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Thread: OAT value error

  1. #1

    OAT value error

    i noticed on last flight the OAT was reading 103 F at 3000 Feet which was totally incorrect as airfield temp was only around 8 degrees centigrade.

    Aircraft is a RV7 with a dual AFS 3000 installation not built by me. I have located the OAT sensor under the left wing root and it looks undamaged before I investigate further does anyone know the best way to test whether the sensor is faulty?

  2. #2
    What is it reading on the ground?

    What are your OAT calibration values?

    Is this a new problem?
    Rob Hickman
    Advanced Flight Systems Inc.

  3. #3
    Hi Rob

    Checked out the following today, to be honest I am unsure whether this is a new issue in as much as it was cld outside when I last flew and a reading OAT value of 103 F at 3000 feet was clearly not right!

    I powered up the EFIS which showed a OAT value of 57 F.

    By the time I found a pen and paper and then opened up the instrument calibration mode and selected OAT settings it showed:-

    Configuration OAT 19.5 deg C / 67.0 deg F

    1. Instrument OFF/ON OFF
    2. Units FAHRENHEIT
    3. Shift Adjust 0.0
    4. Num Cal Points 2

    AD_VALUE : 10144

    Display. (deg F) AD VALUE
    32.0 9571
    66.0 10202


    What stood out was the instrument showed 'OFF'. I changed that to ON and units to Centigrade and pressed 'Save' and exited back to the main display however this still showed a value of 67 F so I repeated the changes and ensured save was selected however when back to the main EFIS display this still showed a Fahrenheit reading but now 57 degs

    The EFIS displays are AF-3000 Series Software Version 7.3.7-MVIS

    The temperature in the hanger taken from a aircraft type OAT gauge was 6 degs C which should show a value of at least 42.8 degs F

  4. #4
    1. The F / C change is done from the EFIS > SETTINGS >OAT C/F main menu so you can switch it in flight.

    2. Your calibration numbers look about right.

    The software does a linear interpolation between the calibration points, using your numbers:


    AD-VALUE : 10144
    32.0 9571
    66.0 10202

    It should be showing 63F

    Are you sure you are not getting heat from the engine on the probe when in flight?

    Have you checked for loose OAT connections?

    You might want to try replacing the probe.
    Rob Hickman
    Advanced Flight Systems Inc.

  5. #5
    Thanks rob for your info.

    I will checkout the C/F selection from the Settings > OAT menu.

    The OAT sensor is mounted underneath in the left wing root area so ought to be out of the engine exhaust.

    You say the Instrument Calibration points appear right and that 63 F should be showing on the EFIS display is that suggesting then that the AD-VALUE of 10144 is being read as an input from the sensor by the software? If so that would imply the sensor is faulty as the temp in the hanger was more like 42 F.

  6. #6
    Adjust the second calibration point to the current temp and AD value and see what happens when you fly it.
    Rob Hickman
    Advanced Flight Systems Inc.

  7. #7
    Okay Rob I'll do that and let you know although I'm a little confused as to how I will know what the current AD value is going to be or does that get computed automatically when the second calibration temp which in this case is currently showing as 66.0 F is changed?

  8. #8
    The AD_VALUE in yellow in the OAT calibration menu is the current digital reading from the sensor
    Rob Hickman
    Advanced Flight Systems Inc.

  9. #9
    Okay Rob I will make some changes and feed back in due course what happens as it might be a while before I can do an air test.

  10. #10
    Hi Rob - had a session at the airfield

    Followed up on your advice and made some OAT configuration changes.

    I determined from a thermometer today's temperature in the hanger was 10 degs C / 50 degs F

    There are two EFIS displays in this RV7, on the screen at pilots side the existing configuration values were:-

    Instrument Calibration
    19.8 deg C / 67.6 deg F

    1. Instrument OFF/ON ON
    2. Units CELSIUS
    3. Shift Adjust 0.0
    4. Num Cal Points 2

    AD_VALUE: 10747

    DISPLAY (degF) AD_VALUE
    32.0 9571
    66.0 10202

    After a bit of trial and error the following changes were made to the first calibration point setting only to get the actual Instrument Calibration values at the top to be 10.0 deg C / 50.0 deg F

    DISPLAY (degF) AD_VALUE
    50.0 11448


    Confusingly the AD_VALUE: which started out at 10757 changed during the process to become 11448. Once the changes were completed the AD_VALUE: number fluctuated which I assumed was in response to temperature variations at the OAT sensor.

    The changes were saved and returning to EFIS display the OAT value now showed up as 10 C

    Given that the two EFIS units are the same and connected I had expected the changes made to be synchronised onto the second EFIS but this did not happen and it was necessary to make similar changes to the OAT configuration on that unit.

    On the second unit the changes were made again only to Calibration Point Number One in order to get the Instrument Calibration values at the top to be 10.0 deg C / 50.0 deg F and to achieve this the actual AD_VALUE: became just 42 and once saved the OAT value on the EFIS screens also showed up as 10 C.

    I did not undertake an air test so I have no idea whether the OAT value is going to remain static or change in line with the current outside temperature on the ground or what it will be when airborne.

    Rob is what I have done the correct way to configure the OAT?

    Why is there the need for a second calibration point and what will be the impact of making no changes to the settings given for that calibration point?

    Why is there such a difference between the numbers of the two AD_VALUES: on either EFIS and also why don't they synchronise?

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