Position Plots

This is an article I wrote in 2000 for ATA Airline pilots operating in long range, overwater airspace, primarily the North Atlantic and Pacific oceans. It assumes that the pilots reading it have completed the ATA navigation ground school and have passed a navigation check ride. Explanations for some of the terms that they would have understood but others might not have been included in brackets. Anyone navigating out or range of land based nav aids can use position plotting to verify the accuracy of his or her navigation. September 18, 2007.

Position Plots

One of the things I’ve always liked about flying at ATA is that we don’t have much tolerance for Captains who want to do things their own way. Non-standardization does happen, of course, as every crewmember who has ever served as a copilot or flight engineer knows. The reasons usually given are, “That’s the way I was taught to do it in flight school,” or “That’s the way we did it at Brand X,” or “That’s just the way I’ve always done it,” none of which are good enough reasons, but fortunately, we don’t have a lot of it. We may not always agree with or like the ATA way of doing it, but by and large we realize that there has to be one way for everybody, and if we really don’t like it we try to work within the system to change it by calling a fleet manager or writing him a letter or talking to a check airman or sim instructor and running our suggested change through one of them. I think that is one of our real strengths, and has helped to make us as good as we are.

What we do have, though, is a lot of very bright pilots and engineers who are always working on better ways to do things, who in the process sometimes introduce elements of non-standardized individualism masquerading as techniques. These individuals feel that what they are suggesting is within the limits of variation that we commonly describe as technique, and that in so doing they are improving things. In fact, in many cases there may well be a reason they haven’t thought of for why their change is not a good idea, and in most cases their variation goes beyond technique and is a change in our procedures. While trying to improve on our procedures is always a good idea, changing them on an individual basis is not.

There are obviously some gray areas here, and the point of this article is not to categorically draw the line between technique and standard procedures. That would be very difficult to do. (One airline, who shall remain nameless but whose first initial is D, as in Delta, defines technique as anything not in the book; that assumes a pretty good book, to me.) An example might help, though, and one that I have noticed is that pilots often have different ways of plotting their position when doing MNPS [Minimum Navigational Performance Standards—applies to the North Atlantic] crossings. There really is only one correct way to do it, and anything else is not technique but is non-standard. But the reason people are doing this, as I have discovered in talking to them about it, is not that they’re just stubborn and want to do it their way, but that they think they have come up with a better way to do it that is not so different as to be non-standard. And almost invariably this thinking, while good in itself, results from a misconception about how and why we do position plots. So let’s take it from the beginning.

We are required to prepare a plotting chart showing our cleared oceanic track anytime we operate in MNPS, NOPAC [Northern Pacific], or CEPAC [Central Pacific] airspace. We are also required to plot our position on that chart 10 minutes after passing each waypoint along the track. The purpose of that position plot is to verify that we are indeed operating along the cleared track. Very simple, really, and possibly because of that, subject to several misconceptions.

The first and most common misconception is that the purpose of the position plot is to verify that the nav system in use is tracking properly—following the cleared, loaded, and plotted track. In other words, that it is working properly. This is the first example of how something fairly simple gets fuzzy and complicated if we let it.

All we do when we plot our position on the plotting chart is verify that the position plotted—our location—is on the desired track. It doesn’t tell us anything about how well the nav system in use is working. It could, in fact, be in error by a considerable amount, but if it thinks it is working properly (there are no warning or error messages), then it will also think it is where it is supposed to be. An analogy might be to tell someone to drive 20 miles due east and stop. If that person’s compass were off 90 degrees but he or she didn’t know it—if, for instance, when it read due east it was actually indicating due south—that person would drive off due south for 20 miles and report in. “Here I am. I made it. According to my compass and odometer, I am 20 miles due east, just like you wanted.” He or she could even plot that point on his AAA highlighted road map to make sure he went to the right place, and sure enough, 90 degrees east for 20 miles would fall right on the map where it should. Remember, all we do when we plot our position is verify that we are where we’re supposed to be—on the desired track. If the nav system in use is not accurate, but does have proper data entered, it will still think it is where it is supposed to be, even if it isn’t. There are other ways to check on the accuracy of the system, but this isn’t one of them.

Another misconception that comes from this is that if the nav system in use always thinks it is where it’s supposed to be, whether right or wrong, what’s the point of plotting it? It will always be “on the line.” And, again, if the point of position plotting were to identify errors in accuracy or in the operation of the nav system in general, there would be no point. But that isn’t the point. The point is to verify that the aircraft is actually navigating along the desired track—the cleared route.

So then, why wouldn’t it ever not be on the plotted course? There are three possible reasons why the position plotted might not fall exactly on the desired course; two of these are fairly inconsequential and one is very consequential and is the reason we do it.

The first and most common reason the plotted position might not fall on the line is because the line is wrong. Those little tick marks can be hard to see and hard to count in the dim light of the cockpit and it is also easy to mix up latitude and longitude. You might plot 50 North and 51 West, for instance, instead of 51 North and 50 West. (You would never be cleared over a longitudinal line in MNPS airspace that wasn’t an interval of five or 10—20 West, 30 West, etc—but it is still easy to mix coordinates up.) In almost every case where the plot doesn’t fall on the line, it is because the line was drawn wrong. If this happens, have somebody else in the cockpit verify that that is indeed the problem, redraw the line, and the line should fall on the plot.

The next most common mistake is that the plot was done wrong, either written down wrong, or simply not plotted in the correct spot. Very easy to do, and again, if you think that is the problem, have somebody else verify it and correct it.

