# How does a G1000 equipped with a GDC 74 calculate IAS from dynamic pressure?

How exactly does a G1000 system equipped with a GDC 74 air data computer calculate the indicated airspeed that is displayed on the PFD from difference between the total and static pressure (i.e. dynamic pressure) measured from the pitot-static system?

The reason I ask is because I want to reverse the calculation and convert KIAS back to dynamic pressure, i.e. $$q$$ as defined below:

$$q = \frac{\rho u^2}{2}$$

My objective is to calculate dynamic pressure from the KIAS value that is logged to the data logs written to the top SD card by the MFD. The problem is that there are multiple ways to perform this calculation -- i.e. does the G1000 display an indicated airspeed that is uncorrected for instrument and position error (IAS, in the traditional sense) or corrected for instrument and position error (CAS)?

• The ADC uses pressure transducers inside the unit to convert the pitot and static pressures to electrical voltages that are digitized and processed by software. How you would tap into the raw signal, dunno. There might be a ground maintenance function in the unit that allows the real time raw signal data to be displayed. – John K Mar 27 '19 at 2:32
• I am trying to calculate dynamic pressure after the fact from the KIAS value that is logged once per second to the G1000 data logs. – Joe Crivello Mar 27 '19 at 2:43

I'm not sure if it is the same for all G1000 installations, but according to this Integrated Flight Deck Pilot's Guide the flight data logging feature records both IAS in knots and TAS in knots.

Do you have the Pilot's Guide for your aircraft? It would likely indicate the values saved in there as well.

• Yes, but my concern is that it is not unheard of for an indicated airspeed that is corrected for instrument and position error (CAS) displayed on an EFIS to be referred to as IAS. – Joe Crivello Mar 27 '19 at 3:04
• I just realized that there was a really easy way to check this all along... just look in the POH and see if the prominent airspeed limitations such as Vne/Vmo are shown as IAS or CAS on the airspeed tape. They are shown in IAS on the airplane that I am interested in, so you are correct. Thanks! – Joe Crivello Mar 27 '19 at 3:06
• Quite true. If it is corrected for position error, how would you propose to remove the position error error correction from the indicated airspeed? – selectstriker2 Mar 27 '19 at 3:08
• For every certified airplane I know of there is a table in the POH with correction factors for converting between IAS and CAS. I wanted to be 100% sure that I was starting with IAS or else it would throw off the dynamic pressure calcuation. – Joe Crivello Mar 27 '19 at 3:09