The International Cessna^® 170 Association

Got an unusual problem. My 170B static system when connected to the static port and the A/S, V/S, Altimeter and encoder will show 20 mph low, 300 fpm down in level flight and about 200 feet low on the altimeter. When disconnect from the encoder and cabin air is the static source everything works just fine. I have blown out the static port, run a .032 safety wire thru the port, put all new static lines in and nothing helps. We put a small doughnut shaped chunk of putty, about 1/8 of an inch high around the static hole and everything works fine again. We have ruled out the encoder and figure now that there was some kind of ram pressure getting into the static port and causing the problem. It is a factory static port. The doughnut seemed to alleviate the problem. What's up?

Thanks.

How was it determined the altimeter and encoder are indicating 200' low in flight? (and how was the A/S determined to be reading 20 mph slow?)
Was another aircraft used? (Another aircraft is not a proper method of comparison for lots of reasons.)
Comparison of static pressure to cabin pressure is also invalid because the cabin pressure is influenced by ram-air inlets, cabin ventilations/heating systems, and tailcone suction.

If the airplane is not flying "sideways" (out of rig) then any/all of the issues mentioned come into play.

Your instruments should be individually tested/calibrated by an instrument shop on the bench, then re-installed, and the pitot-static system should be leak-tested. (A leaking instrument case in any of those static-resource instruments will influence the static pressures observed by the others. Your airspeed is especially suspect, as a leak from the pitot across the aneroid will leak into the static system and influence the other guages, since they are all ganged together on the static side. This would also explain the low A/S reading.)

It's unlikely to be the static port itself that is causing any errors, and the putty doughnut is neither a valid method to test for leaking instruments, nor a proper method to qualify a static port. (If the altimeter reads correctly on the ground with the engine at cruise rpm while the airplane is not moving,... then it should be equally accurate inflight if it's not connected to any other gauge. But if a leaky A/S is introduced to the system both the altimeter and the VSI will indicate a descent with a proper static port. Without a proper static port ...(the instruments disconnected from the static port and resourcing the ambient cabin)...low cabin pressures caused by relative wind at the tailcone will cause errors on the high side....which is what you seem to be indicating.)

My '52 had almost the exact problem. All the instruments individually checked OK, but hooked up to the static port they were seriously off. I installed an alternate static switch (valve) vented to the cabin and everything worked fine. Never could figure what was wrong or what caused it. Cabin static pressure seemed very accurate.

See the following story from the Cessna 140 Assoc's web site. The complete story is at the following link;

http://www.cessna120-140.org/forum/view ... atic+error

It's a pdf file so have fun. Seems like we went thru this same thing a few years back. Might want to do a search on our site too.


Cessna found that often the plane's instruments depending on static pressure would give more accurate indications if the static port (s) were ignored and the instruments removed from the static system and vented to cabin static.

They put out a letter to dealers (no owners allowed) which told the story above and after the noted serial number, only the airspeed indicator was hooked to a static port.

The reality is...sometimes, the airspeed works better hooked to cabin static, too.

The letter from Cessna is included in the article.

Neal

gahorn wrote:... (A leaking instrument case in any of those static-resource instruments will influence the static pressures observed by the others. Your airspeed is especially suspect, as a leak from the pitot across the aneroid will leak into the static system and influence the other guages, since they are all ganged together on the static side. This would also explain the low A/S reading.) ... But if a leaky A/S is introduced to the system both the altimeter and the VSI will indicate a descent with a proper static port. ...

Cessna 120-140 site, page 3, wrote: "4. And the lesson recently that instruments can leak internally, affecting all other devices plumbed to the same system."

I still believe the most likely culprit is a defective/leaky airspeed indicator.

There seem to be two different things going on here.

One: ASIs reading low, fixed by opening the entire static plumbing to cabin pressure.

"The fix? Horrors!!! Break one of the static fittings in the cockpit and watch the ASI jump to 10X and 81 and 65 as they should be...." Isn't that the classic symptom of a blocked static line?

Two: the subject of Cessna's letter, indicating that instruments (including the ASI?) were more accurate when only the ASI was connected to the static port, and the rest open to the cabin. Wouldn't they have tested and/or replaced the instruments in question before making an engineering change? And if the static pressure port was suspect, why leave the ASI hooked up to it? Because they couldn't get FAA approval in that case? (Note that many Piper Cubs, albeit slower than Cessna 120/140s, are certified without static ports.)

