E6B Whizz Wheel: Online Flight Computer & How-To Guide

What is an E6B whizz wheel?

Parts of the E6B flight computer

Outer scale (fixed)

The logarithmic scale around the rim. It carries distance, fuel quantity, true airspeed and any other known quantity.

Inner scale (rotating disc)

The same logarithmic scale on the disc you turn. It carries time in minutes, calibrated airspeed and the second number in any ratio.

Hours ring

Printed inside the minutes scale, it converts minutes to hours and minutes for you, so 100 reads as 1:40.

▲60 rate index

The big arrow at 60 on the disc. Anything per hour (knots, litres per hour, gallons per hour) is set against it.

▲36 seconds index

The arrow at 36 represents 3,600 seconds. Use it when the answer should come out in seconds rather than minutes.

Conversion arrows

The NAUT, STAT and KM, LITRES, US GAL and IMP GAL, KG and LBS, and METRES and FEET arrows around the rim. Set a value under one arrow and read the converted value under another in the same family.

Airspeed & density altitude windows

Cut-outs in the disc. Set pressure altitude against air temperature, and the main scales read true airspeed over calibrated airspeed. The density altitude window reads out at the same time.

Wind side: rose, card and grommet

The back of the computer. A compass rose turns over a sliding card of speed arcs and drift lines, and the grommet is the small circle in the centre where groundspeed is read.

How the circular slide rule works

1. Time, speed and distance

The most-used calculation on the whizz wheel, and the base for everything else on the slide rule side.

1. Find groundspeed. You pass two fixes 96 NM apart in 48 minutes. Put 48 on the inner scale under 96 on the outer scale. The ▲60 index now points to 120 kt.
2. Find time en route. A 175 NM leg at 105 kt groundspeed. Set ▲60 under 105, find 175 on the outer scale and read 100 minutes beneath it (1:40 on the hours ring).
3. Find distance flown. 27 minutes at 140 kt. Set ▲60 under 140, find 27 on the inner scale and read 63 NM above it.

Tip: Once ▲60 sits under your groundspeed, leave the disc alone. Every distance on the outer scale is lined up with its time, so you can read off every leg of the trip from one setting.

2. Fuel burn, endurance and reserves

Fuel works exactly like speed: burn rate against ▲60, quantity on the outer scale, time on the inner scale. It works in any unit (litres, US gallons, pounds or kilograms) as long as you don't mix them.

1. Find the burn rate. You used 54 L in 90 minutes. Put 90 on the inner scale under 54 on the outer scale. ▲60 points to 36 L/hr.
2. Find endurance. 144 L usable at 32 L/hr. Set ▲60 under 32, find 144 on the outer scale and read 270 minutes (4 h 30 m) to dry tanks.
3. Take off the reserve. Under CASA Part 91 MOS Table 19.02(2), a light piston aeroplane keeps a 30-minute final reserve by day VFR and 45 minutes at night. The FAA's 14 CFR 91.151 uses the same 30 and 45 minutes for aeroplanes. That leaves 240 minutes of planned flying by day.

Tip: Reserve fuel is planned to still be in the tanks when you land. It is never part of the en route time.

3. Pressure altitude and density altitude

Take-off distance, climb rate and engine power depend on the density of the air, not on the height shown on the altimeter. On a hot day the aeroplane performs as if it were much higher.

1. Pressure altitude. Aerodrome elevation 2,100 ft, QNH 1008 hPa. Allow about 30 ft per hPa below 1013: 2,100 + (1013 − 1008) × 30 = 2,250 ft.
2. Density altitude. In the density altitude window, set the air temperature (+32 °C) against the pressure altitude (2,250 ft). The window reads about 4,800 ft. ISA at 2,250 ft is about 10.5 °C, so it is 21.5 °C hotter than standard, and about 120 ft per degree adds roughly 2,550 ft.

Tip: Hot, high and humid all reduce performance. Check the take-off and landing charts in the flight manual against density altitude, not against aerodrome elevation.

4. Calibrated airspeed to true airspeed

The air gets thinner as you climb, so the airspeed indicator under-reads your true speed through the air mass. The flight plan and the wind triangle need true airspeed (TAS).

1. In the airspeed window, set the pressure altitude (5,500 ft) against the outside air temperature (+10 °C).
2. Without moving the disc, find the calibrated airspeed (120 kt) on the inner scale. Read TAS directly above it on the outer scale: about 132 kt.

Tip: Sanity check: TAS is roughly 2% higher than CAS for every 1,000 ft of altitude. At 5,500 ft that's about 11%, so 120 kt CAS gives about 133 kt TAS.

5. True altitude and cold-temperature correction

The altimeter assumes ISA temperatures. In air colder than standard the column of air is compressed, so you are lower than the altimeter shows. Pilots remember it as "high to low, look out below".

