I have a '56 Cesssna 172 straight tail. My altimeter is showing 100' high of actual and my airspeed indicator seems to be indicating slow by a noticeable amount as suggested by a knowledgeable CFI who used to own a straight tail 172. Given that airspeed is off the ram air side, and altimeter off the static side, is there much of a chance that I have a piping problem or a bit of crud in a tube, or am I looking at overhauling 2 instruments?

Thanks.

Frank

Frank,

Not an expert by any means, but I'm studying for my PPL and just went through altimeter and airspeed indicator today.

From my notes: A pitot blockage will result in indicated airspeed increase in climb, decrease in descent.

A Static port blockage will give exactly the opposite reactions (airspeed decrease in climb...).

A partially blocked pitot will result in your IAS going to 0.

No mention of what a partially blocked static port would do.

Not much help but it does rule out a couple things.

/a

air speed is hooked to both static air and pitot tube, Altimeter to static port

Sounds like instrument overhaul. Gunk in the static tubes would cause your altimeter to read either the same altitude all the time (total blockage) or react slowly to aircraft altitude changes.

Your airspeed indicator subtracts total pressure from the pitot tube from static pressure, so a partially blocked static port might look like a blocked static port on the airspeed indicator, then slowly returning to more accurate indications.

There's not a pilot-static issue I can think of that would cause your issues, outside of the gauges. A pitot-static check could rule out a lot of things, though.

Risking sounding terse but, my answer is:

Pitot/Static check +1

The altimeter can easily be reset to read correct altitude. You set the altimeter to the current correct barometric pressure and with a small screwdriver it takes about about 5 minutes. The ASI pitot tubing can be checked for leaks easily too. Ask your mechanic to reset the altimeter and about doing this pitot check.

From AC-90-89A
"a. Airspeed check: Slip a long rubber hose
over the pitot mast (surgical tubing is recommended).
As one person reads the airspeed, the other should
very slowly roll up the other end of the tubing. This
will apply pressure to the instrument. When the airspeed
indicator needle reaches the aircraft’s approximate
recommended cruise speed, pinch the hose shut,
and hold that reading. The airspeed needle should
remain steady for a minute if the system is sound.
A fast drop off will indicate a leak in the instrument,
fittings, lines, or the test hose attachment. NEVER
force air in the pitot tube or orally apply suction
on a static vent. This will cause damage to the
instruments."

For more see page 12 here. http://www.faa.gov/documentLibrary/medi ... 90-89A.pdf

Adjusting the altimeter is a 15 minute job if you have never done it before. It is rare that a unit will not read correctly at other altitudes then your field elevation after this adjustment, but age can cause sticky movements. Be sure to tap on the faceplate lightly during adjustment.

The airspeed is a little different. You can remove the unit and perform an easy water column manometer test on it (I used google to learn how when I did mine). Some airspeed indicators have an adjustment screw. Others do not. But you can at least check the IAS accuracy of the unit in an hour or less with some vinyl hose from the hardware store.

A static check technician will perform similar tests with better equipment, and check the full range of both imstruments, for 200-300 bucks. But you will get mostly the same info as performing these simple tasks yourself.

You can check your airspeed against your GPS on a calm wind day. Fly all four cardinal directions, or fly into the wind, against it, then 90 degrees to either side of it. Write down your IAS & your GPS groundspeed. My airspeed indicator seems to read 4 or 5 mph fast at cruise, but right on the money at approach speed. My last airplane was right on the money at cruise, but a little high at approach speed. So there are no hard & fast rules abut how they might be off. I wouldn't worry about the IAS being a bit off, but it'd be nice of the altimeter was closer than 100'.

hotrod180 wrote:I wouldn't worry about the IAS being a bit off...

I'd worry about it. I try to immediately stay on top of everything that aint perfect. Just good form. If you dont, eventually you have a shit tone of small squawks that you are just living with. Never know what or what combination of "no big deals" are gonna bite you.

I had a similar issue with the AS. Faster I went, the larger the error... I had just had a perfect P/S test. Sent the AS indicator off twice, both times saying it was perfect. Turns out a hole was drilled to mount a side pocket and nicked the pitot line. Faster I went, the more the error was. Wasn't till I re-inspected all the lines that I found the insult.

Fact is doing it right is cheap enough. Have the P/S system checked, then send both off for overhaul for peace of mind. Its probably been forever since they were calibrated so I bet it wouldn't hurt.

Perfect is a good goal, but it can be expensive. I like to say that I try for perfection, that way I can usually at least achieve mediocrity.
I do shoot for perfect when it's required, but good enough is often just that (good enough) and is a lot cheaper. In this case, does a 5 mph error in IAS constitute a real problem? To me, no. To someone else, maybe.

Personally for me I consider a faulty ASI/Altimeter dangerous.

Opinion: do NOT settle for "oh well" someone posted I shouldn't screw with.

Get it fixed, be legal and be smart.

00.02 USD

8GCBC wrote:Personally for me I consider a faulty ASI/Altimeter dangerous.

Opinion: do NOT settle for "oh well" someone posted I shouldn't screw with.

Get it fixed, be legal and be smart.

00.02 USD

+1

Not sure about your airplane... but in mine 5mph can make a difference. Say from 38 to 33. Good enough may be ok in the Chevy, but not in airplanes. If $$$ is in the rubric for safety, than its time to get a different hobby. Just sayin...

At 38 mph the last thing I'm doing is looking at my ASI. I'm flying by how the plane feels. So as far as I'm concerned, I wouldn't call an ASI that's off a couple mph a safety hazard.

A1Skinner wrote:At 38 mph the last thing I'm doing is looking at my ASI. I'm flying by how the plane feels. So as far as I'm concerned, I wouldn't call an ASI that's off a couple mph a safety hazard.

+1

I have never seen anything that is wacked on an airplane get better only worse.

Engines, instruments, controls they talk ... Listen to them before they screw something else up.

8GCBC wrote:If have never seen anything that is wacked on an airplane get better only worse.

Engines, instruments, controls they talk ... Listen to them before they screw something else up.

I agree. But I fail to see how a ASI that reads a bit off can screw anything else in the plane up.

A1Skinner wrote:

8GCBC wrote:If have never seen anything that is wacked on an airplane get better only worse.

Engines, instruments, controls they talk ... Listen to them before they screw something else up.

I agree. But I fail to see how a ASI that reads a bit off can screw anything else in the plane up.

It may get worse in flight and bite you.

Fly by the seat if your pants and you should be fine. I flew a bunch of partial panel during flight training, including ASI covered. I hardly ever look at my ASI unless I'm comparing speeds with another plane. But heck, I'm a rookie and don't know much. Also can't afford to have everything perfect.

A1Skinner wrote:Fly by the seat if your pants and you should be fine. I flew a bunch of partial panel during flight training, including ASI covered. I hardly ever look at my ASI unless I'm comparing speeds with another plane. But heck, I'm a rookie and don't know much. Also can't afford to have everything perfect.

VFR conditions and a familiar aircraft not that big a deal for some. Just saying I would not recommend it. I like to know my air speeds: Indicated, calibrated and true. Personally I like them correct and will spend my wife's money to fix it, if wrong

I'm in agreement... a slight error may not matter much if understood by a competent pilot.

My take: sounds like a pitot/static check is in order, and that'll tell you also if you need a new ASI or altimeter. If you're going to use the airplane IFR at all, it's required. But personally I like everything to work, the way it's supposed to work. I've never known anything mechanical to heal itself--they usually get worse.

Cary