A-VASI or A-PAPI

[JABBARA]

Can someone explain the difference between VASI and A-VASI (Abbreviated-VASI) or PAPI and A-PAPI ?

Many Thanks

[square leg]

AVASIS are installed only on one side and are VISUAL approach slope indicators.

APAPI are installed only on one side and it stands for PRECISION approach path indicator.

The latter (PAPI) is installed on ILS approaches, the former (VASI) on runways that would typically have a circling approach.

A Circling is done when a precision approach cannot be flown because of the wind direction for example and the VASI would assist in the latter stages of the final to give one an indication of the glide path.

[fireflybob]

Not quite the full story.

VASI consist of two sets of light units, one which is upwind of the touchdown zone (ie beyond) and another which is downwind of the TDZ (ie before it). When the aircraft (or more accurately the pilots eye) is below the glideslope defined by an individual unit that unit will show red and when above white (there is actually a transition pink zone but that is in the detail). This means that when an a/c is on the correct approach path the further (ie top as seen by the pilot) VASI installation will show White and the nearer (ie lower unit) will show Red. When the a/c is below the defined glidepath all units will show Red and when above all units will show White.

Note airports which serve long bodied a/c have three sets of VASIS either side of the runway so a long bodied aircraft would like to see Red/White/White whereas other a/c want to see Red/Red/White.

PAPIS only consist of one set sited at the ideal touchdown point. This means that, as opposed to VASIS, because they are a point source aid they become more accurate the nearer you get to them which is why they were invented in the first place! VASIS are less accurate and there is more scatter wrt to touchdown accuracy.

When on the slope PAPIS should show 2 reds and 2 whites both sides of the runway. Extra Reds (one or two) mean you are low and extra whites mean you are high.

I hope this is clear - as PAPIS are more accurate then it is easier to align them with the ILS glideslope.

If you do a circling approach then you might end up using either VASIS or PAPIS - it all depends where you are and then there are T-VASIS in Spain still (I think) which is yet another system!

[A Comfy Chair]

The T-VASIS is prevalent in Australia, and I stand to be corrected, but I don't belive there are any VASIS in Aus.

The T-VASIS has the added benefit of allowing an adjusted Threshold Crossing Height. So, where a PAPI allows you to follow a path to a specific point on a runway, it can somewhat depend on what sort of aircraft you are in. EG - ILS aimed at 1000ft into the runway. This may suit one type of aircraft, but may not suit another. Another issue is the pilots eye height. As you can appreciate, in a 737 the pilots eye is similar to the height of the wheels, and so his eyes crossing the threshold will mean the weels cross at 50 ft (or so). In a 747, the pilots eyes are over 40ft above the wheels, so if he flies the same indictation, the wheels will only be 10ft above the threshold, which is too low.

The T-VASIS allows you to fly, for example, 2 dots fly down, and this will give you a higher crossing over the threshold. The PAPI doesn't allow this, as by flying one dot fly down on it, you are simply flying a steeper angle to the same point.

Having said that, PAPI's are the more accurate aid, and are probably the better option, assuming they are positioned for a suitable aiming point for touchdown.

A bit hard to describe without diagrams, but thats the gist of it anyway.

(P.S there is also an AT-VASIS, which is just the T-VASIS on one side of the runway only)

[seacue]

And then there is the VASI for Rwy 19 at KDCA (Washington National). The approach to 19 requires about a 20-degree turn about one mile from the threshold. See: http://www.airnav.com/airport/KDCA

quoting:
Visual slope indicator: 12-box VASI (3.00 degrees glide path)
VASI ON WEST SIDE OF RY 19 REALIGNED 20 DEG WEST OF RY CNTRLN. UNITS ON EAST SIDE OF RY 19 ALIGNED ALONG RY CNTRLN.

[WhiteKnight]

This is quite a good explanation of the T-VASIS principle .


http://www.airwaysmuseum.com/T-VASIS%20Pilots%20Notes.htm

[Charley]

Hmmm... at the risk of stoking controversy I would beg to differ with the definition of 'abbreviated' with respect to the VASI and PAPI system. The explanations as to how each system works is entirely correct however. But as far as I was aware (and I'm more than happy to be corrected if need be), the term 'abbreviated' is applied to each on the basis of the number of lights involved in each system, rather than whether it is deployed on each side of the runway or not.

A traditional PAPI consists of 4-lights arranged horizontally, whereas an abbreviated system consists of only two lights. Similarly, the VASIS consists of two pairs of lights (the upwind pair and the downwind pair) while the abbreviated system again consists of just two, the upwind light and the downwind light. I've been wrong before and could be again! ;)

A picture (http://www.famous5.com/appaids.gif) paints a thousand words, that one being my interpretation of things. Which seems to be backed up by these links:


Transport Canada (http://www.tc.gc.ca/CivilAviation/RegServ/terminology/glossary/a1.htm)
Here (http://www.aviatool.com/standards/slope.htm) is some lightweight technical gumph with angles.


Perhaps the confusion arises because of the T-VASIS system. That system appears on both sides of the runway and the AT-VASIS system becomes 'abbreviated' when installed on one side only.

And just to throw into the mix: the tri-colour visual approach slope indicator! Anyone seen one?

(edited for link problems and a missing space)

Cheers, Charley.

[JABBARA]

Thank you all guys! taking your time to respond the topic.

Charley, links were very helpful, thank you again.

[Spitoon]

CAP 637 is a good overview of the lighting and markings that you'll find at a UK aerodrome. It's a big download - 1.8mB - but very useful as a reference.

CAP 637 (http://www.caa.co.uk/docs/33/CAP637.pdf)

From what I can recall of VASIs, the A-VASI system only had lights on one side (left) of the runway whereas, as Charley says, an A-PAPI system uses only two boxes.