Poor Engine- Yet Airworthy

OK, I'll bite.

So -- how do I know the tach is reading exactly right at 2200, or 1800, or 2350 RPM? How do I know it's not 2180 or 1825?

I know book value at max RPM in level flight (2550), but I don't see that on a ground static run (nor should I).

IMO, if the RPM on application of full power equals 21-2200 and then maybe hits 2300 once airborne, I know I'm getting expected performance.

We just completed a very thorough annual, and there was no calibration of the tach required (or course we did static runups, etc, but we didn't strobe the prop and compare to inside readout because that's not required.
 
Tom-D said:
What is the owner / operators requirement to have any other instrument except the altimeter calibrated ?
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None set forth except that he is to ensure the aircraft is airworthy. It's another of those issues where the FAA gives you plenty of rope to hang yourself with and it's up to you not to fall from the scaffold. If you do have an accident where the accuracy of the instrument shows to be involved in the accident chain, you will get dinged for flying an unairworthy aircraft. If you don't have an accident, the only limiting factor will be when the IA does his runnup and says "We have a problem...."
 
dmccormack said:
OK, I'll bite.

So -- how do I know the tach is reading exactly right at 2200, or 1800, or 2350 RPM? How do I know it's not 2180 or 1825?

I know book value at max RPM in level flight (2550), but I don't see that on a ground static run (nor should I).

IMO, if the RPM on application of full power equals 21-2200 and then maybe hits 2300 once airborne, I know I'm getting expected performance.

We just completed a very thorough annual, and there was no calibration of the tach required (or course we did static runups, etc, but we didn't strobe the prop and compare to inside readout because that's not required.
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You check it against an optical tach. Due to the nature of how mechanical tachs operate, typically they will lose RPM with age, so the first indicator you will have is that you aren't making static RPM. So before tearing into the engine to find out why it's not making power, the first thing you do is check the tach calibration. There is no operational requirement to check the tach calibration. If everything is reading as it should, you've met the operational burden. It's when things don't read as they should that one checks the tach calibration as the opening salvo in diagnosing "why."
 
It was my "job" to go out with the pilots and strobe the props before the Air Race Classic a couple years ago. I was surprised at how inaccurate and hard to read some of the analog tachs were compared to the digital tachs which were almost always right on.
 
Ron Levy said:
When you check the shop manual on the tach itself, let us know what it says.
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Hee hee, this is fun.

Reducto ad absurdum, and all!

I guess if I'm back under the panel sometime, I'll try to get a manufacturer and part number off of the tach and see if I can locate a shop manual.

Neither the PA-32 shop manual, the POH nor the flight manual list the instrument P/Ns or manufacturers.
 
Ron Levy said:
91.7(b) does not say it's "pilot's discretion" to decide whether something is airworthy or not. It says it's the pilot's responsiblity to ensure that it's airworthy. Very big difference.
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I'll give ya that, but it is still their decision.

what criteria would they use, to say it is not airworthy?
 
Tom-D said:
I'll give ya that, but it is still their decision.
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...and the pilot's responsibility if s/he guesses wrong.

what criteria would they use, to say it is not airworthy?
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Whatever criteria apply to the issue in question, and it may take research to be sure what criteria those are. For a mechanical tach, what's the limit? 50 low? 100 low? 200 low? 500 low? I don't know. The FAA pubs refer to SAE AS404B, but I'm not spending $65 just to find the answer.
 
Jaybird180 said:
Hypothetical question:

A plane with a weak running engine goes in for annual. The plane is personally owned and operated strictly for pleasure.

Is this plane considered airworthy?
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Real question though is was it airworthy when you brought it to him? Determination of airworthy is PIC duty on every flight. If you are running down the runway full throttle @ 50 kts on your way to take off and you're in a 172 making 2100rpm, is it legal for you to rotate?
 
Ron Levy said:
Whatever criteria apply to the issue in question, and it may take research to be sure what criteria those are. For a mechanical tach, what's the limit? 50 low? 100 low? 200 low? 500 low? I don't know. The FAA pubs refer to SAE AS404B, but I'm not spending $65 just to find the answer.
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Didn't we have a long discussion in here about a year ago using either fluorescent lighting or the mercury vapor lights at the airport at night to check tach accuracy? :yesnod:

Jim
 
Ron Levy;670219Whatever criteria apply to the issue in question said:
There is none, now what?.[/COLOR]

and it may take research to be sure what criteria those are.

