KMSY ILS29 GS Too High

KMSY ILS29 IHOX aircraft indicates on glideslope when crossing runway threshold at 220 ft above runway. Your glideslope is located incorrectly. The glide path appears correct but you seemed to have placed the glideslope near the centroid of the runway. According the FAA NASD the glideslope should be at N29 59 27.9656 W090 15 16.7865. I have not decompiled your AIRAC 2202 rev 1 to confirm the misplaced GS but the measurement at the threshold is the tell.

AIRAC 2202 rev. 1, we use following coordinates for the GS antenna:
ILS29 (IHOX) 29.99110278, -90.25466389

That is (according google maps) - exactly the position of the GS antenna:

Comparing with the FAA values:
29.99110155, -90.25466291

… and here, last the coordinates from the stock data:
29.991102665662766, -90.25466367602348

All three positions are exactly the same. Sorry, I can´t see any difference here.

The most common reason for this is a wrong runway threshold (and the MSFS has a lot of such wrong thresholds). Also in this case.

The runway-center-position for 11/29 is according the MSFS stock data
29.99292690306902, -90.26634827256203

The runway-length is 10104 ft (approx. 3079 meters), that means the threshold should be (from the center point 5052 ft away.

Distance to the threshold RW11 - 5043 feet

Distance to the threshold RW29 - 4737 feet

That means when I take really the half of the runway-length (5052 feet), the threshold for RW29 is here (red line):

Thats approx. 300 feet before and thats the reason why you mean you´re too high. The center-position of the runway 11/29 is misplaced/wrong … there are approx. 430 airports with ILS approaches, where this is the case.

It´s not a navdata issue - as you see the navdata are correct - you also see the runway threshold positions (directly from the Jeppesen source), also correct.

By the way, that same should happened with the stock navdata because when they really use the FAA data without any “transformation” you should end it to high too.


Thank you for the quick response Richard. Yes, I agree with you and the explanation. I have wrestled with that runway center point from the beginning of my testing with ADE alpha. Note that the VASI are also located as offsets from the runway center rather than the threshold so if the center is wrong then the whole kit and kabodle is wrong LOL.

Minor note: My measurement above threshold was above your green line, the Jeppesen point, where typical AAL is 50 ft; however, this is a CAT I approach so the chart doesn’t provide the threshold crossing altitude. My brain doesn’t understand how I can be at 220 ft when the threshold relationship to the GS antenna is correct unless that is the way the approach was designed from a TERPS perspective.

As well as FSX and all P3D versions…
The only solution here for having a correct altitude profile in the simulator is to recalculate a “fake” glideslope antenna position from REAL GS offset distances from threshold and apply them to the simulator runway data. Certainly not easy to widely implement (because it requires some mathematical calculations) but can solve the most significant discrepancies

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