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I have been doing the Pitot Static testing using the MGL Explorer light IEFIS TAS values against its GPS ground speed values flying up and back a coarse perpendicular to the wind direction and averaging the speeds.

I am finding the following:
- Low speeds 50KTS the TAS reads slightly low 1 to 2 knots difference after averaging out.
-65 knots has no error.
-80 knots the TAS reads 7 knots high.
-120 knots the TAS reads 16 knots high.

I am using the standard Sonex pitot/static probe in the standard location.

I had the system leak tested and calibration checked before first flight and all was leak free and 100% accurate.

I tried opening the alternate static port and saw no change in the speed readings.

Given the above, what is the most likely cause of the speeds reading slightly low at lower speeds and reading high at the higher speeds?

I thought if the static was not right it would read either too low or too high at any speed? I also feel that when the calibrations were done, all speeds were 100% accurate, so it shouldn't be possible for the pitot probe to be feeding more pressure in than available or do I have this theory wrong?

Any advice on where to start is greatly appreciated.

Murray Parr wrote:I have been doing the Pitot Static testing using the MGL Explorer light IEFIS TAS values against its GPS ground speed values flying up and back a coarse perpendicular to the wind direction and averaging the speeds.

I am finding the following:
- Low speeds 50KTS the TAS reads slightly low 1 to 2 knots difference after averaging out.
-65 knots has no error.
-80 knots the TAS reads 7 knots high.
-120 knots the TAS reads 16 knots high.

I am using the standard Sonex pitot/static probe in the standard location.

I had the system leak tested and calibration checked before first flight and all was leak free and 100% accurate.

I tried opening the alternate static port and saw no change in the speed readings.

Given the above, what is the most likely cause of the speeds reading slightly low at lower speeds and reading high at the higher speeds?

I thought if the static was not right it would read either too low or too high at any speed? I also feel that when the calibrations were done, all speeds were 100% accurate, so it shouldn't be possible for the pitot probe to be feeding more pressure in than available or do I have this theory wrong?

Any advice on where to start is greatly appreciated.

TAS is a function of CAS corrected for Density altitude.

CAS is IAS corrected for position and instrument error.

The MGL efis has a compensation factor for IAS at various airspeeds to account for position error. Have you done that part of the setup yet?

MGL is using IAS to calculate TAS, which means you need to make sure it's calibrated.

TAS is a function of CAS corrected for Density altitude.

CAS is IAS corrected for position and instrument error.

The MGL efis has a compensation factor for IAS at various airspeeds to account for position error. Have you done that part of the setup yet?

MGL is using IAS to calculate TAS, which means you need to make sure it's calibrated.[/quote]

Thanks Chris,

I haven't done that part of the setup yet; however, it seems quite a bit off at higher speeds, I would like to try to get it closer before compensating. I think it must be the static or perhaps OAT gauge inaccuracy, unless it is getting some prop wash into the pitot. Maybe I should explore the altitude accuracy which would indicate if it is the static location or not. I will have to study up on how to do that.