Tachometer reads low

brien23

brien23

Cleared for Takeoff
Joined
May 31, 2005
Messages
1,489
Location
Oak Harbor
Display Name
Display name:
Brien
How low can a tachometer read 100, 150, 200 at what point must it be replaced or repaired. I have seen something like 5% is the max error allowable but cant find it.
 
Last edited:
Found in a previous thread: "The regs require your tach to be operating properly at takeoff (91.213(a)) and during flight (91.7(a)). Further, if it fails in flight, you are required to "discontinue the flight" (91.7(b)). Lest we get into a "correct" versus "inoperative" discussion, the FAA maintains that malfunctioning is the same as "inoperative" (Administrator v. Bass, Administrator v. Bernstein)."

The only thing I can't tell you is how far off it has to be before it reaches a legally unairworthy condition. I've flown planes with tachs that were known to be off 100-150 RPM after checking them with a strobe tach so I knew what I had based on what the instrument read -- kind of like a compass correction card. I'm not entirely sure the FAA would say that's within the letter of the law, but the errors were consistent and repeatable, so I felt comfortable with the safety factor.
 
Very timely question. One of our club airplanes has an inaccurate tach (according to the chief instructor) and so we "idle that one at 900 because the tach is off a little." I wondered where to find the information as to what makes it airworthy or un.

John
 
Operating the engine and propellor in excess of the manufacturer's limits should be more of a concern than complying with FAR airworthiness regs.
 
3393RP said:
Operating the engine and propellor in excess of the manufacturer's limits should be more of a concern than complying with FAR airworthiness regs.
Click to expand...

No argument from me. But this is a fixed pitch C-172 so you'd have to work at it.
 
There is an app for about $10 that determines RPM from sound. Good for checking tachs.
 
In Canada the allowable error is 4%. I haven't seen any FAR specs. These tachs use a magnetic drag to make them work, and the magnet weakens with age, making the tach under-read.
 
In the TCDS it lists the maximum static RPM for an engine/propellor combination. It also lists a minimum static RPM. If the static RPM isn't within the range, an investigation by a mechanic is necessary before the airplane leaves the ground. This is something that I'm going to include in my personal checklist for any airplane that I'm not familiar with.
 
Stewartb said:
Do you routinely do full power runups?
Click to expand...

Most don’t even though you probably should. A full power takeoff is close enough most of the time. Just glance over once your advanced the throttle all the way. 5-10mph want change very much
 
It changes a great deal. Static means static. In a conversation about inaccurate tachs the value of static rpm is suspect.

The last two mechanical tachs I've bought indicated 50-75 rpm less than my TruTach in cruise. Not a big deal.
 
A video RPM smartphone app is a very accurate (I think) way of determining your static RPM. And it is totally free, can be accomplished safely from inside the airplane in 2 minutes. I can’t see how there could be much error as the computing power of a phone is high enough to be able to figure out frame rate to many decimal places. My app has 10 rpm steps in the setup, but you can pay a few bucks for 1 rpm accuracy.
 
Among other things, an inaccurate tach leads to incorrect fuel flow calculations, incorrect %power estimates for leaning, as well as risk for prop overspeed. There are lots of phone apps that seem to be pretty accurate. I check my tach vs the phone app on a regular basis, and it seems spot on now. Before I had it reconditioned it read about 50-100 rpm low, which explained the slightly high fuel flows and better than book speeds I was getting.
 
When a Tach reads LESS than actual most folks will increase RPM per indicated.

In addition to previous post it may make ADs, 100 hr Inspections and MOH come

around sooner.

Sorta makes a Tach look cheap.
 
Matthew Rogers said:
A video RPM smartphone app is a very accurate (I think) way of determining your static RPM. And it is totally free, can be accomplished safely from inside the airplane in 2 minutes. I can’t see how there could be much error as the computing power of a phone is high enough to be able to figure out frame rate to many decimal places. My app has 10 rpm steps in the setup, but you can pay a few bucks for 1 rpm accuracy.
Click to expand...
Although it sounds good, but it's about as legal as flying a timed instrument approach using my Naval Observatory synchronized wristwatch.
 
Jaybird180 said:
Although it sounds good, but it's about as legal as flying a timed instrument approach using my Naval Observatory synchronized wristwatch.
Click to expand...
What is illegal about double checking your tach using a phone app? Are you not allowed to double check your "high tech" wind up panel installed "IFR certified" clock with your phone either?
 
