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Plotting

Navigation

There is a fair amount of misinformation out there on whether or not we are required to plot in this day of GPS and glass cockpits. Most of the fault lies with us pilots, since we are always looking for the most effective (and easiest) way of getting the job done. But part of the blame belongs to the U.S. FAA who left something out of the most recent change to Advisory Circular 91-70. I fell for it too. So that's the first thing we need to tackle: You Have to Plot. But, in these days of glass cockpits and GPS, there is more than one way to plot.


Do You Have to Plot?

Yes, you have to plot. On April 6, 2017, there was a great deal of confusion when the 2010 version of AC 91-70A was replaced by AC 91-70B and some key language was replaced. But I have confirmed that the requirement has not changed.

Yes — Legally speaking

It says so, right here:

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Photo: "The Rule," from FAA Order 8900.1, Vol 4, Ch 1, ¶4-80

Click photo for a larger image

If all you read was the current Oceanic and International Operations advisory circular, you would be led to believe plotting is optional:

[AC 91-70B, ¶6.4.8.2] Up to now the only recommended method of cross-checking aircraft position in the oceanic airspace environment was manual plotting on a chart. However, a panel of aviation industry and FAA personnel completed an Operational Safety Assessment of methods for cross-checking oceanic flight navigation. The panel determined that an alternative to manual plotting, by which aircraft position could be checked through use of aircraft FMS-driven navigation displays and indications, would provide for an equivalent level of safety.

[AC 91-70B, ¶6.3.1.11.1] Plotting your route on your chart will increase your situational awareness as you execute your trip through oceanic and remote continental airspace.

[AC 91-70B, ¶6.3.1.11.1] You should use a chart, of appropriate scale, to provide yourself with a visual presentation of your intended route, regardless of your type(s) of long-range navigation system (LRNS). Plotting your route on your chart will increase your situational awareness as you execute your trip through oceanic and remote continental airspace.

[AC 91-70B, ¶6.3.1.11.2] Your chart should include, at a minimum:

  1. The route of your filed flight plan or currently effective route clearance.
  2. Clearly depicted waypoints using standardized symbology.
  3. Graphic depictions of all ETPs.
  4. Alternate airports.
  5. Proximity of other adjacent tracks.
  6. Note: For certificated operators, if OpSpec/MSpec A061 has been issued authorizing use of an Electronic Flight Bag (EFB) and the principal inspector (PI) has authorized “interactive plotting for oceanic and remote continental navigation,” the EFB application may be used in place of a paper plotting/orientation chart. The current edition of AC 120-76, Guidelines for the Certification, Airworthiness, and Operational Approval of Electronic Flight Bags, provides guidance for operators to develop associated EFB procedures. For part 91 operators, an EFB may be used, provided the criteria and considerations of the current edition of AC 91-78, Use of Class 1 or Class 2 Electronic Flight Bag (EFB), are observed.

[AC 91-70B, ¶6.3.2.6.3] You can use various additional techniques in order to verify that the correct points are loaded for your planned route. Verify the total route distance in your FMS against your master document to help find embedded mistakes. You should also cross-check course/headings and distances between each waypoint to ensure the FMS routing matches your master document. Referencing your plotting or orientation chart here can also be beneficial.

[AC 91-70B, ¶D.2.9.2] Ten Minutes After Waypoint Passage. Cross-check navigational performance and course compliance by one of the following methods:

D.2.9.1 The “plotting” method is appropriate for all aircraft navigation configurations.

  1. Verify your plotting/orientation chart reflects the currently effective route clearance.
  2. Plot your present latitude/longitude and record the time on your chart.
  3. You should plot your position using coordinates from the nonsteering LRNS.
  4. Investigate/take corrective action if your plotted position does not agree with your currently effective route clearance.
  5. Using the steering LRNS, verify the next waypoint is consistent with the currently effective route clearance.
  6. Verify your autopilot steering mode is in LNAV/VNAV or other appropriate mode to ensure steering to the next intended waypoint.

D.2.9.2 The “navigation display” method is appropriate for and available for use in aircraft equipped with an operable FMS:

  1. Confirm the aircraft symbol is on the programmed route on the navigation display (at smallest scale).
  2. Check system-generated cross-track deviation or similar indication of any deviation from the programmed route of flight.
  3. Using the steering LRNS verify the “TO” waypoint is consistent with your currently effective route clearance.
  4. Investigate/take correction action to address any anomalies or unexpected deviations.
  5. Verify your autopilot steering mode is LNAV/VNAV or other appropriate mode to ensure steering to the next intended waypoint.

