Tri-bar VASI vs. PAPI

[3WE]

Supposedly with a two-bar VASI, jumbo jets, on a "proper" descent path came exremely close to snagging stuff just short of the runway.

Why doesn't the PAPI (one single bar) have the same problem?

I think I know the answer, but thought I'd check for confirmation.

1) If you happened to fly the closest VASI light, right on the edge of being too low, you are flying a 3-degree slope to 500 feet from the threshold- not the TDZE. (and if you fly the middle one, right on the edge it's 3 degrees to 1000 feet and the far is 3 degrees to 1500 feet).

2) Conversely, the PAPI is locaed 1000 feet from the threshold.

Bottom line is that the VASI has 500 feet of slop on where your descent path intersects the runway, while the PAPI leads you "only" to the 'target touchdown point'

Yes? No? Maybe?

Footnote- The VASI doesn't really tell you if you are on a precise descent angle per se- just that you are within a reasonable window, wheras the PAPI is giving you some angle information- though not YOUR descent angle.

[Peter Kesternich]

As the name suggests, the Precision Approach Path Indicator is more precise. It incorporates five different stages (going from 4 reds to 4 whites) as compared to four on the 3-bar VASI. Since the PAPI lights are in the same location, close to the 1000ft marker, which typically is the beginning of the touchdown zone, it is easier for the pilots to take in the information at a glance, since there view doesn't need to shift.
Both systems are set to specific angles, as far as I know, which can vary from airport to airport, but normally wouldn't change depending on which aircraft is on short finals.

[3WE]

As the name suggests, the Precision Approach Path Indicator is more precise....

That doesn't really answer my question.

The problem with the VASI is that the wheels of a jumbo jet are running well below and behind the pilots eyes- so the third bar was added to make the pilots aim further down the runway- so the wheels hit "more where we want them to".

The PAPI doesn't fix that factor- it's aiming you to the exact same place and the jumbo's wheels are still hanging down there a good distance behind and blow the pilots eyes- but this time- there is only one approach path- and that would tend to make jumbo jets touch down sooner than smaller planes...

Shouldn't the PAPI aim a jumbo further down the runway? The glide slope antenna's are specifically located near the wheels to address "the pilots eye" factor!!!!! But how can it? It can't- it's a set of lights in a single place....it would seem to be putting jumbo jets down too soon.

I guess the accuracy might be an indirect fix- that earlier touchdown + VASI slop can equal some broken threshold lights and scuffed piano keys-whereas an early touchdown done accurately is better....... but I was hoping to dig deeper.

I'm wanting to hear that "yeah, the PAPI does lead a jumbo to touch down earlier, but the issue was that the VASI closest to the runway could guide you to X distance........

(Maybe some real numbers or something and discussion that the PAPI doesn't really fix the eye vs. wheels thing).

[MCM]

OK - well to start with, yes you're right, the PAPI does not necessarily fix the eye vs wheel thing.

The first acronym we need is "minimum eye height", or MEHT.

This value changes between different PAPI set ups, and is a critical piece of information for a pilot. It varies based on exactly where the PAPI is located relative to the threshold. Remember on slope usually gives a three degree path (but can be different).

The MEHT is the minimum height over the threshold that will give an "on slope" indication (ie 2 red, 2 white).

At airport A, the MEHT is 74ft. This means that the lowest the pilot's eyes can be is 74ft above the threshold given 'on slope'. As you correctly identify, the subsequent height of the wheels is then dependent on how high the pilot's eyes are above the gear.

So, in the 767 for example, the pilot sits around about 24ft above the wheels. Which means that at the lowest part of 'on slope', his wheels are 50ft above the threshold. In the real world, 74ft is a little high for the 767 - but you get the idea. In the real world with a PAPI at 74ft the correct three degree path to the touchdown zone in this aircraft would result in just about seeing three red lights on short final. The 747, with its higher wheel to eye height, would see on slope.

At airport B, the MEHT is 50ft. This would mean that a 767 pilot, with an on slope indication, could have the wheels cross the threshold at 26ft. Bit on the low side. In this case, flying the correct visual aim point, the pilot would see three white lights as you neared the runway, rather than the two and two.

The VASI is a little different. The VASI is usually aligned so the first two bars work for 1000', and the second two are for larger aircraft (but have a higher approach angle). They can result in longer landings - but at least will keep your wheels clear.

The key is that pilots have to take that into consideration when using such an aid. But at the end of the day, these indicators are guides to assist a pilot fly to a visual aim point.

[3WE]

Thank you.