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Navigation Accuracy Check

International Operations Appendices

It's getting harder and harder to justify the need for a navigation accuracy check, with GPS redundancy and hybrid IRS, ADS, and all the rest. Most of these techniques came from the days where flying without a navigator was an act of faith and have stubbornly remained. The book — AC 91-70B — no longer requires you do these, but still says you should. I think you really should.



Figure: Example Plotting Chart (Arrival), from Eddie's notes.

The Requirement


Figure: RVSM/Nav Accuracy Log Example, from Eddie's notes.

The requirement to do a navigation accuracy check prior to coast-out and prior to coast-in went away with the adoption of AC 91-70B in 2016, but it remains a very good idea. We are continuing to see aircraft violated for things a good navigation accuracy check would have caught. In that spirit, I offer the following techniques. You can get into the nuts and bolts of plotting here: Plotting.


The navigation accuracy check compares FMS position against a ground-based navigation aid and can be accomplished in several ways, depending on aircraft avionics. Depending on aircraft FMS capabilities, the following methods are presented in order of probable accuracy.

  1. FMS Cross-Points
  2. Latitude Only Math
  3. FMS Bearing/Distance
  4. FMS Lat/Long Plotting versus NAVAID Plotting
  5. Overhead VOR Fix
  6. Radar Fix

FMS Cross-Points


Photo: Nav Index, Page 1, from Eddie's Aircraft.

Some FMS installations include a cross points function that computes the position of the aircraft relative to a waypoint or VOR. The applicable VOR is inputted and compared to a raw data instrument tuned to the same VOR.

Using the G450 as an example: Select the Nav Index by pressing the NAV key.

Select CROSS PTS (LSK 5L).


Photo: Crossing Points, Page 1, from Eddie's Aircraft.

Select PPOS DIR (LSK 1L)


Photo: PPOS Direct, Page 1, from Eddie's Aircraft.

Enter the waypoint, YQX in our example


Photo: PPOS Direct, Example YQX, Page 1, from Eddie's Aircraft.

In the example photo, we see that we are on the 270° radial, 112 NM from YQX.


Tuning the YQX VOR we can see the raw data on the EBDI.

In the example photo, we see that we are on the 270° radial, 113 NM from YQX.

Of course the raw data distance is greater than the actual distance because it is the slant range.

Latitude Only Math


Photo: iPad example north of Gander, from a friend of Eddie's aircraft.

You can simply take the aircraft's FMS latitude/longitude position, subtract the VOR's latitude/position, and compare the result to the VOR radial/DME, easy. Right? Well, yes it is possible to do that, but if you plan the VOR radial to be along a line of longitude you can eliminate the hardest part of the math, the direction. Then it is a matter taking advantage of the relationship 1° latitude equals 60 nautical miles.

Plotting procedures are fairly straight forward but require a level of precision and perhaps some practice. Need a refresher? See: Plotting for some basic instruction.

Note that you cannot do this along a line of latitude since the distances per degree vary with latitude. The higher north or south you go, the fewer miles per degree.

A friend of Eddie submits this example, taken on a flight crossing just north of the Torbay (YYT) VOR.

  1. Determine the magnetic variation of the VOR from your en route chart or, for some aircraft, the FMS database. In our example, the YYT magnetic variation is 19° West.
  2. Draw a line to true north or true south by converting the VOR's 360° or 180° magnetic course. West variation is added, east variation is subtracted. If you draw this on your chart, it should be straight up and down from the VOR, it will be parallel to any line of longitude. In our example, 360° + 19° = 019° on the VOR's magnetic compass rose.
  3. Set a VOR CDI to this course line. When the aircraft crosses the course and the CDI centers, record the FMS latitude and the VOR DME. You will not need the longitude for reasons about to be clear. In many FMS you can turn to the page that presents current position and enter the value into the scratch pad with a single key stroke, making it easier to record without memorizing a long string of digits. In our example, we get N48°59.6' latitude and 90 DME. (The iPad presents positions in terms of degrees, minutes, seconds but most FMS will use degrees and minutes up to a tenth in accuracy. The seconds can be converted into decimal minutes thusly: minutes = seconds / 60.)
  4. Determine the VOR's latitude from the chart or another source. Notice that the VOR's longitude will be equal to the aircraft's longitude any place along the chosen radial, hence we can disregard it for the purposes of a navigation accuracy check. In our example, the YYT latitude is N47°29.1.
  5. Subtract the two latitudes. This might involve a "carry the one" type of operation where you realize 1° = 60 NM and 1' = 1 NM. In our example: N48°59.6' - N47°29.1' . . . 48 - 47 = 60 NM and 59.6 - 29.1 = 30.5 NM so you end up with 90.5 NM.
  6. Compare the latitude difference to the VOR DME. In our example they are within a half a mile.
  7. Note: the VOR DME should normally be greater than the distance over the ground because of the slant range. While this is not the case in this example, they are close enough. The VOR DME for this aircraft was reported to the nearest mile and quite often the magnetic variation could introduce errors of this magnitude.

FMS Bearing/Distance

Some FMS installations can provide a course and distance to fly to a selected waypoint. The applicable VOR is inputted and the reciprocal of the course is compared to a raw data instrument tuned to the same VOR.

FMS Lat/Long Plotting versus NAVAID Plotting


If the FMS doesn't provide a means for providing a track and distance to a VOR, the check can still be accomplished by recording the VOR radial and DME and the primary FMS latitude/longitude.

For example, the Gander VOR can be used for a coast-out NAV accuracy check.

The VOR should be drawn on the plotting chart with a magnetic north flag which would be drawn in this case 22° to the west. (This flag points to magnetic north and can be labeled “360°M / 348°T”)

The latitude and longitude from the FMS is plotted directly on the chart. A compass plotter measures the angle from the magnetic north flag to determine the aircraft is on a 270° radial and the distance is measured to be 112 nm. (We were cleared direct 51°N 050°W so didn't overfly YJT, CYMON, or DENDU.)

More about how to plot a VOR radial/DME: Plotting.

Overhead VOR Fix.

If the route of flight is directly over a VOR, the FMS latitude and longitude can be checked against the VOR latitude and longitude when directly overhead. This method is not as accurate as a radial/DME fix because of the geometric error associated with an overhead passage.

Radar Fix.

If all other means are unavailable and the aircraft is still under radar contact, ATC may be able to provide a radar fix in terms of a radial/DME from a known NAVAID which can be plotted or compared to an FMS cross points solution.

Book Notes

Portions of this page can be found in the book International Operations Flight Manual, Part VIII, Chapter 27.


Advisory Circular 91-70B, Oceanic and International Operations, 10/4/16, U.S. Department of Transportation

Revision: 20180418