Aviation Asked by Michael Hall on September 26, 2021
I hope this isn’t a dumb question, but I haven’t flown a whole lot of GPS approaches and today I observed something that I cannot explain: There was a 2 degree split between the Desired Track (DTK) displayed on the GPS, and the inbound course shown on the approach plate.
If it matters, the approach was the RNAV 16 Y approach at KRNT, and I was using my club airplane’s Garmin GTN 650. The database in the GPS was up to date, and my approach plate in Foreflight was current. DTK showed 159, and the plate showed 157.
Not a big difference, but they ought to be the same, right?
At first I thought it might be displaying track direct from my present position, but I had early turned the fix a little coming from the west, and was lined up a bit right which would have made the course needed to correct a few degrees less than 157.
And as I made minor course corrections back and forth across the final approach course the DTK never wavered.
Is there a good explanation for this difference that I’m just not aware of?
Here is the final answer handed down directly from the Garmin technical support department. After a conversation with one of their aviation technicians, he told me that the error seems to be in their database. The same discrepancy (a track of 159 instead of 157) appeared whenever he reviewed the KRNT 16 RNAV Y approach from his end. He even rolled back the database to check previous versions and it has been this way for quite awhile. When asked about a possible magdev discrepancy, he said they they acquire all of their deviation data from the same governmental source that the chart makers do. He said that they are releasing a magdev update in the near future that MIGHT fix the problem, but said that it was most likely just entered wrong in their database. He opened a case file to make sure it is addressed and said that he will notify when it has been taken care of.
Correct answer by Aaron Holmes on September 26, 2021
@Ralph J was on the right track with the hidden decimals behind the degrees. This should solve the mystery:
You’ve probably noticed that GPS course guidance is often a degree or three different from the courses shown on the chart. The reason is the magnetic variation used by your GPS, the chart designer, and the VOR defining an airway may all be different.
Approach designers calculate to the fraction of a degree. This means a transition route might start at 141.4 and end at 141.2. The whole route appears as 141 on the chart, but those tenths of a degree measured at the actual turning fix might be the difference between a legal 89.9-degree turn, and a forbidden 90.1-degree turn.
The article is entitled A Bit Over Ninety written by Jeff Van West at IFR Magazine.
Answered by Aaron Holmes on September 26, 2021
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