Tachometer reading incorrect

[Pilot DAR]

Good for you, being interested is probably the best way to be safe!

Pilot DAR

[Molesworth 1]

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.

I have finally got round to reading this.. and it's a real gold-mine - especially in regard to emergency procedures!

I also found a picture taken by my passenger while the a/c was cruising at aprox 90 knots.

[BackPacker]

Well, the red line is definitely there, right where we predicted (at 2550 rpm) and you were definitely over it. Not good.

What makes it worse is that you apparently have a habit of setting your cruise power based on the ASI (you wanted to get 90 knots) instead of the RPM gauge. That's not as it's done by the book but it's actually a bit dangerous too: Suppose there is a block in your pitot/static system somewhere (insect in pitot tube - it happens!) then the ASI is underreading and you would be overspeeding the engine, even with a correct RPM gauge, to get the desired airspeed. The exact same thing would happen if your airframe is somehow "dirtier" than normal, either because of bugs and other dirt, or something else that increases the drag (wheel pants taken off for maintenance for instance).

Next time, at the top of the climb, first set the cruise attitude, let the aircraft accelerate (under full power) to an approximate cruise speed, then set cruise RPM. Trim, and then verify that your cruise RPM gives you the desired cruise performance (ASI).

[Molesworth 1]

Backpacker

I would expect to see about 2200 rpm in the cruise. That's when I first noticed the tach was out (should have noticed it earlier, but as I have said, I had no previous experience or instruction on this kind of fault - it was totally unexpected). Of course, as you say it could have been the ASI that was overreading, but given the sound of the engine, performance and so on I decided the tach was the faulty instrument.

Mistakenly assuming the tach as correct (which indeed is what we are taught) and suspecting the ASI could in this case have lead to a stall on final.

I made the right decision.

Any instructors out there - maybe make a note to mention the red-line issue to your students?

[BackPacker]

Mistakenly assuming the tach as correct (which indeed is what we are taught) and suspecting the ASI could in this case have lead to a stall on final.

I would certainly hope you were taught better than that!

Go fly a few circuits with an instructor with the ASI covered up. Or even better, go out and do some airwork (stalls particularly, but also steep turns) with the ASI covered up, then go and do some circuits.

Relying on the ASI as your sole indicator of an imminent stall is very dangerous, and is not making use of all other clues that precede a stall.

I made the right decision.

Continuing the flight with an instrument consistently indicating out of the limits? Not sure I agree with you on that statement. You didn't break anything, this time, but that doesn't mean the decision was right.

[Molesworth 1]

Backpacker

I meant - the decision to trust the ASI rather than the tachometer.

I don't agree with you about it being the wrong decision not to continue, either. I had to make a quick assessment of the situation. Adjusting the power to 2200 rpm in cruise meant I was unable to maintain height. So what does that tell me? Obviously if there was any other indication of the engine being outside limits it would have been a different story.

Without any other indication of engine malfunction and given that the flight was short in any case I didn't see that returning to the departure airfield immediately was necessary.

I can't believe that you really want me to fly by RPM readings on final approach? Especially considering that all the evidence pointed to the tachometer reading incorrectly. I know it's possible to fly without the ASI on final, but honestly - air speed is absolutely critical at this stage.

Being PIC means having the privilege and responsibility of making a decision and standing by it. Sometimes with hindsight and more information one discovers that a better course of action could have been taken. Not this time!

[BackPacker]

Adjusting the power to 2200 rpm in cruise meant I was unable to maintain height. So what does that tell me?

It tells you that the aircraft is not performing as it should. It doesn't tell you what causes that. As said, it can be an ASI malfunction, an RPM gauge malfunction, or an airframe that's somehow "dirtier" than normal, for instance because of a flap failure, failure to detach the tie-downs before taking off, a piece of cloth blown onto the tail, the previous pilot forgetting to remove the dead horse from the back, you name it. The aircraft we fly typically don't have dual-everything so there is no way to cross-check and *know* that a single instrument is failing.

Better find out what's wrong on the ground, not in the air.

And if you could not maintain altitude with 2200 rpm, why not limp home at 2550 (redline), which should give you about 80 knots? Why continue the flight over the redline limit at 2625?

Obviously if there was any other indication of the engine being outside limits it would have been a different story.

The next indication of an engine overspeed would most likely be a spectacular and catastrophic failure. A propellor blade detaching, a piston working its way out of the cowling due to a broken conrod, that sort of thing. But an engine overspeed doesn't lead to a gradual increase/decrease of oil pressure/temperature, which are the only other indicators that you'd have available.

