Tip: I Failed the Static Test ? So Might You!

Awhile back I took my RV-8 in for the bi-annual pitot/static and transponder check. My Garmin 327 transponder checked out perfectly. (The avionics technician said he NEVER sees problems with Garmin transponders.) The static system test was another story: The test equipment (vacuum line) is attached to one of the static ports via a rubber cup that fits around the static port on one side of the aircraft and is held in place by a tripod with suction feet that stick to the side of the airplane around the static port. The port on the opposite side of the tail is covered with a piece of electrical tape to seal it shut. Several tests are done to check the accuracy of your altimeter and I won?t go into that, but a leaky static system will spell FAIL right away. For the leak test the altimeter is taken up to 1000 feet above the ambient pressure altitude and a valve is closed at that point to isolate the static system from the pump. The altimeter must not lose more than 100 feet in one minute. Well, mine went down like a two-bit streetwalker on Friday night! FAIL! I didn?t have time to chase down the leak right then and the tech was nice enough to only charge me for a transponder check. My bird was not legal to fly IFR until I got the leak stopped and a valid test completed.
Today I borrowed a vacuum pump from a buddy, put a high-tech needle valve (Wal-Mart aircraft aisle, fish tank supplies) in the vacuum line and attached the line to the static port with some sticky tape that is used to seal vacuum bags. The cobbled together test equipment worked great and the static leak was evident as I brought the altimeter up 1000 feet then closed the needle valve. The altitude dropped like that same skanky chick, again!
Now I won?t bore you with all that ensued for the next five hours as I disconnected every joint in the system to isolate the leak, starting at the panel and working my way back to the static ports. Suffice to say that I removed a lot of interior panels and instruments and said a lot of words that my grandchildren are quite sure I don?t even know! Remember that I started at the panel and ended at the static ports. You know where I found the leak, don?t you? Right, the static ports themselves. And I?m writing this after-action report because I?ll bet a lot of you will have the same problem or have already been there.
I used Van?s pop rivet static ports. They are simple, work great and last a long time. However, the leak in my static system was actually not in the system at all. The vacuum line from the test equipment is attached with a rubber cup that covers the entire pop rivet static port. Turns out that the air was leaking AROUND the pop rivet. To fix the problem I cut a tiny hole in a piece of electrical tape and placed it over the hole in the center of the static port rivet. The rest of the tape sealed the area around the periphery of the rivet. If I was just installing the static system I think I would seal the rivet with a glob of proseal or something similar.
If this helps just one of you, I hope you will let me know. I?d hate to think I blew an entire afternoon just for my benefit.

Thanks

Ron,

Thanks. I will not have a leak in that spot because I installed real static ports. Now, I may have a leak elsewhere but not at the port. I've never understood how a rivet could be used as a static port. I know some must use it because the plans say to do it that way but how do you securely attach the tubing to the rivet anyway? Maybe it's just me but I don't think a person should install anything on their aircraft unless it's needed, and if it's needed it should be robust, well made, and properly suited to the task. Even if you could attach the tubing securely, I think the domed shape will introduce errors. Rivets are good for a lot of things but for static ports? YMMV but I've never understood this. Go with real ports, they're important.

Bevan
RV7A wiring

Bevan said:

Even if you could attach the tubing securely, I think the domed shape will introduce errors. Rivets are good for a lot of things but for static ports? YMMV but I've never understood this. Go with real ports, they're important.

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Actually the "domed" head is what makes the rivet work so well. Many people have installed "real" static ports that are flush only to find that they produce an error in flight.

I got the shop to do the test from the main line passed the alt static connection, not at the outside ports. We did find a leak at the GRT AHRS, but easy to fix on site. The shop passed our setup without having to test at the static ports on the outside of the plane ... since it's not a certificated plane.

ronschreck said:

I used Van?s pop rivet static ports. They are simple, work great and last a long time. However, the leak in my static system was actually not in the system at all. The vacuum line from the test equipment is attached with a rubber cup that covers the entire pop rivet static port. Turns out that the air was leaking AROUND the pop rivet.

