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GrummanDude
Joined: 15 Jan 2006
Posts: 926
Location: Auburn, CA
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 Posted: Tue Sep 08, 2009 11:10 pm Post subject: Tach accuracy |
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Is there a regulation, FAR, saying the tach cannot be placarded and must be replaced if the tach is more than 150 rpm wrong? I can't find it if there is one. [quote][b]
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Gary
AuCountry Aviation
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az_gila
Joined: 17 Jun 2006
Posts: 99
Location: Tucson, AZ
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| Posted: Wed Sep 09, 2009 7:50 am Post subject: Tach accuracy |
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Gary,
I don't think so, since this came from
AC 20-37D AIRCRAFT METAL PROPELLER MAINTENANCE
on care of metal props - note the bit on no post-manufacture accuracy.
It might be hidden in the SAE standard referenced by TSO C94 for
tachometers, but the SAE has deemed it necessary to charge $$ for all
of their documents.
..but this really old FAA TSO gives the original 1954 accuracy as 25
rpm at room temp....
http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgTSO.nsf/0/985307e8eb1b9a1986256e930061198b/$FILE/c49.pdf
The link above is to the TSO and also to SAE AS404A
This SAE $$ document is now at rev. C - but, like the text below
says, no post-manufacture stuff is probably in rev C....
I seem to remember the 150 rpm from one of the FAA Mechanics AC
books, but I can't presently find it - but it was not a FAR --
gil A
-------------------- AC 20-37D text extract -------------
8. TACHOMETER INSPECTION.
Due to the exceptionally high stresses that may be generated by
particular propeller/engine
combinations at certain engine revolutions per minute (RPM), many
propeller and aircraft
manufacturers have established areas of RPM restrictions and other
restrictions on maximum
RPM for some models. Some RPM limits have never exceed values as
close as 3 percent of the
maximum RPM permitted, and a slow-running tachometer can cause an
engine to run past the
maximum RPM limits. Since there are no post-manufacture accuracy
requirements for engine
tachometers, tachometer inaccuracy could be a direct cause of
propeller failure, excessive
vibration, or unscheduled maintenance. Tachometer accuracy should
always be checked during
normal maintenance intervals or sooner if indicators such as
excessive vibration or aircraft
performance changes might indicate inaccurate RPM readings.
---------------------------
At 12:01 AM 9/9/2009, teamgrumman(at)aol.com wrote:
| Quote: | Is there a regulation, FAR, saying the tach cannot be placarded and
must be replaced if the tach is more than 150 rpm wrong? I can't
find it if there is one.
|
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GrummanDude
Joined: 15 Jan 2006
Posts: 926
Location: Auburn, CA
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| Posted: Wed Sep 09, 2009 1:04 pm Post subject: Tach accuracy |
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I couldn't find anything either. The buyer wants a new one not realizing it is probably 100 rpm wrong to begin with.
--
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Gary
AuCountry Aviation
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az_gila
Joined: 17 Jun 2006
Posts: 99
Location: Tucson, AZ
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| Posted: Wed Sep 09, 2009 4:45 pm Post subject: Tach accuracy |
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Yep... I sent my mechanical one for overhaul because it was about 125 rpm out, It came back with the same error...:^)
The airpark pilots tell me that the local Tucson speedometer shop can re-calibrate it without telling the FAA, but I bought a digital one instead.
The old one went on e-bay for $50.... gil A
At 01:58 PM 9/9/2009, you wrote:
[quote]I couldn't find anything either. The buyer wants a new one not realizing it is probably 100 rpm wrong to begin with.
--
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az_gila
Joined: 17 Jun 2006
Posts: 99
Location: Tucson, AZ
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| Posted: Wed Sep 09, 2009 7:22 pm Post subject: Tach accuracy |
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Century Instruments IIRC - the problem is that they can't legally overhaul it, they can just inspect and repaint the dial.
Something to do with not having the full data from Stewart Warner.
The speedometer shop fairies might do better....:^) gil A
At 06:01 PM 9/9/2009, you wrote:
[quote]Gil: Recently discovered that I have an Tach accuracy problem, was referred to a shop in Houston. But, was warned about overhaul problems with numerous shops around the country..so, will probably go with a new mechanical Tach with a warranty. Who overhauled your Tach?
Thanks,
Roger
Roger Rucker
AA5A '78 HWO
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flyv35b(at)minetfiber.com
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| Posted: Thu Sep 10, 2009 4:42 am Post subject: Tach accuracy |
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I gave up on mechanical tachs years ago. If you really want to know what your RPM is (accurately) there is no substitute. If you want to evaluate performance changes or whether or not you have the prop pitched the way you want it or not an electronic digital readout tach is essential. For instance what improvement in static rpm and full throttle rpm do you get with the addition of a Power Flow exhaust or HC pistons in a Cheetah. How does full throttle rpm change with altitude and other similar questions you many want answers to.
It amazes me sometimes that people don't mind spending $10K on a new Garmin 430W installation but they won't spend an extra $200 to buy an electronic tach over a mechanical one!
Cliff
[quote] ---
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Discover
Joined: 26 Feb 2007
Posts: 429
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| Posted: Thu Sep 10, 2009 7:07 am Post subject: Tach accuracy |
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Hi Gil,
Just curious what you used to check the accuracy of your tach?
ned
[quote] ---
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az_gila
Joined: 17 Jun 2006
Posts: 99
Location: Tucson, AZ
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| Posted: Thu Sep 10, 2009 8:17 am Post subject: Tach accuracy |
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Hi Ned,
I used a borrowed digital model rpm gauge - worked great. Just pointed it forward from the cockpit when in flight.
They can be checked for accuracy against the 60/120 Hz component of mains driven lights.
Cheap too --
http://www.towerhobbies.com/products/towp1221.html
It will even work on three bladed props....:^)
gil A
At 08:05 AM 9/10/2009, you wrote:
[quote] Hi Gil,
Just curious what you used to check the accuracy of your tach?
ned
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