So any single indication that's showing over the limit is reason to abort the flight, as far as I'm concerned.

I can't believe that you really want me to fly by RPM readings on final approach? Especially considering that all the evidence pointed to the tachometer reading incorrectly. I know it's possible to fly without the ASI on final, but honestly - air speed is absolutely critical at this stage.

What is the length of the runway you were flying from? Was that anywhere near the minimum required? Otherwise the airspeed on final, assuming a sufficient margin above the stall (no pre-stall buffet, no stall warner, no significant pull on the stick/yoke), is not nearly as critical as you think. That is, if you learned to land properly, bleeding off any excess speed in the flare.

Being PIC means having the privilege and responsibility of making a decision and standing by it. Sometimes with hindsight and more information one discovers that a better course of action could have been taken. Not this time!

So what you're saying is that next time you'll happily boost the engine over its redline RPM again?

[bjornhall]

of a flap failure, failure to detach the tie-downs before taking off, a piece of cloth blown onto the tail, the previous pilot forgetting to remove the dead horse from the back, you name it.

Getting silly now...

Meanwhile, in the real world, the pilot had no problem correctly diagnosing the problem and taking the appropriate action... Bit strange if someone flying a familiar airplane could not tell 2200 RPM from 2600 RPM from the sound of the engine. Bit stranger if engine RPM was 2600 RPM with the throttle pulled way back simultaneously with the ASI underreading (2 simultaneous independent failures). So there are indeed ways to crosscheck the indications.

[Molesworth 1]

Thanks for that, bjornhall.

No one at my flight centre (one of the biggest in the UK, at that) were too excited about the problem. (Mind you if you told them the wings fell off they would just yawn)

That C152 POH is fascinating. Is that based on the a real Cessna POH, Backpacker? It has a novel approach for controlling the aircraft if inadvertantly entering cloud -let go of the yoke and keep the little airplane in the turn co-ordinator level with the rudder. I'm going to try it on my next flight (not in cloud though)

Small mistake though (I think). When braking on a short runway it says push back on the stick - surely it should be forward? (Next action in the former case would be - apply full power - as the aircraft will be back in the sky!)

[worrab]

Pull back on the (yoke) to relieve some of the strain on the relatively fragile nosewheel assembly.

[SNS3Guppy]

It has a novel approach for controlling the aircraft if inadvertantly entering cloud -let go of the yoke and keep the little airplane in the turn co-ordinator level with the rudder.

Not too novel...this has been the standard counsel since prior to the second world war.

The concept is idiot proofing the maneuver, and trying to keep the pilot as much out of the process as possible. The pilot continues to be the most dangerous component in the aircraft, and it's the pilot who causes the loss of control in instrument conditions.

The safest counsel is to steer well clear of instrument conditions until you're properly trained, and thoroughly proficient. Inadvertant VFR into instrument conditions still continues to be a consistent killer of pilots and a destroyer of aircraft.

When braking on a short runway it says push back on the stick - surely it should be forward?

It should say pull back on the control yoke.

The bottom line with your aircraft, all other references is aside, is that it's not functioning properly. You need to refuse to fly it until it is functioning properly. There's no grey area. If the aircraft isn't right, don't fly. Period.

[Molesworth 1]

Not too novel...this has been the standard counsel since prior to the second world war.

No word of it in my PPL training that I recall. It's a great idea though - it's a pity it's not more widely taught as it could save lives.

[SNS3Guppy]

There's an enormous amount left out of training in that part of the world, from what I gather reading these posts. I'm quite amazed at the number of posters who suggest they've never been taught to properly lean an engine, or the proper use of carburetor heat...but these have been required knowledge for any student pilot from the very early days of flying.

What we have in flight training today is a heritage of inexperience. One low-time, inexperienced pilot teaching another. One pilot is taught by another who knows nothing more than what the low-time pilot before him taught when he got his first instructing job...it's a chain in which each up and coming instructor doesn't know or think for himself, but only repeats and rabbits what his own instructor did...and his own instructor did only what he was taught...ad nauseum.

Even today concepts such as ground effect being a cushion of air beneath the wing are still passed ignorantly from one instructor to another, taught to far too many students...when the truth is much more simple, and much more important to understanding flight.

In your Cessna 152, you may try another experiment some time. Run the trim full up with a power off descent, simulating a glide with the engine out to a landing...see what airspeed the airplane will hold in a glide. Then try it from level with power, trimmed hands off. Pull the power to idle with the trim given three full trims up (grasp the top of the wheel and pull it down to the bottom, three times). See what airspeed it holds. Somewhere between the two, closer to full up trim, you'll find that without any input from you, once trimmed, the airplane wants to glide all by itself. Then do the glide with rudders and see what happens.

