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I know how the analog airspeed indicator shows the airspeed from the ram air pressure from the pitot tube. But I'd like to know how an air-data computer converts the raw pressure data to airspeed that is displayed on a PFD.

  • What sensor is used for this purpose?
  • What exactly happens inside the computer?
  • Does the sensor and/or procedure differ from unit to unit?
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  • $\begingroup$ Are you asking how the conversion from analogue to digital happens? $\endgroup$
    – Rory Alsop
    Nov 1 '17 at 11:50
  • $\begingroup$ @RoryAlsop: Yes - what is the sensor being used, and is it standard in all ADIRUs? Also, how does the actual conversion actually happen? $\endgroup$
    – user18035
    Nov 1 '17 at 11:53
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    $\begingroup$ The sensor is the transducer into the pitot-static probe. A calibrated voltage is sent to the air data computer which uses some algorithm to compute the indicated airspeed (using temperature). Sensor are not standard, nor transducers. If the probe is only a pitot (total pressure), then an individual static pressure sensor is used and its signal goes to the AD unit too. See Wikipedia and Nasa $\endgroup$
    – mins
    Nov 1 '17 at 13:33
  • $\begingroup$ Nice! What happens inside the transducer? i.e. Is there a membrane that is pushed / pulled (like a microphone) depending on the pressure difference? $\endgroup$
    – user18035
    Nov 1 '17 at 13:38
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    $\begingroup$ That was the case with aneroid capsule ASI, but in modern avionics the transducers are piezoresistive semiconductors on a Pyrex glass which generate a voltage. Try to not use the comments as a chat (better to refine your question) $\endgroup$
    – mins
    Nov 1 '17 at 13:47
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Usually there is an intermediate module ADM (air data module) that makes the conversion from analog pressure into digital numerical value. One ADM will do this for the ram air pressure, another ADM will do the same for the static pressure. Their data are sent on Arinc numerical buses to the Air Data part of the ADIRU. Airspeed is basically the difference of the received data. Please refer to the following diagram:

https://upload.wikimedia.org/wikipedia/commons/f/ff/Airspeed_indication_system_-_fly_by_wire.png

Inside the ADM you have: first of all a PIEZO that will convert the mechanical pressure into electrical voltage, a piezo is somehow what you have in a balance you may use to get your weight. In the balance you have too an analog digital converter since the weight is displayed as a number.

Similarly in the ADM, the next item is an analog/digital converter that will convert the electrical voltage into a binary number that will be sent on the ARINC bus to the ADIRU

You might wonder why the ADM is apart and not inside the ADIRU, it is done like that to have the shortest possible pneumatic tube. I do understand I haven’t been enough clear in my previous answer that has been deleted, therefore it is important to understand the following:

The ADIRU managing Air Data and Inertial data, is located in an electronic bay, the pneumatic piping is not sent to the electronic bay, that will make a long distance, therefore no pressure conversion is done by the ADIRU, but by intermediate modules, the ADMs

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  • $\begingroup$ Now this is a much better answer! +1 $\endgroup$
    – DeltaLima
    Jun 2 '19 at 7:37
  • $\begingroup$ The somehow is that a piezoelectric material is one that produces an electrical charge when it is mechanically deformed. This means that you can measure the voltage to determine the mechanical deformation and thus the pressure. Better yet unlike piezoresistive materials no external current is needed to do this the electrical charge is produced directly from the mechanical force being exerted on it. For reference this is also how electric igniters without batteries usually work the applied pressure causes a charge build-up until it gets large enough to arc between the contacts. $\endgroup$
    – MttJocy
    Sep 14 at 0:25

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