David
You were right. I had an altitude error plus 130ft comparing to my GPS.
I put a half wash behind each whole of the static. Then I got an altitude error minus 200ft, and the IAS was VERY slowly.
So the job is to trim the thickness of the wash to match the GPS.
ERROR - GPS altitude and barometric altitude are different things, and you shouldn't expect them to be the same. The GPS altitude should be fairly close to the actual altitude above sea level, plus or minus various errors in the GPS system. But, barometric altitude only agrees with the actual altitude if the air temperature matches the standard atmosphere air temperature at all altitudes between the source of the altimeter setting and the altitude of the aircraft.
If the air temperature differs from standard temperature, the difference between barometric altitude and true altitude may be quite large. This difference may easily be more than 1000 ft in the winter.
Info on cold temperature effects on altimeters.
Info on the laws of physics behind the cold temperature effects on barometric altimeters.
See the
diagram at the bottom of the page showing how the pressure levels are scrunched closer together in cold temperatures.
Info on an incident where an airliner missed terrain by about 150 ft due to cold temperatures.
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How to check for errors in your airspeed system:
Step 1 - Check for static and pitot system leaks (you've already done this).
Step 2 - Confirm EFIS ASI and OAT instrument errors are low. The EFIS ASI can either be checked at an instrument shop, or you can make a water manometer to check it against. The OAT probe can be checked in some ice water, and you can compare it against the reported temperatures at your airport. Be aware that some OAT systems may have errors due to poor placement of the OAT probe - it may be heated by the exhaust or by warm air coming from the cowling, or the back side of the probe may be heated by warm air in the cockpit.
Step 3 - Do a flight test. Set the altimeter to 29.92 (or 1013.25 mb). Fly a GPS box pattern with four legs, being sure to hold exactly the same IAS and altitude on each leg. Record the GPS ground speed and track, IAS, pressure altitude and OAT on each leg.
Step 4 - On the ground, crunch the data, using this
Excel spreadsheet I created for a Kitplanes article.
I wrote two articles for Kitplanes magazine earlier this year. They go into much more detail on how to check the accuracy of your static system.