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?

I think you will find most engineering setups will have a strobe they can use to check RPM.:ok:

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?

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.

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.

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

I suppose what I am asking is how does one spot this accurately during a normal "A" and power check?

After a certain time flying a certain aircraft, you know the sound an engine makes at a certain RPM. That will give you a rough indication.

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.

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.

What was the tach reading on take off? If I was taking off in a SEP with a Lycoming type engine and the tach read over 3000 I'd probably return and land.

Surely if it overread it would have redlined on the takeoff roll. What rpm did you have on the climb out?

Crikey!

Ground check: at take-off the tachometer off the end of the green arc when full power commanded?

From what I recall it was 2700 - 2800 rpm - yet I had to cruise at 2500 to get my 90 knots.

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.

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.:E

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?

You should.

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.

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

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.)

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 done - except for the last bit of final.

So do that as well.

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.

The only figures you can use are the type certificate data figures. These are what must be determined when performing an inspection. Lycoming may have engine limits, but this says nothing about propeller limits, and propeller limits say nothing about engine-propeller limitations.

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.

"Untwisting" fixed pitch propellers?

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.

..... when you run it through soft ground perhaps.

If it is fixed pitch, and airworthy, you can stake your life on the blades being where they were last manufactured or overhauled to be. Because you are staking you life on it.

In today's technological world, forget the inaccuracy of charts, and just use a portable optical tach to check the installed one. The optical tach can be calibration checked when pointed at a flourescent light.

Here is the wording of the Canadian requirement for tachometer airworthiness:

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.
(amended 2000/12/01;

[quote]
As in the example on the 152 for overspeeding the manual says refer to the figures provided by Lycoming that are higher, Not happy with that I faxed Cessna who came back to me and confirmed that if it was over the type Cert figures refer to what the manual is telling you for overspeeding ....... I have it in black and white. it tells you no action is required for the amount we had and to refer to Lycomings engine figures and overspeed data, I then contacted Lycoming who confirmed the figures and spoke at length with the Civil Aviation Authority with all the information at hand and they concurred.


Fix pitch props do untwist, that is why there is a requirement when overhauling a fixed pitch prop to check the angles and retwist it. We in the UK now have to overhaul fixed pitch props at their designated overhaul periods and they are no longer on condition.

If you have a known aircraft on your fleet that is not over revving or known to be it is a simple job to swop tachos if you do not have the facility to calibrate it to check its reading.

A surprising topic.

If you think the tacho is over reading by 300 to 500 rpm then you should never have left the runway. Given the average spamcan's static rpm it would have been on the redline early in the takeoff run.

Even more importantly if it had been under reading, the engine could have been severely down on power. This is the last chance you have to check the aircraft is going to perform the way it is supposed to and clear the far hedge, fence, wall, hill.

Obviously if you are chasing rpm settings you do not have much experience - but do read the flight manuals and KNOW the expected static rpm from the aircraft at full throttle - if you do not get that do not fly. It can save more than just embarassment!

:ok:My mechanical tach failed, so I tried a "Tiny Tach" digital tach bought on ebay for £25. It is spot on accurate , easy to fit and takes up minimum space.

Even more surprising was that I had been chatting to the instructor who had flown the a/c just before me and he never mentioned a word about it - nor did he report it in the tech log.

er.. except they have a post-it note system and nothing gets written in the tech log. I really need to have a chat to someone at the CAA about this sort of thing sometime:E

If that sort of practice is really happening then your first step might be the club - then the CAA.

But for your own sake learn how to assess whether the aircraft is flyable or not. Yes you might expect a club to go a long way towards helping you. But there are any number of mechanical and other issues which can all occur with little warning - from one flight to the other.

If you cannot make a 'go, no go' on your own initiative and knowledge then you should perhaps consider whether flying is a smart recreation?

I'm not trying to be especially smart, but for your own and your passengers health this is a skill you need. There is little more annoying than being stranded somewhere with a 'duff' aircraft - unless it is 'being up there, wishing you were down here'.

Perhaps you should be aware of it. In any aircraft with a fixed-pitch prop it's very easy to exceed redline by failing to reduce power in a shallow descent. Just as pulling too much G can cause permanent damage that later kills someone, overspeeding can cause damage that later results in engine failure or the catastrophic loss of part or all of a prop blade. (In some, the V-tail Bonanza for example, you can get the opposite problem, a shallow dive with cruise power can make you exceed Vne and pull the tail off.)

You make an assumption that RPM = Power setting for a given aircraft type, which is not a good idea. For a given power output, all other conditions being equal, RPM will vary according to the prop fitted (and of course its condition).

The prop is every bit as much of an airfoil as the wings.

(edited for peer spolling)

I remember vividly my first lesson in landing: "One hand on the power, one hand on the stick and I want to see your eyes moving from the air speed to the runway. Use the stick to adjust your speed and your power to sort out the height". Now I know there's plenty of debate about the last bit, but unless it's completely calm with no ups and downs, I can't really see how you can always give yourself a half-decent chance of a good landing without changing/adjusting the power in the final descent.

