Eddie sez:

It is a common fear of inexperienced oceanic travelers: the FMS is programmed, the charts are plotted, you are wondering what to do with yourself for the next three hours. Then the oceanic clearance comes in and it doesn't look anything like what you had planned; you have been rerouted. You know a reroute is an invitation to a Gross Navigational Error so you obviously want to take all of this seriously and if you ever needed to do navigation accuracy checks, the time is now. But how you are you going to do all this without a computer flight plan?

I used to wax poetic about how now is the time to get out your plotter and a sharp pencil and get to work. But these days you are better off picking up the phone and calling your flight planning service provider. See: Preferred Method.

If you can't do that for some reason, there is still no reason to panic. You can get this done manually. You really ought to practice this once or twice. What follows is a walk through on how we used to do this routinely.


Figure: Oceanic reclearance, from Eddie's notes.

Last revision:


Preferred Method: E-mail, App Upload, Fax, Phone

The easiest and most accurate method is to email or phone your flight planning service and ask them to run the flight plan using the reroute points. If you have the technology you can have them upload the new flight plan to your App or email the PDF to you. Even if you don't have an Internet connection, you can have them read you the particulars for the oceanic legs that matter and save yourself from having to do the math.

As is true with your original flight plan, you should understand which legs the numbers apply to and if they are based on initial or midpoint course lines. In our case, we as a flight department asked for midpoint true courses. More about this: Initial vs. Midpoint Course.

With the example shown:

  • The data given on a particular line are the numbers needed to fly from the previous line.
  • The course line is measured from the midpoint.
  • Specifically: Flying between 55°N 050°W and 58°N 040°W will require a true course of 062° as measured at 045°W (the midpoint) and is 378 nautical miles in length.

At this point, you should plot the new course and you are ready to continue as before. If you can't print it or are not using an iPad Master Document, you really should transpose the data to a navigation worksheet, shown below, as a part of your master document. If you don't have the data, you will have to compute it on your own. Read on . . .


Figure: GV Reroute flight plan, from Eddie's notes.
Click photo for a larger image

Classic ("Back in the Old Days") Method

If you can't get a copy of the reroute using the preferred method, above, you will have to write one of your own. You should keep a number of these blanks for just this purpose. You can download a blank: here.


Figure: Navigation worksheet, from Navigation Worksheet.

It isn't hard if you are methodical about it.

Step 1: Plot the New Oceanic Points


Figure: Reroute plotted, from Eddie's notes.

Plot the new points on your existing chart unless it would be too hard to read, in which case you should get a fresh chart. More on this: Plotting.

Step 2: Enter Known Data Onto Navigation Worksheet


Figure: Known data entered onto navigation worksheet, from Eddie's notes.

You can guess the winds from your existing flight plan or download them from a data link if available. The variation can be read directly from the en route chart. Note that the chart's plotted variation lines are likely to be out of date, your FMS may provide more timely variation data.

Step 3: Determine True Courses

Using 10 Degree Tables


Figure: 55° True Course 10° Table, from True Course 10 Degree Tables / 55°.

Using True Course 10 Degree tables you can find courses and distances between major latitudes and longitudes. A major latitude is one to a whole degree and a major longitude is one to the nearest 10 degrees. More about this: True Course 10 Degree Tables.

In the example we see that flying from 55°N 050°W to 58°N 040°W will require a true course of 062° as measured from the midpoint and a distance of 376 nautical miles. (The 050°W and 040°W are not needed in the table, the answer will be the same at any pair of latitudes 55° - 58°N in the world, so long as the longitudes are 10 degrees apart.)

Using a Chart and Plotter


Figure: Reroute en route chart, plotting first leg, from Eddie's notes.

If you do not have 10 degree tables or if your waypoints are not covered in the tables, you can determine the courses and distances right off the chart. More about how to do this: Plotting.

In our example we measure a course of 062° and a distance of around 395 nautical miles.


Figure: True courses and distances added, from Eddie's notes.

In our example we've taken the courses and distances to 040°W, 030°W, AND 020°W from the ten degree tables. We used a plotter and the en route chart to find our course and distance to GOMUP.

Step 4: Determine Magnetic Courses


Figure: True / Variation / Magnetic / Deviation formula, from Direction.

Compute each leg's magnetic course by adding the magnetic variation to the true course. Westerly variation is added, easterly is subtracted. More about this: Variation.


Figure: Magnetic course added, from Eddie's notes.

The navigation worksheet now includes magnetic courses based on the variation we provided. (The plotting chart was a few years out of date and some of the variation figures were off by a few degrees. We need to keep that in mind when checking navigation performance.)

Step 5: Determine Drift, Heading, and Ground Speed

The higher your TAS in relation to your wind speed the less of a factor it will be, in fact the harder it will be to visualize on a wind computer such as the military CPU-26A or a civilian version such as the Jeppesen CR-2. For the purpose of demonstrating the method, we will use the highest wind in the example, on the leg to 59°N 020°W: 220/20. The method is similar for each leg.

CPU-26A Example


We start by spinning the inner slide to the wind direction (220°) and place an "X" on a gradient line separated from the inner grommet equal to the wind speed.


We then spin the compass to place our true course under the true index. We also adjust the slide so our TAS is under the X.

We can read the drift angle directly under the X, in our example it is 2 degrees right.

We can read our ground speed directly under the center cursor, in our example it is just over 490 knots.

CR-2 Example


The method in a standard circular navigation computer, such as the Jeppesen CR-3, is similar to the military CPU-26.

  1. Set the true airspeed (i.e., 480) under the TAS index.
  2. Set the true course (i.e., 090°) under the TC index.
  3. Find the wind direction (i.e., 220°) on the wind slide.
  4. Draw a dot over the windspeed on the wind direction line. You may have a choice of scales. In our case, we use the largest scale available for ease of legibility.
  1. Draw a line from the wind dot parallel to the horizontal scale to read the tailwind on the vertical scale (i.e., 12 knots, which means our groundspeed will be 490 + 12 = 192 knots)
  2. Draw a line from the wind dot parallel to the vertical scale to read the crosswind on the horizontal scale (i.e., 16 knots)
  3. Find the crosswind, in knots, on the outer scale. Remember that since we entered this outer scale with 480 knots, our 16 knot crosswind is actually 1.6 when read against the 16 number.
  4. Read the corresponding drift angle on the inner scale. (i.e., 1.9°)
  5. Since our true course is 090° and our magnetic variation is +14° we have a magnetic course of 090 + 14 = 104°. Adding our drift angle gives us a magnetic heading of 104 + 2 = 106°)

Figure: Drift, heading, ground speed added, from Eddie's notes.

The navigation worksheet now includes drift and groundspeed. We can then figure magnetic heading by adding the drift to magnetic course.

Note: Each step of this process involves the width of your pencil on the wind side of the circular computer and small errors early magnify themselves. This navigation worksheet will get you in the ballpark, which is what you need to do a navigation accuracy check. That is, after all, the entire reason for doing this. More about this: Navigation Accuracy Check.

What about the rest of the worksheet? If the leg distances are close, I'll just transpose those from the original flight plan.

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