papis? how do they work??

[flyfish]

Just curious really; how do PAPIS,work.Sorry
if i have spelt it incorrectly.I mean the landing aids with the changing lights,red/white.
I have recently started flying, and used them a couple of times,its just bugging me on how they are able to assess your height.
(perhaps i should get out more !! ).

[Dan Winterland]

Each PAPI light is like a slide projector with a red bottom segment white top segment. The four lights are set at a different angle, so that when you are on the correct glideslope you see two reds and two whites. The slope set corresponds with the ILS glideslope angle and will lead you to the radar touchdown point, usually 1000' in. If it isn't an ILS runway, the PAPIs are set at 3 degrees.

When you are on the slope, the red PAPIs are next to the runway. That may not seem important, unless you are a CAA examiner. I was asked that in my air law exam, and couldn't remember despite having stared at them regularly for over ten years!

[shakespeare]

The thing to remember with PAPI lights is that they bring you to the same touchdown point. i.e. 2 whites and 2 reds = 3 degree glide path. 3 reds and 1 white (flown constantly) reduces the angle but you will touch down at the same point. The same applies flying 3 whites and 1 red, however with a higher angle.

Hope that helps!

[flyfish]

Thanks for the answers chaps,as with everything , if you understand how something works, using it practically always seems easier.Cheers.

[reynoldsno1]

Don't forget that if they appear green, then the lights are shining through the grass and you are VERY low....:?)

[Fokjok]

Just a point of interest, which I hadn't realised until a while ago:

PAPIs go quite far out of true, quite quickly. This is hardly surprising considering that they are precision optical devices placed very close to a bit of tarmac which people keep throwing heavy bits of metal onto at high speed.

PAPIs are regularly re-aligned by a simple process using a sighting device a short distance in front of the installation.

This is one reason why the GS is sometimes not completely co-incident with the PAPIs, and a mention of this to the tower will enable them to send the man out with the kit to put the situation right.

[Luftwaffle]

Also, they misread when there is ice or frost on them, so beware of PAPI indications when using pilot-activated lighting on cold days/nights.

[Yoeman]

Precision Approach Path Indicator (PAPI) was invented in England .. the name Tony Smith is often associated with this invention .. which is probably true ..

PAPI was adopted by the International Civil Aviation Organization (ICAO) and was to replace the older Visual Approach Slope Indicator (VASI) by 1995 ... this was based on the premise that the life of VASI was 15 years, so that when it came time to replace VASI, it could be done with PAPI ... however, the reality was that some countries did not have funding arranged to do this, so VASI continues at many sites ...

PAPI is the same as VASI in the sense of the light units ... all that has been done is to relocate the units to the "theoretical touchdown point" and to vertically align the four units A, B, C, and D separately at about 20 minute increments of A=2'30", B=2'50", C=3'10" and D=3'30" ... the units B and C define the "approach corridor" with which the approach path (slope) is (B+C)/2=3'00" ... pilots fly the "approach corridor" (two whites and two reds) and not the approach path since the path itself is not seen ...

the PAPI signal is not coincident with the ILS path ... it can only be made to be approximately close to the ILS path through increasing the width of the approach corridor which is indicated by the color signals ... the PAPI is a visual aid

information on PAPI is available in the ICAO Annex 14, FAA advisory circulars and Transport Canada manual TP312 ... as well as in the pilot flight manuals ...