Once this advisory circular was published, I wrote that the FAA was giving us latitude not to plot. Someone from the FAA called me soon thereafter to tell me that is wrong. Just because the language was removed from the advisory circular doesn't mean you don't have to plot. The language still exists in the 8900.1 series. I've had a few friends go through FAA table tops since then and they were both expected to be plotting experts. So legally speaking, the U.S. FAA expects you to plot.

Yes — Practically speaking

In the old days — when we were entering latitude and longitude into an inertial navigation system that didn't understand named waypoints — the typical reason given for plotting was the one degree error.

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Photo: The classic one-degree error

Click photo for a larger image

These days we are more likely to hear about oceanic re-routes that crews missed or entered in error.

  • Oceanic Reroute — Reroutes are common and except in rare combinations of aircraft/controller CPDLC capabilities, will require crews to manually enter FMS waypoints. I often read about crews that somehow mishandled the data entry. Plotting could have saved them.
  • An FMS error — yes, these happen. A few years ago the FMS makers didn't all agree on what nomenclature was needed for half-degree latitude separation and the result was chaos. The pilots who plotted caught the error. Another cause, albeit rare, is a coding error that waited until the database was updated. The first airplane to fly the route when the new database went into effect was caught. I've caught two such errors, but both were over continental Europe, not oceanic.

No matter the reason, I like to think of it this way. When you are flying domestically all of your airplane's wizardry is keeping you separated from other airplanes and air traffic control's radar is backing everything up. When you are flying oceanic you don't have ATC's radar to help you out. It is up to you to back up the airplane's computers.

When do you have to plot?

[FAA Order 8900.1 Volume 4, Chapter 1, Paragraph 4-80.A]

  • Plotting Procedures. Plotting procedures have had a significant impact on the reduction of gross navigational errors. There is a requirement to plot the route of flight on a plotting chart and to plot the computer position, approximately 10 minutes after waypoint passage. Plotting may or may not be required, depending upon the distance between the standard ICAO ground-based NAVAIDs.
    • Plotting procedures are required for all turbojet operations where the route segment between the operational service volume of ICAO standard ground-based navigational aids exceeds 725 NM.
    • Plotting procedures are required for all turboprop operations where the route segment between the operational service volume of ICAO standard ground-based navigational aids exceeds 450 NM.
    • 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, or proximity to potentially hostile border areas.
    • Any existing approvals that differ from the plotting requirements in this paragraph and Class II navigation procedures should be reviewed and revised as necessary. Direction and guidance is available from the navigation specialists in coordination with AFS-400.

What is the service volume of a navigation aid?

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Figure: Standard High Altitude Service Volumes (Aeronautical Information Manual, Figure 1-1-1.)

You could 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:

  • Standard High Altitude Service Volume between 18,000 and 45,000 ft: 130 nm
  • Standard Low Altitude Service Volume between 1,000 and 18,000 ft: 40 nm
  • Standard Terminal Service Volume between 1,000 and 12,000 ft: 25 nm
  • NDB HH Service Volume: 75 nm
  • NDB MH Service Volume: 25 nm

What about fixed airways?

I often hear from crews that want reassurance that there is no need to plot the routes between California and Hawaii because they are fixed and appear on their charts. They are wrong.

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Photo: The Hawaii "Romeo" Routes

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They are much more than 725 nm outside the service volume of the nearest navigation aid, they have to (and should) plot.

But what if I'm doing everything perfectly?

Just because you are doing everything "by the book" doesn't mean another aircraft's errors can't impact you. How is plotting going to save you from another airplane's error? Here is a scenario for you. Let's say you filed Mach 0.80 as did the airplane in front of you, but the airplane behind you filed M0.83. Shanwick Oceanic planned on all this by placing the minimum spacing between you and the lead aircraft and allowed extra space between you and the third. Now let's say there is an unforecast increase in the tailwind and all three airplanes start to miss their next forecast ETA. (You are early.) You and the third aircraft revise your ETAs, as required, while maintaining your filed Mach Numbers. The lead aircraft, however, slows down to avoid having to revise his ETA and there is a loss of separation between you and the lead aircraft. Shanwick will be asking for all three aircraft to provide their paperwork and having shoddy paperwork may implicate the wrong crew.