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So; the system didn't actually fail. The failure was in the test method.

Mel said:

So; the system didn't actually fail. The failure was in the test method.

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That was on my mind as well after reading Ron's post.

I put a dab of sealant where the tubing goes onto the rivets but have never leak checked the system. Seems to work OK for my VFR flying and ATC
personnel, though.

rivets work well

I'm with Mel on this one!
My first 4 planes had the rivets for static ports and my airspeeds were always dead on. I went with the SafeAir ports/tubing this time for the ease of compatibility with Dynon probes and instruments. Safe Air changed their ports from flush to protruding and thats the reason I switched. Nothing wrong with the Vans "Rivet" port though.
My airspeeds with these new ports are dead on also.
Mel, I have to chuckle every time I read that Vans design needs "improving".

ronschreck said:

If I was just installing the static system I think I would seal the rivet with a glob of proseal or something similar.

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If I remember correctly, the instructions that come with the static system kit specifically say to seal the tube to the rivet with proseal or RTV after connecting the tube. I would think that this would have sealed the leak you encountered.

I also agree with Mel, there are a lot of RV's flying with fancy looking static ports that are nearly worthless. They might make it easier to get a leak free system but if the static pressure is not correct, it was wasted effort.

Bevan said:

Even if you could attach the tubing securely, I think the domed shape will introduce errors. Rivets are good for a lot of things but for static ports? YMMV but I've never understood this. Go with real ports, they're important.

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I can't speak to the errors on other models, but on the RV-8 Van's recommended "pop rivet" static ports are very accurate. My testing showed less than 1 kt error from 55 kt to 160 kt, with the error increasing to 1.7 kt at 180 kt IAS.

Under Wing?

Does anyone have any feedback for this type, where the static port is at same location as the pitot?

rvbuilder2002 said:

If I remember correctly, the instructions that come with the static system kit specifically say to seal the tube to the rivet with proseal or RTV after connecting the tube. I would think that this would have sealed the leak you encountered.

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You are correct, and I did just that but the leak was not through the rivet/tube connection. It was around the rivet itself, between the rivet and the skin of the empenage. I should have put a dab of proseal in there at the time I set the rivet.

GrayHawk said:

Does anyone have any feedback for this type, where the static port is at same location as the pitot?

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Understand that a static port must be at a location where the pressure is the same as the ambient pressure, or you will have errors. Also understand that the whole purpose of a wing is to create high pressure below it, and low pressure above it, so that lift is created. Thus it is difficult to find a location below the wing where the pressure is the same as the ambient pressure throughout the whole flight envelope. Many manufacturers manage to find a location that has acceptably small errors, but it can take many hours of flight testing, moving the static port location for each test, before they find a suitable location.

I was witness to a test campaign that Diamond Aircraft did when they created the DA-20-C1 Katana. They had a good working static port on the DA-20-A Katana, using a Piper type pitot-static probe under the wing, but they changed the spar design on the DA-20-C1, and that meant the pitot-static probe had to move a bit to avoid interference with the spar. They spent a lot of time flight testing, trying different part number Piper probes (each part number has a different angle on the bottom face, which fine tunes the pressure seen at the static port). They eventually created a probe with a custom angle on the bottom, and finally had acceptable errors in the static system. Some of the configurations they tested had very large errors - one of them gave very low indicated stall speeds, and an ASI that read about 15 kt too fast in cruise. We joked that they should sell that one as a high performance option.

I strongly recommend that people stick to Van's recommended static port, or a port that has the same external shape. The location is also important on aft fuselage ports, so stick to what Van recommends.

