Issues with TILS directions?

[TLTeo]

So I've gotten back in the Viggen after a break, and I noticed that the instructions from the TILS seem to be slightly off. The aircraft is consistently guided to the right of the runway, and right before the threshold the indications move to the left. Screenshots attached. Am I missing something obvious?

 

 

[Machalot]

I have noticed this too and have been using the TILS indication only for pitch, while visually compensating for it in yaw. I thought it might be a function of nav drift, but I don't think it is.

[Snappy]

It’s stated somewhere in the manual, that the TILS guidance will not be exactly onto the runways centerline but slightly offset to one side, due to it being a deployable mobile system, as the mobile ground unit for the combined localizer & glideslope antennas is parked offset to one side of the runway’s beginning.

For obvious reasons it cannot be parked on the centerline ;) or beyond it (because then the Glideslope guidance would be wrong, which probably would present much larger issues)

 

I think traditional commercial airport ILS avoid this by using two antenna systems installations for localizer and glideslope , one offset to the side at the runways beginning for GS and one on the extended runway centerline beyond it for LOC guidance .

 

As for the late sideways shift of the guidance, I‘d have to check again , had not noticed it before myself , but I also was under the impression that you were to use visual landing mode for the final part of the landing once below cloud ceiling and with runway in sight.

 

Have to re-read the manual on this.Hope I could help a bit.

 

kind regards,

 

Snappy

[TLTeo]

That would make sense, if the system was guiding me at the right heading, but at a point off the centerline. The result from that is that as you go in you are more and more off the touch down point, but at long distances it should be more or less ok.

 

Instead, the instructions from a few km point quite nicely to the runway threshold, but at the wrong heading, as the first screenshot shows clearly. That incorrect heading is also always on the right side of the glide slope. That's why I suspect more than just the vehicles being off is at play.

 

edit: also, when you catch the TILS signal the nav system automatically performs a fix, so it's not drift either.

[Snappy]

Hi TLTeo,

 

no , I think theres a misunderstanding or more likely I formulated badly/incomplete in my explanation :

 

The ground unit that’s offset to one side of the runway will NOT transmit the localizer signal on the correct runway heading with only a slight parallel discrepancy.

 

With „Offset“ I meant, the localizer signal direction angle is offset, (by 3 degrees from runway heading) , so the antenna actually transmits its signal slightly „across“ the extended centerline of the runway.

 

The localizer signal actually intersects the runway centerline approx 900m before the runway threshold.

Thats why you get that sight picture from the cockpit in your screenshots.So at around a little less than 1km from the runway you would be exactly on centerline but heading is off by some 3 degrees if you would fly a perfect approach according TILS guidance.

 

I just read it up again, its page 140 of the RC2 Viggen manual, if you want to look it up.

Thats also the page where it mentions the system is / should not be used for the last part of the approach, due to the above system design .

 

kind regards,

 

Snappy

[TLTeo]

Aaaaah that makes perfect sense, so basically you should follow the glide slope with the expectation of making a turn ~1km from the runway. Thanks! That must have been interesting in the middle of a Swedish forest in dodgy weather...

[Eldur]

Eeyup. And it's always been like that. Fly the thing at 500m, Pitch down, when it does and as you close in on the RWY, you're always slightly coming in from the right, crossing the "extended runway" where you'll have to turn into it while staying on the glideslope.