The E6B and the CX-3 are the two flight computers most student pilots use, and the fastest way to master them is to learn what each solves: the manual E6B (paper or metal) computes density altitude, true airspeed, time-speed-distance, fuel burn, and wind components, while the electronic CX-3 does all of that plus weight and balance. Learn the E6B first, then add the CX-3 as an upgrade.
Part of our Aircraft Performance guide.
Check your work online: after you spin the wheel, confirm the answer with PilotMall’s free online E6B flight computer. See also our guides to the online E6B and the crosswind component calculator.
E6B vs CX-3: what each tool actually does
Both computers are slide-rule descendants built for the same job: turning raw numbers (temperature, altitude, airspeed, wind, fuel) into the answers you write on a nav log. The honest difference is narrow but important.
- The manual E6B (paper or metal) solves density altitude, pressure altitude, true airspeed, calibrated-to-true conversions, time-speed-distance, fuel consumption, and the wind triangle (crosswind and headwind components, wind correction angle, and groundspeed). It has no batteries, no screen, and nothing to fail on a hot day in the run-up area. What it does not do is weight and balance. The slide rule has no way to sum moments for you.
- The ASA CX-3 is an electronic flight computer that does everything the E6B does and adds weight and balance, plus it does the arithmetic for you and shows the units. You type the inputs; it returns density altitude, TAS, wind components, fuel, and a loaded CG. The trade-off is that it runs on batteries and, at this writing, stock runs low.
The one thing the E6B does that the CX-3 cannot is run with no power source at all. Everything else the CX-3 matches or beats. That single fact, plus price and reliability, is why nearly every training program starts students on the manual E6B.
| Task | Manual E6B | ASA CX-3 |
|---|---|---|
| Density altitude | Yes | Yes |
| True airspeed | Yes | Yes |
| Time-speed-distance, fuel | Yes | Yes |
| Wind (crosswind/headwind, WCA, groundspeed) | Yes | Yes |
| Weight and balance | No | Yes |
| Needs batteries | No | Yes |
| Allowed on FAA knowledge test | Yes | Yes |
How to find density altitude on the E6B
Density altitude is pressure altitude corrected for nonstandard temperature, and high density altitude is what robs your airplane of lift, power, and climb. The slide-rule side of the E6B computes it in a few seconds. Follow these steps in order.
- Find your pressure altitude. Set the altimeter to 29.92 in Hg and read the altitude, or add (29.92 minus your current altimeter setting) times 1,000 to the field elevation.
- Note the outside air temperature (OAT) in degrees Celsius.
- On the slide-rule side, find the window labeled for airspeed and density altitude computations.
- Rotate the inner wheel until your OAT on the temperature scale lines up over your pressure altitude on the pressure altitude scale.
- Read the density altitude in the density altitude window, without moving the wheel.
Want a sanity check? Use the rule of thumb: ISA temperature for a pressure altitude is 15 minus (2 times PA in thousands), and you add about 120 ft of density altitude for every 1 degree C the OAT sits above that ISA value. If your E6B answer and the mental estimate disagree wildly, you read a scale wrong. For the aerodynamics behind the number, see density altitude.
How to find true airspeed on the E6B
True airspeed uses the very same window you just used for density altitude, which is why the two are taught together. Once your OAT is aligned over your pressure altitude:
- Leave the wheel where it is.
- Find your calibrated airspeed on the inner (rotating) scale.
- Read true airspeed directly opposite it on the outer scale.
As density altitude climbs, TAS rises relative to indicated airspeed by roughly 2 percent per 1,000 ft, so your groundspeed and fuel planning both depend on getting this right. TAS is the airspeed you carry into the wind triangle and into flight planning to solve groundspeed and time en route. Remember that the indicated airspeed you read in the cockpit comes from the pitot-static system, and the E6B is what converts it to a true value.
The wind side: crosswind and headwind components
The wind side of the E6B (the rotating azimuth with the sliding grid card) is where you split a reported wind into the piece that pushes down the runway (headwind or tailwind) and the piece that pushes you sideways (crosswind). This is a calculation, and it is separate from the stick-and-rudder crosswind landing technique you fly once you know the number.
To solve wind components on the wind side:
- Rotate the azimuth so the wind direction sits under the true index at the top.
- Put the grommet (center hole) on a reference line and mark a dot straight up from it at the wind speed.
- Rotate the azimuth so the runway heading is now under the true index.
- Read the headwind or tailwind component as the vertical distance from the grommet to the dot, and the crosswind component as the horizontal distance from the centerline to the dot.
