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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 www.asa2fly.com/CX3.

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5 Comments

  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 CFI@asa2fly.com 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.

  4. Posted March 27, 2018 at 8:03 pm | Permalink

    Thank you !!!
    Thank you !!!
    Thank you !!!

    I was able to apply your presentation regarding the required indicated airspeed question to my Sporty’s E6B.

    David R.

  5. Len
    Posted May 19, 2018 at 7:06 am | Permalink

    Anyone know how to use CX3 to calculate True Altitude? E.g., While flying above a ridge which is 5,000′ AMSL, your altimeter indicates an altitude of 7,000′ with an altimeter setting of 29.42 received from a nearby airport which is at an elevation of 3,500′. The outside air temperature is -20C. What is your clearance above the ridge? Appreciate if you could lead me through it.

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