PAPI LIGHTS

[Maverick2167]

Just a random question- is there a value or thumb rule where the perceived PAPI lights will change based on your ground speed or distance if you fail to follow the correct glide path? What I am trying to infer here is say, you're on the correct 3-degree glide path (visual or ILS) and PAPI shows 2REDS 2whites, and then a sudden increase, and constant Tailwind follows, assuming you don't compensate your VS for it, and don't bother to see the glide slope indicator slowing moving down. At what point or distance will the PAPI show 3whites and 1 Red or vice versa?

[B2N2]

https://flightlight.com/app/uploads/2017/04/PAPI-Instruction-Manual-ZA737-757-Style-A-RevC.pdf

Thats the reference I found the fastest.
Your answer is on page 7.
It’s not just distance by the way, it’s angle and distance.
At 10 miles and 3000’ AGL you’re on a three degree angle and should see two white two red. At the same 3000’ AGL but 20 miles you’ll see 4 red.
Looking for a runway VFR at night you’ll generally pick up the 4 red before you’ll see the approach lighting system.

[Maverick2167]

https://flightlight.com/app/uploads/2017/04/PAPI-Instruction-Manual-ZA737-757-Style-A-RevC.pdf

Thats the reference I found the fastest.
Your answer is on page 7.
It’s not just distance by the way, it’s angle and distance.
At 10 miles and 3000’ AGL you’re on a three degree angle and should see two white two red. At the same 3000’ AGL but 20 miles you’ll see 4 red.
Looking for a runway VFR at night you’ll generally pick up the 4 red before you’ll see the approach lighting system.
Thanks a lot, let me check it out

[alf5071h]

A quick web search could have directed you to a Wiki page; in particular see the external references:-
https://en.wikipedia.org/wiki/Precision_approach_path_indicator

PAPI is a very adaptable and versatile system which can be applied to many situations and uses.

The system can be set up for a range of transition angles - angular difference between individual boxes, which a pilot might relate to the 'tightness' of the combined beams - how easy is it to use. These settings depend on location and chosen aircraft type / operation.
These values will also require adjustment for steeper values of glideslope - consider the vertical distance between transitions as the GS angle increaces.

The system should be harmonised with other approach aids - see extensive previous Pprune discussion. This area has to consider aircraft types, threshold crossing height, and in some instances length of runway.
Thus any attempt to find a simple relationship will depend on knowledge of the specific installation - there will not be a rule of thumb.

A better strategy is to learn about the system, practice using it, and when able experiment with GS deviations, including speed and height deviations; noting that transitions become progressively tighter with decreasing height - as related to range. i.e. because of the nature of angular systems the geometric relation will change according to the distance from the origin.

[compressor stall]

Have a think about the MEHT for your aircraft and that of the runway.

There’s more potential variability there than anything you’ve suggested.


edit… removed the word route. Fat fingers and autocorrect.

[Maverick2167]

Have a think about the MEHT for your aircraft route and that of the runway.

There’s more potential variability there than anything you’ve suggested.
What I'm trying to deduce here (which was not clear in my initial post) is whether there is any rule of thumb based on my aircraft GS (consider a medium wake aircraft) to calculate at what distance or time on a normal 3-degree glideslope, will I see a change in lights i.e. from the lights going from 2RED 2 WHITES to 3 RED or 3 whites, if the glide was not maintained.

Also, Considering the Glidepath of the PAPI and ILS glideslope exactly matched, will I see the change at half dot deviation of the Glide slope indicator, or since the PAPI is a visual reference it would require a little more time to show or perceive the change?

[STBYRUD]

Well - as per usual definition three white/three red should normally show if the deviation from the glide path exceeds 0.17 degrees, and four white or red at a deviation greater than 0.5 degrees. Thus - a dot on the glide slope equals (assuming PAPI and GS transmitter are colocated and aligned, and set for a 3 degree path) four red or four white.
​​​​​​
​​​​​​Then again none of this is secret knowledge, and can be found within seconds with a Google search...

[alf5071h]

Maverick, you must add the PAPI setup transition angle between each beam to your assumptions.
Even then, whatever relationship you determine between PAPI and ILS, in practice when descending and deviating from the required GS you have to consider both vertical and horizontal components of flight path, which in turn depend on range. Thus it is difficult to determine which component of flightpath relates to the change.

The advantage of PAPI over most other visual guidance systems is that it provides a sharp transition between colours. This because each beam uses a lens to focus the colour separation - like an old style picture slide projector, and therefore is very precise.

[321XLR]

Interesting from FAA FAR/AIM: https://www.faa.gov/air_traffic/publications/atpubs/aim_html/chap2_section_1.html

4 NM ?


https://cimg3.ibsrv.net/gimg/pprune.org-vbulletin/1402x367/vasi_f9c57bbd93726ca5b341ad6a535d4f084844489f.png

https://cimg9.ibsrv.net/gimg/pprune.org-vbulletin/1470x332/papi_73b88433c07b21360e499d876955ee7a64800bd2.png