This note describes an example flight using the charting sextant. The Installation Instructions, and an Operators Manual are separate links. First, let's preface the example with a review of some things to be showcased pertaining to the sextant chart.
To understand this example, and to enjoy using the sextant-chart gauge, you must be thoroughly familiar with the nomenclature and procedures in the Sextant Manual. Please take the time to re-read the Sextant Manual.
The "NM" display at the top of the chart gives the distance and direction from the center of the chart to the green circle. Its obvious function is to show the distance and direction from the assumed position to the fix location at the cocked hat. The reverse of this direction is the direction from the fix to the waypoint.
The chart can also be used for several other simulation functions.
1. If you fly at a constant speed from the previous waypoint directly toward (no WCA) the waypoint that is to be your assumed position at the time of the next fix, at the time of the fix, the direction in the NM display is the direction the wind is blowing from. If you divide the NM displayed distance by the time in hours that it took you to fly from your previous known position to the fix, you will know the wind speed.
2. You can simulate ruler and protractor functions on the chart:
The location (latitude/longitude) of the center of the chart is set by setting the assumed position on the sextant worksheet. The location of the green circle is the lat/longitude of the display on the chart. To find the distance and direction from point A to point B, just set the latitude/longitude of point A as the assumed position on the sextant worksheet, and move the green circle until the latitude/longitude display on the chart reads the latitude/longitude of point B. A typical use would be to set the next waypoint into the Sextant as the assumed position, make a fix, look at the direction to the waypoint, and if it is more than 20 degrees course change, decide not to fly to that waypoint but to the next one. So, by moving the assumed position and the next waypoint, the "ruler and protractor" function gives the new course and distance from your present position to the next waypoint.
3. By setting the previous waypoint (not the next one) as the assumed position, and moving the green circle to the center of the top hat (fix), the NM display shows the direction (track) the plane flew, and the distance traveled. Dividing the distance by the time will give the ground speed.
4. Usually between waypoints you fly with a WCA, so the determination of the wind speed and direction is more complicated than as described in 1. above. The "W" at the lower right corner of the chart is a hot button that uses the plane course, heading, TAS and Ground Speed to compute the average wind for that leg. To measure the wind, use the previous fix as the assumed position. At the moment,this only works correctly if the elapsed time from the previously known position (fix) and the current fix is exactly one hour. The ground speed is available as the distance covered in one hour. Therefore the "W" only produces correct results between fixes one hour apart.
The example flight will use a C-47 equipped with the standard 4 wing tanks plus 8 bladder/ferry tanks to fly from Goose Bay, NF to Prestwick, UK. Guy Goddard describes such a flight in a C-54 as NA08. In the C-47, at 150 KTAS, the flight takes 12h 40m, and burns 82 gph for consumed fuel of 1040 gallons, plus reserves for headwinds etc. The date is 14 Feb 1942, and for navigational purposes the plan calls for arrival at Prestwick at dawn, which is at 0745 GMT on that date. With a full load of fuel (1600 gal), less 2 hours reserve the plane has 1436 gallons for flight, which at the standard 550 HP gives 17.5 hours of flight. To fly the 1900 miles, you therefore need to make a ground speed of 109 Knots. This implies that you can have up to 39 Knots of headwind. This would then delay your arrival from dawn to about noon, so you might have to fly down a sun line of position if radio DF near Prestwick was not available.
Real Weather updated every 15 minutes was selected, and the take off scheduled for 1900Z. The great circle route with fixes every hour (150 nm apart if no wind) was calculated using formulas found on Ed Williams site shown below:
The fixes are shown below:
This information is needed in flight, so it is made available on the briefing page of the electronic kneeboard.
Using the times and locations as stated above, the USNO website
was used to come up with suitable stars and their azimuth and elevation for the hourly fixes. A typical display of that navigational data is shown below:

