OAT problems

[Petr Javorsky(Guest)]

We have installed EFIS D10A, incl. AOA, OAT, EDC.
EFIS displays OAT value minus 28 deg.C.
No change of temperature when probe is warmed up.
Wiring OK (tested). Probe type 2 (set 2). Another EFIS unit connected - the same result: -28degC. Another EDC connected - OAT not found for either probe type setting. So original EDC installed back - still minus 28 deg.C.
What might be the cause?
Is there any way how to test OAT probe (voltage, current measurement etc.) prior to be mounted into an airplane?
Thank you for help.
Petr Javorsky, Evektor-Aerotechnik

 

[dynonsupport]

Petr,
This may be a case of the EFIS and EDC having firmware that are not the same. Please try updating the firmware in the EFIS while the EDC is connected. The program updates the EDC if it is present, but if it is disconnected, they can become out of sync with one another.

The OAT probe can be tested. It's a resistance between the ground wire and the signal wire. It should be about 10K ohms at room temp and will vary as you heat or cool it. You could also measure the voltage on the signal wire when it's hooked to the EDC and running. If I remeber right, this should be about 1.6V at room temp.

Please let us know if these suggestions work.

 

[Petr Javorsky(Guest)]

Thank you for advice. We disconnected OAT probe from EDC and measured resistance - probe is OK. Then connected probe back to EDC and now OAT works! Nobody knows why....but works. (We did the same yesterday i.e. probe diconnection and connection, but did not solve the problem). Today yes. Software version is 2.11.
Another question: I sent yesterday an e-mail to info@dynonavionics.com regarding EFIS D10A instrument error at low airspeeds and AOA calibration problems we have. Should I send that e-mail to another address? Thank you for your help.

 

[dynonsupport]

Petr,
The person who answers the info@dynonavionics.com emails is out at an airshow this week. He will get back to you shortly, or if you need help quickly, please post your question here.

 

[Petr Javorsky(Guest)]

Thank you for information.
I will try to describe our problem (more details incl. some charts etc. in e-mail I sent).
Our first airplane with EFIS has also standard airspeed indicator (ASI). Both EFIS and ASI are supplied with the same static pressure from our standard pitostatic probe under left wing (known position error measure by pitotstatic bomb). Pitot pressure to EFIS is brought from Dynon probe under right, Pitot pressure to ASI from standard pitotstatic probe under left wing. We expected both EFIS and ASI will show similar airspeeds because of the same static pressure. However there was difference up to 10km/h between the EFIS and ASI at low airspeeds (70-120). So we brought to EFIS also the pitot pressure from our standard probe to have both ASI and EFIS supplied with the same static and pitot pressures. The result was the same - still 10 km/h difference at low airspeeds. We demounted both ASI and EFIS and measured instrument errors in our Instrument Test Lab. The measurement has shown there is indeed minus 10 km/h instrument error of the EFIS at low airspeeds (ASI max error was +/-5km/h) and that the lowest indicated speed by the EFIS is 48 km/h then skip to 0. To be sure, we measured another EFIS - almost the same results (more details in an e-mail I sent). So now we are sure the difference 10 km/h between IAS indicated by EFIS and ASI at low airspeeds is due to big instrument error of the EFIS. It might be confusing for the pilot to have different indicated airspeeds by the standard ASI and EFIS. The EFIS instr. error is decresing with the increasing speed so no problem within normal range (130-230 km/h), but problem at low airspeeds.
My questions were:
- Have all produced EFISes similar instrument error or are there diffirence between them?
- Is instrument error -10km/h at low speeds normal for all produced EFISes? (2 EFISes measured)
- What is the reason of big error at low speeds (mechanical or possible error in software)?
- Is any chance how to minimize instrument error?
- Would be possible to have a software adjustment of instrument error, e.g. similar like have the digital fuel quantity indicators (fuel tank calibration)?
Thank you for help.
Petr

 

[dynonsupport]

Petr,
First, we do not display airspeed below 30 knots, which is about 50 km/h, so this is expected.

Second, we do not have a way for the user to calibrate the EFIS airspeed. We calibrate each and every unit here at the factory using our own calibration system. As you can see, both EFIS units read the same, so they have been calibrated to the same standard, but they were each calibrated seperatley.

At low airspeeds, the pressures are very low and can be hard to measure. The best way to test an airspeed indicator is using a manometer. Are you sure that the mechanical ASI you have is correct and that the EFIS is incorrect? We've been calibrating EFIS units for many years and haven't had any systematic complaints of the airspeed being off at low speeds. When we are done calibrating, we check each unit at many airspeeds and they have to pass within a tight tolerance in order to ship them.

We'll go and check that our system is still doing a good job at low airspeeds, but we would also suggest that you double check your test bench and make sure that it is accurate as well. The difference between 50 and 60 km/h is only .007 PSI! The difference between 230 and 240 km/h is .35 PSI, or 500 times greater. As you can see, small errors make a huge difference at low airspeeds.

We generally think in Knots over here in the US, and 10 km/H is only 5.4 knots. We allow an accuracy of 3 knots, so it is possible that your system has a slight error one direction while we have an error the other direction, making our system look 10 km/h off.

 

[Petr Javorsky(Guest)]

Both ASI and EFISes tested in our Certified Test Lab using by means of Water column SDU-1, S/N 235, Calibration validity: 12.4.2005-12.4.2007.
Tested range 70-300km/h (38-162 knots). At 70km/h (38 knots) the ASI error was +3 knots (+5.5 km/h), EFISes -5.5 knots (-10 km/h). I attached an excel file with tables/charts to my e-mail I already sent. Accuracy of 3 knots which you allow does correspond to our 5.5 km/h given in ASI specification. However we measured 5.4 knots on both tested EFISes which seemed to us too much. And of course the IAS difference between ASI and EFIS was also due to opposite direction of errors.
Thank you for your support.