Figure: Plotting Chart Example, from Eddie's notes.
We plot to avoid the classic one degree error, to double-check the other pilot's FMS entries, to ensure the flight plan uplink was accurate, and to make sure the database itself is accurate.
But wait, you say. The database itself has been QC'd, you've already checked the waypoints in the FMS, and your G450's graphical presentation makes any kind of graphical plotting completely pointless!
Well, you might have a point. You aren't going to get the standard "Ten percent of the traffic is general aviation but they are responsible for ninety percent of the gross navigational errors!" from me. That is a lie. Most of the GNEs belong to the airlines but they pay the bills and their errors are smoothed over by their companies. See, I can make a bold statement with no facts to back me up too.
But, back to the task at hand, the book says plot and maybe there is something out there lurking we don't know. Besides, lives could have been saved had plotting procedures been used. Just ask those on Mishaps / Korean Airlines Flight 007.
You may think you know all you need to know about plotting, if so skip to the section that interests you. But before you do, a quiz. When is plotting required? If you said when a route segment "between the operational service volume of ICAO standard ground-based navigational aids exceed 725 nm," then you probably do know it all. Otherwise, you might want to study what follows:
What follows comes from the references shown below. Where I think it helpful, I've added my own comments in blue.
Figure: Pacific Organized Track System (PACOTS), from Jeppesen En Route Charts.
Just because the route doesn't change on a daily basis doesn't mean you or your FMS will not make mistakes. The distance between ground-based navigation aids far exceeds those required by FAA Order 8900.1, Volume 4, Chapter 1, Paragraph 4-80.A, you must plot. More information: When is Plotting Required?.
A quick look at a few pilot blogs and postings confirms that few airlines require plotting over fixed track systems. Listening to the HF I've heard one or two chewed out for getting it wrong and I've heard from a POI or two that they get it wrong more often than we in the corporate world. (They get fewer violations because their companies pay most of the cost of operating the system.) But in this day and age, how are these navigation errors possible?
I think the leading cause of these navigation errors in complacency. More about this: Pilot Psychology / Complacency. But you could find yourself in trouble from another source. Chances are you are getting your oceanic waypoints via an uplink and those waypoints are coming from a computer. The computer got them from someone. If that someone made a mistake, chances are it will have been caught by another airplane before you. But what if you are the first after a database update? In the years since I've flown with uplinked waypoints I've caught three database errors, though none were oceanic. In my view, taking a few minutes to plot is a small price to pay to keep from hitting another airplane.
Figure: Standard High Altitude Service Volumes, from Aeronautical Information Manual, Figure 1-1-1.
You can have a navaid tuned and identified far outside its service volume, which means it doesn't count when making the plot / don't plot decision. Typical service volumes from [Aeronautical Information Manual, ¶1-1-8:
Plotting requirements have never been tied to Class I or Class II airspace, but the issue is doubly moot now that Class I airspace is virtually worldwide for aircraft with GPS. More about this: Airspace / Class I vs Class II.
[FAA Order 8900.1 Volume 4, Chapter 1, Paragraph 4-80.A]
[Advisory Circular 91-70A, ¶3-6.a.]
(2) Turbojet Operations. All turbojet operations, where the route segment between the operational service volume of ICAO standard ground-based NAVAIDs exceeds 725 NM, require plotting procedures.
(3) Turboprop Operations. All turboprop operations, where the route segment between the operational service volume of ICAO standard ground-based NAVAIDs exceeds 450 NM, require plotting procedures.
(4) The Administrator requires plotting procedures for routes of shorter duration that transit airspace where special conditions exist, such as reduced lateral and vertical separation standards, high density traffic, proximity, or potentially hostile border areas.
Where did these numbers come from? They appear to be arbitrary. The original version of this advisory circular came out September 6, 1994 and has the same 725 and 450 nautical mile requirements. There does not appear to be an ICAO specification, most international regulations recommend plotting procedures and North Atlantic regulations appear to assume they are being used. My earliest navigation manuals recommend plotting every 30 minutes or after a significant change in heading, speed, or altitude. My last few jobs where navigators were involved required a position plot every hour. As I continue to search for the evolution of the 725 number I remind myself that I don't know everything and if the book tells me I must plot beyond 725 nautical miles from the service volume of an ICAO standard ground-based navaid, that's what I do.
Figure: One-degree error example, from Eddie's notes.
[Advisory Circular 91-70A, ¶3-6.a.(6)] The FAA requires crews to use a plotting chart to provide themselves with a visual presentation of the intended route. Regardless of the type of LRNS in use, operators must use plotting charts. Plotting the route will increase SA and reveal errors or discrepancies in the navigational coordinates that flight crews can correct before such errors can cause a deviation from the ATC cleared route. As the flight progresses, plotting the position approximately 10 minutes after passing each waypoint helps confirm that the flight is on course. If the plotted position indicates off track, the flight may have deviated unintentionally and the flight crew should investigate at once.
