Originally Posted by
MechEngr
Ahh, true heading - OK.
What is unclear is why there is also a connection to latitude and longitude.
I looked at the airports - based on data from
https://www.airnav.com these are the deviations in degrees from 270:
82V -0.009377087
KBJJ 0.001984966
KCIU -0.024015687
KCNM 0.017476942
PABR -0.00926391
AirNav provides Lat/Long for each end of the runway which I converted with ATAN to a degrees variation. It did not list the last two.
SKLM, is listed as runway true heading: 272.4 on
SKLM - Jorge Isaac Airport and
SYCJ is listed at 271.9
So it's a mystery. While I can imagine truncating the true heading to one place would be a problem, I can't see where they would truncate the true heading by more than 2 degrees.
It probably depends which FMS database you use - I've just run some numbers from the one I use (not AirNav) and the true headings for the 7 runways in question range from 269.968° to 270.012°.
As for the FAA's assertion that "only seven runways worldwide, as identified in this AD, have latitude and longitude values that cause the blanking behavior", I suspect that only runways of over a certain length (5000 feet?) at civil airports have been considered. There are around 150 runways in total with true headings in the above range, but once short runways and military fields are excluded (and, for some reason, airports with parallel runways) there are only about a dozen left worldwide.
I don't think that mag variation has anything to do with the criteria.