Tachometer reading incorrect
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Tachometer reading incorrect
The a/c I hired yesterday had a tachometer which was over-reading by between 300 - 500 rpm, making it just about useless - my passenger wondered why I kept changing the power settings on final.
Is there any way of checking for this on the ground besides putting on full power in the run-up checks?
Is there any way of checking for this on the ground besides putting on full power in the run-up checks?
Preplanned RPM settings can only ever be approximate on final, the setting required depends on a number of factors including position in the glideslope, a/c loading, headwind etc etc so I don't see myself why an incorrect tach could cause any significant problem on approach.
However, if it did indeed over-read to that extent, surely this would have been apparent at both full power and idle power checks?
Could it just be that the prop on that aircraft has a finer pitch to others you are used to flying hence requires a higher rpm for a given glideslope?
However, if it did indeed over-read to that extent, surely this would have been apparent at both full power and idle power checks?
Could it just be that the prop on that aircraft has a finer pitch to others you are used to flying hence requires a higher rpm for a given glideslope?
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Was it placarded as overreading, or did you find out yourself. And if so, how?
And how much did you pay when the flight was over? If the tacho RPM overreads significantly, then so will the tacho counter. If the rental price is based on tacho hours, then you'd be charged 1.2 hours or so for each hour flown.
And how much did you pay when the flight was over? If the tacho RPM overreads significantly, then so will the tacho counter. If the rental price is based on tacho hours, then you'd be charged 1.2 hours or so for each hour flown.
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foxmoth
I don't think checking the tachometer with a stobe would be practical every time I fly.
Mariner9
The approach was still flyable and of course the power setting depends on a number of factors, but without the tachometer one doesn't know what power setting one has selected and that doesn't make life any easier. It's quite possible to do an approach without an ASI too, but I'm not going to try!
Yes - I probably should have realised there was a problem when doing my power checks, but the mag checks were fine. It's even possible the problem developed during take off. The a/c had been flown previously that day by an instructor and he obviously hadn't noticed it.
It's possible too that the error was not linear over the whole range - being greater at the high end.
The propellor was of the fixed pitch type.
BackPacker
I found it out myself - some time after leveling off after take off. This would have meant I had done my power checks at 1200 rpm rather than 1700 rpm. Also if the engine had not been producing full power on take off I would have been misled into thinking it was. I probably would have realised this by sound alone, but one can't always be sure.
The flight was charged by time and not tacho hour so I wasn't over-charged.
I don't think checking the tachometer with a stobe would be practical every time I fly.
Mariner9
The approach was still flyable and of course the power setting depends on a number of factors, but without the tachometer one doesn't know what power setting one has selected and that doesn't make life any easier. It's quite possible to do an approach without an ASI too, but I'm not going to try!
Yes - I probably should have realised there was a problem when doing my power checks, but the mag checks were fine. It's even possible the problem developed during take off. The a/c had been flown previously that day by an instructor and he obviously hadn't noticed it.
It's possible too that the error was not linear over the whole range - being greater at the high end.
The propellor was of the fixed pitch type.
BackPacker
I found it out myself - some time after leveling off after take off. This would have meant I had done my power checks at 1200 rpm rather than 1700 rpm. Also if the engine had not been producing full power on take off I would have been misled into thinking it was. I probably would have realised this by sound alone, but one can't always be sure.
The flight was charged by time and not tacho hour so I wasn't over-charged.
Moderator
If the tach is really out that much, it should be replaced. In Canada, a 5% error is the most which will be tolerated to be placarded. Such errors can allow engine or propellr speed limits to be exceeded in some cases. Not good. They can also really make some performance charts inaccurate.
If in doubt, have the tach check optically - it's easy. Generall older tachs cannot practically be adjusted, as their cases are crimped shut. Buying a new tach is generally the only soultion.
Pilot DAR
If in doubt, have the tach check optically - it's easy. Generall older tachs cannot practically be adjusted, as their cases are crimped shut. Buying a new tach is generally the only soultion.
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I suppose what I am asking is how does one spot this accurately during a normal "A" and power check?
If it's a recent failure of the tacho, then the idle RPMs will be very abnormal, while the engine idle settings have not changed. Likewise, when starting the take-off roll, there are minimum and maximum RPMs that you should see when applying full power.
Another way is by setting cruise RPM and not getting the associated cruise performance (IAS). But then again that might be caused by a faulty pitot/static system or faulty ASI too, so double-check with DME/GPS and correct for wind.
But indeed, those are general indications and not very accurate.
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Backpacker
Thanks, that's useful.
I fly a different aircraft (although the same type) very time. Idle indicated in excess of 1000 rpm which I should have noticed as far from normal (although the setting does vary considerably).
It was in the cruise that I first noticed it and it was possible then to gauge fairly accurately the extent of the over reading.
Thanks, that's useful.
I fly a different aircraft (although the same type) very time. Idle indicated in excess of 1000 rpm which I should have noticed as far from normal (although the setting does vary considerably).
It was in the cruise that I first noticed it and it was possible then to gauge fairly accurately the extent of the over reading.
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Surely if it overread it would have redlined on the takeoff roll. What rpm did you have on the climb out?
I insist all my students know what the static RPM limits (fixed pitch prop) are for the aircraft they fly. It is considerably lower than the redline. For example the C 150 has a 2700 RPM redline but the POH lists the static RPM as 2460 to 2560 RPM. I teach all my student that as soon as the throttle has reached the full in/forward position (after being slowly and smoothly advanced at the start of the takeoff role) they check the RPM (looking for static RPM value) and engine gauges (looking for all in the green) and verbalize the call "good engine". In molesworth 1 case I would expect my students to have rejected the takeoff at the engine check point because the RPM would have been at or over the redline RPM and therefore not at the expected static RPM value.