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Photo: Mach Number Technique

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Even if you've flown the route a hundred times (an example of a good pilot gone bad)

You don't know every quirk of your FMS and sometimes that can bite you. For example, the Gulfstream GV FMS originally had a 100 waypoint limitation; that's a lot of waypoints. Well, no, it isn't. It is easy to have more than 100 when flying through Europe. One of our TAG Aviation pilots in 2005 was an old pro at flying from London to White Plains, over and over again. On one of his trips they departed Italy instead. He simply downloaded the flight plan and headed west. The crew didn't plot, didn't check the FMS, and ran out of waypoints at 30° West longitude. At that point, the airplane turned directly to White Plains. How do you spell Gross Navigational Error? Plotting would have saved them.

How is a Common Plotting Chart Laid Out?

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.

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Figure: Plotting chart layout, (Eddie's notes)

[ICAO Annex 4]

  • 2.1.7 Recommendation.— The charts should be True North orientated.
  • 2.15.1 True North and magnetic variation shall be indicated. The order of resolution of magnetic variation shall be that as specified for a particular chart.
  • 2.15.2 Recommendation.— When magnetic variation is shown on a chart, the values shown should be those for the year nearest to the date of publication that is divisible by 5, i.e. 1980, 1985, etc.
  • 2.18.1.1 World Geodetic System — 1984 (WGS-84) shall be used as the horizontal (geodetic) reference system. Published aeronautical geographical coordinates (indicating latitude and longitude) shall be expressed in terms of the WGS-84 geodetic reference datum.
  • 2.18.2.1 Mean sea level (MSL) datum, which gives the relationship of gravity-related height (elevation) to a surface known as the geoid, shall be used as the vertical reference system.

There are some basic guidelines you should know for the chart you are using:

  • The horizontal lines are "parallels of latitude," usually a line for every degree with a major line every five degrees.
  • The vertical lines are "meridians of longitude," with a line for every degree with a major line every five degrees.
  • Longitude and latitude are subdivided into 60 parts known as minutes and labeled with a single quote mark (').
  • Minutes are further subdivided into 60 parts known as seconds and labeled with double quote marks (").
  • Bedford (KBED) airport, for example is at 42°28'11.8" N, 71°17'20.4" W; which is pronounced "forty-two degrees, twenty-eight minutes, eleven point eight seconds north, seventy-one degrees, seventeen minutes, twenty point four seconds west.

More about this: Navigation: Coordinates.

How to Plot a Position

There are many techniques on how to do this correctly, here is mine:

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Figure: Position plotting example, (Eddie's notes)

  • Ensure you are in the correct quadrant: In the north the latitudes increase as you go up, in the south they increase as you go down. In the west the longitudes go up as you head west, in the east the go up as you head further east. Locate the nearest five degree line in the general area of your point, in the example 30° N.
  • Locate the five degree line just above your latitude, in our example 35° N.
  • If your chart has one degree markings locate the nearest degree below your point, otherwise count the degree lines. In our example 33° N.
  • Count the tick marks between degree lines on the chart you are using. In our example there are six so we conclude each tick mark represents 10 minutes of latitude. Counting up four tick marks we identify 30°40' N latitude.
  • Notice that a line connecting 33°40' N 150° W and 33°40' N 160° W does not cross 33°40' N 155° W, it runs high. That's because the chart is a Lambert Conformal Projection which bends toward the poles. To get a more accurate position, we need to find the 33°40' point on the scale closest to our position.
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Figure: Position plotting example, (Eddie's notes)

  • When dealing with longitude, we look for the nearest five degree line under our desired position, in our example 155° W.
  • Then we look for the nearest five degree line greater than our desired position, in our example 160° W.
  • We can now locate the nearest degree under our position, in our example 156° W.
  • Counting the tick marks, we see there are six so 30' of longitude will be three greater.
  • Unlike latitude, lines of longitude appear parallel between lines of latitude, so we can draw a line at 156°30' W between tick marks and it will remain accurate.
  • We then transpose our mark identifying 33°40' N to this line of longitude and viola, we have our position.

How to Plot a True Course

For a tutorial on headings, courses, variation, and the like, see: Navigation - Direction.

Measuring Course

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Figure: Jeppesen Plotter, (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:

  • Decide if you want the start, mid, or ending course. Most pilots will need the start course. For more about this, see: Start / Min / End Point Differences.
  • Place the hole in the center of the compass rose over a line of longitude near the desired point, in our example we've used 40°W which is also the longitude of our start point.
  • Align the line along the 0° mark to your desired course. You may find it helpful to insert your pencil or pen point in the hole while rotating the plotter to line up with your course.
  • Read the true course along the line of selected longitude, 085° in our example.
  • Note: You will have two choices aligned with the line of longitude, 085° and 275° 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.