Any optical tach checker can be calibrated against a fluorescent light. Just hold it a couple of feet from the light, aimed at it of course, and see if it reads 3600 on the two-blade setting. The 60-Hz of the grid is normally maintained to +-0.01Hz, or about 0.017%. Close enough for a tach checker, I think.
 
https://www.amazon.com/dp/B0006N72U2/
Handheld optical tach, less than $30. It will work from inside the cabin through the windshield. Above post of Dan Thomas is spot on.

Not unusual for OEM mechanical tachs to read 100-200 RPM low at cruise to max.
 
Domenick said:
https://www.amazon.com/dp/B0006N72U2/
Handheld optical tach, less than $30. It will work from inside the cabin through the windshield. Above post of Dan Thomas is spot on.

Not unusual for OEM mechanical tachs to read 100-200 RPM low at cruise to max.
Click to expand...
Or again, as I mentioned, the FREE (for +/-10rpm, or $1.99 for +/--1rpm) tachometer app on your iPhone that is just as accurate and with you all the time......but you guys do what you want.

https://apps.apple.com/us/app/video-tachometer/id1492583587
 
How much does an iPhone cost?
:)
 
Domenick said:
Snarkiness aside, the question of smartphone operating system market share is an interesting one. Only in the US and Japan does IOS dominate Android. Worldwide Android:IOS is about 87:13.

https://www.pcmag.com/news/ios-more-popular-in-japan-and-us-android-dominates-in-china-and-india
Click to expand...
Ok, but the same apps are available for android phones too. So which pilots these days do not have any smartphone? Can afford to fly, but haven’t updated your phone in the last 5 years?
 
Matthew Rogers said:
Can afford to fly, but haven’t updated your phone in the last 5 years?
Click to expand...

What do smartphones have to do with "updating"? Nothing. They are still ****ty phones that do a whole bunch of other things poorly.

People who choose not to spend $1k on the truly worthless devices can afford to fly more. To the tune of about 250 gal of 100ll, or perhaps get a year of insurance, or most of an annual inspection. All more useful expenditures than a new "phone".
 
Ravioli said:
What do smartphones have to do with "updating"? Nothing. They are still ****ty phones that do a whole bunch of other things poorly.

People who choose not to spend $1k on the truly worthless devices can afford to fly more. To the tune of about 250 gal of 100ll, or perhaps get a year of insurance, or most of an annual inspection. All more useful expenditures than a new "phone".
Click to expand...
It is getting a little off topic, but the sensors in most smartphones $100 and up include accelerometer, gyroscope, barometer, proximity sensor, ambient light sensor, and a digital compass. These sensors are much more sensitive and accurate than those found in legacy airplanes, $30 digital tachs, and even most legacy AP mechanics equipment. It is often the case that a 10 year old $600 Snap-On tool will be obsolete because of a $20 Amazon device using new ***ty sensors. Just look at digital inspection cameras. Those used to cost hundreds for a crappy camera with bad resolution and now are $19.99 with bluetooth and HD.

The argument was that somehow a smartphone is not a sensitive enough piece of equipment to accurately tell if your tach is reading correctly. That argument is false.
The argument that it costs more than a $30 Amazon optical tach is also false as the fact is that 99% of all pilots have a smartphone capable of running a FREE video tachometer app. So the net cost is $0.
The argument that a smartphone is full of ****ty sensors is false, as those sensors are more advanced than the best stuff around 5 years ago.
 
I have several compass apps in my Samsung android. None of them are worth anything. They are confused by the tiny magnetic fields generated by the circuitry.
 
Dan Thomas said:
I have several compass apps in my Samsung android. None of them are worth anything. They are confused by the tiny magnetic fields generated by the circuitry.
Click to expand...
Ok, so get an iPhone. Mine works as well as any wet compass. Excellent repeatability over and over if I take a measurement off a fixed point, walk around for a while and come back to the fixed point, it is always the same. You may still need a calibration card, just like in an airplane, especially if you always wear a big belt buckle!

But this is about the tach, and how an optical tachometer app on a phone is just as good or better than most other tachometers. Prove me wrong, because I have also checked it against the 60hz grid power and at 3600 rpm it is steady, but slowly starts to flash at both 3610 and 3590 rpm. It is steady again at 7190 rpm, but not 7200, so it looks like the grid is just barely under 60hz right now. (I didn’t pay the $1.99 upgrade to get -/+ 1 rpm precision).
 
Matthew Rogers said:
Ok, so get an iPhone. Mine works as well as any wet compass. Excellent repeatability over and over if I take a measurement off a fixed point, walk around for a while and come back to the fixed point, it is always the same. You may still need a calibration card, just like in an airplane, especially if you always wear a big belt buckle!