Worked example. You are landing Runway 09 (heading 090) and the tower reports wind 120 at 15 knots. The angle between the wind and the runway is 30 degrees. Set 120 at the true index, mark 15 up from the grommet, then rotate 090 to the index. You read about a 7 or 8 knot crosswind and about a 13 knot headwind.
The mental cross-check is the clock rule: a wind 15 degrees off the runway gives about one quarter of its speed as crosswind, 30 degrees gives about one half, 45 degrees about three quarters, and 60 degrees or more is essentially all crosswind. At 30 degrees, half of 15 is 7.5 knots, which matches the wheel. Compare that crosswind component against the airplane’s demonstrated crosswind value and your own personal limit before you commit. Many E6B models also print a dedicated crosswind component grid on the card, so you can look the answer up directly if you prefer a chart to the azimuth.
Weight and balance on the CX-3
Weight and balance is the job that sends students to the CX-3, because the manual E6B cannot do it. The math itself is simple in concept but tedious by hand: moment equals weight times arm, and CG equals total moment divided by total weight. The airplane has to be at or below max gross weight and inside the CG envelope, both at once.
On the CX-3 you enter each station (empty airplane, front seats, rear seats, baggage, fuel) as a weight and an arm, and the calculator sums the moments and returns the loaded weight and CG for you. That removes the arithmetic errors that creep in when you add long moment columns on paper. The relationships still matter, though: a forward CG makes the airplane more stable but raises stall speed, adds pitch (elevator) force, and lengthens the takeoff roll, while an aft CG lightens the controls and lowers stall speed but degrades stall and spin recovery, sometimes dangerously.
Whichever tool you use, the reference data comes from the airplane’s POH. There is no separate FAA weight and balance handbook to buy; the theory lives in the weight and balance guide and in PHAK Chapter 10, and the loading limits, datum, arms, and station diagram come straight from your aircraft manual. If you fly a Skyhawk, that is the Cessna 172 manuals collection.
Which tool should a student buy first?
Buy the manual E6B first. It is inexpensive, it is always legal on every FAA test, it never runs out of battery, and, most importantly, working the slide rule teaches you the relationships (why density altitude rises with heat, why TAS climbs with altitude, why a 30 degree wind gives half its speed as crosswind) instead of hiding them behind a keypad. Instructors and examiners want to see that you understand those relationships, not just that you can type.
Add the CX-3 later as an upgrade when you want faster arithmetic, unit handling, and built-in weight and balance, especially heading into cross-country season or the checkride. Plenty of pilots carry both: the E6B as the always-works backup, the CX-3 as the fast everyday tool. For deeper buyer’s-guide detail on the paper vs metal vs electronic choice, see the E6B gear guide. And remember that once you are certificated, an EFB app handles most of this in flight, though apps are not allowed on the knowledge test; see GPS and EFBs for how that fits in later.
What you'll need
The flight computers, plotter, and reference book below are the exact tools this page walks you through, all from PilotMall.com.
Frequently asked questions
What can an E6B do that a CX-3 cannot, and vice versa?
The one thing a manual E6B does that a CX-3 cannot is run with no batteries or screen, so it can never lose power on a checkride. Otherwise the CX-3 does everything the E6B does (density altitude, true airspeed, time-speed-distance, fuel, and wind) and adds weight and balance, which the manual E6B cannot compute at all.
How do you find density altitude on an E6B?
On the slide-rule side, align your outside air temperature over your pressure altitude in the airspeed and density altitude window, then read density altitude in the density altitude window. Find pressure altitude first by setting the altimeter to 29.92 or adding (29.92 minus your altimeter setting) times 1,000 to field elevation.
How do you calculate a crosswind component on an E6B?
On the wind side, set the wind direction under the true index, mark the wind speed straight up from the grommet, rotate the runway heading to the true index, and read the crosswind component as the horizontal distance from the centerline to the dot. For example, Runway 09 with wind 120 at 15 knots (a 30 degree angle) gives about a 7 to 8 knot crosswind and about a 13 knot headwind.
Are the E6B and CX-3 allowed on the FAA written and checkride?
Yes, both the manual E6B and the ASA CX-3 are approved for the FAA knowledge (written) test and the practical test. Phone and tablet apps are not permitted on the FAA knowledge test, which is another reason to own a dedicated flight computer.
Should a student pilot buy an E6B or a CX-3 first?
Buy the manual E6B first, because it is inexpensive, always allowed, never runs out of battery, and teaches you the relationships behind every answer. Add the CX-3 later as an upgrade when you want faster math and built-in weight and balance.