Thanks, Kevin,

Sounds like some testing is in order as this location is on a bought flying (built 1991) RV-6A. I rather doubt that extensive testing was done in 1991 to choose location.
So choices are to check it now, move it when I can.

ronschreck said:

You are correct, and I did just that but the leak was not through the rivet/tube connection. It was around the rivet itself, between the rivet and the skin of the empenage. I should have put a dab of proseal in there at the time I set the rivet.

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I'm still not picturing that; probably just my lack of vision. When my static was checked, the device covered the entire rivet. Are you saying the leak was through the rivet/skin joint to the inside of the aircraft? Because I would have expected the RTV covering the rivet/tube joint inside would seal that - it did on my aircraft. Just curious; I'd have expected the rivet to seal anyway. I've seen an RV with the tube/rivet joint secured with safety wire and no leaks. Apparently the expansion of the rivet into the drilled hole was enough to seal it.

flion said:

Are you saying the leak was through the rivet/skin joint to the inside of the aircraft? Because I would have expected the RTV covering the rivet/tube joint inside would seal that - it did on my aircraft. Just curious; I'd have expected the rivet to seal anyway.

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Yep. That's what I'm saying. I would think that the RTV would seal that gap as well, but it didn't on my aircraft. YRMV

Quote:
Originally Posted by ronschreck
You are correct, and I did just that but the leak was not through the rivet/tube connection. It was around the rivet itself, between the rivet and the skin of the empenage. I should have put a dab of proseal in there at the time I set the rivet.

I see some talk about "setting" the rivit.

Maybe I interpreted the Van's instructions wrong. I did not "set" the rivet. I pulled to pin in the rivet and "glued" to the fuselage with JB wield. then I installed the flex tubing from inside the fuselage and worked some silicon around the flex tube inside the fuselage.

Is that what you mean't or were we to actually set the rivet?

Carl

don't set the rivet

carl nank said:

I see some talk about "setting" the rivit.

Maybe I interpreted the Van's instructions wrong. I did not "set" the rivet. I pulled to pin in the rivet and "glued" to the fuselage with JB wield. then I installed the flex tubing from inside the fuselage and worked some silicon around the flex tube inside the fuselage.

Is that what you mean't or were we to actually set the rivet?

Carl

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That's what I did too.

carl nank said:

Quote:
I see some talk about "setting" the rivit.

Maybe I interpreted the Van's instructions wrong. I did not "set" the rivet. I pulled to pin in the rivet and "glued" to the fuselage with JB wield. then I installed the flex tubing from inside the fuselage and worked some silicon around the flex tube inside the fuselage.

Is that what you mean't or were we to actually set the rivet?

Carl

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I guess my communication skills are rusty. English is my second language.
It is, however a blind rivet and I SET it per Van's instruction, which says:

"The vent ports are SD-42-BSLF blind rivets SET (my emphasis) into the fuselage side skin. The mandrels driven out, making them hollow, and a 1/8" i.d. clear plastic tube is slipped over the protruding ends of the rivets. The joint is sealed with RTV silicone sealant."

I did not, however seal the rivet to the skin as this was not called for in the instructions. I later found that a leak between the rivet and the skin made it difficult to test the static system even though the system itself is air tight. If you sealed the rivet in place with JB Weld, proseal or some other sealant I believe you will avoid a similar problem. Good on you!

I believe the instructions say to slightly set the rivet, but not enough to break the mandrel. Tap out the mandrel to leave a hollow rivet. Push the static hose over the rivet end and seal it in place with a smear of proseal or RTV around the perimeter.

I could be wrong...but thats how I have done it about a dozen times and have never had a leak (in that location anyway )

I believe that what Ron is saying is that when the suction cup tester is attached over the pop rivet, air leaked between the rivet and skin and into the tester, thereby negating the static test. There is nothing wrong with the static system, only the test results as the tubing was sealed to the rivet. Sealing the rivet to the skin would make the test a valid proceedure.