IOWs you assume that your tach is correct due to what you see.

For a mechanical tach, what's the limit? 50 low? 100 low? 200 low? 500 low? I don't know. The FAA pubs refer to SAE AS404B, but I'm not spending $65 just to find the answer.
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What you are saying is you have no clue as to what makes a tach airworthy or not. For as long as you have been flying you made your decision upon what you saw and what you though was right. Just like every one else, simply because there is on requirement to do other wise.
 
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Ron Levy said:
If you don't know if the tach is accurate, how do you know the engine is performing within spec?
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Ron -- as you have admitted in this thread, there is no (none, not any) accuracy requirement for the tachometer beyond the indications required for engine performance to meet the manufacturer's numbers.

In other words, if the tach indicates what's expected on static runup and all other indications agree (oil pressure, MAP, whatever else you have on board) you have satisfied the accuracy - and therefore airworthiness -- requirement.

That said, if the pilot suspects the tach indication isn't correct or not indicating correctly, the pilot can declare the airplane un-airworthy and go down whatever expensive analytics he/she chooses.

Back to my earlier question -- if the static runup indicates 2150 per the Lycoming Manual, then that's that. If at cruise the tach indicates 2300 and the actual, precise RPM is 2317.002 -- who cares?

As long as the max RPM is not exceeded at max power (this is limited by the prop), and the idle RPM meets book expectations -- what's the difference?

The only truly accurate tach's I've seen have been the digital type (at least I guess they were accurate -- I couldn't count that fast). All the rest have small enough dials it's hard to tell the difference between 2125 and 2150.

Except in my huge Art Deco counter-clockwise Tach -- that sucker is HUGE! And it bounces some. So I guess I'm at 2223 and 2257 at cruise.

Fall_2009_143.jpg


:dunno:
 
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Ron Levy said:
If you don't know if the tach is accurate, how do you know the engine is performing within spec?
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How do you know it isn't right?

Here we go again,
 
Ron Levy said:
I've got plenty of clues. You're the one who said that accuracy didn't matter in determining airworthiness.
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You have not told us how you determine the tach is accurate, You can't because no one else can either.

I have a hand held electronic tach that I use when the onboard Tach is in doubt. At annual time is the only time it matters, Plus the fact that I can't require an owner to replace the Tach because there is no requirement to have it with in certain allowances, nor is there a requirement that the onboard tach be used as the out of annual turn up too see the engine RPM.
 
dmccormack said:
Ron -- as you have admitted in this thread, there is no (none, not any) accuracy requirement for the tachometer beyond the indications required for engine performance to meet the manufacturer's numbers.
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Never said that, and it isn't true. In fact, I told you where to find it, but apparently you (like I) aren't willing to pay $65 to get the answer.
 
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dmccormack said:
In other words, if the tach indicates what's expected on static runup and all other indications agree (oil pressure, MAP, whatever else you have on board) you have satisfied the accuracy - and therefore airworthiness -- requirement.
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That's like saying you wouldn't care if your torque wrench was 15 ft-lb off if you didn't know it was off that much. How would that go down if you were torquing someone's spark plugs?

Likewise, what if the tach is reading 150 RPM low on that 2150 RPM minimum run-up? That would mean the engine is really turning 2000 RPM, not the minimum 2150 for airworthiness, and that airplane would be way short of power no matter what the MP (which is going to read ambient pressure at full throttle regardless) or oil pressure (which doesn't drop noticeably between full throttle and 75% power) showed you.
 
Ron Levy said:
If you don't know if the tach is accurate, how do you know the engine is performing within spec?
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By also referencing general performance. If you're off the runway the same spot as usual, everything sounds as it usually does and the tach is reading what it should, you don't have much to be concerned with. It might be a good idea just for general principle to check them against optical on every fifth annual or so. Typically if you are paying any attention to your plane, it will become obvious when the tach is off. If you aren't paying attention, then it doesn't matter anyway.

Also I've never seen a tach gain RPM in a failure that wasn't blatantly obvious. The insipid failure mode is to slowly lose rpm as the magnets weaken and the spring loses elasticity.
 
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Henning said:
By also referencing general performance.
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That can give you hints, but static RPM is the standard of measurement, and if the tach isn't accurate, neither is the measurement. Also, a tach degrading to the slow side can mask overspeeds in cruise and especially in a descent with a FP prop.
 
