CFI Brief: CX-3 Flight Computer – Indicated Airspeed

Some questions on the FAA Knowledge Exam will require you to determine approximate indicated airspeed. These types of problems are best solved with the use of a CX-3 Flight Computer. Today, we will work through a sample knowledge test question requiring us to run several different calculations on the CX-3 to determine indicated airspeed from the given information. Problems like the one below are a great way to get use to using your new go to tool in the flight bag.

On a cross-country flight, point A is crossed at 1500 hours and the plan is to reach point B at 1530 hours. Use the following information to determine the indicated airspeed required to reach point B on schedule.

Distance between A and B 70 NM
Forecast wind 310° at 15 kts
Pressure altitude 8,000 ft
Ambient temperature -10 °C
True course 270°

The required indicated airspeed would be approximately

A. 126 knots.
B. 137 knots.
C. 152 knots.

Step 1 is to determine the Ground Speed it will take to cover 70 NM in 30 minutes (1500 – 1530 hours), this information is given in the question.

From the FLT menu select the Ground Speed Function.

Input a Distance (Dist) of 70 NM and Duration (Dur) of 0.50 HR to get a Ground Speed (GS) of 140 KTS.

Step 2 we need to find the True Airspeed using our equated ground speed, forecast wind, and true course.

From the FLT menu select Wind Correction.

Input a Ground Speed (GS) of 140 KTS, True Course (TCrs) of 270°, Wind Speed (WSpd) of 15 KTS, and Wind Direction (WDir) of 310°. The CX-3 will show a TAS of 151.8 KTS.

Step 3 we can now determine the required indicated airspeed using the true airspeed determined in step 2 and the pressure altitude and ambient temperature given in the question. Note that the Indicate Airspeed is shown in the CX-3 as Calibrated Airspeed (CAS).

From the FLT menu select Airspeed.

Input a True Airspeed (TAS) of 151.8 KTS, Outside Air Temperature (OAT) of -10°C, and Pressure Altitude (PAlt) of 8,000 FT. The CX-3 will show a CAS of 137.15.

The correct answer to the above question is B: 137 knots.

For additional information on using your CX-3 Flight Computer check out the complete Users Guide at

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  1. NordicDave
    Posted January 4, 2018 at 10:58 pm | Permalink

    Really helpful. The new CX-3 is best electronic E6B, but the documentation is a little thin for showing how to work through basic calculations.

    Super appreciate this blog entry on deriving Indicated Airspeed. It was extremely helpful.

  2. Posted January 5, 2018 at 8:04 am | Permalink

    Hi NordicDave, if you have a particular problem you would like us to work through on the blog we would be happy to do so. And you can always email if you have questions regarding your CX-3 Flight Computer.

  3. NordicDave
    Posted January 17, 2018 at 10:22 am | Permalink

    I cannot say enough how ASA has leapfrogged Sporty’s computer and the previous ASA CX-2. It’s a whole new level of user friendliness and capability.

    The FAA written has several questions about computing wind angle. Ex: wind from 45 degrees with a x-wind component of 25kts, what’s the total wind. Lots of questions about fig. 37 in FAA-CT-8080-2F supplement worked from different angles.

    I’ll jot down a few more and email.

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