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Thread: Airspeed Indicator accuracy and the importance of a true static "system"

  1. #1
    Subsonic's Avatar
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    Default Airspeed Indicator accuracy and the importance of a true static "system"

    I bought a Dynon D2 pocket panel and have been able to keep my eye on the GPS speed on final approach to landing vs. my original 1956 AI. With the D2 on the panel, I can scan quickly enough to still land nicely. I've learned my landings are really at about 50 to 55 mph on GPS and my AI is showing around 68 to 75. I always wondered why my indicated landing speed was so high, as compared with what others on this site have reported. Turns out I've been slower than I thought the whole time.

    I fly with an iPad mini on my yoke with Foreflight too, but could never look back and forth between it and the AI quickly enough to get a good estimate of the differential, because it was too hard to do that and land the plane. Anytime I tried to slow it down much more than about 68 -74 on my AI, I'd stall out of the flare. Now I know I'm really somewhere around 48 or 50 mph when stall occurs.

    I made 13 landings at three different airports on the same day and found this relationship was pretty consistent. When I'm indicating 80 to 90 on GPS, the AI shows about 88 to 97, and the needle is sort of bouncing around. The faster I go, the closer the indications are to matching until about 100 mph and above when they are virtually identical. I went as fast as 125 at altitudes between 1,000 to 2,000 feet MSL, and the speeds indicated on both instruments above 100 always matched. I spent over an hour and a half doing this.

    Poking around with the glare shield removed I found my VSI, AI, and altimeter gauge static ports are all disconnected from the static system line. They just vent to the atmosphere behind the panel. So much for having a static system. Also, I recently had a Mode C altitude encoder installed and noticed that port was likewise left with the port vented to the local atmosphere by the Avionics company installer and not spliced into the static system like I thought it would be. Now I know why. He couldn't find a static system.

    I'm thinking I should I get my static system really hooked up first and make speed comparison measurements again since the designers of the avionics system intended that system to be in place. It probably got clogged up at some point in the last 61 years, and somebody did a simple fix and disconnected it. If needed, where can I get my AI recalibrated? Thanks.

  2. #2

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    Default Re: Airspeed Indicator accuracy and the importance of a true static "system"

    I would connect everything the way it should be first then worry about adjustments. Just a reminder, if a line needs blown out, NEVER blow air in the line towards the instruments.
    Last edited by Jeff J; 10-27-2017 at 09:15 PM.

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    Stephen's Avatar
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    Default Re: Airspeed Indicator accuracy and the importance of a true static "system"

    I check the accuracy of my Airspeed by flying a triangle course, changing my heading 120 degrees for each leg. I average my AS and GPS reading then compare. I repeat this task for a variety of speeds down to just above stall. My AS is reading about 5 mph fast at lower speeds.
    "You can only tie the record for flying low."

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    Administrator Steve Pierce's Avatar
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    Default Re: Airspeed Indicator accuracy and the importance of a true static "system"

    GPS is giving you ground speed, airspeed indicator is giving you airspeed to relative wind. When landing at places without wind socks sometimes I compare gps grounds speed to indicated airspeed to determine if I have a tailwind or not. It is usually easy to tell just by feel because I feel like I am fast when I have a tailwind. Static systems are not real accurate. Vented to the back of the panel are the worst, on the static on the pitot works ok but the static ports on the belly of the latest Tri-Pacers seem to be the most accurate.

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    Default Re: Airspeed Indicator accuracy and the importance of a true static "system"

    I wonder how the avionics shop did the 91.413 check. Without the static system hooked up, you can't do the required data correspondence check!


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    Default Re: Airspeed Indicator accuracy and the importance of a true static "system"

    I noticed that our static ports are all connected together but then the end of the line is simply open behind the panel. You can really tell the difference when you open the cabin vent down by your leg. The airspeed really jumps around.

    We have the pitot that incorporates both the pressure and the static together in kind of an upside down F shaped tube. However it appears that they never bothered to run the static line to it. Only the pressure side is connected. It's on the list to correct it but we've kind of gotten used to it as is. Might be a winter project.
    “Seek advice but use your own common sense.”
    ― Yiddish Proverb

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    Gilbert Pierce's Avatar
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    Default Re: Airspeed Indicator accuracy and the importance of a true static "system"

    Quote Originally Posted by dgapilot View Post
    I wonder how the avionics shop did the 91.413 check. Without the static system hooked up, you can't do the required data correspondence check!


