VASI vs PAPI

N18040

N18040

Well-Known Member
I was looking through part 91 today, and was wondering if anyone could shed some light on way 91.175 permits you to descend below the MDA if you have the VASI in sight, but says nothing about PAPIs. I can't really come up with a situation where you could see the VASI or PAPI, and not see any of the other things listed, so this doesn't seem like it would have too much practical application, but I was just curious as to why they would let you descend with the VASI, and not the PAPI.
 
N18040 said:
I was looking through part 91 today, and was wondering if anyone could shed some light on way 91.175 permits you to descend below the MDA if you have the VASI in sight, but says nothing about PAPIs. I can't really come up with a situation where you could see the VASI or PAPI, and not see any of the other things listed, so this doesn't seem like it would have too much practical application, but I was just curious as to why they would let you descend with the VASI, and not the PAPI.
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I believe the term "visual approach slope indicator" is generic; it can apply to any form of VASI or PAPI configurations.

And really, on an approach down to minimums with limited approach lights (or none at all), the PAPI or VASI will penetrate better than anything else you'll see. They're often the first things you see, followed usually by the threshold lights or REILs.
 
N18040 said:
I was looking through part 91 today, and was wondering if anyone could shed some light on way 91.175 permits you to descend below the MDA if you have the VASI in sight, but says nothing about PAPIs. I can't really come up with a situation where you could see the VASI or PAPI, and not see any of the other things listed, so this doesn't seem like it would have too much practical application, but I was just curious as to why they would let you descend with the VASI, and not the PAPI.
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According to 91.175....

First off.... you may descend to 100 feet above TDZE if you have the red side row bars or red terminating bars in sight. The systems that have these are the ALSF-1 and -2.

(i) The approach light system, except that the pilot may not descend below 100 feet above the touchdown zone elevation using the approach lights as a reference unless the red terminating bars or the red side row bars are also distinctly visible and identifiable.

To descend from MDA and land though, the requirements are:

(ii) The threshold.
(iii) The threshold markings.
(iv) The threshold lights.
(v) The runway end identifier lights.
(vi) The visual approach slope indicator.
(vii) The touchdown zone or touchdown zone markings.
(viii) The touchdown zone lights.
(ix) The runway or runway markings.
(x) The runway lights.


Of course, what you're referring to.... VASI, encompasses all "systems" that are found on the left or right side of the runway that provide a visual glide path.
 
dasleben said:
I believe the term "visual approach slope indicator" is generic; it can apply to any form of VASI or PAPI configurations.
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That is what I was wondering, but in the AIM 2-1-2, VASI and PAPI are listed seperately under the heading of "Visual Glideslope Indicators. If they meant it to include both, why didn't they just say Visual Glideslope Indicators instead of VASI?

That letter of interpretation also seems to agree that the PAPI doesn't count, but does anybody have any reasons why it wouldn't count?
 
N18040 said:
That is what I was wondering, but in the AIM 2-1-2, VASI and PAPI are listed seperately under the heading of "Visual Glideslope Indicators. If they meant it to include both, why didn't they just say Visual Glideslope Indicators instead of VASI?
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It's a government bureaucracy.

That letter of interpretation also seems to agree that the PAPI doesn't count, but does anybody have any reasons why it wouldn't count?
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I have no reason, why or why not it doesn't count.

However, the visual slope indicator on precision runways is by the touchdown zone. In the few really hard approaches I've shot, I don't remember seeing that before I see other stuff.

Maybe on an approach without lead in lights of some sort you'd see that first, but man I'd be hard pressed to think of that scenario. Maybe a non-precision approach of some sort with a broken base of clouds.
 
Polar742 said:
It's a government bureaucracy.
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Agree. Makes no real sense. I've always used whatever is there, good enough for me.

I have no reason, why or why not it doesn't count.

However, the visual slope indicator on precision runways is by the touchdown zone. In the few really hard approaches I've shot, I don't remember seeing that before I see other stuff.

Maybe on an approach without lead in lights of some sort you'd see that first, but man I'd be hard pressed to think of that scenario. Maybe a non-precision approach of some sort with a broken base of clouds.
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I never did see VASIs as the first thing in approaches I remember. Even on a 100-1/2 PAR, aside from the ALS, it was usually the edge lights or REILs first before the VASI/PAPI/OLS whatever popped up.
 
MikeD said:
Agree. Makes no real sense. I've always used whatever is there, good enough for me.



I never did see VASIs as the first thing in approaches I remember. Even on a 100-1/2 PAR, aside from the ALS, it was usually the edge lights or REILs first before the VASI/PAPI/OLS whatever popped up.
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I don't even know that we'd look outside once the C123 is up and running anyway. Feel the ground effect, pull the power, stop. Probably where we'll be flying into won't have area for a GA either, so we're landing anyway.

Probably lit by smudge pots.

:beer:
 
Polar742 said:
I don't even know that we'd look outside once the C123 is up and running anyway. Feel the ground effect, pull the power, stop. Probably where we'll be flying into won't have area for a GA either, so we're landing anyway.

Probably lit by smudge pots.

:beer:
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Smudge pots....thats full up! Need those and that "alignment of elements" type VASI system.....ever seen one of those? Definitely low cost.

As for 91.175......FARs what? No application where we'll be going.
 
I always like to pull out my FAR/AIM just before minimums to double check that what I see will make me legal to complete the approach....
 
MikeD said:
I never did see VASIs as the first thing in approaches I remember. Even on a 100-1/2 PAR, aside from the ALS, it was usually the edge lights or REILs first before the VASI/PAPI/OLS whatever popped up.
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I've mostly noticed this at night. I've noticed that on certain approaches, the PAPI or VASI lights cut through the fog much better than other lights (assuming no ALS). A great example is the ILS 15L into BWI. 400' DH without approach lights; it's been my experience that the first thing you see is the PAPI system, at least at night.
 
dasleben said:
I've mostly noticed this at night. I've noticed that on certain approaches, the PAPI or VASI lights cut through the fog much better than other lights (assuming no ALS). A great example is the ILS 15L into BWI. 400' DH without approach lights; it's been my experience that the first thing you see is the PAPI system, at least at night.
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Excellent. Thanks for that.

I figured one of ya'll have needed it. I just couldn't think of a situation where I saw that first.
 
I shot a VOR approach in moderate rain and saw the PAPI before the REILs or threshold lighting.
 
bLizZuE said:
I shot a VOR approach in moderate rain and saw the PAPI before the REILs or threshold lighting.
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I imagine it does depend on the particular WX conditions, coupled with the intensity settings. Interesting seeing the differences.
 
I went in to McAlester, OK a couple of weeks back and there was a pulsing VASI! Or throbbing, whatever you call it. I was trying to remember back to my PPL written to know which thing meant what.
 
SpiraMirabilis said:
I went in to McAlester, OK a couple of weeks back and there was a pulsing VASI! Or throbbing, whatever you call it. I was trying to remember back to my PPL written to know which thing meant what.
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PVASIs are easy, and a simple low-cost system. KDVT had them on the north runway, but they may be gone now. Pulsing white, steady red, pulsing red (or alternating red/white) that quickens in intensity the lower you get. The AF actually evaluated these at one time and found them to be more accurate than a 2-box VASI system and the same as a PAPI.

Tri Color VASIs are still at a few airports around too.
 
I can't remember which airport in southern Alabama I flew into, but it had a freakin "alignment of elements"...I was like WFT?! Had to go check the AIM to make sure it was there, and by God, it was...
 
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