But this is about the tach, and how an optical tachometer app on a phone is just as good or better than most other tachometers. Prove me wrong, because I have also checked it against the 60hz grid power and at 3600 rpm it is steady, but slowly starts to flash at both 3610 and 3590 rpm. It is steady again at 7190 rpm, but not 7200, so it looks like the grid is just barely under 60hz right now. (I didn’t pay the $1.99 upgrade to get -/+ 1 rpm precision).
Click to expand...

Nope. No iPhone extravagance for me. If I plan to muddle around the bush I take my hiker's compass, which doesn't have batteries to go dead on me, and a cellphone compass will be no good inside the cockpit of an airplane unless it's calibrated and kept accurate and set in exactly the same place it was calibrated. If I need to check prop RPM I have an optical tachometer that cost me around $40. I do use the other apps I have like a collection of GPS accessories and an inclinometer/level program; they're handy once in a while. The GPS does need to be within cellphone range, or I have to scan the territory first before I get out of range so I have the map. I have a handheld GPS (non-map) that gets me where I want to go---if I even need it---and doesn't need cell service. I have a barometer app. A flashlight app. An altimeter app. A speedometer app. I use the scientific calculator more than all the rest put together.
 
Can't teach an old dog.....

Guess I understand why GA is in the state that it is. Go buy the old tech unitasker optical tach. I'll keep using the newest (and consequently cheapest) and most advanced tech out there.

I think the only piece of old tech that I would consider over a newer unit is an AOA indicator. Since predicting airflow correctly is still in the realm of super computers and even Boeing has trouble with AOA indicators, a dirt simple airflow-based AOA indicator *may* be better than an advanced unit like the AV-30 with its predictive AOA. But I bet that those will soon catch up as there is more data for the computers to use to predict the AOA better in all regions of flight and couldeventually be better than a single probe AOA indicator that could also have blocked airflow in certain unusual circumstances.

I will still install an AV-30 or AV-20 the next time I need to replace some legacy gauge in my plane.
 
Matthew Rogers said:
Ok, but the same apps are available for android phones too. So which pilots these days do not have any smartphone? Can afford to fly, but haven’t updated your phone in the last 5 years?
Click to expand...
To each their own. I certainly do not begrudge you your app.

I tried an acoustical tachometer for Android on my phone (Samsung Galaxy S7) some time ago. It sucked, which was why I went with the $30 optical tach. I keep it in my flightbag. It works perfectly and is easily checked against a fluorescent light as described by Dan Thomas above (#17).
 
Domenick said:
To each their own. I certainly do not begrudge you your app.

I tried an acoustical tachometer for Android on my phone (Samsung Galaxy S7) some time ago. It sucked, which was why I went with the $30 optical tach. I keep it in my flightbag. It works perfectly and is easily checked against a fluorescent light as described by Dan Thomas above (#17).
Click to expand...
The one that I use is an optical tachometer app (video tachometer). So it does not suffer from noise issues and can also be checked against grid power (interestingly it even works with led lights in addition to fluorescent lights).
It benefits from the video processing in that it can determine the rpm of any object at any distance, something that the handheld optical tachometers can’t do (I assume they have to be held close to the object being measured. So apart from being safer by staying farther away from your spinning prop, you could tell the rpm of the helicopter flying overhead, or the guy at the run up area across the airport.
 
Matthew Rogers said:
The one that I use is an optical tachometer app (video tachometer). So it does not suffer from noise issues and can also be checked against grid power (interestingly it even works with led lights in addition to fluorescent lights).
Click to expand...
I missed the fact your app was optical.
...something that the handheld optical tachometers can’t do (I assume they have to be held close to the object being measured. So apart from being safer by staying farther away from your spinning prop, you could tell the rpm of the helicopter flying overhead, or the guy at the run up area across the airport.
Click to expand...
The optical tach I have works from up to twelve feet away and works perfectly from inside the cockpit through the windshield. No danger.
 
Domenick said:
I missed the fact your app was optical.

The optical tach I have works from up to twelve feet away and works perfectly from inside the cockpit through the windshield. No danger.
Click to expand...
Excellent, I figured that a handheld optical tach would be more limited in its range and not be able to measure through clear plastic either. Nobody wants to be close to a spinning prop, especially in front of one.
 
Matthew Rogers said:
Excellent, I figured that a handheld optical tach would be more limited in its range and not be able to measure through clear plastic either. Nobody wants to be close to a spinning prop, especially in front of one.
Click to expand...
Most of them work just fine through the windshield.
 
You must log in or register to reply here.
Back
Top