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    Several different avionics shops have done the VFR transponder checks on my Clipper over the years. None ever worried about the static systems checks as they are not required for a VFR check. Clippers came from the factory with the ASI and ALTIMETER static ports open.
    No static checks are required under the below VFR test.
    See:
    14 CFR Appendix F to Part 43, ATC Transponder Tests and Inspections.
    Last edited by Gilbert Pierce; 10-28-2017 at 09:12 AM.

  8. #8
    Homer Landreth's Avatar
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    Default Re: Airspeed Indicator accuracy and the importance of a true static "system"

    Quote Originally Posted by smcnutt View Post
    I noticed that our static ports are all connected together but then the end of the line is simply open behind the panel. You can really tell the difference when you open the cabin vent down by your leg. The airspeed really jumps around.

    We have the pitot that incorporates both the pressure and the static together in kind of an upside down F shaped tube. However it appears that they never bothered to run the static line to it. Only the pressure side is connected. It's on the list to correct it but we've kind of gotten used to it as is. Might be a winter project.
    Your plane raises a question I have had and never spent the time to get the "gospel word". I have dealt with a lot of no static "system" planes, and on a ratio of about 40 % did and 60% didn't, they have no instrument connectivity, but each instrument had a brass plug in it's port with just a pin hole in it. Does anyone have a reference for this plug? To me it would obviously somewhat dampen pressure chances from slipping and skidding, open windows etc so those changes would be less of an effect than if the instruments just had an open hole.
    Relative your plane having all instruments connected together but open behind the panel, I don't ever recall seeing this, but the fact that you do have a pitot/static capable sensor on the wing sort of leads to the suggestion that maybe your plane should have that static line connected to the tube on the wing and someone just didn't do it.

  9. #9
    Jim Hann's Avatar
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    Default Re: Airspeed Indicator accuracy and the importance of a true static "system"

    Quote Originally Posted by smcnutt View Post
    I noticed that our static ports are all connected together but then the end of the line is simply open behind the panel. You can really tell the difference when you open the cabin vent down by your leg. The airspeed really jumps around.

    We have the pitot that incorporates both the pressure and the static together in kind of an upside down F shaped tube. However it appears that they never bothered to run the static line to it. Only the pressure side is connected. It's on the list to correct it but we've kind of gotten used to it as is. Might be a winter project.
    Steve, Gilbert, et al might be able to add to this but based on my research the only PA-22 that came with the static "tube/boom/probe" was the -135. I am assuming the -125 had nothing but open ports behind the panel. The 150-160 has the late model system that has previously been referenced. Based on that smcnutt which model did your airplane start out as? Or am I way off and you have a PA-20?

    Also there is a requirement, even for VFR, of a check for a new installation (or maintenance) like I just had done, 14CFR91.413(b). All I know is that my radio inspector had more boxes out than in the past with the old Mode A/C setup and that he signed it off as passed. I didn't ask for details (and I paid him in cash.)

    Jim
    1957 PA-22/20 "Super Pacer" based 1H0
    Lifetime EAA member
    Vintage Aircraft Association member
    Lifetime EAA Chapter 32 member


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    Default Re: Airspeed Indicator accuracy and the importance of a true static "system"

    Quote Originally Posted by Gilbert Pierce View Post
    Several different avionics shops have done the VFR transponder checks on my Clipper over the years. None ever worried about the static systems checks as they are not required for a VFR check. Clippers came from the factory with the ASI and ALTIMETER static ports open.
    No static checks are required under the below VFR test.
    See:
    14 CFR Appendix F to Part 43, ATC Transponder Tests and Inspections.
    To actually complete the check, they are required to verify that the transponder Mode C altitude output remains within 125' of the altimeter reading up to the service ceiling of the airplane even for a VFR check. Read paragraph (c) of 91.413. Even a VFR airplane with a transponder with altitude encoder needs to comply with 43 Appendix E paragraph (c) to meet the 91.413 requirement.


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