Archived

This topic is now archived and is closed to further replies.

Alcides Segovia

VASI inaccurate

Recommended Posts

Hello everyone. 

 I have been flying FS since 2004. I have gone through quite a learning curve ever since I started. I remember when I discovered that planes fly normally at 28000 or above for cruise. I used to level off at 3000  :lol: .

 

 Today I come because I have discovered something else that is troubling me. 

 I normally fly PMDG NGX. CS 757 and Level D 767. Those are my aircraft to fly. My landing descent rate averages about 250 ft/M. 

 I normally fly the approach as a intercept to an instrument approach then about 7 NM out I disengage and continue following the glide slope and the Localizer. 

 I always try to land at the TDZE or the White Blocks.. Anything over that for me and it seems to me like by the time I flare and finally touch down it will eat up alot of runway and normally ends me in the lines but way to far from the blocks in my opinion.

 

 The thing is that both my ILS and VASI are showing me below the glide slope at about 500 ft above the ground when I am approaching the runway. 

 

 So my question is. When do I stop following the ILS or the VGSI?

 

 Do I strictly follow it regardless and land where ever it takes me to land as long as it is safe to stop or can I brake off when I see prudent to land at the TDZE.?

 

Thanks

Share this post


Link to post
Share on other sites
Help AVSIM continue to serve you!
Please donate today!

I think your observation is absolutely valid.

 

You should disregard both (VASI and ILS) when below 200 ft AGL.

Share this post


Link to post
Share on other sites

I don't have enough RW experience to say, but, in FSX, I do disregard the VASIs when I'm over the approach lights.  If I didn't I'd be way down the runway before I touched down.  I just keep the runway 'picture' looking the same and fly it down at the correct speed.

Share this post


Link to post
Share on other sites

I should say this...in theory at least, if you fly the VASIs to the runway, you would flare right as you're abeam the VASIs so you might float for a while.  In most cases, for myself, I'm not trying to 'paint the numbers' but I do like to put it down at a reasonable spot for a decent rollout.

Share this post


Link to post
Share on other sites

Think about the tools you're using - ILS is an instrument system, meaning eyes inside until visual, say theoretically you're visual at the minima you're just going to land the thing. Even in the real world PAPI and VASI don't always align perfectly with the ILS glideslope. To the previous example, you've flown the ILS and become visual at 2000' AGL, you can see the runway and the PAPI/T-Vasis, as you are now visual, you can fly accordingly, use the PAPI and T-Vasis over the instruments.

 

Different tools for different purposes, both perfectly legal, do what you like, but then it will come down to company policy, if you're following the ILS and have on slope indications but the PAPI show 4 red, are we going around?

 

But yes when you transition from the approach phase to the landing phase, around 300' is when you're more than likely committed to land, is a good time to pick your touchdown point and plant it there.

Share this post


Link to post
Share on other sites

It seems that (from my recollection, not a statistically significant sample or anything like that) it is not uncommon for the approach plate to state that the ILS is not coincidental with the visual glide slope indicator.

Take the now infamous Rwy 28R at KSFO. The approach plate states clearly that the ILS and VGSI are not coincidental. Also shows the ILS touch down to be 1,091 ft from the end of the (11,870 ft long) runway. So for an ILS approach the landing distance available is 11,870 - 1,091 = 10,779 ft. But the published LDA is 11,570 ft (consistent with the identified 300 ft displacement). So - for an ILS approach the glideslope puts you 791 ft farther down the runway than you might make if you were doing a visual landing and touched down exactly at the displacement point.
 

The thing is that both my ILS and VASI are showing me below the glide slope at about 500 ft above the ground when I am approaching the runway.

 

If both are showing you below the glideslope than short of an error in one or both systems then you are below the glideslope.  Maybe this is the case - because you state that you are descending at about 250 FPM - at first I thought that was your touch-down vertical speed, which is about right, but now I'm thinking that you are too low because your entire descent is at -250 FPM so naturally you would be below the glideslope the entire time since this equates to a descent angle of a bit less than one degree vs. the ~ 3 degree angle you should be descending at.

Is this a specific airport / runway where there seems to be a discrepancy?

Share this post


Link to post
Share on other sites