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 91-70A — still says you have to do these and I think it is a good idea.
Figure: Example Plotting Chart (Arrival), from Eddie's notes.
Figure: RVSM/Nav Accuracy Log Example, from Eddie's notes.
Coast-Out Navigation Accuracy Check.
[AC 91-70A, Appendix 2, ¶2.f.(1)] Gross Error Accuracy Check. Before oceanic entry, check the accuracy of the LRNS against a ground-based NAVAID. Record the results of the accuracy check with the time and position. A large difference between the ground-based NAVAID and the LRNS may require immediate corrective action. Operators should establish a gross error check tolerance based on the type of LRNS. It is not advisable for crews to attempt to correct an error by doing an air alignment or by manually updating the LRNS since this has often contributed to a gross navigation error (GNE).
Before you stray out of ground-based NAVAID range — sooner actually, before you leave the service volume of an appropriate NAVAID — you need to compare what it is telling you versus what your FMS is telling you. The procedure can be checking a VOR Radial/DME plot versus your FMS latitude/longitude on your plotting chart, or using an FMS "Cross Points" function versus the VOR Radial/DME.
Coast-in Navigation Accuracy Check.
[AC 91-70A, Appendix 2, ¶2.l.(1)] Compare Ground-Based NAVAID to LRNS. When departing oceanic airspace and acquiring ground-based NAVAIDs, crews should note the accuracy of the LRNS by comparing it to those NAVAIDs. Note any discrepancy in the maintenance log.
The coast-in navigation accuracy check is conducted in the same manner as for coast-out, except that the earliest possible navigation aid is sought for the first opportunity to check navigation performance, keeping in mind the service volume of the navaid is limited.
[AC 91-70A, Appendix 2, ¶2.n.(1)] Navigation Accuracy Check. When arriving at the destination gate, crews should note any drift or circular error in the LRNS. A GPS primary means system normally should not exceed 0.27 NM for the flight. Some inertial systems may drift as much as 2 NM per hour. Because the present generation of LRNSs is highly accurate, operators should establish a drift tolerance which, if exceeded, would require a write-up in the maintenance log. Required Navigation Performance (RNP) requirements demand close monitoring of drift.
Latest generation Hybrid IRS, such as those in the G450, will may do this for you automatically.
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.
- FMS Cross-Points
- Latitude Only Math
- FMS Bearing/Distance
- FMS Lat/Long Plotting versus NAVAID Plotting
- Overhead VOR Fix
- Radar Fix
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.
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.
- 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.
- Convert the north (360°) or south (180°) magnetic course from the VOR to true. 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°.
- 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.)
- 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.
- 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.
- Compare the latitude difference to the VOR DME. In our example they are within a half a mile.
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.
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 101.
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.
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.
Portions of this page can be found in the book International Flight Operations Manual, Part VIII, Chapter 27.
Advisory Circular 91-70A, Oceanic and International Operations, 8/12/10, U.S. Department of Transportation