Tachometer reading incorrect

[Pilot DAR]

Amother reason is that a prop can "untwist"

..... 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.

[Pilot DAR]

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;

[NutLoose]

[quote]

Quote:
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 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.

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.

Quote:
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.
"Untwisting" fixed pitch propellers?

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.

There's little point swapping tachometers if you can't verify the calibration of the replacement tachometer.

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.

[gasax]

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!

[Croqueteer]

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.

[Molesworth 1]

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

[gasax]

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'.

[Mike Cross]

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.

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)

[worrab]

my passenger wondered why I kept changing the power settings

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.

[BackPacker]

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.

[Molesworth 1]

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?

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.

[BackPacker]

No instructor has ever mentioned a red line and I don't recall reading about it in the C152 hand book.

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.

[asyncio]

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

[Molesworth 1]

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

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.

[gasax]

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.

[Molesworth 1]

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.

[IO540]

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....

[Pilot DAR]

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

unless the fault is minor.

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

[TheGorrilla]

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.

[Molesworth 1]

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.