Tom
RV-7A N175TJ flying

What the captain means is......

tacaruth said:

I believe that what Ron is saying is that when the suction cup tester is attached over the pop rivet, air leaked between the rivet and skin and into the tester, thereby negating the static test. There is nothing wrong with the static system, only the test results as the tubing was sealed to the rivet. Sealing the rivet to the skin would make the test a valid proceedure.

Tom
RV-7A N175TJ flying

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AMEN

pitot static check

The vacuum line from the test equipment is attached with a rubber cup that covers the entire pop rivet static port. Turns out that the air was leaking AROUND the pop rivet. .[/QUOTE said:

sometimes a little grease around the edges of the rubber cup along with it being taped down solves this problem.....had the same thing happen to me the first time around....

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pitot static check

Ron,
Your explanation is right on. I too discovered a leak around the pop rivet. Sometime in the future I will seal the area from the inside using proseal as you suggest.

I recommend

The www.safeair1.com pitot static system. Its excellent and if you cut the tubes with the correct tool and square, you will not get leaks. I suffered from the same problem and was very happy with my upgrade.

Hugh.

tacaruth said:

I believe that what Ron is saying is that when the suction cup tester is attached over the pop rivet, air leaked between the rivet and skin and into the tester, thereby negating the static test. There is nothing wrong with the static system, only the test results as the tubing was sealed to the rivet. Sealing the rivet to the skin would make the test a valid proceedure.

Tom
RV-7A N175TJ flying

Click to expand...

I understood what Ron was saying

What I meant to say was

rvbuilder2002 said:

Push the static hose over the rivet end and seal it in place with a smear of proseal or RTV around the perimeter.

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If you follow the instructions and smear a bead of sealant around the hose after the hose has been pushed on to the rivet, the only place for the sealant to be in contact is the perimeter of the hose and the skin. This makes a fillet of sealant between the hose and the skin surface. If you do this you have sealed the hose to the rivet and the rivet to the skin since the smear of sealant is in contact with the skin and the hose.

And I think that is what Vans static system instructions tell you to do but I might be wrong

hughfi said:

The www.safeair1.com pitot static system. Its excellent and if you cut the tubes with the correct tool and square, you will not get leaks. I suffered from the same problem and was very happy with my upgrade.

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Have you tested this in flight yet? Others have reported large static system errors with their static ports.

It seems like Van's ports are _spot on_.

kevinh said:

Have you tested this in flight yet? Others have reported large static system errors with their static ports.

It seems like Van's ports are _spot on_.

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Part of it depends on how you mount the SafeAir1 ports.

Because of all the talk about flush mounting them, I elected to mount mine on the outside of the skin and have no issues with errors in my airspeed.

My experience has been that as a whole and almost universally, the flush mounted static ports do result in discrepancies. Some have ended up ok, but by and large the majority end up having errors - most don't even know to do have errors, but they usually do. Many people end up gluing a pop rivet head over the flush ports to end up with a near perfect system. Sealing the rivet to the skin is something I'm rather ambivolent about because in my opinion, other than the static tester, I don't know if there is a huge benefit...but it can't hurt either.

Also, we too have one of those nifty vacuum powered suction cup thingies for doing pitot/static testing...BUT, I have a whole assorment of rubbe tips for the static port...if you have the standard Van's rivet for the port, there is no real reason the shop doing your check needs to have a giant cup over the whole thing - just a big/small enough cup to cover the rivet heat and it ends up working great.

Overall, it's worth your time to make sure the connection between the port and the hose is good. You wouldn't believe how many people tell us their system is "tight and doesn't leak", until we hook the pitot/static box up to it and see some really fantanstic leaks!

Cheers,
Stein

Accuracy of pop rivet static ports

CAFE tested two of Van's factory airplanes (RV-8A and RV-9A) using a calibrated pitot-static system mounted to the wing to measure CAS. They also used a cabin barograph to measure IAS via the aircraft pitot-static system. In both cases CAFE reported that the speeds measured using the Van's pitot-static system were too high across the entire range of airspeeds tested. The maximum errors were about +5 mph for the RV-8A and +7 mph for the RV-9A.

