Always shows a headwind
- Thread starter cwood2000
- Start date
1 - If you are seeing higher than actual (your altimeter shows you at 1,000ft but you are actually at 900ft) it means your static source is in an area of "low" pressure so you need to increase it. In this situation you need the tape to be behind the tiny hole. This causes more of the air to press against the tiny hole which increases the pressure felt at the hole. Kind of like the air is running into a "dam".
2 - If you are seeing lower than actual (your altimeter shows you at 1,000ft but you are actually at 1,100ft) it means your static source is in an area of "high" pressure so you need to lower it. In this situation you need the tape to be in front of the tiny hole. This causes less air to press against the tiny hole (Bernoulli effect) decreasing the pressure felt at the hole. Kind of like the air is flowing over a wing.
In my case the altimeter was showing me higher than I really was (situation 1) so I had to put the tape and subsequent "wedge" behind both of the tiny holes in my static system. You can see this in the photo I posted (Post #18). BTW, even some certificated aircraft have static ports with cut outs on them to compensate for this exact situation.
Yes, static port(s) must be on the outside of the fuselage. Just think of what happens if you have the typical "vented inside the cockpit" alternate static source and do an alternate static check while in flight. The altimeter jumps and displays a higher altitude and IAS than when connected to the primary (external) static source. Air pressure inside a flying fuselage is almost always lower than actual (again the Bernoulli effect) which will always show you higher than you actually are, again situation #1. This can be a particularly dangerous situation.
Last edited:
Steveden
Member
Wow! 5 kts, that surprises me for a tube and fabric cub, but I've never done that test. I'll have to give it a try. Thx
Steveden
Member
Thanks Galin, great information.
Does not surprise me at all. The faster the air flows (airspeed), the greater the Bernoulli effect, the greater the error becomes. In my particular case, at approach speed (85Kts) IAS and CAS were within 1kt of each other while altitude was within 10ft of actual. At normal cruise (143Kts) the Airspeed Indicator had me flying 13Kts faster than actual and the Altimeter indicated I was over 100ft higher than I really was. Since the error got bigger the faster I flew, the main suspect was the Static Port location.
Last edited:
If the ADAHRS has a leak then for all practical purposes your static system is "vented inside the cockpit" typically an area of low pressure. This would explain what you are seeing. Let us know what you find.
Last edited:
I'll have to put this on hold for a couple of weeks, added the tape, during pre-flight I noticed a crack in the spinner back plate. Removed the prop and will have to replace the back plate and (aluminum) spinner. Was not a new spinner/backplate, crack is right in the bend on the back plate. As Roseann Roseanna Dana use to say....It's always something
So far the tape has not been the answer, but I am still working with it. Just to note. All other systems are working properly, Autopilot works perfectly, Altitude is with a few feet of GPS altitude and ATC reports my altitude correctly. Here is a clip of the issue. Lower left hand corner, XW indicator, as I complete a 180, note it flipping back to a headwind. Small clip of the issue at
1 - Most of the time GPS altitude will be different than the altitude from your altimeter so don't use that as a verification method.
2 - ATC only sees what is being reported by your EFIS. So if your EFIS has the wrong altitude then ATC will see and report the wrong altitude. IOW, "Garbage in, Garbage out."
3 - The autopilot only follows what is being sent to it from the EFIS so just another case of "Garbage in, Garbage out."
Did you see ANY difference in IAS with a few layers of gorilla tape (at exactly the same power setting and same pressure altitude) as with no tape? Also, have you verified your altitude at cruise speed (with the ILS method or overflying an airport at cruise speed) and/or verified your TAS with a three heading GPS speed test? Those are the only methods you should use for these altitude and TAS verification.
FWIW: I did a flight to KBQK on Friday and used the ILS method just to re-check my altimeter. While at my normal cruse speed (143Kts) and the Glideslope centered at the FAF (YOKHO), my altimeter displayed 1,700ft which is exactly the altitude of this particular location. So I again verified my altimeter is displaying correctly using a method completely independent of my EFIS.
2 - ATC only sees what is being reported by your EFIS. So if your EFIS has the wrong altitude then ATC will see and report the wrong altitude. IOW, "Garbage in, Garbage out."
3 - The autopilot only follows what is being sent to it from the EFIS so just another case of "Garbage in, Garbage out."
Did you see ANY difference in IAS with a few layers of gorilla tape (at exactly the same power setting and same pressure altitude) as with no tape? Also, have you verified your altitude at cruise speed (with the ILS method or overflying an airport at cruise speed) and/or verified your TAS with a three heading GPS speed test? Those are the only methods you should use for these altitude and TAS verification.
FWIW: I did a flight to KBQK on Friday and used the ILS method just to re-check my altimeter. While at my normal cruse speed (143Kts) and the Glideslope centered at the FAF (YOKHO), my altimeter displayed 1,700ft which is exactly the altitude of this particular location. So I again verified my altimeter is displaying correctly using a method completely independent of my EFIS.
Last edited:
Pardon me for commenting, but do the speed tests you have done compensate for altimeter (static source ) error? I don’t think they do, but I could be wrong. If so I apologise for muddying the water.
One way - possibly clumsy and maybe dangerous, to test for static error might be to adjust the altimeter setting in flight and observe if there is a setting, close to the reported qnh, that produces a stable wind reading that does not flip, but again I might be wrong.
One way - possibly clumsy and maybe dangerous, to test for static error might be to adjust the altimeter setting in flight and observe if there is a setting, close to the reported qnh, that produces a stable wind reading that does not flip, but again I might be wrong.
I have not recorded any changes in IAS with the amount of tape, I was focused on the indicator and what is was or was not doing. I'll check on the altitude via a ILS, good idea. Attached is a spreadsheet of two runs I did today. Same course, 3000 msl, 30.00", 24.5/2450 on the power settings. Still scratching my head. What do you see?
Thanks Fish, I have adjusted the setting but it still flips. It is a new install, so I have probably done something wrong, just don't know what it is. One note that it will show a tailwind if I am turning, but as soon as the wings are level, back she goes.
Thanks again,
CW
Using the gps to wind and tas spreadsheet from contrails suggests that your TAS airspeed is off by about 12 knots, it should be around 167-168 unless i’m horribly wrong, which is always possible.
The TAS on the right column came from checkride.com they are pretty close to Dynon’s
According to the numbers you posted I calculated your TAS to be 167Kts (for both runs) not the 181Kts you are displaying which agrees with what SUNFISH posted. This is a +14kt difference in EFIS calculated TAS to GPS only calculated TAS. Take the same measurements at a slow but stable speed (below 100Kts) and see what the numbers are.
I see you fly out of Quincy. I am in St. Augustine so if you want to, take a quick flight over here and we can do some extensive flight testing. Your problem has me curious.
I see you fly out of Quincy. I am in St. Augustine so if you want to, take a quick flight over here and we can do some extensive flight testing. Your problem has me curious.
Last edited:
Sounds like maybe a static error - too low air pressure artificially increasing indicated airspeed. Using your check ride.com calculator, to see a 168 TAS at 3000 ft, 30.00’ and about 12C, you should be seeing an IAS around 160 kias. You are showing an ias around 172.
....Anyway, I’m jealous! 160 knots is about twice as fast as me
....Anyway, I’m jealous! 160 knots is about twice as fast as me
That would be above and beyond, but welcomed. I can come your way as well. I have family coming in for the holidays Wednesday but tomorrow, or Sunday Monday would work, or Wed-Thurs next week.
I sent you a private message. BTW, I am assuming you have an VOR/ILS/LOC receiver in your airplane.
Last edited: