The Navigation Computer: Distance, Speed and Time

For distance, speed and time problems the outer scale is always distance or speed and the inner scale is always time.

Example. How far do you go in 48 minutes at 120 knots ground speed?

First, set what you know. Set the ground speed. Bring the ‘60’ black triangle against 120 knots on the outer scale.

Now find 48 minutes on the minutes (inner) scale. Read off the answer - 96 NM on the outer scale.

Example. In 30 minutes you have covered 90 NM over the ground. What is your ground speed?

Set what you know on the computer. Set 30 minutes (inner scale) against 90 NM (outer scale).

Now read off the ground speed against the ‘60’ black triangle. The answer is 18.0.

What order of magnitude? At 18 knots, you would stall. At 1800 knots, you would strip the wings off. Common sense tells you that this must be 180 knots - which is the right answer.

Example. Your ground speed is 220 knots and you have 57 NM to go. How long will it take you?

As before, set what you know. Move the black ‘60’ triangle against 220 knots.

Now bring the cursor round to 57 NM (outer scale, because it is a distance).

Read off the time on the inner scale. The answer is 15.5. Are we expecting 15 minutes or 150 minutes? 57 NM is about a quarter of 220, so about a quarter of an hour is obviously the right answer. It is 15.5 minutes.

Next: The Yellow Scale

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