1. Indicated altitude 6,000 ft over a station at 2,000 ft elevation, OAT −10 °C. ISA at 6,000 ft is about +3 °C, so it is 13 °C colder than standard.
2. The correction is about 4 ft per 1,000 ft of air column per °C: 4,000 ft × 0.004 × −13 ≈ −210 ft. True altitude is about 5,790 ft, which is 210 ft lower than the altimeter shows.

Tip: Over high terrain in winter, check your lowest safe altitude with cold-temperature correction applied.

6. Wind triangle: heading, drift and groundspeed

The wind side of the E6B solves the navigation triangle mechanically, with no trigonometry. Switch the simulator above to the wind side and follow along.

1. Set the wind direction. Turn the rose until the forecast wind direction (320°) sits under the TRUE INDEX.
2. Plot the wind speed. Mark a dot 20 kt straight up from the grommet. The Plot from the numbers below button does this for you.
3. Set the course. Turn the rose until your true course (270°) sits under the index.
4. Set the TAS. Slide the card until the wind dot sits on the TAS arc (110 kt). This is what Slide to my TAS does.
5. Read the answers. Groundspeed reads under the grommet: about 96 kt. The dot sits about 8° right of the centre line, so fly a true heading of 278°.

Tip: The wind correction angle always points into the wind. Wind from the right means you add degrees to the heading; wind from the left means you take them off.

7. Finding the actual wind in flight

When your groundspeed and track aren't what you planned, work the wind triangle backwards to find the wind you are actually flying in.

1. You are holding a true heading of 095° at 120 kt TAS, and your fixes show a track made good of 100° at 108 kt groundspeed.
2. Put the track (100°) under the index and slide the grommet to the groundspeed (108 kt). Mark a dot where the 5° drift line (heading 095° is 5° left of track, so the drift is 5° right) crosses the 120 kt TAS arc.
3. Turn the rose until the dot is straight above the grommet. The wind direction reads under the index and the distance from the grommet to the dot is the wind speed: about 058° at 16 kt.

8. Crosswind and headwind components

Check the landing wind against the aircraft's maximum demonstrated crosswind in the flight manual before you commit to a runway.

1. Runway 17 (170° magnetic), with the ATIS giving a wind of 210° at 18 kt. The wind is 40° off the runway heading, from the right.
2. Crosswind = 18 × sin 40° = 11.6 kt from the right. Headwind = 18 × cos 40° = 13.8 kt.

Tip: Quick mental method: at 30° off, the crosswind is half the wind speed; at 45°, about 0.7 of it; at 60° or more, treat it as the full wind speed.

9. Top of descent and rate of descent

Plan the descent so you arrive at circuit height on time, without a steep, fast arrival.

1. Cruising at 7,500 ft with a 2,000 ft circuit height, you have 5,500 ft to lose. A 3° path is about 300 ft per NM, so start down about 18 NM out.
2. Rate of descent for 3° ≈ groundspeed × 5. At 110 kt that's 550 ft/min. On the wheel, set ▲60 under 110 and read about 10 minutes against 18.3 NM: 5,500 ft in 10 minutes is 550 ft/min.

10. The 1-in-60 rule for track errors

1 NM off track after 60 NM flown is a 1° track error. This is the fastest way to get back on track without a chart plotter.

1. Leg length 90 NM. After 30 NM you fix your position 3 NM left of track, which leaves 60 NM to go.
2. Track error = 3 ÷ 30 × 60 = 6°. Closing angle = 3 ÷ 60 × 60 = 3°.
3. Alter heading 9° right to fly direct to the destination, then take off the closing angle (3°) once you are back on track.

11. Unit conversions

The conversion arrows on the outer rim let you convert units without doing any sums. Set the known value under its arrow and read the answer under the other arrow in the same family.

1. Distance: 50 NM = 57.5 statute miles = 92.6 km.
2. Fuel volume and weight: 100 L of avgas = 26.4 US gal ≈ 72 kg at the usual 0.72 kg/L planning density (Jet A-1 is about 0.80 kg/L).
3. Temperature: the one scale on the computer that isn't logarithmic. °F = °C × 9/5 + 32, which the strip under the wheel above shows directly.
4. Seconds: to time a 3.5 NM final at 140 kt, set ▲36 under 140 and read 90 seconds under 3.5.

Common whizz wheel mistakes

• Misplacing the decimal point. Always estimate the answer before you read the scale.
• Reading from the wrong index. ▲60 gives minutes; ▲36 gives seconds.
• Mixing true and magnetic directions. Forecast winds and the wind side are true; runways and ATIS winds are magnetic.
• Using indicated altitude in the density altitude or airspeed window instead of pressure altitude.
• Applying the wind correction angle the wrong way. The heading always turns into the wind.
• Planning to use reserve fuel en route. It must still be in the tanks when you land.

Using the E6B in CASA and FAA pilot exams