How to Use the CX-3 Flight Computer for DGCA Air Navigation Exams

What is the ASA CX-3 flight computer?

The CX-3 is a handheld electronic flight computer made by Aviation Supplies & Academics (ASA). It does the same job as the familiar manual E6B or CRP-5 wiz-wheel, but instead of lining up scales and reading off a pencil mark, you type in the numbers and the unit gives you the answer on a colour screen.

According to ASA's published product information, it covers time, speed, distance, heading, wind, fuel, altitude, cloud base, standard atmosphere, glide, climb and descent, weight and balance and holding patterns, plus a calculator, clock, stopwatch and countdown timer. It also handles unit conversions in 12 categories with 38 conversion factors, and runs on four AAA batteries.

For a DGCA CPL student, the main appeal is speed and fewer reading errors on Air Navigation and flight planning questions. The flip side is that you must still understand what each number means, because the computer will happily give a precise answer to a wrongly entered problem.

Main menus and the functions you will actually use

The CX-3 has five main menu keys: FLT (the E6B flight functions), PLAN (multi-leg trip planning), TIMER, CALC and W/B (weight and balance). SET opens preferences such as units, theme, backlight and time. Inputs and outputs are shown separately on screen, with units, so you can see at a glance what you entered and what was calculated. Exact screen labels can differ slightly between firmware versions, so check against the manual for your unit.

• Wind Correction (heading and groundspeed): enter true course, TAS, wind direction and wind speed to get wind correction angle, true heading and groundspeed.
• Wind Component: enter runway (or track) direction and wind to get headwind or tailwind and crosswind. The ASA manual notes that runway winds should be magnetic.
• Airspeed: Planned TAS and Actual TAS from CAS, pressure altitude and OAT; Required CAS; Mach number.
• Altitude: Pressure Altitude from indicated altitude and altimeter setting, Density Altitude, Cloud Base and Standard Atmosphere.
• Ground Speed and Estimated Time of Arrival: the classic time-speed-distance problems.
• Fuel: Fuel Burn, Fuel Rate and Endurance.
• Compass Heading: apply variation and deviation to go from true to compass heading.
• Unit conversions: the CONV key converts the value on screen, for example NM to km, US gallons to litres, or inches of mercury to hPa.
• TIMER and clock: count-up stopwatch, countdown timer, and a clock that shows UTC, local and destination time and keeps running when the unit is switched off.
• CALC: a general calculator that also does time arithmetic in hours, minutes and seconds.

Set it up correctly before you solve anything

Most wrong answers on an electronic computer come from units, not arithmetic. The CX-3 was designed in the United States, so check every default before you start practising for an Indian syllabus.

• Open SET and check default units: speed in knots, distance in nautical miles, altitude in feet, temperature in degrees Celsius, pressure in hPa (millibars), and fuel in litres or kilograms as the question demands.
• Inside a function, the UNIT key changes the unit of the line you are on. Make sure you are changing the input you think you are changing.
• Decide whether a problem is in true or magnetic. Winds in a METAR or forecast are true; winds passed by ATIS or the tower for take-off and landing are magnetic. Keep course and wind in the same reference.
• Enter times with the colon key, for example 1:31:35 for 1 hour 31 minutes 35 seconds.
• Fit fresh batteries before the exam. The Quick Start Guide notes that removing the batteries erases memory and clock settings, so do it a few days before and recheck your unit settings afterwards.

Worked example 1: heading and groundspeed from wind

Question: True course 245 degrees, TAS 140 kt, wind 290/25 kt (true). Find the true heading and groundspeed.

On the CX-3, go to FLT, choose Wind Correction, then enter course 245, TAS 140, wind direction 290 and wind speed 25. The unit should show a wind correction angle of about 7 degrees right, a true heading of 252 degrees and a groundspeed of 121 kt.

Check it by hand so you trust the answer. The wind angle relative to the course is 290 minus 245, which is 45 degrees. WCA = asin(25 x sin 45 / 140) = asin(0.1263) = 7.3 degrees. The wind is from the right, so you turn right into it: heading = 245 + 7.3 = 252.3, or 252 degrees true. Groundspeed = 140 x cos 7.3 minus 25 x cos 45 = 138.9 minus 17.7 = 121.2 kt.

