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GRT TAS calc WAY off

edclee

Well Known Member
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PROBLEM FOUND!!!!! GRT TAS calc WAY off

I cant figure out why my TAS is being reported way over what it actually is. My IAS is corrrect and verified numerous ways. The problem could be something else I cant figure out. When I go the the AHRS maintenance page and check the velocity value, it is 35 ft/sec while sitting still. Recycling the EFIS doesn't change it. I is always this or very close. When in flight, it changes and increases to much higher numbers so it is working, but obviously something is wrong if it shows 35 ft/sec (about 24 mph) sitting still.

Anyone else run into this?
Ed
 
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I run Dynon not GRT - but Dynon has a zero-airspeed calibration routine that you run in a closed hangar with a cover on the pitot tube, so it knows where true zero is. I suggest you look for that in a setup menu somewhere.
 
I cant figure out why my TAS is being reported way over what it actually is. My IAS is corrrect and verified numerous ways. The problem could be something else I cant figure out. When I go the the AHRS maintenance page and check the velocity value, it is 35 ft/sec while sitting still. Recycling the EFIS doesn't change it. I is always this or very close. When in flight, it changes and increases to much higher numbers so it is working, but obviously something is wrong if it shows 35 ft/sec (about 24 mph) sitting still.

Anyone else run into this?
Ed

How did you confirm that the IAS is accurate? I thought you had to convert to TAS nad compare to GS.

GRT has an offset for adding or subtracting from the TAS before presentation to the pilot. You sure there is not a number in there? It is table that adjusts the offset by speed range, I think.

It would be quite odd that an AHRS flaw provided accurate IAS but inaccurate TAS. That conversion is just math, using ALT and OAT.
 
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I agree that the AHRS data shouldn’t have anything to do with TAS - it is OAT, altitude, and IAS cooking into an equation - lots of people have very poor OAT locations that pick up engine heat, so check that first.
 
More information is needed (numbers). As already posted, it is hard to directly verify IAS; usually it is calculated from TAS.
 
I’d check pitot static system first. As an aside, do you also show weird winds, i.e., always a big headwind, or crosswind?
 
TAS

More information is needed (numbers). As already posted, it is hard to directly verify IAS; usually it is calculated from TAS.

My AHRS page says my velocity is 35 ft/sec sitting still powered up for several minutes. I have cycled the power in several locations over several hours and it comes up the same. That is likely the problem with my TAS indications as well. The TAS reported is way in excess of what the TAS actually is. For instance at 2000 ft MSL and OAT of 69 degrees altimeter 30.05 and IAS 137, TAS is shown as 160 mph. Clearly wrong. Also, wind correction information is WAY off as one would expect if the TAS is that far off. My IAS is correct and verified by another Airspeed I have used in this system for a long time. I checked the magnetometer for accuracy again this morning at a large airport nearby. Taxiing on all four compass points the error in GPS track and heading is within 1 to 2 degrees.
 
Recently had this conversation with GRT, Jeff Dufouw. My question was, how dose my EIS4000 and Sport SX with OAT temp probe installed compute TAS? A friend that I fly with says mine is off compared to his Dynon. My probe is in a bad spot, 3 degrees off compared to others in flight. If I subtract MSL at a point from my altitude, multiply IAS by +2% per thousand , I get almost my TAS as indicated on my Sport SX ( old one, slow processor version). 14OKTS IAS , MSL 1000, 5000 AGL = 140 x 1.08= 151TAS It appears to me that temp is not playing a big role in getting to TAS. Here is what Jeff GRT said.

That sounds like a good approximation. The EFIS uses the full TAS calculation including OAT, except the EFIS doesn't currently compute true OAT or apply a recovery factor. One place to find this is the Aviation Formulary web page.

http://www.edwilliams.org/avform.htm
 
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.... Also, wind correction information is WAY off as one would expect if the TAS is that far off.

