Set up the CR-3 computer with 10 on the outer scale over 20 on the inner scale.
Next, find the factor 80 on the outer scale and read the value for "X" directly below
on the inner scale. The number below 80 is 16. This could represent 1.6, 16, 160 or
Since you have "estimated" your answer to be approximately 16, you now
read the value for "X" as 16. (Figure 4.3-15)
Figure 4.3-15 Ratio 2
Remember that determining the correct position for the decimal point is a major challenge
in solving a ratio for the unknown value. Always estimate the approximate answer before
interpreting the computer.
TIME - SPEED - DISTANCE
In aviation the unit of measurement for distance is usually the nautical mile, which is 6080
feet. Time is measured in hours, minutes and seconds. Speed is in nautical miles per hour
or "knots." On the CR-3 computer, the time scale is on the moveable disk (inner scale) and
is graduated in minutes. Since most TIME, SPEED, DISTANCE, and FUEL
CONSUMPTION problems are expressed in units per hour, we will use the RATE INDEX.
Time, speed, distance, and fuel consumption problems are simply ratios that deal with time
(rates). The unknown values are found by transferring the known values of the ratio directly
to the outer (DISTANCE) and inner (TIME) scales. Keep in mind that the RATE INDEX (▲)
represents 60 minutes and is used as the basis for what is happening per hour.