This particular example is for 0500 GMT at fix 10 (fx10), and the three stars selected were Mirfak for the course line, Deneb for the check line, and Rasalhag for the speed line.
The winds aloft can be measured during the flight by using the following procedure. Set the most recent known fix (Goose bay at the start) in the sextant as the assumed position. Then fly at constant speed and direction toward fx1 for one hour and make the first sextant fix, using the sextant chart and moving the green circle to the center of the cocked hat. Clicking on the NM in the bottom left corner of the chart will provide you with a Ground Speed (the NM traveled in one hour is the ground speed in Knots), and the direction you traveled (the ground track). These two parameters, plus the Aircraft TAS and the true heading (the AP setting plus the magnetic variation (available on the sextant) will provide the four parameters needed to compute the wind direction and speed. The results are available by clicking the "W" in the bottom right corner of the chart. Note that it (at the moment anyway) has to be exactly one hour, and the sextant assumed position has to be an accurate representation of where the plane was an hour ago. Also the plane cannot change speed or direction and still provide a good estimate of the wind.
If you had a physical chart in front of you, you could now plot the fix on the chart (where the green circle is), and use a ruler to get the distance to the next waypoint, and use a protractor to get the true course. To use the sextant chart to get this information, update the sextant assumed position to the current fix position (caution, write the latitude/longitude down temporarily since changing the sextant assumed position will change the displayed values of latitude and longitude on the chart. Then move the green circle on the chart until the next assumed waypoint (latitude and longitude) show on the chart. Now the NM display at the top will give you the distance and true direction from wherever you are to the next waypoint.
The E6B can now be used, since your wind direction and speed are now known, to calculate the WCA. Apply that correction to the true course to the next waypoint, and convert from true to magnetic heading, and that becomes the autopilot heading to the next waypoint, which you fly for the second hour. (Note that the distance usually won’t be 150 nm, so you aren’t flying exactly to where you think the next waypoint is, but on the route to or thru it). At the end of the second hour, you repeat the process of obtaining a fix, recording the latitude/longitude and the new wind update of speed and direction. Repeat the changing of the assumed position to the fx2, and moving the circle to the waypoint.3’s latitude and longitude.
The example flight shown below will explain how the sextant chart gauge may be used to navigate. The example starts with the plane being iin the run-up area in Goose Bay prior to take off.


GMT Elevation Azimuth 1855 24° 49' 226° 1900 24° 16' 228°Note that the LOP moves 33 NM in 5 minutes, so the timing of the shot is critical to the process. The snap below shows the cockpit in the take off position. Note the Ike Slack timer below the Sperry AP which will mark the timed events during the flight.

In the example flight, fx1 was measured at 54° 10' N, 55° 26' W using the green circle. Then the NM hot button produced 148 NM and 70 degrees as the distance traveled, and the true course. (The nav error, due primarily to the wind, but also the slow climb to 7500 ft, was about 13 miles south, and 30 miles east of the waypoint). Click on the "W" and see the wind guestimate is 352 degrees true, at 16 knots. Updating the sextant assumed position to these numbers, and then moving the green circle so the latitude on the chart reads 55° 19' N and 52° 34' W (fx2). Note the new course is 73 degrees. Use the E6B to figure the WCA at 6 degrees left, or 67 degrees true. The sextant worksheet shows the magnetic variation at 11 degrees, so you should fly from the fx1 to fx2 at 78 degrees magnetic.
Note that fx1 was taken at dusk. While most stars are not visible, in this case, the Sun, Moon, Jupiter and Saturn were all visible for navigation. In the worst case, only the Sun would be visible, and it would provide only a speed line.
The snap below shows the flight at sunset, at about 2030Z:

Do not change the sextant assumed position (yet). At the end of the second hour, use the sextant and chart to locate fx2. In the example flight it was 54° 50' N, 52° 5' W, and the distance was 121 nm on the same course of 70 degrees. Clicking "W" we see 180 degrees true, at 37 knots which depicts a serious shift in the wind. As you can see the difference between waypoint.2 and the fix position is about 29 miles south, and 12 miles West. This process is repeated each hour. Adjust the sextant assumed position to the new fix 54° 50' N, 52° 5' W (fx2), and move the green circle until the latitude and longitude on the chart showed the location of the next waypoint, waypoint.3 (56° 7' N and 48° 22' W) to obtain the distance and course to follow to waypoint.3. Use the E6B to determine the WCA to fly for the next hour, and the process continues to repeat.
The snap below shows the chart after fx8 has been made. It is about 0300Z. The "NM" display is enabled, and shows the ground speed = 133 knots, and the course is 102 degrees true. The "W" display has also been enabled, and shows the wind is from 161 degrees true, and the average wind speed over the previous hour is 35 knots.

The snap below shows the chart after fx9 has been made. Compare the results with the results on the screen above, made an hour ago.

The snap below shows the chart after fx10 has been made.

The snap below shows the outside view at daybreak. The time is approximately 0730 GMT.

The snap below shows the plane on descent to Prestwick. The approach was an entry to a right downwind circuit to RWY 13.

The snap below shows the plane on short final for runway 13. Note the elapsed time of 13:23.

The snap below shows the plane parked with Number 1 shut down, and the cargo doors open.

The elapsed time is 13:26, and 474 gallons remain in the tanks. The filled in navigation log might look something like the snap below. Note the flight planner assumed there would be about a 15 knot tailwind, shortening the planned time from 12h 40m to 11h 14m:

Epilog: At Waypoint.11, the plane was 80 nm south of it's assumed position. The Pilot didn't want to approach EGPK from a direction other than expected for security reasons, so a decision was made to fly at 30 degrees Magnetic for 40 minutes to get back on track. At the end of that time, another fix was made that showed the plane to be North of the track by about 30 NM. the snap below shows the track of the aircraft during the whole flight. This is shown for historical purposes, and explains the reason why the last leg of the flight took longer than expected.

Copyright March 2008 Dave Bitzer