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Well its one more issue to add to the list of things I now know can go wrong which I didn't learn in my PPL course. My present flying club has been very helpful in this respect.
The older Cessna's don't have a green arc - they might well have a red line although I've never been consciously aware of it.
Is there any other reason the tacho could be over the red line? The propeller falling off?
The older Cessna's don't have a green arc - they might well have a red line although I've never been consciously aware of it.
Is there any other reason the tacho could be over the red line? The propeller falling off?
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It's quite possible to do an approach without an ASI too, but I'm not going to try!
People have killed themselves by screwing up an approach with a dead ASI, which is quite unnecessary.
Fly a circuit with an instructor and the ASI covered up.
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Amother reason is that a prop can "untwist" if its not the tacho this happens over a time so its not working so hard and the revs rise slightly....... Overhauling the prop you can have it set to be a climb prop or a cruise prop... If they can, ask them to swop tachos with another one if they have 2 or more aircraft, see it it makes any difference.
Have had one over revving in the past and took it up with both Cessna, Lycoming and the CAA.
Cessna stated read the book that says go by Lycomings figures. ( which I had)
Lycomings figure for the static rated RPM for the Engine was higher than the Cessna figure by quite a large amount and even swopping props with an overhauled one or renewing the RPM gauge I was able to reduce the figure...
The Overspeed limits took it well over what it was ever achieving so It was left alone and quite happily revved through the redline on take off but was below Lycomings acceptable figure. So it lived like that for a thousand of hours or so until the prop was eventually replaced again at overhaul, and now it is back where it should be.......... oddly it's prop went on another aircraft and is fine on that one, go figure.
If the rpm gauge is fluctuating this can often be a worn tacho drive cable as the inner whips around inside the outer allowing it to happen.....
Lycomings figures and determination of overspeed / overboost are here on their website for you all.
http://www.lycoming.textron.com/supp...dfs/SB369J.pdf
So as an example a Cessna 152 that is on the Type cert as
http://www.airweb.faa.gov/Regulatory...$FILE/3A19.pdf
on Lycomings figures the O-235-L2c as an example is actually rated at 2800 RPM
Have had one over revving in the past and took it up with both Cessna, Lycoming and the CAA.
Cessna stated read the book that says go by Lycomings figures. ( which I had)
Lycomings figure for the static rated RPM for the Engine was higher than the Cessna figure by quite a large amount and even swopping props with an overhauled one or renewing the RPM gauge I was able to reduce the figure...
The Overspeed limits took it well over what it was ever achieving so It was left alone and quite happily revved through the redline on take off but was below Lycomings acceptable figure. So it lived like that for a thousand of hours or so until the prop was eventually replaced again at overhaul, and now it is back where it should be.......... oddly it's prop went on another aircraft and is fine on that one, go figure.
If the rpm gauge is fluctuating this can often be a worn tacho drive cable as the inner whips around inside the outer allowing it to happen.....
Lycomings figures and determination of overspeed / overboost are here on their website for you all.
http://www.lycoming.textron.com/supp...dfs/SB369J.pdf
So as an example a Cessna 152 that is on the Type cert as
Engine limits S/N A1500433, A1520735, 681 through A1521014
For all operations, 2550 r.p.m. (110 hp.)
S/N A1521015 and on
For all operations 2550 r.p.m. (108 hp.)For all operations, 2550 r.p.m. (110 hp.)
S/N A1521015 and on
http://www.airweb.faa.gov/Regulatory...$FILE/3A19.pdf
on Lycomings figures the O-235-L2c as an example is actually rated at 2800 RPM
Last edited by NutLoose; 6th Jul 2009 at 21:06.
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Have done - except for the last bit of final.
Yes you will be coming in a bit faster than usual, yes you will need a decent length runway, no you won't know exactly how fast you're going.
So you just fly along the runway a couple of feet up until it stops flying and you land. No problem, just don't try it on 450m of grass, if it happens to you for real divert to somewhere with a couple of kilometres of tarmac, that's all really.
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Cessna stated read the book that says go by Lycomings figures. ( which I had)
Lycomings figure for the static rated RPM for the Engine was higher than the Cessna figure by quite a large amount and even swopping props with an overhauled one or renewing the RPM gauge I was able to reduce the figure...
Lycomings figure for the static rated RPM for the Engine was higher than the Cessna figure by quite a large amount and even swopping props with an overhauled one or renewing the RPM gauge I was able to reduce the figure...
These are spelled out in the type certificate data sheets for the airplane. That Lycoming states the engine may be run to a particular RPM is meaningless. The only figure that has meaning is the one established for the engine-propeller combination, specifically the one found in the TCDS.
Amother reason is that a prop can "untwist" if its not the tacho this happens over a time so its not working so hard and the revs rise slightly....... Overhauling the prop you can have it set to be a climb prop or a cruise prop... If they can, ask them to swop tachos with another one if they have 2 or more aircraft, see it it makes any difference.
There's little point swapping tachometers if you can't verify the calibration of the replacement tachometer.
The answer here is to verify the actual propeller RPM, first. If you don't know the actual propeller RPM, then you don't know anything, and no matter what you do from then on, it's purely guesswork with no baseline for comparison.