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GRT HX & Dual AHRS ASI reading

V

vluvelin

Well Known Member
During pitot line test with water manomerer noticed diferance in ASI reading
On one display +3,+4kts on lower speeds +2 higher speeds
On other display -3,-5kts on lower and - 2 on higher

Is this normal with no leaks in the line?

There is no calibration for Indicated AS then what do you do???
The correction card like on compass would look stupid
 
Do you have one AHAR or two? If you have one, then some thing is wrong as you have one source of airspeed with two different reading. If you have two, then the AHARs are not matching and would talk to GRT about it. Although it might not be enough to cause a split and therefore you would see exact same IAS on both screens.
Also check your static port as that could cause a difference if there is a leak in the static port.
 
Dual AHRS and pitot and static lines split just before entering both AHRS
No leaks on Pitot or Static How much difference everyone else have????

Thank you all
 
Water Manometer Test

I have HX with dual AHARS similar to you. I did a water manometer test also and I?m indicating about 9 MPH fast at stall speed and 3 MPH at cruse speeds. I didn?t compare my different screens. I figured they would be the same.

I have a slight leak in my pitot line but I don?t think enough to cause this error. It is my understanding there isn?t anything you can do. The cruse error isn?t that big a deal and I have set my stall speed 9 MPH fast to match my test.

Maybe someone else has a better idea.

Also, Static has nothing to do with this test.
 
Calibration for Airspeed

The airspeed errors mentioned appear to be those caused by changes in the bias of the sensor. Because airspeed is calculated roughly as the square root of the differential pressure, bias errors have the largest influence on the indicated airspeed at lower speeds. For example, a bias error that causes a 3 mph error at 70 mph, will only represent 1.5 mph error at 140 mph.

The EFIS does provide an adjustment to account for the aging of the sensor that can result in a change in the bias. The setting is made on the last entry on the altimeter calibration page.

Ideally the AHRS should be have been on for at least 15 minutes so that temperature gradients in the AHRS have stabilized. You could also perform this step at the end of a flight.

With the airplane in a hangar (so that the pitot and static pressure are equal...no wind on either port), adjust the airspeed bias until the airspeed registered on the AHRS maintenance page is less than +/-25 fps. (A 25 fps bias error will contribute only 2 mph IAS error at 70 mph).

Thanks
Greg Toman
GRT Avionics
 
Greg_Toman said:
The airspeed errors mentioned appear to be those caused by changes in the bias of the sensor. Because airspeed is calculated roughly as the square root of the differential pressure, bias errors have the largest influence on the indicated airspeed at lower speeds. For example, a bias error that causes a 3 mph error at 70 mph, will only represent 1.5 mph error at 140 mph.

The EFIS does provide an adjustment to account for the aging of the sensor that can result in a change in the bias. The setting is made on the last entry on the altimeter calibration page.

Ideally the AHRS should be have been on for at least 15 minutes so that temperature gradients in the AHRS have stabilized. You could also perform this step at the end of a flight.

With the airplane in a hangar (so that the pitot and static pressure are equal...no wind on either port), adjust the airspeed bias until the airspeed registered on the AHRS maintenance page is less than +/-25 fps. (A 25 fps bias error will contribute only 2 mph IAS error at 70 mph).

Thanks
Greg Toman
GRT Avionics
Click to expand...

Great info, I will check mine but during each IFR certification for altimeter, mine has very close both on altitude and IAS.
 
Thank you for Info. Today got both AHRS to read same. To get this result one of the AHRS had to be set Bias at -117 the limit is -128 does it meen the unit have to go for checkup soon?

Manometer KTS-----IAS KTS
210-----------------215
200-----------------205
170-----------------173
80-------------------80
60-------------------59
 
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