Estimating an ETA, and a Groundspeed using proportional method.

Sometimes you will need to work out times and distances to find Estimated Elapsed Times (EET), ETA's, or Groundspeeds.

Scenario:

You left 'A' at 1200 NZST for B.

You are overhead 'X' at 1230 NZST.

What would be your ETA at B?

Given:

Distance   A - X  35nm

Distance   X - B  42nm

Time         A - X   30 minutes

Using the Navigation Computer:

• Place the 1st “Distance” (35nm) on the outer wheel against the “Time” (30 min) on the inner wheel.
• Look on the outer wheel for the 2nd “Distance” (42nm) then read the 2nd “Time” (36 min) on the inner wheel.

1st leg takes 30 minutes, 2nd leg will take 36 minutes.

1230 NZST + 36 minutes = 1306 NZST.

Answer:

ETA         1306 NZST

And while you have the Navigation Computer set to those numbers, you can also find the Groundspeed.

Answer:

GS      70kts

The 1 in 60 rule

The 1 in 60 rule is a method to quickly estimate small angles in navigation, especially when quick approximations are needed.

It works based on the assumption that 1 nautical mile subtends an angle of 1 degree at a distance of 60 nautical miles.

It might sound a bit complex at first, but once the concept is grasped, it becomes a handy and straightforward method for estimating angles.