There is to me, a fairly rigorous/complex setup to make the wind speed and direction data on my sport sx accurate. I’m not flying that far, to really care that much to know this information, so I’ve put no effort into it. I just ask my friend off my wing 😀
 
Recently had this conversation with GRT, Jeff Dufouw. My question was, how dose my EIS4000 and Sport SX with OAT temp probe installed compute TAS? A friend that I fly with says mine is off compared to his Dynon. My probe is in a bad spot, 3 degrees off compared to others in flight. If I subtract MSL at a point from my altitude, multiply IAS by 2% per thousand , I get almost my TAS as indicated on my Sport SX ( old one, slow processor version). It appears to me that temp is not playing a big role in getting to TAS. Here is what Jeff GRT said.

That sounds like a good approximation. The EFIS uses the full TAS calculation including OAT, except the EFIS doesn't currently compute true OAT or apply a recovery factor. One place to find this is the Aviation Formulary web page.

http://www.edwilliams.org/avform.htm

The real fly in the ointment is WHY does my SX AHRS data show 35 ft/sec when I am standing still? It does this whether I am in the hangar, engine off, or outside in no wind. If I consider the roughly 23 mph this equates to about what my errors in TAS and wind direction readings are. The TAS is so far off that in todays light wind conditions, the wind direction arrow showed a headwind no matter what my direction of flight was! :eek::confused:
 
It’s broke, send tech support email, or call. If they decide, yes broke, create a RMA , and send it in for repair or replace
 
The real fly in the ointment is WHY does my SX AHRS data show 35 ft/sec when I am standing still? It does this whether I am in the hangar, engine off, or outside in no wind. If I consider the roughly 23 mph this equates to about what my errors in TAS and wind direction readings are. The TAS is so far off that in todays light wind conditions, the wind direction arrow showed a headwind no matter what my direction of flight was! :eek::confused:

You have yet to indicate that the TAS calibration table in the AHRS menu is clear of any entries. Until that is stated as being clear of all entries, the obvious assumption is that there is bad data in that table resulting in the erroneous TAS presentation. Cycling power will have no effect on this table as it is for permanent non-volatile adjustment.

If you need directions on how to manage the table, download a user manual and scroll to the TAS calibration section near the end. Not all user manuals have the procedure included even though the function is likely available on all GRT EFIS/AHRS. It is highly likely to be the same as the original WS user manual procedure.

My RV7 has zero offset data in the table and all is fine. I imagine I am not alone in this.
 
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The real fly in the ointment is WHY does my SX AHRS data show 35 ft/sec when I am standing still? It does this whether I am in the hangar, engine off, or outside in no wind. If I consider the roughly 23 mph this equates to about what my errors in TAS and wind direction readings are. The TAS is so far off that in todays light wind conditions, the wind direction arrow showed a headwind no matter what my direction of flight was! :eek::confused:

You should stop looking at that raw data, as you have no idea what it is used for or what it is supposed to show on the ground. I doubt anyone but GRT knows what that data is and what it is used for. For all we know, 35 is the default output for 0 IAS.

GRT, like ALL EFIS' first use pitot and static pressure to determine IAS. They then use simple mathematical computations to determine TAS (using IAS, OAT and baro altitude). If you have determined that IAS is accurate, then you DO NOT have a problem with your AHRS. The issue is in the s/w or configuration or the OAT or ALT data.
 
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Problem Found

Well after much head scratching, flying, and many gallons of fuel looking at the problem, I found it. I have the SX system with both the primary S200 and the Slave S100. I have been using the S100 for nearly a year, but only recently acquired the S200, which has all the smarts and air data. The interdisplay link sends all the EIS stuff which is wired to the Slave S100 which is exactly what I have been using it for. During my setup of the new (to me) S200 primary, I of course checked to be sure the engine info was there, which it was, but I always use the S100 for only engine stuff. On this last flight I decidedd I would see if there was any difference in values and found none...EXCEPT the OAT read 44C on the S200, not the 16C that the S100 said. It turns out there is a parameter to select EIS none, Chelton EIS, or 4000/6000/8000. I have the EIS 4000, which is what the S100 was set to, but the setting on the S200 from the previous owner was Chelton EIS. I changed it and Voila! All is good, temps are the same and the TAS works and is accurate, and of course wind now works properly.
Ed
 
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