A simple, time-based plotter for light aircraft navigation

April 15 2005 edition

Steve Seibel

steve at aeroexperiments.org

www.aeroexperiments.org

I created this time-based plotter back in the days before
affordable, hand-held GPS units were available. I still use it from time to time, both for pre-flight planning and in the air. With the advent of real-time groundspeed information from inexpensive GPS units, anyone who still uses an actual paper map in flight from time to time will find the time-based plotter to be more useful than ever. (Additional photo).

The purpose of the time-based plotter is to allow a pilot to
quickly estimate the time required to fly between 2 points on a map, at a known
or estimated groundspeed, without having to use a calculator or flight
computer.

The time-based plotter is just a paper overlay that is glued
or taped to standard, distance-based plotter.
The time-based plotter is designed for a specific map scale and a
specific set of groundspeeds: the one we'll describe here is designed for use with a standard VFR sectional chart, and is optimized for a target groundspeed of 90 knots, with additional markings corresponding to
groundspeeds of 80 or 101 knots, 70 or 116 knots, and 60 or 135 knots. The plotter includes the following markings: a set of closely
spaced vertical lines that are spaced at one-minute intervals (at a groundspeed
of 90 knots), a set of closely spaced, slanting, parallel lines that are also
spaced at one-minute intervals (as measured along the bottom edge of the
plotter, at a groundspeed of 90 knots), and the three long diagonal lines that
correspond to the 6 extra groundspeed choices as noted above.

How to use the plotter:

Let's assume we want to measure the distance required to fly
from point A to point B, where point A falls at the "zero" mark on
the bottom edge of the plotter and point B falls at the "26" mark
on the bottom edge of the plotter.

If our estimated groundspeed is 90 knots--the speed for
which this particular plotter is optimized--then our estimated time enroute can
be read directly off the bottom edge of the plotter: 26 minutes.

If our estimated groundspeed is slower than 90 knots, then
we'll read the plotter in the "slower" mode. We have three choices for "slower"
groundspeeds: 80 knots, 70 knots, and 60 knots. Let's assume that our estimated groundspeed is 60 knots. From the "26" mark on the bottom
edge of the plotter, we move straight up until we hit the long diagonal line
labeled "60 or 135 knots". We
then move forward and downward along the nearest of the closely spaced, slanting,
parallel lines until we are back at the bottom edge of the plotter. We arrive at the bottom edge of the plotter
at the "39" mark, so our estimated time enroute is 39 minutes.

Of course, we can also interpolate. For example, if our estimated groundspeed is
65 knots, then from the "26" mark on the bottom edge of the plotter,
we'll move straight up until we are halfway between the long diagonal line
labeled "70 or 116 knots", and the long diagonal line labeled
"60 or 135 knots". We'll then
move forward and downward along the nearest of the closely spaced, slanting,
parallel lines until we are back at the bottom edge of the plotter. We arrive at the bottom edge of the plotter
at the "36" mark, so our estimated time enroute is 36 minutes.

If our estimated groundspeed is faster than 90 knots, then
we'll read the plotter in the "faster" mode. We have three choices for "faster"
groundspeeds: 101knots, 116 knots, and 135 knots. Let's assume that our estimated groundspeed is 135 knots. Now instead of starting by moving upward
along a vertical line, we'll start by moving upward and rearward along the
nearest of the closely spaced, slanting, parallel lines. From the "26" mark on the bottom
edge of the plotter, we'll move upward and rearward along the nearest of the
closely spaced, slanting, parallel lines until we hit the long diagonal line
labeled "60 or 135 knots".
Then we'll move straight down until we are back at the bottom edge of
the plotter. We arrive at the bottom
edge of the plotter between the "17" and "18" marks, so our
estimated time enroute is 17.5 minutes.

Again, we can also interpolate if necessary. For example, if our estimated groundspeed is
125 knots, then from the "26" mark on the bottom edge of the plotter,
we'll move upward and rearward along the nearest of the closely spaced, slanting,
parallel lines until we are halfway between the long diagonal line labeled
"70 or 116 knots", and the long diagonal line labeled "60 or 135
knots". We'll then move straight
down to the bottom edge of the plotter.
We arrive at the bottom edge of the plotter halfway between the
"18" and "19" marks, so our estimated time enroute is 18.5
minutes.

This all may sound a bit awkward when described in detail,
but in practice I've found the time-based plotter to be extremely handy in
flight. It's very convenient to be able
to rapidly come up with a rough estimate of the estimated time enroute to a
distant waypoint or airport simply by laying the plotter against a map. Even when I'm using a GPS unit as my primary
navigation instrument, I still keep the time-based plotter in my flight bag, and use it quite often.

Anyone can make their own time-based plotter for any scale
of map and any desired range of groundspeeds.
The angle of slant of the closely spaced, parallel, slanted lines will
determine how many of the long, diagonal lines for "extra"
faster/slower pairs of groundspeeds will fit on the plotter. The closely spaced, parallel, slanted lines
should be spaced so that they intersect the bottom of edge of the plotter in
intervals that are exactly one minute apart.
So should the closely spaced vertical lines. Once these two sets of lines are in place, the proper position
for the long diagonal lines, corresponding to the additional groundspeed
choices, can be determined.

This particular plotter has been laid out in such a way that the full length of the bottom edge of the plotter is available at the 90-knot, 101-knot, and 116-knot airspeeds. The useable portion of the bottom scale becomes progressively smaller at groundspeeds below the target groundspeed of 90 knots, and the useable portion of the bottom scale also becomes progressively smaller at groundspeeds above 116 knots. If desired, the plotter could be designed in the opposite manner--with the closely spaced, slanting, parallel lines running up and to the left instead of up and to the right. This would favor the groundspeed range immediately below the target groundspeed, rather than the groundspeed range immediately above the target groundspeed.