# 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.