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8
Time, Speed, and Distance Problems
The rate arrow on the disk is always set to indicate a
value per hour on the outer scale. There are three
basic time-speed-distance problems. In two of these
problems you know the rate, while in the third prob-
lem, the rate is part of the answer you are looking for.
To find the Time En Route, let’s assume you
know your airspeed is 150 knots (nautical miles per
hour).
1. Set the rate arrow to 150.
See
Figure 3.
2. You have determined the distance to your
destination to be 245 nautical miles. Speed
and distance are always on the outer scale;
245 is halfway between 24 and 25.
3. Look directly opposite to that value on the
inner scale to find the Time En Route. It is
between 1:35 and 1:40. There are five cali-
bration marks on the middle scale between
1:35 and 1:40, and 245 NM on the outer scale
is closest to the third calibration, or one hour
and 38 minutes.
To find out how far you can go if your fuel
endurance is 4.5 hours and your ground speed is
known to be 125 knots:
1. Set the rate arrow at 125 on the outer scale.
See
Figure 4.
2. Find 4:30 on the inner scale.
3. The value on the outer scale is slightly more
than 55. You know that 4 hours at 125 knots
should cover 500 miles, so the outer scale is
read as 500, not 50 or 5,000, which makes
each large calibration mark worth 10 nautical
miles. The answer is 564 nautical miles.