But the third reason the plot might not fall almost exactly on the plotted track is that you aren’t where you are supposed to be—the track is right, the plot is right, but the aircraft is not navigating along the desired track—uncorrected, you are headed toward a track bust. There are really only a couple of ways this can happen, and plotting an accurate position is the only way to catch them.

The most obvious cause for a navigational error is that the nav system in use does not have the correct coordinates for the next waypoint. (The waypoint behind has to be right, or you would have had a deviation off track on the last 10 minute plot.) If this occurs, immediately check the next loaded waypoint against the oceanic clearance: it should be readily clear to everyone in the cockpit whether it is correct or not. Reenter the correct next waypoint coordinates in the nav system in use, and reintercept the correct track as expeditiously as possible. Then, of course, replot it to be sure everything is back to normal.

Other causes for nav errors would be that the autopilot is in heading instead of nav, or in radio nav instead of long range nav, or has no lateral mode at all, or isn’t even coupled—the latter unlikely but possible. (But being left in heading mode instead of nav is not at all unlikely, and was the reason for one of our very rare track busts.) It is also possible that the autopilot is “cross-coupled” (the captain using the B autopilot, the first officer the A autopilot) and the other side is set up for something else, FMS nav, for instance, with only an abbreviated or previous route entered, or Route 2 on the opposite GPS. Another cause might be that a reroute was entered into the non-flying side only, or didn’t transfer over correctly. Finally, in a triple INS aircraft, the INS in use might be in manual waypoint change instead of auto. The only way you’re going to catch these kinds of oversights—mistakes that have not been caught already as part of the normal check and double check process—is with a position plot.

At the risk of seeming to be a bit obsessive, let me add that there is also a right way and a wrong way to plot the position. The right way is to locate the latitude and longitude along the grid marks to either side of the track, mark the top and bottom and left and right side on those grid marks with short tick marks, then put the plotter first along one set of marks and draw a line across the track, then do the other set, also using the ticked grid marks. The two lines should fall on the track. In other words, plot both the latitude part of the position fix and the longitude part independently using ticked grid lines. Anything else sets you up to see what you want to see. If, for instance, you just lay the plotter along the grid marks for the longitude and mark that point on the track, and then put the plotter along the latitudinal grid marks and mark that on the track, the temptation to put the plotter where it should be to fall on the track, rather than on the actual noted position, is very, very strong. This may seem like nit picking, but it is not. A careful, accurate position plot is your last chance to catch a gross error in navigation. This is something you do for yourself, to stay out of trouble, not something you do for the company because they said so (although they do). It’s in your own interest to do it right. Otherwise this last good chance to catch a major error—flying off track or along the wrong track—is going to be missed.

Finally, which nav system position is the best one to plot? The nav manual says to use the nav system that is coupled to the autopilot, and it says that for a reason. What I see is a lot of guys using the third system, which would be either the Litton INS in a GPS/FMS/INS aircraft or the third INS in a triple INS aircraft, for their plots. The reason given is always, “The nav system in use always thinks it is where it is supposed to be, so what’s the point of plotting that one? The third system is independent. That’s a good cross check on the one in use.” And they are right in that respect: comparing the calculated position of one nav system to another, particularly two completely different nav system types, is an excellent way to check on the functioning of those systems. If everything is working properly they should agree on where they are, within the limits of system accuracy. But just because they agree on where they are doesn’t guarantee that the nav system in use is going where it should. Only an accurate position plot of the system in use—the one that is doing the driving—can do that. Plotting the third system, one that is not coupled, simply tells us its position, which is nice to know, but really doesn’t mean anything because it’s not doing anything. (It may be supplying position information to the FMS and to the other two INS’s in a triple mix configuration, but it isn’t driving the airplane.) There is absolutely nothing wrong with comparing the position, track deviation, and track angle error of one system to another—in fact, that is an excellent idea and one that is provided for in the way we set up our nav systems using the appropriate MNPS checklist—but doing that won’t tell you, not for sure anyway, that you are actually navigating along the cleared track. An accurate position plot taken off the nav system in use—the one coupled to the autopilot—will. Every time.

So to review, position plotting tells you nothing about the accuracy of your nav system; position plotting does tell you whether you are navigating along the cleared track or not. The only valid and meaningful position plot is one taken off the nav system in use; anything else is “nice to know” and might point to an error, but in and of itself is irrelevant since it isn’t doing the driving. The right way to plot position is to hold the position on the nav system in use 10 minutes after passing the waypoint, mark the corresponding latitude and longitude on the grid marks to either side of the track, connect them with a plotter, and see where the intersecting lines fall. Anything other than exactly on the line (within the limits of plotting accuracy), means that the line was drawn wrong, the position was plotted wrong, or you are off course.

Think of your position plot as a waypoint triple check. The first check is done on the ground after all waypoints have been loaded (then a circle is drawn next to each on the flight plan). The double check is a recheck of an approaching waypoint’s coordinates, the distance between the next two waypoints and the course between the next two waypoints (then a line is drawn through the circle). The triple check is the position plot—your actual position relative to the cleared track. This is your last chance to catch a deviation while it is still small enough to correct without adverse consequences, meaning before enough time has passed that you could have wandered over to an adjacent track. Remember, this is also air traffic control—strategic navigation, or separation. We could all probably find Shannon with much simpler procedures than this if that was all there was to it. But this is much more important than just simple navigation: this is what insures that we stay inside the little bit of protected airspace that has been reserved for us over a very big ocean honeycombed with many other little bits of protected airspace, each with a relatively tiny aluminum airplane inside it, full of people. It’s not brain surgery (for one thing, brain surgeons only have one life in their hands), but it isn’t tiddly winks either. For all kinds of reasons, it’s worth the effort to do it right.