Papi / Vasi

[2close]

I'm hoping one of you learned folk can point me in the right direction. I've searched the web but can't find the technical information I'm looking for.

I'm trying to find out at what point (DME / Altitude) PAPI and VASI, etc. becomes critical to flight safety.

Is there a maximum and / or minimum distance from TDZ that the aids are designed to be effective at?

Under what circumstances are these aids beneficial to flight safety? Obviously, they are of no use in IMC conditions. In a non instrument VMC approach where are they factored into the safety margins? I presume during night approaches where the paths may have obstacles.

Grateful for any pointers.

Cheers,

2close

[Zeffy]

FAA AIM 2-1-2:

http://www.faa.gov/airports_airtraffic/air_traffic/publications/ATpubs/AIM/Chap2/aim0201.html#2-1-2

"The visual glide path of the VASI provides safe obstruction clearance within plus or minus 10 degrees of the extended runway centerline and to 4 NM from the runway threshold."

Exceptions are noted on charts and/or airport facility directories -- e.g. "VASI Rwy 31 use limited, unavbl byd 4° rgt of centerline due to trees."

[chevvron]

PAPI's at my airport are supposedly useable from 8nm. I've tried them from 6 with no problem, but then I am talking of an approach speed in the order of 55-60kt!

[2close]

Thanks guys.

Much appreciated.

Interesting that the FAA state the VASI is designed to be used from 4nm.
The PAPI at my own field is also easily usable from 6nm and on a 3 deg slope that would equate to a 1824' height AGL.

As a significant number of Precision Approaches start from 2000' AGL and the PAPI is designed to be a supplementary aid to glideslope navigation I calculate that the PAPI should therefore be visible from approx. 6.6 nm for a 2000' Final Approach to landing and 8.2 nm for 2500'.

Does this sound about right?

2close

[Spitoon]

There's a spec for PAPI in ICAO Annex 14 Vol I Chapter 5. After quickly skimming it I can't find an answer for you but there are references to more details in other docs - maybe it's there but I didn't see it or it's in the other docs. A couple of relevant points are:
5.3.5.28 The system shall be suitable for both day and night operations.
5.3.5.32 Suitable intensity control shall be provided so as to allow adjustment to meet the prevailing conditions and to avoid dazzling the pilot during approach and landing.
It also talks about obstacle clearance criteria for a code 4 non-instrument runway going out to as much as 15km.

I'm no IAP designer either so I don't know the tie up between DH/MAP and AGL but I think there is a relationship which may include PAPIs - I guess it'll be in Doc 8168.

Don't know much about VASIs either - I thought they had long since been replaced as a standard by PAPI. I noiticve Annex 14 talks about T-VASIs but I've never come across them in my part of the world.

Not much help I know, but maybe some pointers to something of interst?

[Zeffy]

The FAA 4nm limitation is predicated on an obstacle survey, not a distance from the runway at which the VASI lights are visible.

In other words, there could be a mountain ridge at 4.2 nm from a runway, extending well above the angle of the VASI setting without a requirement for a warning on charts or in facility directories.

Scottsdale, AZ has 4.0 deg PAPI's.
http://www.naco.faa.gov/pdfs/sw_30_23NOV2006.pdf
On one of those spectacular desert nights with unlimited visibility, the lights of the city/airport/PAPI may be in sight at higher altitudes well beyond 20 miles to the Northeast. But I assure you that if one lines up with the runway 21 centerline at 20 miles with all four PAPI lights WHITE/WHITE/WHITE/WHITE and quickly descends to intercept the PAPI path, the lights at the airport will disappear due to the presence of terrain. (Adjust the descent profile or prepare to eat the big dirt sandwich).:ooh:

Airports located at the bottoms of mountain canyons frequently have VASI/PAPI's for only one end of a runway -- not due to budgetary issues, but due to significant terrain on or near the approach.
For example, runway 07 at Eagle County, Colorado - KEGE
http://www.naco.faa.gov/pdfs/sw_195_23NOV2006.pdf
Runway 13 at Hailey, Idaho - KSUN
http://www.naco.faa.gov/pdfs/nw_16_23NOV2006.pdf

I hope this clarifies -- obviously, there are significant differences between viewability and obstacle protection.

[chevvron]

Makes me wonder why PLASIs never caught on for smaller airports. Much easier to interpret, only one unit to look at (and for the airport operator to maintain!)