Inside the FMS
Lines of latitude and longitude have been the cornerstone of navigation for hundreds of years. Even with the incredible leaps in technology, from triple-installation hybrid IRS to satellite-based navigation with augmented position inputs, an entry of latitude and longitude in the Flight Management System (FMS) can still be a necessary part of a flight plan.
Further complicating the issue, shorthand inputs and abbreviations have emerged to simplify entries, but we’ll look at how they may add confusion. Combined with the overall infrequency of lat/long inputs, it can be difficult to remember how to enter latitude and longitude correctly using degrees and minutes.
It’s worth pointing out that the FMS does not recognize seconds, but minutes can be entered in tenths or hundredths by inserting it after a decimal (N32°20’30” would be N3220.5). The purpose of this article is not to reteach everything you’ve learned from your early days in ground school; the purpose is to review making latitude and longitude inputs into the flight plan. This article will review:
Think of the Globe in Quarters
To begin, remember the earth is divided into quadrants (think of an orange sliced into fourths). The earth is divided based on the Equator and the Prime Meridian (See Figure 1).
|Figure 1 – The Globe Divided by the Equator and Prime Meridian, (Wikimedia Commons)|
Any point north of the equator can be referenced as north latitude and south of the equator as south latitude. The Prime Meridian works the same way only in the vertical axis. West of the Prime Meridian becomes west longitude and east of it becomes east longitude. Looking at it this way, the coordinate system begins to make sense (See Figure 2).
|Figure 2 – Lines of Latitude and Longitude, (Wikimedia Commons)|
Make Sure to Include All the NEWS
Latitude and longitude can be expressed in several formats depending on the source. Some basic rules: latitude precedes longitude and the N,E,W,S cardinal headings describe which quadrant the coordinates apply to. Latitudes range from 0 to 90° and longitudes range from 0 to 180°. Lat/longs can be written in several formats (see Figure 3) but usually consist of degrees and minutes.
|Figure 3 – Various Depictions of a Lat/Long Position, (reproduced with permission 30West IP)|
Because the FMS uses dd°mm.mm’ format, the seconds are converted to minutes by simply dividing them by 60. For example, N33°20'30" is converted to N33°20.5' (30”/60 = .5’). So if the defined waypoint has dd mm’ ss” it will need to be converted to tenths (and/or hundredths) prior to entering in the FMS (See Figure 4).
|Figure 4 – Lat / Long Entry for N33°20’30” and W111°52’30”|
Once placed in the flightplan using a line select key, the MCDU will display the lat/long as a temporary waypoint using the format *LL01, where *LL means it’s a lat/long and 01 means it’s the first temporary waypoint in the flightplan. Additional entries will be sequentially numbered. (See Figure 5).
|Figure 5 – Lat / Long Entered in the Flight Plan|
Lat/longs will also appear with the same format on the MFD (See Figure 6).
|Figure 6 – Lat / Long depiction on the MFD|
It is important to note that lat/longs can be abbreviated during the entry, but doing so causes the FMS to make assumptions that may not be what the pilot had intended. If a leading or trailing 0 is omitted, the FMS may misinterpret the desired location. Let’s look at the following example that was provided by a customer. The crew wanted to enter N21°02.6’ and W86°52.0’. Can you see what happened here? The crew entered and received the following (Figure 7):
|Figure 7 - Entry Did not Produce Desired Lat/Long of N21°02.6’ and W86°52.0’|
The result shown above is not what the crew had intended. This is because when entering degrees and minutes, the pilot entered 3 characters instead of 4, leaving the FMS to determine where the additional character (in this case a 0) should be placed. When the pilot abbreviates rather than entering the full coordinates (13 characters), the FMS must make assumptions based on the number of characters entered before and after a decimal point. In the example above, it inserted a leading 0 and came up with a valid but different coordinate more than 1000 miles from the intended location. What they should have entered was (See Fig 8):
|Figure 8 – Correct Entry for N21°02.6’ and W86°52.0’|
For this reason, Honeywell recommends always entering the full lat/long coordinates (6 characters for latitude and 7 for longitude). The position should then be verified against the MFD and the Master Document.
When It’s OK to Take the Shortcut
Another option when entering waypoints in oceanic regions is the ARINC 424 shorthand. ARINC 424 shorthand is an additional way of simplifying lat/longs. Its primary purpose is to name the commonly used waypoints for oceanic crossings to reduce the risk of gross navigational errors. Data providers create named waypoints using a five-character combination of the lat/long with a N, E, W, or S deliberately placed to define the quadrant in which it’s located (See Figure 9). They are coded in the navigation database just like a conventional waypoint and can be retrieved when the pilot enters the appropriate name. Some points worth mentioning: the FMS is not converting the shorthand to lat/long, it is a defined waypoint just like any that would normally be entered in the flightplan. Also, not all oceanic waypoints are in the navigation database (to save space), so if the FMS doesn’t recognize the shorthand entry, the crew will have to enter it manually using the full lat/long format described above.
|Figure 9 – ARINC 424 Shorthand by Quadrants, (reproduced with permission 30West IP)|
Twice the Fun, Half the Spacing
One final piece of information to complete the article. A little over 5 years ago, half-degree grid waypoints were implemented in the North Atlantic to increase efficiency in the core tracks and reduce spacing. These are discussed in detail in the ICAO NAT OPS Bulletin, but for the purpose of our discussion it’s important to be aware of the existence of half-degree coded waypoints on charts and in the Nav Database. The basic difference is the cardinal headings (North and West) are removed from the shorthand and the letter “H” precedes the abbreviated lat/long. For example, North 53°30’ W040°00’ would be coded as a half-degree waypoint with the label H5340 at 3L(See Figure 10).
|Figure 10 – Example of Half-Degree Waypoints|
In summary, latitude and longitude entries can be entered either by using the full latitude and longitude or by using the ARINC shorthand for oceanic waypoints that are in the navigation database. When entering manually, Honeywell recommends entering the full coordinates in the formats listed above and cross checking carefully against the Master Document. For more information or specific questions, please email FTS@Honeywell.com.
Program Pilot David Rogers supports EPIC and NG FMS-equipped Cessna and Gulfstream aircraft for Honeywell Flight Technical Services. He can be reached via email at David.Rogers@honeywell.com.