A second check: true course 090, TAS 120 kt, wind 030/20. The wind is 60 degrees off the nose from the left, so WCA = asin(20 x sin 60 / 120) = 8.3 degrees left, true heading 082 degrees, and groundspeed = 120 x cos 8.3 minus 20 x cos 60 = 118.7 minus 10.0 = 108.7 kt.

If the question asks for magnetic heading, apply variation after this step (east is least, west is best), and apply deviation as well if it asks for compass heading.

Worked example 2: wind components, TAS, pressure and density altitude

Wind components. Runway 27 (270 degrees magnetic), tower wind 310/18 kt. The angle between wind and runway is 40 degrees. Headwind = 18 x cos 40 = 13.8 kt; crosswind = 18 x sin 40 = 11.6 kt from the right. In FLT, Wind Component, enter runway 27, wind 310 and 18. As a cross-check, the CX-3 manual's own example (runway 03, wind 350/10) gives 7.66 kt headwind and 6.43 kt crosswind from the left, which matches the same formulas.

True airspeed. CAS 120 kt, pressure altitude 6,000 ft, OAT +5 degrees C. In FLT, Airspeed, Actual TAS, enter these three values. The answer is about 132 kt (131.6 kt using the standard atmosphere with compressibility). The old rule of thumb of adding 2 percent per 1,000 ft gives about 134 kt, which is close but not exact, so expect the computer and the answer options to agree more closely than a mental estimate.

Pressure altitude. Aerodrome elevation 2,000 ft, QNH 1003 hPa. With QNH set, the altimeter reads elevation, so enter indicated altitude 2,000 ft and altimeter setting 1003 hPa. The CX-3 should return about 2,281 ft. The common exam rule of 30 ft per hPa gives 2,000 + (1013 minus 1003) x 30 = 2,300 ft. Read the question: if it tells you to use 30 ft per hPa, the expected answer is 2,300 ft.

Density altitude. Same aerodrome, OAT +35 degrees C, pressure altitude 2,281 ft. ISA temperature at that level is about 10.4 degrees C, so the aircraft is about ISA +25. The CX-3 should give a density altitude of about 5,040 ft. The 120 ft per degree rule gives about 5,230 ft, so pick the method the question asks for. Both show the same practical point: a hot Indian afternoon can make a 2,000 ft aerodrome perform like one at 5,000 ft. The CX-3 manual's example (5,900 ft, 29.75 inHg, 75 degrees F giving PA 6,059 ft and DA 8,427 ft) can be reproduced with the same standard atmosphere formulas.

True altitude. In the CX-3 version 1.3 manual we checked, the Altitude menu lists pressure altitude, density altitude, cloud base and standard atmosphere, but not a separate true altitude function. Use CALC with the standard 4 percent per 10 degrees rule. Example: altimeter reads 6,000 ft on a sea-level QNH, OAT at that level is minus 5 degrees C, ISA is plus 3 degrees C, so ISA deviation is minus 8. Correction = 6,000 x 0.04 x 0.8 = 192 ft, so true altitude is about 5,808 ft. Colder than ISA means you are lower than indicated.

Worked example 3: time, speed, distance and fuel

Using the groundspeed from example 1 (121.2 kt), how long does a 185 NM leg take? Time = 185 / 121.2 x 60 = 91.6 minutes, or 1 hour 31 minutes 35 seconds. In FLT, use the time-speed-distance function (Ground Speed or ETA), enter distance and groundspeed and read the time.

Fuel for that leg at 32 litres per hour: 1.526 hours x 32 = 48.8 litres. Endurance with 150 litres usable at 32 litres per hour: 150 / 32 = 4.6875 hours, or 4 hours 41 minutes 15 seconds. Use FLT, Fuel, then Fuel Burn or Endurance.