If you were cynical enough, you could suspect that Cessna made this change because leaks in the pitot-static instruments were very common, and they wouldn't have to fix as many of them if they weren't connected together.

I'm gonna try a couple of things next week including checking the integrity of the A/S indicator. I'll let you all know what I come up with, if anything. Thanks.

I had a similar problem after installing a second venture to run new gyros. The second venture was installed next to the existing venture on the left side of the plane which is the same side the static port was on. Before the modification everything worked fine, after the installation I experienced basically the same problem you mention. The A&P tried without success to solve the problem and eventually relocated the static port to the right side of the plane. That fixed the problem.

markeg1964 wrote:I had a similar problem after installing a second venture to run new gyros. The second venture was installed next to the existing venture on the left side of the plane which is the same side the static port was on. Before the modification everything worked fine, after the installation I experienced basically the same problem you mention. The A&P tried without success to solve the problem and eventually relocated the static port to the right side of the plane. That fixed the problem.

I wonder why his didn't simply move the venturiis to the right side of the airplane per the Cessna dwgs? That wasy there'd be no approval-basis issues.

This problem must be airplane specific. My 1951 170A has the static port in the factory position exactly half-way between where the two venturis used to be on the left side 8" behind the firewall. The A/S, VSI, and altimeter all indicate correctly when compared with the GPS groundspeed on a calm day and altitude readout on the transponder. The plane has had an IFR static/pitot/transponder certification since 1979 with at least three different encoders and two static hose re-routes and has never seen the reported problems.

Curiosity got the best of me tonight. I went out and flew 1715D with the intention of flying it uncoordinated left and right in order to induce error in the static system due to ram pressure or lower pressure at the static port.

Holding the plane level in cruise, I skidded it to full ball deflection both ways and could not get more than 50' altitude change on the altimeter. The transponder/encoder altitude did not change. The VSI would jump down when skidded right about 500 fpm and jump up about 500 fpm when skidded left but in both cases would go back to zero after a few seconds while holding the skid. The altitude decreased 50' in a right skid and increased 50' in a left skid.

Because of what I saw tonight, I don't think a plane (mine anyway) could be out of rig far enough to cause error in the static system.

In addition to the static problems that I had, as mentioned in my above comments, I also installed a salvaged Cessna static port on the right side of the airplane and teed them together to no avail. There was absolutely no difference.

blueldr wrote:In addition to the static problems that I had, as mentioned in my above comments, I also installed a salvaged Cessna static port on the right side of the airplane and teed them together to no avail. There was absolutely no difference.

Don't know how your instruments were checked "OK" but I'll still venture you have a leaking Airspeed indicator. It's the only intrument in the cluster that will give exactly this sort of failure.

George,
I'm getting too old to have to climb down off of the turnip truck to answer these things.
The first thing I did was to send the air speed to the shop since it's the only instrument common to both the pitot and static systems. When I sent it to the shop, I told them what the problem was and that I thought it might be leaking across. They said it checked OK.
If the air speed meter was leaking, why wouldn't it effect the system when it was vented to cabin static?
I have to get back up on the truck now, George.

blueldr wrote:...
If the air speed meter was leaking, why wouldn't it effect the system when it was vented to cabin static?....

Because it could be leaking into the exact same environment (the cabin) as the other instruments were sampling (no isolation exists.) If the airspeed were leaking only across it's diaphragm...and not into the cabin thru it's diaphragm and case.... then your hypothesis would be correct.
I still believe your instrument shop made an error in testing the instrument (and/or a combination of the instrument had a leaking static line fitting.)

Cabin air as a static source is tricky. (Most airplanes with alternate static sources also have published correction tables relating to speed, gear and/or flap settings, and sometimes other parameters. That's why cabin air is not an ideal static source. Open air vents can pressurize a cabin giving lower-than-actual altitude readings at speed....and then slower airspeeds (higher deck-angles) can result in a "vacuum" thru the stinger and result in a higher-than-actual reading.) A leaking airspeed throws so many variables into the equation that all readings become suspect. Without telemetry it's almost impossible to determine accurate altitude/speed due to changing baselines, so the best thing is to test static system for leaks, and test instruments for leaks, hysteresis (a type of variable error influenced by elasticity of bellows.)

I was raised in the South. I like turnips. With cornbread and fresh onion and sweet milk.