta love homebuilts. It's adjustable, too, if it ever needs it, and it will read accurately no matter the direction of cable rotation. No backward scale needed for the Continental A-series engines.

Dan
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this man is a genius:cheers:
 
Ron Levy said:
You might want to read the Lycoming SB on overspeeds.

http://www.lycoming.textron.com/support/publications/service-bulletins/pdfs/SB369J.pdf
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I'm aware of the service bulletin Ron but lets use a little common sense here. We all know many airplane tachs are off by 100 RPM or so - we also know that many people are idiots and have no idea how to run their engines. Most of these engines still last a very long time - hence why I said less than a 5% overspeed is not likely to have any immediate effect. If over-speeding by 5% did destroy the engine most of the engines in the rental fleets would be destroyed because that is how they are commonly operated.

You'll also notice there is nothing too concerning int he service bulletin as a result of a 5% overspeed. Obviously these things are written on the side of caution but any knowledge of real world operations would make it clear that such things happen all the time.

On one hand you're beating Tom up for saying that he doesn't think a tach being inaccurate is airworthy and on another your jumping on me for saying a 4% overspeed isn't going to cause significant damage. I was aware of the service bulletin, I'm double aware because you already posted it, and now I'm triple aware since you decided to show it to me again. Could you perhaps now write something reasonable about your opinion on this matter instead of just beating up everyone else's? If you can't do that then perhaps you shouldn't post at all, because what you're doing isn't helpful.
 
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jesse said:
I'm aware of the service bulletin Ron but lets use a little common sense here. We all know many airplane tachs are off by 100 RPM or so - we also know that many people are idiots and have no idea how to run their engines. Most of these engines still last a very long time - hence why I said less than a 5% overspeed is not likely to have any immediate effect. If over-speeding by 5% did destroy the engine most of the engines in the rental fleets would be destroyed because that is how they are commonly operated.

You'll also notice there is nothing too concerning int he service bulletin as a result of a 5% overspeed. Obviously these things are written on the side of caution but any knowledge of real world operations would make it clear that such things happen all the time.

On one hand you're beating Tom up for saying that he doesn't think a tach being inaccurate is airworthy and on another your jumping on me for saying a 4% overspeed isn't going to cause significant damage. I was aware of the service bulletin, I'm double aware because you already posted it, and now I'm triple aware since you decided to show it to me again. Could you perhaps now write something reasonable about your opinion on this matter instead of just beating up everyone else's? If you can't do that then perhaps you shouldn't post at all, because what you're doing isn't helpful.
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Slow down, Jesse. Tom is saying the tach must be accurate to be airworthy on an annual, but he has no standard by which to judge that, and he says he doesn't even check it unless there is another problem which leads him to it. That is absurd on its face. If it needs to be accurate to be airworthy, he should be checking it on every annual, and he doesn't, and when he checks it, he should have an approved standard for accuracy, and he doesn't.

I do believe that one should know how many RPM the engine is turning based on what the tach is reading, and I believe I said that, but I don't think that a tach is rendered unairworthy merely because it isn't "dead nuts" accurate. I was hoping Tom or one of the other IA's here could come up with a standard for that, since I was unable to find one, but so far nobody has.

Second, you opined without any apparent basis other than your own gut feeling that a 4% overspeed "is not going to have a serious effect," and I merely pointed to Lycoming's official guidance on point.

Finally, the fact that you are aware of that SB doesn't mean everyone else who reads your post is equally aware. I'm sure you've answered the same question more than once here.
 
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Ron Levy said:
Slow down, Jesse. Tom is saying the tach must be accurate to be airworthy on an annual, but he has no standard by which to judge that, and he says he doesn't even check it unless there is another problem which leads him to it. That is absurd on its face.

I do believe that one should know how many RPM the engine is turning based on what the tach is reading, but I don't think that a tach is rendered unairworthy merely because it isn't "dead nuts" accurate. I was hoping Tom or one of the other IA's here could come up with a standard for that, since I was unable to find one, but so far nobody has.

Second, you opined without any apparent basis other than your own gut feeling that a 4% overspeed "is not going to have a serious effect," and I merely pointed to Lycoming's official guidance on point.

Finally, the fact that you are aware of that SB doesn't mean everyone else who reads your post is equally aware. I'm sure you've answered the same question more than once here.
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How absurd would it be for you to fly any aircraft that would not show you the numbers required by the AFM?

And you as a pilot have no method of telling what is wrong with the aircraft. I'm sure the FAA/NTSB will have no problem jamming you with several parts of FAR 91.