Okay, I'll bite. It's true that the approach technique that's currently taught in powered flying is to approach *below* the zero-power glidepath, and use engine power to maintain your desired glide path. But that's by no means the only technique. You can also approach *above* the zero-power glidepath, reduce the throttle to idle and vary the drag (flaps, airbrakes, sideslips, forward slips, s-turns, you name it) to maintain the desired glide path. Gliders do this all the time and although this technique might not be desirable in a busy (power) circuit, it's a good technique to master.

Not a very appropriate comment, me thinks!

Gasax, I'm a bit confused by your previous post as you seem to be saying I should have noticed the tach red lining and aborted take off yet in the next breath you seem to imply that if I was more experienced I wouldn't be relying on the tach.

I checked my PPL literature and there's no mention at all about tach failure or misreading. Presumably not all aircraft even have one. On the take off roll the emphasis is on checking the tach to see if full power is being generated. It doesn't take much experience with an aircraft type to sense intuitively whether the aircraft is producing full power - if you're airborne and not even a third the way down the runway can one not reasonably assume all is well? No instructor has ever mentioned a red line and I don't recall reading about it in the C152 hand book. I'm pretty sure there isn't one in this aircraft's tach or if there is it is so faded as not to be noticeable.

While I appreciate the advice given by others in this thread and it is certainly helpful I get a bit annoyed by ppruners who see it as a chance to have a go at anyone they feel is not as "experienced" as they.

Try the "limitations" section of the POH. Under "engine".

I did not find a full C152 POH online quickly, but stumbled onto this thing which is an (uncertified) summary: http://www.ginecoweb.com/Piloto%20Pr...a152Manual.pdf. Page 3. 2550 RPM.

You can get the full 152 POH here
Cessna FREE Manuals
Which confirms the 2550 RPM limit.

I need to check first with the CAA as to the correct procedure. For all I know the "post-it note" system might be perfectly legit.

For the avoidance of confusion - and nothing else!

As you open the throttle one of the first things that should be checked is whether the aircraft is making the 'full power' static rpm. With a fixed pitch prop and at the density altitudes we get in the UK that will be in the range of 2100 to 2400 rpm across virtually all spamcans. The specific rpm for your model/year etc should be known to you.

If you do not get that rpm then shut the throttle and find out why. Could simply be the carb heat left out - or the engine may be knackered. Over reading is rarer but if for instance an O-200 has more advanced timing the rpm goes up - but only some O-200s can withstand this without cracking!

In the approach configuration the rpm is whatever you need. If could be anything between the idle and near full throttle depending what else is happening - so as long as the engine is running, the tacho will tell you nothing about the state of the engine.

Have a look at a couple of recent accident reports where deterioration of engine performance has had a major influence - the recent Sandown Cherokee one springs to mind.

It is unfortunately a fact of life that the standard of and content of, much of what you are taught during a PPL does not give you the information you really need to operate outside a club environment.

If the club are also 'sailing close the the wind' think seriously about going somewhere else.

thanks gasax

Just had a word with an Airworthiness Surveyer from the CAA. Without my prompting his response was that the tach was "erring on the safe side" but would make setting the power setting on approach difficult.

Also I can insist on entering the "Defects" column of the tech log - which is what I will do from now on unless the fault is minor.

I'd get this fixed. Near the top end, the engine power output varies drastically with small changes of RPM and if the RPM is not set accurately, the fuel flow will be nowhere near the POH value. Combine this with somebody pushing the boundaries a bit on fuel planning....

Hey Molesworth,

You are entitled to grow and learn in aiviation, as others of us have - to whatever degree (it can be difficult to determine on PPRuNe, particularly from just a few posts).

That said, it is your obligation to grow and learn.

The PPL is a "license to learn". There is just no way that a person can learn much beyond just being safe, during such a short training time. You'll have upwards of a thousand hours, and still find yourself learning. I have many times that, and learn new things regularly - even here! Though happily, the learning curve seems to be flattening out now!

You will find that as flying becomes "second nature" the actual readings of instruments at any moment will be less important to you (though you still must know and operate within limitations, whether properly marked or not). "Second nature" opens the door to complacency, but that's beyond the scope of this post.

As for that's a Pandora's box for new pilots. New pilots probably cannot correctly judge "minor" vs. "major" correctly. An informal chat with the maintainer of the aircraft you fly, will bring you up to speed on what people are looking for. By the way, the absence of a correctly located, visible red line on a tach would definately be "major" and snag worthy.

For your reference, here's a guidline for the "major minor" determination in Canada. It can be carried across to un modified aircraft condition fairly well too, just read "suspected defect" in place of "modification"

Canadian Aviation Regulations - Part V, Standard 571, Appendix A

Pilot DAR

While flying around the local area my friend in the front seat of my Cub pointed at the taco/rpm guage, which had sprung from 2100 up to 3600.

She said "that doesn't seem right?!" (while pointing furiously at the thing - she's a pilot too!)... I said "no, it isn't. But that's a lovely view and down there and I reckon that pub would be a great place for lunch". After an hour or so we landed.... Happy days. :)

Few weeks later we changed the taco drive cable.

Gasax and Pilot Dar,

You are absolutely right. I am indeed learning quite a bit post-PPL - and none of it from an instructor either!

It's good - I can't understand pilots who give up flying - for me it becomes more interesting with every flight.