This works great in calm conditions. Inside a cloud is very seldom calm. It's usually bumpy, and turbulent. The airplane doesn't want to stay wings level, and certainly doesn't want to hold a pitch attitude or an altitude. It doesn't want to maintain a stable descent, and it usually doesn't want to go straight. Add to this the illusions and disorientation that comes with entry or flight in the clouds, and you've got a recipe for disaster...hence the constant and urgent counsel to avoid flight in instrument conditions until properly certified and instructed.

[Gertrude the Wombat]

No word of it in my PPL training that I recall. It's a great idea though - it's a pity it's not more widely taught as it could save lives.

Perhaps TPTB think that even more lives could be saved by spending the time teaching "do not get into cloud, it will kill you" instead.

[BackPacker]

That C152 POH is fascinating. Is that based on the a real Cessna POH, Backpacker?

I honestly have no idea. It was the first or second Google hit and I only scanned it quickly until I found the 2550 number I was looking for. But I would assume that if someone goes to the trouble of summarizing a POH, that they would base it on the POH, yes.

What's more important to note is that the summary (or indeed a complete POH downloaded from the internet) may not be right for your model or even your specific airframe. So you should always verify the information with the actual POH of the actual aircraft.

Small mistake though (I think). When braking on a short runway it says push back on the stick - surely it should be forward?

Pull back on the (yoke) to relieve some of the strain on the relatively fragile nosewheel assembly.

Actually, for a short-field landing you pull back on the yoke/stick not to relieve stress on the nosewheel, but to put as much weight on the main wheels as possible. This allows for the most effective braking without skidding. Same reason you retract your flaps as soon as possible in such a situation: more weight on the wheels.

What you need to realize is that the main wheels are behind the center of gravity on an aircraft with a nosewheel. This means that anything that reduces the lift of the wings, or increases the downforce generated by the elevator, will increase the weight on the main wheels. And more weight on the main wheels means more effective directional control and more effective braking. In contrast, if you move the stick/yoke fully forward, the elevator will generate an up-force, effectively reducing the weight on the mainwheels.

There is a caveat though, which you rightfully pointed out. If you landed too fast and touched down with a speed above Vs, pulling back on the stick/yoke when still above Vs will normally result in getting airborne again. So for a short-field landing speed control on final is indeed critical and you should not attempt a short-field landing for real if you have any doubts about the ASI accuracy.

The bottom line with your aircraft, all other references is aside, is that it's not functioning properly. You need to refuse to fly it until it is functioning properly. There's no grey area. If the aircraft isn't right, don't fly. Period.

A few weeks ago I was preparing to fly from Rotterdam to Fenland for the aeros contest over there, but I was in serious doubts about being able to make it over the Channel, given the weather conditions and the equipment of the plane. A very experienced flight instructor told me: "Breathing and eating is mandatory or else you die. Paying taxes is mandatory because otherwise you'll be thrown in jail. Everything else is optional." Wise words.

[Molesworth 1]

for a short-field landing you pull back on the yoke/stick not to relieve stress on the nosewheel, but to put as much weight on the main wheels as possible

Thanks for the explanation, BackPacker - that makes complete sense.

[smarthawke]

I do hope that the rest of the machine is kept in finer fettle than that of the instrument panel condition...

With a potentially over-reading tacho or too fine a pitch prop on the front and it being flown in this condition without anyone seeming to care about your concerns make me think your money might be better spent at some other establishment.

Incidentally, all the C152 tachos I've seen/flow maintained have a green arc from 2000 - 2550 as the normal engine range of operation so the tacho fitted may be a 'non-genuine' one. Exactly what should infromation be displayed on the instrument panel, instruments and elsewhere on the airframe will be in the Placards section of the POH for that specifice aircraft (by registration/serial number not aircraft type).

[Molesworth 1]

I do hope that the rest of the machine is kept in finer fettle than that of the instrument panel condition...

Most of the rest of the plane is held together with duct tape

[Pilot DAR]

What we have in flight training today is a heritage of inexperience.

Best line I've ever read on PPRuNe!

[Maoraigh1]

Trust your instructors. They will have preflighted it. They know the two basic parts of an aircraft:
The parts that are meant to move -if they don't, they'll have WD40'd them until they do.
The parts that are not meant to move - if they do move, they'll have duct tape them until they don't.