point it has been specially marked with a triangle, called the RATE INDEX. Realize this is
0.6, 6.0, or 60 MINUTES, NOT 1.
Beneath this scale is a smaller scale marked in hours. This scale directly reads hour values
that correspond to the minute scale. For example 120 minutes = 2:00 hours and 1200
minutes = 20:00 hours. The hour circle converts this for us. Below the number 12 (Figure
4.3-10) the value 2:00 is found above the hour circle and 20:00 below the circle.
The small marks between the hour values on the upper side of the hour circle represent
ten-minute intervals. As an example, notice the value 15 (here 150 minutes) on the TIME
(minutes) scale (figure 4.3-2) and directly below it is 2:30, or 2 hours and 30 minutes, on
the hour scale. Notice the small mark to the right of the 2:30, directly below the number 16
(here 160 minutes). This represents the next ten-minute interval, or 2:40 (2 hours and 40
minutes). The value 168 on the minute scale will read 2:48, or 2 hours and 48 minutes on
the hour scale.
SECONDS AND MINUTES
Seconds have the same relationship to minutes as minutes do to hours (60 seconds is one
minute; 60 minutes is one hour). Since the numbers and relationships are the same, the
same scales can be used to measure these values; just remember which units are being
used. For instance, the TIME scale is assigned to read seconds, the hour circle will read
minutes. Referencing the above example, with 150 minutes on the TIME scale, directly
below it is 2:30, or 2 hours and 30 minutes, on the hour scale. If 150 seconds is on the
TIME scale, directly below it is 2:30, or 2 minutes and 30 seconds, on the hour circle
(which now reads minutes).
There is a special mark (the RATE INDEX ▲) for 60 minutes because it equates to one
hour. Since one hour is an important value, a special mark denoting the second's
equivalent to one hour is needed when the TIME scale represents seconds. Since there
are 3600 seconds in one hour this special mark is at the "36". The very small arrow with
"SEC" beneath it is referred to as the "seconds bug" or "high speed" index (figure 4.3-11).
This "high speed" index is used when the large numbers on the TIME scale are to
represent SECONDS (rather than minutes), and the inner hour circle is to represent
minutes (rather than hours). The "high speed" index is used in rate problems involving
seconds as the time flown or to be flown.