These data don't seem to support the claim that the pop-rivet static ports are extremely accurate. However, they perhaps don't rule it out either, since there could be additional error from other sources (such as the pitot tube). I'm also not sure how to square these results with Kevin Horton's recent testing (using different methods) that shows a maximum error of about +1.5 kts.

Has anyone reported similarly rigorous tests for an RV with flush-mounted static ports? Anecdotal reports are much less convincing, since the test and data reduction procedures usually aren't given.

Alan Carroll said:

CAFE tested two of Van's factory airplanes (RV-8A and RV-9A) using a calibrated pitot-static system mounted to the wing to measure CAS. They also used a cabin barograph to measure IAS via the aircraft pitot-static system. In both cases CAFE reported that the speeds measured using the Van's pitot-static system were too high across the entire range of airspeeds tested. The maximum errors were about +5 mph for the RV-8A and +7 mph for the RV-9A.

These data don't seem to support the claim that the pop-rivet static ports are extremely accurate. However, they perhaps don't rule it out either, since there could be additional error from other sources (such as the pitot tube). I'm also not sure how to square these results with Kevin Horton's recent testing (using different methods) that shows a maximum error of about +1.5 kts.

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I'd love to know how the CAFE guys addressed the influence of the wing's pressure field on the pressure from the static source on their boom. NACA TN 2311 found that a static port on a wingtip mounted boom had an error equal to about 1% of the dynamic pressure, even if the boom was well ahead of the wing tip (as reported in NASA Reference Report 1046 - Measurement of Aircraft Speed and Altitude - available in this 14 MB pdf file). That would cause an error on the order of 1 mph, if the NACA data is applicable to their probe location and the RV airfoil.

At 180 kt, their results for the RV-8A differ by about 2.8 kt from mine. Some of that difference could be due to static pressure error due to their boom location. Some of it could be due to errors I could have made when I calibrated my EFIS ASI. Some of it could be aircraft to aircraft differences.

Kevin Horton said:

XXsnipXX
That would cause an error on the order of 1 mph, if the NACA data is applicable to their probe location and the RV airfoil.
XXsnipXX

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Is the error + or - ?
Concerned mostly about errors at low air speed.

GrayHawk said:

Is the error + or - ?
Concerned mostly about errors at low air speed.

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Whose error?

I found that on my RV-8 the static system position error would cause the ASI to read almost perfectly at 55 kt IAS. Between 60 kt and 120 kt the ASI would read slightly too low. At higher speeds the ASI would read a bit too fast. All errors at speeds lower than 160 kt were less than one kt. The instrument error in my ASIs is greater than one kt at many speeds, so it is a bigger contributor to the error in the airspeed system than is the static system position error.

The CAFE foundation reported that the static system position error in Van's RV-8A prototype would make the ASI read too high at all speeds.

NACA found that the pressure reported by wing tip booms (as used by the CAFE Foundation in their testing) would cause the sensed pressure to be too high, which would make the ASI read too low. The CAFE Foundation apparently calibrated their boom in a wind tunnel, but it isn't clear whether it was mounted ahead of a wing in the tunnel, nor which airfoil that wing had, nor if they tested at the full range of AOAs in the tunnel.

Kevin Horton said:

Whose error?

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The error that you alluded to in the quote snippet I provided.

But I already felt that if a wing mounted static port results in higher than ambient static pressure and with no wing induced effect on the pitot pressure, result with be lower than actual air speed (or - error). Indicating slower than actual is better than indicating higher than actual in certain cases IMHO.

good timing!

Hi Ron.

I'm about get the pitotstatic system checked by a certified airplane shop (requried on Ex-planes here in Norway) so your post came just in time!

If the thing leaks, I'll try the trick with tape over the static port and see if it helps.

Thanks for sharing!

IFR check

Thanks for the point about the rivet possibly leaking.