In the example chart, the crew has made a one-degree error in the FMS. After ten minutes the plotted position clearly shows something is wrong, allowing the crew to fix things before they stray too far off course and possible in another aircraft's path.
|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.|
Figure: Gross navigational error report example, from an Eddie source.
These errors are caught . . . here is a two degree error by a G-V:
Figure: Plotting chart layout, from Eddie's notes.
Apart from saying the chart should be north up, must be based on WGS-84 and mean sea level, the chart printers are pretty much given discretion on how to print their plotting charts. There are some basic guidelines you should know for the chart you are using:
More about this: Technical / Navigation: Coordinates.
Figure: Position plotting example, from Eddie's notes.
There are many techniques on how to do this correctly, here is mine:
Figure: Position plotting example, from Eddie's notes.
For a tutorial on headings, courses, variation, and the like, see: Technical / Navigation - Direction.
Figure: Jeppesen Plotter, from Eddie's collection.
A plotter is nothing more than a circular instrument designed to give angular differences between lines. A navigation plotter, such as the Jeppesen model shown, will typically have a hole in the center of a compass rose. To use:
Note: You will have two choices aligned with the line of longitude, 085° and 265° in our example. In most cases it will be the number on top, but when dealing with courses near vertical it can be confusing. Always remember to give your answer a common sense check. In our example, we are headed to Europe and the answer should be generally easterly.
Figure: Dividers, step 1, from Eddie's collection.
[AFM 51-40, Page 5-8.]
If you have a set of dividers and a flat surface you aren't afraid to scratch, find the distance between waypoints is quite easy. First, place the points of the dividers on the start and end waypoints.
Figure: Dividers, step 2, from Eddie' collection.
Next, find a line of longitude near the course line. It is important to use a line of longitude about the same latitude as the course, since these will change over great distances. Each degree of latitude equals 60 nautical miles.
Figure: Post It Measuring Tool, from Eddie's notes.
If you don't have a set of dividers — do you really want to have such a sharp instrument in the cockpit? — you can construct your own with a PostIt note or other straight-edged paper. You will be accurate within a nautical mile if you do it this way:
Note: You can also determine the distance using a set of "10 Degree Tables," where we would see the exact distance is actually 369 nautical miles. (Our PostIt note was off by 1 nautical mile, or had an error rate of 0.27 percent, not bad. More about this: International Operations / 10 Degree Tables.
Figure: Starting vs Mid vs Ending Points, from Eddie's notes.
Your plotting chart is based on a Lambert Conformal projection, the lines of longitude converge near the poles. Except for the equator, the lines of latitude are not straight, they curve toward the equator. The measurement of your true course depends on where you place the center of your plotter and it does make a difference. In the figure shown, flying from 33°N 160°W to 33°N 150°W should, intuitively, require a 090° true course. The actual course, however, depends on what you want: the starting, mid, or ending course.
Most flight planning services offer either the starting or midpoint courses. Some pilots want to know what their initial course will be, others want the average course on the entire leg. It is a matter of personal preference.
I prefer using the starting course, since that is what the FMS will be showing prior to crossing the waypoint. (Following waypoint passage the course will update to reflect the "current" great circle route to the next waypoint. This course will constantly update as you progress, only reaching the midpoint course when you are actually at the midpoint.)
More about this: Technical / Navigation: Initial vs. Midpoint Course.
Figure: Variation Example, from Eddie's notes.
To determine the magnetic course, you will need to add or subtract the variation:
TC + West Variation = MC
TC - East Variation = MC
In our example, it appears our midpoint is very close to the "16°W" line of variation and no interpolation is necessary. Based on this we add 16 to our 270 true course and verify that our FMS and flight plan show a 286° magnetic course plus or minus a few degrees.
For a tutorial on magnetic variation, see: Technical / Navigation - Direction.
Note: It is not uncommon to find differences of 2 or 3 degrees in the magnetic course determined from a plotting chart and that reported in a computer flight plan. The magnetic variation changes over the years and your chart may be dated or hasn't been updated with the correct variation.
Figure: VOR/DME Plotting, from Eddie's notes.
There are many ways to turn a VOR radial/dme into a latitude and longitude, the best of these may very well be inside your FMS. If you need to do this on a plotting chart, this method works well:
If you are doing this to check the accuracy of your FMS, you might have easier tools at your disposal. More about this: International Operations / Navigation Accuracy Check.
Portions of this page can be found in the book Flight Lessons 1: Basic Flight, Chapter 23.
Portions of this page can be found in the book International Flight Operations, Part II, Chapter 5.
Advisory Circular 91-70A, Oceanic and International Operations, 8/12/10, U.S. Department of Transportation
Air Force Manual (AFM) 51-40, Air Navigation, Flying Training, 1 July 1973
ICAO Annex 4 - Aeronautical Charts, International Standards and Recommended Practices, Annex 4 to the Convention on International Civil Aviation, July 2009