Emergency Do-Overs & Dynamic Learning

By Scott Spangler on January 18th, 2010

FAA_Safety-Team A subscriber to the FAA Safety Team notices, I immediately open and read emails with “Emergency Notice” in the subject line. On Saturday, January 16, Emergency Notice NOTC 2101 said:

“The FAA has been made aware of an issue with an update to a navigation database that became effective on 14 January 2010.  To date, the known affected models are: Honeywell Bendix/King KLN 35, KLN 88, KLN 89, KLN 89B, KLN 90, KLN 90B, KLN 94, KLX 100, KLX 135, KLX 135A, KLN 900.”

It provided a link to a Honeywell website for more information. It described the problem succinctly in bold red letters:

After production and distribution of the Cycle 1001 databases (effective date 14-Jan-2010), Honeywell was informed by our data supplier, Jeppesen, that the data file delivered to Honeywell contained incorrect Dynamic Magnetic Variations for all terminal and en route waypoint records. For that reason, it is imperative that the incorrect databases NOT be used for arrival, departure, or approach operations.

In other words, airplanes with avionics that use the affected database are grounded until pilots download and install the corrected replacement. Fortunately, correcting a database error is a lot easier and expedient than correcting the same error on paper charts. It’s a pain, for sure, but imagine the inconvenience if pilots had to wait for new charts?

Magnetic variation is a term familiar to all pilots, the result of incorrect data is clear, an increased chance of metal meeting something hard, with an unfortunate outcome. But what in the heck is Dynamic Magnetic Variation?

FAA-IPH On my first pass through Google I learned that Dynamic Magnetic Variation plays an important (but not clearly understood) role in the operation of MRI machines. Interesting, but not what I was looking for. I had better luck in the Instrument Procedures Handbook, FAA-H-8261-1A, which was better suited to my dynamic knowledge quest.

The glossary says: “Dynamic Magnetic Variation – A field which is simply
a computer model calculated value instead of a measured value contained in the record for a waypoint.”

Appendix A on Airborne Navigation Databases provides a better explanation. Specific points on earth—airports, navaids, waypoints, and intersections—are known as “fix records.” Smaller than a Tweet, each one contains 132 characters divided into fields, some of which are common to all such as lat, long, and magnetic variation.

Because avionics and instruments use this data to display magnetic courses in the cockpit, how the database deals with magnetic variation depends on the type of fix. For airports  “the magnetic variation is given as the difference in degrees between the measured values of true north and magnetic north at that location.” A VOR navaid fix record uses “Station Declination,” which is “the angular difference between true north and the zero degree radial of the navaid the last time the site was checked.” And waypoint fix records use “Dynamic Magnetic Variation,” a computer model calculated value instead of a measured value.”

unique fields 

The subject gets even more interesting later in Appendix A, under the heading Issues Related to Magnetic Variation. I already knew that the amount of variation changes over time because stuff is slowly moving—drifting—deep in the earth. But aviation database publishers apparently don’t use annual drift values, they acquire their variation data from NOAA in the “ Epoch Year Variation” format. “Theoretically, this value is determined … and published for public use every five years.”

In some cases, it seems, “the variation for a given location is a value that has been calculated by the avionics system. These “Dynamic Magnetic Variation” values can be different than those used for locations during aeronautical charting.” (Still, it’s computer based, so GIGO–garbage in, garbage out.)

Thankfully, the manual clarifies this: En route, “magnetic courses are computed by airborne avionics using geodesic calculations based on the latitude and longitude of the waypoints along the route. Since all of these calculations are based on true north, the navigation system must have a  way to account for magnetic variation.” Scott Spangler


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One Response to “Emergency Do-Overs & Dynamic Learning”

  1. MPetey Says:

    Fascinating. Does Honeywell currently offer a software fix or a date when one will be available?

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