Watch the fuel units. Many DGCA questions mix litres, US gallons, imperial gallons and kilograms. Use CONV for volume, and remember that volume to mass needs a specific gravity or density, which the question must give you. The CX-3 manual's own endurance example, 38 gallons at 9.5 gallons per hour, gives exactly 4 hours (displayed as 3:59:59 because of rounding), a useful reminder that the last second on screen does not matter in the exam.

CX-3 versus the manual E6B or CRP-5

The manual computer is a circular slide rule on one side and a wind slide on the other. It needs no batteries, works the same way in every exam, and makes you see the wind triangle physically, which helps understanding. Its weaknesses are reading accuracy (typically to the nearest degree or knot, with some estimation between graduations) and time spent setting up the wind side.

The CX-3 is faster for long chained problems, gives more decimal places and removes scale-reading errors. Its weaknesses are batteries, menu navigation, unit settings and the risk of trusting a wrong entry.

A sensible approach is to learn the wind triangle on the CRP-5 or E6B first, so you can recognise a sensible answer, and then use an electronic computer, if one is permitted, for speed. Whichever you use, practise with the same tool you will take into the exam.

Exam tips and common mistakes

Permission first. We could not find an official DGCA or Pariksha portal document that lists which calculators or flight computers are permitted in the exam hall, so we are not claiming the CX-3 is allowed or banned. Before your exam, read the latest instructions on the DGCA Pariksha portal notice board, your admit card and any public notice from the Central Examination Organisation. If you are unsure, ask your flying school or the exam centre in writing, and be ready to work with a manual computer or the on-screen tools if needed.

Practice on CPLGrid. CPLGrid now has a built-in flight computer inside Navigation practice and mock tests, so you can solve wind, TAS and time-speed-distance questions without switching devices, and free calculators are available on the resources page.

• Mixing true and magnetic: entering a magnetic runway wind into a true-course problem, or forgetting variation at the end.
• Wrong wind convention: winds are given as the direction they blow from. Never enter the direction the wind is blowing to.
• Unit traps: statute miles instead of nautical miles, Fahrenheit instead of Celsius, inches of mercury instead of hPa, US gallons instead of litres.
• Using IAS or CAS where TAS is required in the wind triangle.
• Using indicated altitude instead of pressure altitude for TAS or density altitude.
• Over-precision: the CX-3 shows decimals, but answer options are usually rounded. Pick the nearest option and do not waste time on the last digit.
• Not sense-checking: a headwind should reduce groundspeed, and you should turn into the wind. If the answer disagrees, re-check the entries.
• Practise speed: time yourself on 20 mixed problems. Aim to finish a wind triangle in well under a minute so you have time left for the chart and theory questions.

Frequently asked questions

• Is the CX-3 the same as an E6B? It is an electronic E6B. It solves the same problems as the manual E6B or CRP-5, but by entering numbers instead of reading scales.
• Is the CX-3 allowed in the DGCA CPL exam? We have not found an official DGCA document that says either way. Check the latest Pariksha portal instructions, your admit card and DGCA public notices before exam day.
• Which units should I set for DGCA navigation? Knots, nautical miles, feet, degrees Celsius and hPa, with fuel in litres or kilograms as the question gives it. Recheck the settings after changing batteries.
• Does the CX-3 calculate true altitude? The version 1.3 manual we checked lists pressure altitude, density altitude, cloud base and standard atmosphere under Altitude. For true altitude, use the 4 percent per 10 degrees rule with the calculator.
• Should I still learn the CRP-5? Yes. It builds understanding of the wind triangle, needs no batteries and is a reliable back-up if an electronic computer is not permitted or fails.
• Where can I practise these calculations online? CPLGrid Navigation practice and mock tests include a built-in flight computer, and the CPLGrid calculators page has free tools for quick checks.

Useful resources

Official and manufacturer sources we checked for this guide, plus CPLGrid pages to continue your Navigation preparation.

• DGCA Pariksha portal (exam notices and instructions)
• ASA CX-3 Quick Start Guide (PDF)
• ASA CX-3 User Manual v1.3 (PDF, German edition)
• ASA CX-3 product information (Sporty's)
• CPLGrid Air Navigation subject
• CPLGrid free flight calculators
• How to prepare for DGCA CPL exams