I also suspect that my Vans steam guage is reading low..I think it is outside of the 20' accuracy limit at low altitudes.

Not a problem for the check..I assume I could label it as inopperative and use the Dynon which I can calibrate before i take my airplane to the shop.

Question..Is there an adjustment screw on the Van's steam guage altimeter that would enable me to zero out the error?

Thanks

Frank

frankh said:

Question..Is there an adjustment screw on the Van's steam guage altimeter that would enable me to zero out the error?

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Look and see if there is an adjustment screw right next to the altimeter setting knob. My United altimeter has one.

L.Adamson

L.Adamson said:

Look and see if there is an adjustment screw right next to the altimeter setting knob. My United altimeter has one.

L.Adamson

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That screw is used to make an adjustment between between what the kollsman window scale reads and the altitude.
You turn teh window to read the current B. pressure.
Loosen the screw to disengage the knob from the Kollsman window.
Turn the knob until the altimeter reads the current elevation where you are located.
Tighten the screw to re engage the gears between the kollsman window and the knob.
I of course have never done this my self because I am not a trained and certified aircraft instrument technician, but this is how I understand it works.
If your altimeter has a non linear error (more or less error at one end of the altitude range than the other) I don't believe there is much outside of an instrument shop that can be done.

When you loosen the screw as Scott suggests don't take it all the way out. Just loosen it enough that the inside of the screw head will be flush with the surface of the altimeter. After you do the this set the altimeter to the proper altitude, then slide the screw side ways to the left. This will disengage the internal gears. Pull on the baro set knob it should slide out a bit, turn the inner scale to the proper setting. Push the knob back in then slide the screw to the right and screw in. Don't tighten too much. Everything should all go back together smoothly and no force.

CAUTION WHEN MAKING THIS ADJUSTMENT!

Be aware that when you perform this adjustment, you will be voiding any certification of this instrument and very possibly destroying the linear calibration.
This adjust is only supposed to be done by a certified instrument shop!

I put an Piper blade on the underside of my wing...(pitot & static)... Airspeeds read low at low speeds... about correct at mid to higer speeds... Have lived with it for now 500+ hours..afraid to change... I put my own colored reference markers on my airpspeed gauge that I live by.. It's one on my 'to do' list to install fuselage static ports... So I can confirm that the static port under the wing is not the thing to do.. However, I understand, that experimenting with the blade angle on the blade can improve it...
Sheldon

Sheldon said:

I put an Piper blade on the underside of my wing...(pitot & static)... Airspeeds read low at low speeds... about correct at mid to higer speeds... Have lived with it for now 500+ hours..afraid to change... I put my own colored reference markers on my airpspeed gauge that I live by.. It's one on my 'to do' list to install fuselage static ports... So I can confirm that the static port under the wing is not the thing to do.. However, I understand, that experimenting with the blade angle on the blade can improve it...
Sheldon

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I have one of those heated pitot/static tubes that use to cost around $200 when I bought it, and is now slightly less than a measly $1900.00 (for whatever reason).

However, it does hang down farther than a Piper type, and does work perfectly on the static side.
I also ended up with fuse static ports when I installed the A/P and didn't want to run another static line back, or use fuse static pressure.

L.Adamson

Sheldon said:

I put an Piper blade on the underside of my wing...(pitot & static)... Airspeeds read low at low speeds... about correct at mid to higer speeds... Have lived with it for now 500+ hours..afraid to change... I put my own colored reference markers on my airpspeed gauge that I live by.. It's one on my 'to do' list to install fuselage static ports... So I can confirm that the static port under the wing is not the thing to do.. However, I understand, that experimenting with the blade angle on the blade can improve it...
Sheldon

Click to expand...

It's not very common knowledge, but there are a bunch of different part #'s for the piper pilot blade. Many of the different #'s are for different bottom angles. This is at least part of the reason that some people utilizing used/surplus blades have no error and other people do.