FWIW, Canada has an annual tach calibration requirement. From http://www.tc.gc.ca/eng/civilaviation/regserv/cars/part6-standards-a625c-2460.htm

8. Tachometers The accuracy of mechanical drag cup type tachometers, for fixed wing propeller driven aircraft, shall be checked on site annually, and be accurate to within the tolerances established by the aircraft manufacturer or, where no tolerance has been specified by the aircraft manufacturer, to within +\- 4% of engine RPM at mid-point of the cruise range.

Dan
 
Ron Levy said:
That's like saying you wouldn't care if your torque wrench was 15 ft-lb off if you didn't know it was off that much. How would that go down if you were torquing someone's spark plugs?

Likewise, what if the tach is reading 150 RPM low on that 2150 RPM minimum run-up? That would mean the engine is really turning 2000 RPM, not the minimum 2150 for airworthiness, and that airplane would be way short of power no matter what the MP (which is going to read ambient pressure at full throttle regardless) or oil pressure (which doesn't drop noticeably between full throttle and 75% power) showed you.
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Torque Wrenches have calibration standards, 1 pound at 12 inches is 1 foot pound, show me the formula for a tach.

all the rest of your post is a guess.
 
Ron Levy said:
That can give you hints, but static RPM is the standard of measurement, and if the tach isn't accurate, neither is the measurement. Also, a tach degrading to the slow side can mask overspeeds in cruise and especially in a descent with a FP prop.
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"Hints" is what we mechanics work off of. We look for hints, when hints apear, we dig deeper to see what is being hinted at.
If you're willing to own a plane with a fixed pitch prop that's just something you have to deal with. I'm not aware of many planes with a fixed pitch prop that will dangerously overspeed in cruise, and even in descent, if someones engine management is that that poor, they probably should stick to gliders.

There is no rule stating that you must check the calibration, and there is no rule that says you may not. It's up to you. If you want to leave an optical unit on your glare shield to run continuous reference against, that is also permitted.
 
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Ron Levy said:
That can give you hints, but static RPM is the standard of measurement, and if the tach isn't accurate, neither is the measurement. Also, a tach degrading to the slow side can mask overspeeds in cruise and especially in a descent with a FP prop.
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And varies for each engine and prop combination.

but tachs are called out by part number and stay the same in in the IPB.
 
Ron Levy said:
That's like saying you wouldn't care if your torque wrench was 15 ft-lb off if you didn't know it was off that much. How would that go down if you were torquing someone's spark plugs?

Likewise, what if the tach is reading 150 RPM low on that 2150 RPM minimum run-up? That would mean the engine is really turning 2000 RPM, not the minimum 2150 for airworthiness, and that airplane would be way short of power no matter what the MP (which is going to read ambient pressure at full throttle regardless) or oil pressure (which doesn't drop noticeably between full throttle and 75% power) showed you.
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Torque wrenches are calibrated on some PM schedule (in the Air Force it was monthly trip to PMEL).

Tachometers are not.

Now, re-read my example:

Ron Levy said:
Back to my earlier question -- if the static runup indicates 2150 per the Lycoming Manual, then that's that. If at cruise the tach indicates 2300 and the actual, precise RPM is 2317.002 -- who cares?
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If the static runup (which is limited by the load on the prop) achieves the full static value (for grins, we'll say 2150), then that is good enough.
 
Ron Levy said:
I gotta remember not to let Dan, Tom, or Henning ever work on my plane.
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LOL, you'd be so lucky... Just keep on paying those NE rates to fix things that aren't broke... BTW, just how frequently do you calibrate your tach? Do you think your engine will ingest itself at 50 RPM over redline? 150rpm over? What RPM do you think will lead to catastrophic failure? What RPM do you think virtually stock O-200s turn in Formula racing?
 
dmccormack said:
For someone who is very precise about terminology I find your statement troubling, as you don't need an "A&P ticket" to work on an airplane.


:cornut:
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Nevermind the "errors" in terms of forgotten references... :D
 
Ron Levy said:
"Lucky"? Hardly, unless you got your A&P ticket without telling anyone. Or is that omission another "error" in the FAA system?
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There's no error, an A&P ticket is not and never has been required for working on an airplane. Used to be I could even sign annuals off, but they changed that rule.
 
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