When you do not see the Static RPM required in the AFM you have a discrepancy, you fly the aircraft you risk your ticket. I'm sure if the FAA sited a pilot for a cracked lens they will site you for flying an unairworthy aircraft.

That will be my last post in this thread, I'm sure every body except Ron knows my point. I'm off to a Fly-in at Concrete Wa. for the weekend.
 
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Re-reading the SB indicates that any overspeed (using the definition in the Bulletin) will cause the engine to become subject to increased inspection procedures, depending on the percentage of overspeed. The categories are <5%; 5-10% and >10%. But it also said that any amount of overspeed can result in engine failure. This is for fixed wing. I find it interesting that for rotary wing, any overspeed is intolerable.

Personally, I wouldn't want a mechanic correcting one issue by creating another.

What I'm learning is that there are 3 specifications the pilot can use to determine if the engine is healthy and that is idle speed; static rpm; and max rpm (WOT in-flight). These are all non-subjective.

Things such as hesitation, funny noises and 'it just runs like a dog' (although helpful) aren't sufficient to determine airworthiness. I think a case can be made that the objective numbers are airworthiness issues.

Just because something is run that way in the "real-world" doesn't make it right.

Henning said:
Time to buy a plane.
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And judging by some of the liberal attitudes I've seen towards maintenance I am so thankful to be out of the rental market. Perhaps someday I will also be responsible for
Tom-D said:
Sec. 91.405 — Maintenance required.
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and can follow other good examples

John Baker said:
Two or three years ago when I had to do a SODA flight with an FAA inspector from the local FSDO, they went nuts trying to find something, anything, wrong with my 1978 Warrior. It turned out my appointment was one hour after some clown had smashed his PA-28 into an IRS office over in Texas. They were jumpy to say the least.

They had two of their inspectors going over every inch of my airplane itself. They had another one going through all the aircraft logs looking for anything amiss. Then they had another one going through my log book (I was a student at the time) I was with that one while the other three were working on their part of it. He brought up every instructor I had ever had on his screen. He grilled me about all of them.

After more than two hours the only thing they could find wrong with my airplane, my instruction, me, or my aircraft records, was my right wing tip had a half inch crack that had gone past a stop drill hole. Just one.

The SODA ride covered every single aspect that could be covered in a SODA. It lasted a full hour. I even asked the guy at one point after he had me do a simulated power off emergency landing, if he was giving me a check ride? He said he was not, but I had to meet all of their requirements.

That ride was considerably more extensive than my check ride itself.

So no, they can not find things wrong with an airplane if they want to, at least not if it is a well maintained airplane. They sure as hell wanted to find something, anything, wrong with mine.

They had put me through a full blown FAA grinder. I passed with flying colors, and so did my airplane.
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just my $.02
 
From: customercare@cessna.textron.com
To: Jaybird180
Subject: Density Altitude
Date: Tue, 14 Aug 2012 13:50:47 +0000
Dear Sir,
In regard to your request for a formula to normalize for density altitude, I have never seen any type of formula for this calculation here at Cessna. So there is nothing that I can send you to find this information.
Thank you,
Shannon Selstedt
Customer Service Engineer
Propeller Hotline
Cessna Aircraft Company
316-517-5800

From: Jaybird180
To: productsupport@mccauley.textron.com
Subject: Static RPM question
Date: Mon, 16 Jul 2012 11:30:18 -0400

Hello,
I have a McCauley Fixed Pitch prop on my Cessna 172S and there is a question about the static rpm and Density Altitude calibration. I am hoping that you can provide the calibration formula for normalizing to standard day conditions for comparison to the aircraft TCDS.
I also intend to ask the same question of Lycoming; just not sure if you would have the formula.
Thank you.
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No reply as yet from Lycoming, but I suspect the same answer.

I consider the matter closed, but thought I'd post for posterity's sake.
 
I thought static RPMs were easy...

First, build your binary...

gcc -o foo foo.o bar.o -static -l[library] ...

Heh heh. Sorry. ;)
 
This thread had many of my favorite participants, and I had a question, so I thought I would bump it and add the question: Is maximum static RPM an operating limitation or is it the maximum that can be achieved at full throttle? Example- if an airplane has a maximum static rpm of 2400, and maximum rpm of 2700 is the plane unairworthy if at standard temperature and pressure, no wind and brakes locked, full throttle yields 2450 rpm? If so, what has to be done to restore the airplane to airworthiness? Assume the fixed pitch propeller is the only one authorized for that aircraft. If the plane is in service and a pilot preparing for a short field takeoff in no wind sees an rpm of 2450, can he simply reduce throttle and take off? If maximum static rpm, but not engine redline, is exceeded, what measures, such as inspection, would need to be made to ensure continued airworthiness?
 
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