the learning never stops!

Post Position Plot

International Operations Appendices

A post-position plot is simply a check made after enough time has elapsed since crossing the waypoint to detect a navigation error, but soon enough to fix things before a loss of separation with other traffic occurs. We started doing these after inertial reference systems replaced our navigators, and started wondering about the requirement when we installed our first GPS. Well here we are years later and guys are still getting violated even with hybrid IRUs and six satellites hooked up to two GPS. We still need to do these.




Figure: Two Degree Check Error Example, from Eddie's notes.

[AC 91-70A, Appendix 2, ¶2.j.] Approximately 10 minutes after passing an oceanic waypoint, crews should plot the latitude, longitude and time on the plotting chart. It is advisable to plot the non-steering LRNS. A 10-minute plot can alert the crew to any lateral deviation from their ATC clearance prior to it becoming a GNE. A good cross-check for the position of the 10-minute plot is that it is approximately 2° of longitude past the oceanic waypoint.

[NAT Doc 007, ¶8.4.20.] A position check should be made at each waypoint and the present position plotted 10 minutes after passing each waypoint. For a generally east-west flight, this 10 minute point will be approximately 2 degrees of longitude beyond the oceanic waypoint. It may therefore in fact be simpler to plot a present position 2 degrees of longitude after each 10 degree waypoint. There may be circumstances, (e.g. when, due to equipment failure, only one LRNS remains serviceable) in which additional plots midway between each waypoint may be justified.

The "non-steering LRNS" may not apply to many aircraft, since triple FMS with hybrid IRUs tend to blend everything together. The point, however, is to plot the aircraft's position after about ten minutes to ensure the next waypoint wasn't entered in error. If, for example, the flight plan download (or manual entry) entered the next waypoint as 51N 040W, one degree south of the clearance, it would look perfectly normal on the cockpit displays but plotting the position would alert the crew that something is amiss, as shown on the figure. Don't think this is possible? Modern G550's continue to get caught in just this type of Gross Navigational Error.

Plotting on Paper Charts


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

All of the regulatory guidance, just ten years ago, talked about the ten minute check and made no allowance for a "two degree" check. But the logic, ease of use, and elimination of fifty percent of the error potential made it a winning technique. If "about ten minutes" puts you close to a meridian of longitude, then use it.

Plotting is a basic pilot skill, one thinks, but perhaps there are easier ways.

See: Plotting 101.

Plotting on an EFB


The following comes from Michael Keller, a technically savvy friend. He's also a great photographer, it is his shot on the cover of the International Operations Flight Manual. This is written for a Gulfstream PlaneView cockpit, but the procedures for other avionics should be similar.

You have plotted your waypoints onto the Jeppview when you enter it into the application, display it and crosscheck it against your Universal (or other) flight plan. Now, when you are flying the trip, and you need to plot your 10 minute cross checks against the flight plan . . . go onto the map near where you estimate the plane to be 10 minutes after passing a trip waypoint. On your MCDU, go into the Pos Sensor pages . . . do the position update button (R1 I think) and when you see 10 minutes on your clock after passing the waypoint, touch that position update and drop it into the scratch pad . . . "freezing it."

Back on the Jeppview app . . . at the point where you estimate you would be, touch and hold the screen (just off route, NOT on the line) and a user created way point will arise....You can rename that waypoint such as "NODLE +10" or "NODLE10" or your preference to indicate the waypoint plus 10 minutes. Then touch just below on the user created waypoint and you can quickly edit the lat long it has detected from your touch, but typing in the lat long from your scratchpad (which is now your crosschecking operation.)


The user created waypoint, whatever you named it . . . should jump over to the route and show that you are on the filed/cleared route as you would expect if all is working to order.

Then, before the end of the flight, simply touch the button to save the flight plan to your IPad which you can later email to yourself to put in your files for safekeeping.


These errors are caught . . . here is a two degree error by a GV:

Date: 10/01/01
Error: Gross Navigation Error
Narrative: GLF5 W/B RAN F500 cleared GOMUP 62/20 64/30 observed by radar 6105N022217W (90 NM off track.) A/c reported 62/20 as cleared when actually at 60/20. No L-o-s in Reykjavik OCA*. *Checking separations in Shanwick OCA. Follow-up with operator.

Table: Gross Navigational Error Report, from Eddie's notes.

Book Notes

Portions of this page can be found in the book Flight Lessons 1: Basic Flight, Chapter 23; as well as the book International Flight Operations Manual, Part VIII, Chapter 31.


Advisory Circular 91-70A, Oceanic and International Operations, 8/12/10, U.S. Department of Transportation

NAT Doc 007, North Atlantic Operations and Airspace Manual Doc 007, Edition 2013

Revision: 20161014