The aeronautical slide rule 


1937: the first aeronautical E6B slide rule 
The navigation's problems are always the same from immemorial time and a modern aircraft slide rule is not so different from the graphs drawn in the Middle Ages to determine the ship's position. The pilots have to find their location and perform various conversions of measures with great rapidity: in this task the slide rules are unbeatable. Mr. Spock used the E6B on board Star Trek Enterprise: was considered an irreplaceable instrument even in a technological future. The interface of the aeronautical slide rule has no equal for dead reckoning and its graphic is used in several navigation software. 

2266: the E6B with Mr. Spock onboard Enterprise 

The classic E6B slide rule, one of the most widely used in aviation 

The classic interface of an E6B navigation software 
The aircraft slide rule, knows as E6B flight computer, was invented in the '30s and its use is so instinctive that is often preferred to electronic calculators: solves all the problems of flight, finds the angle of drift caused by the wind and it is essential to convert the jungle of measures in which the pilots must unravel. In fact they use indifferently meters, feet, nautical miles, statute miles, kilometers, liters, gallons, etc. 

Raxon de Martelojo, chart of 1430 to determine 


The presentation of the E6B in the 30s 

Slide rule watches 


The 24 hours "Cosmonaute" and Scott Carpenter in a 1963 advertising 

The paper "Easy E6B" 

Download the template with instruction and virtual E6B emulation 

On the left a real areonautical slide rule, on the right the "Easy E6B" 
It has one mobile outer scale and a fixed inner one with the number 60 (Speed index) marked with an arrow; remember that, as in the standard slide rule, only the numbers are given: "0.9", "9", "90", "900", "9,000" are always read as "9" and how to locate the dot or how to add tenths or hundreds we must find by ourselves, but it is always instinctive to know if we are dealing with tens, hundreds or thousands. Here are some examples, useful in daily life. The complete instructions, with all the aeronautical functions, are included with the template. 
1) Multiplication 
Example: 12 x 15. 
Align 12 on the outer scale with 10 on the inner scale. Then 15 on the inner scale corresponds to 18 on the outer scale. Take into account the position of the decimal point and add one zero to obtain 180. 


2) Division 
Example: 300/15. 
Align 30 on the outer scale with 15 on the inner scale. Then 10 on the inner scale corresponds to 20 on the outer scale. Take into account the position of the decimal point to obtain 20. 


3) Time required 
Example: Obtain the time required for travel 330 kilometers driving at 55 km/h. 
Align 55 on the outer scale with the Speed index (MPH). Then 33 on the outer scale corresponds to 36 on the inner scale. Thus the time required is 360 minutes (6 hours). Can also be calculated in miles instead of kilometers. 


4) Average speed 
Example: obtain the average speed (km/h) needed to travel 120 kilometers in an hour and 30 minutes. 
Align 12 on the outer scale with 90 (minutes) on the inner scale. Then the Speed index (MPH) corresponds to 80. Thus the average speed is 80 kilometers per hour. 


5) Rate of fuel consumption 
Example: obtain the hourly rate of fuel consumption for 5 h running time and a total consumption of 35 liters. 
Align 35 on the outer scale with 30 on the inner scale (300 minutes = 5 hours). Then the Speed Index (MPH) corresponds to 70. Thus the fuel consumption rate is 7 liters per hour. 


6) Fuel required 
Example: obtain the fuel required for a trip with 6 l/h of consumption and a running time of 5 h. Align 60 on the outer scale with the Speed Index (MPH). Then 30 on the inner scale (300 minutes = 5 hours) corresponds to 30 on the outer scale. Thus the fuel required is 30 liters. 


7) Estimated running time 
Example: obtain the estimated running time with 5 l/h of fuel consumption and a tank of 30 liters. 
Align 50 on the outer scale with the Speed Index (MPH). Then 30 on the outer scale corresponds to 360 on the inner scale. 360 minutes = 6 hours thus the estimated running time is 6 hours. 


8) Distance 
Example: convert 45 miles into nautical miles and kilometers. 
Align 45 on the outer scale with STAT on the inner scale. Then NAUT corresponds to about 39 and KM to about 72 km. 




The paper E6B is useful also on helicopters 

The backside side of the E6B 
We have seen how the front of the aeronautical slide rule serves to calculate distances and make conversions, timing or power consumption. The back, inspired by the graphic Raxon de Martelojo * shown on the second page, is used to solve the problems of reckoning: we see how determine the changes of direction and speed needed to compensate the wind, although our paper model does not have these features. 
Example: You have laid out a course on a chart and measured it to be 090° true. The winds aloft forecast calls for the wind at your chosen altitude to be 230° at 18 knots, and the performance data for the airplane says that you can expect a true airspeed of 125 knots at that altitude. 


Fig. 1 Fig.2 


Until the '70 on board the aircraft was only used the slide rule 

Nicola Marras 2008 
If you see this page without the sidebar you come from 