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RV9Otter

Active Member
I have been working with GRT to resolve a artificial horizon issue on my GRT Horizon model W.S. They sent a replacement AHRS and said that if I installed it in place of the old one, the only thing that I would need to do would be a magnetometer calibration.

I installed the replacement AHRS today and went for a test flight. At about 30 knots, the tape and digital airspeed indicated on the EFIS blew right through the white, green, yellow and red and pegged itself at what appeared to be beyond its maximum airspeed capabilities. I have a backup airspeed and it indicated normal, as well as a independent AOA that indicated normal.

I was very careful to reconnect the P/S system lines to the replacement AHRS exactly the same as they were on the original AHRS.

GRT is closed for the holiday, I assume through New Year's, so, any guess as to what might be going on? Do these GRT AHRS's need to have the airspeed calibrated? Thanks, Curt
 
No, there is no indicated airspeed calibration.

I would first check for water between the GRT AHRS and the T that goes to your mechanical ASI.
 
No Water

Bill:

The line from the AHRS to the backup airspeed indicator is very short, 18 inches, as the AHRS is behind the instrument panel. The plane has been in the hangar the entire time so no moisture should have been introduced.

Thanks, Curt
 
RV9Otter said:
Bill:

The line from the AHRS to the backup airspeed indicator is very short, 18 inches, as the AHRS is behind the instrument panel. The plane has been in the hangar the entire time so no moisture should have been introduced.

Thanks, Curt
Click to expand...

The Easy test is to plumb a spare steam Air Speed indicator in the place of the AHRS and take a test flight.
 
Steam Backup Worked Properly

I presume that one of the ports on the AHRS is pitot and the other is static. In looking at the AHRS from the rear (with the front facing the direction of aircraft travel), does anyone know which port is which on the AHRS (you would think the manuals would show this in a diagram but I can't find it)? Is the left one pitot or static?

I don't think I have reversed the connections as both the pitot and static lines go through a T connector and the other end of those T connectors are my backup steam dial airspeed and altimeter, and both of those backups indicated correctly.

I'm beginning to think that the air computer in the AHRS is faulty.
 
You can attach a water manometer (vinyl tube in a U shape, with water in it) attached to the pitot tube to trouble shoot on the ground. About 6" more water on the side away from the pitot should show about 100 KIAS.

There are other things that can be wrong here, such as the digital connection to the EFIS.
 
BobTurner said:
You can attach a water manometer (vinyl tube in a U shape, with water in it) attached to the pitot tube to trouble shoot on the ground. About 6" more water on the side away from the pitot should show about 100 KIAS.
Click to expand...

Yep, just tested my new GRT Mini and Garmin G3X Touch yesterday, between 50 knots and 230 knots at 10 knot intervals, error was 1 knot or less.

Here is the manometer instructions: http://www.iflyez.com/manometer.shtml
 
Thanks...

Thanks all for your ideas. I double checked today that all hoses and the electrical connections are correct.

This was supposed to be a simple swap of the AHRS and airspeed was working just fine when I took my old one out. GRT said that the electrical and Pitot Static were all compatible with this AHRS, no changes necessary.

I suspect that I received an AHRS with something wrong with the air/data system. I'll call GRT when they open after the holiday's and post what the problem was, once resolved. Thanks again. Curt
 
AHRS Version

RV9Otter said:
Thanks all for your ideas. I double checked today that all hoses and the electrical connections are correct.

This was supposed to be a simple swap of the AHRS and airspeed was working just fine when I took my old one out. GRT said that the electrical and Pitot Static were all compatible with this AHRS, no changes necessary.

I suspect that I received an AHRS with something wrong with the air/data system. I'll call GRT when they open after the holiday's and post what the problem was, once resolved. Thanks again. Curt
Click to expand...

I could be mistaking, but I seem to recall that the "newer" low profile single AHRS units have the pitot and static ports reversed from the positions that were used in the original "tall" single AHRS units. You may want to double check the marking on the ports of the replacement unit.

Skylor
RV-8
 
Ports Are Reversed

Skylor:

You were correct...the ports on the newer version are reversed. I should have caught this but was operating on the guidance that I had received that this was an identical replacement, except for the height.

When I reversed the connections, everything seems to be fine.

I'm a little concerned that having the pitot connected to the static port may have caused some damage during the test flight around the pattern. I'll speak with GRT after the holiday to see what they say.

Thanks again to all who weighted in. Curt
 
Hello Curt,

I am glad to hear that you figured out what caused the airspeed error. I wouldn't be concerned about damaging the static pressure sensor. It has an absolute maximum pressure of 58 psia. Even a flight to Vne at sea level on a high pressure day would still be under 16 psia on the sensor.

-Marc Robertson
GRT Avionics
Engineer
 
Thanks Everyone and GRT

Marc:

Thanks...you all at GRT have really been helpful, as well as everyone on this forum. Much appreciated. Curt
 
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