Measuring Distance

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Figure: Dividers, step 1, (Eddie's collection)

[AFM 51-40, Page 5-8.]

  • One of the disadvantages of the Lambert Conformal chart is the lack of a constant scale. If the two points between which the distance is to be measured are approximately in a north-south direction and the total distance between them can be spanned, the distance can be measure on the latitude scale opposite the midpoint. However, the total distance between any two points that do not lie approximately north or south of each other should be be spanned unless the distance is short. All distances should be measured as near the mid latitude as possible.
  • In the measurement of long distances, select a mid latitude lying approximately half-way between the latitudes of the two points. By using dividers set to a convenient, reasonably short distance, such as 60 nautical miles picked off at the mid latitude scale, you may determine an approximate distance by marking off units along the line to be measures as shown [in the figure].
  • The scale at mid latitude is accurate enough if the course line does not cover more than 5 degrees of latitude (somewhat less at high latitudes). If the course line exceeds this amount or if it is crosses the equator, divide it into two or more legs and measure the length of each leg with the scale of its own mid latitude

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.

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Figure: Dividers, step 2, (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.

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Figure: Post It Measuring Tool, (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:

  • Place the straight-edged paper along side the course. In our example, the PostIt note isn't long enough so we've used two, overlapping, notes.
  • Place a tick mark at the starting and ending waypoints.
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  • Move the straight-edged paper to the nearest line of longitude at about the same latitude. Place one tick mark over a convenient line of latitude, 49°N in our example.
  • Read the distance from the ending tick mark, using 60 nautical miles per degree. In our example the distance covers at least 6° of latitude, which comes to 360 nautical miles. The mark goes further by a tenth of a degree, meaning another 10 nautical miles. We conclude the distance between waypoints is therefore 370 nautical miles.

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: 10 Degree Tables.

Start / Mid / End Point Differences

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Figure: Starting vs Mid vs Ending Points, (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: Navigation: Initial vs. Midpoint Course.

How to Determine a Magnetic Course

True Course plus Variation equals Magnetic Course . . .

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Figure: Variation Example, (Eddie's notes)

To determine the magnetic course, you will need to add or subtract the variation:

  • Look for the nearest lines of magnetic variation on the chart before and after the midpoint and interpolate if necessary.
  • If the variation is West, add this value to the true course to determine magnetic course. If the variation is East, subtract this value from the true course to determine magnetic course:
  • 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: 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.

How to Determine the Position of a VOR Radial/DME

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:

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Figure: VOR/DME Plotting, (Eddie's notes)

  • Place the plotter hole over the VOR in question and your pencil point in the hole to hold the plotter centered over the VOR. (CON in our example.)
  • Find the nearest line of variation. (6° West in our example.)
  • Rotate the edge of the plotter to 360* in the northern hemisphere, 180* in the southern hemisphere, and then rotate toward the magnetic pole by the amount of the variation. (Rotate 6* to the west, in our example.)
  • Move your pencil from the center hole to the straight line of the plotter that describes the line to the magnetic pole, move the plotter so the straight edge connects that point to the VOR. Draw a line with a flag on it from the VOR in the direction of the pole. Label this line "360° Mag" if you like. (In our example the flag points slightly left.
  • Now return the plotter center hole to the VOR and rotate it in relation to your 360° Mag line by the number of degrees in the VOR radial. Draw a line as you did earlier by placing a tick mark, moving the plotter, and drawing the line. (In our example the line is 15° clockwise from the Magnetic north line.)
  • Determine the distance using the Distance Measuring techniques shown above. Place a tick mark on the VOR line. (015°/85 DME in our example.)
  • The latitude and longitude can be read directly from the chart using the Plot a Position techniques shown above. (55°15'N 8°25'W in our example.)

If you are doing this to check the accuracy of your FMS, you might have easier tools at your disposal. More about this: Navigation Accuracy Check.

References

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

* This version of AC 91-70 has been superseded but it retained because it contains older guidance that helps place current guidance into perspective.

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

Aeronautical Information Manual

Air Force Manual (AFM) 51-40, Air Navigation, Flying Training, 1 July 1973

FAA Orders 8400 and 8900

ICAO Annex 4 - Aeronautical Charts, International Standards and Recommended Practices, Annex 4 to the Convention on International Civil Aviation, July 2009

Revision: 20190125
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