VASIS intepretation

[fly_boy]

Had a discussion with a former heavy jet transport pilot the other day (we're now flying turboprop together) and he mentioned that we should fly a "3 red/1 white" approach to the VASIS to be on glideslope. The "2 white/2 red onslope" indication is meant for big jets due to a different sight picture from a much higher cockpit seating arrangement.
I haven't seen this written anywhere so could somebody point me in the right direction, pls.

[carbheatcold]

Fly_Boy

It is the filters set at the PAPI lights which determine the angle displayed, whether 3 or 4 deg and I am of the opinion anything other than 2whites/2reds means you are off the set G/S. Could he be confusing the speed differences between a/c types, ie a faster approach speed gives a greater rate of descent than a slower approach?
I am not sure but PAPI/VASIS info maybe specified in CAP168.

Hope it helps

Carbheatcold

[fokker]

Sort of right, BUT....

There is some truth in saying that owing to differences in length, deck angle and others, a T/prop or other small a/c MAY land further down the runway than it needs to in '2 + 2'.However, the only way to be sure that you meet obstacle clearance and performance criteria is to make a '2 red/2 white' approach every time.

When I was a t/prop FO, some used to give just the same advice. Ignore it. They're cowboys, it achieves nothing useful and may end up killing you. :eek:

BTW, it's just occurred to me while being picky..... they're not VASIs but PAPIs these days; VASIs were a quite different presentation and therein may lie the answer. I believe that VASI' COULD be 'flown' differently to accommodate different a/c. Perhaps that's where this comes from. Either way, I stick to the above.

[ 15 November 2001: Message edited by: fokker ]

[Eff Oh]

Next time you are on the ILS, look at the PAPIs. Then fly 3whites 1 red, again check the glideslope, see what difference it makes. I used to fly turboprops too and we always flew a 2 white 2 red approach. It is the ONLY type to fly. Often guys who have flown jets start going on about this kind of rubbish. The truth of the matter is...THEY ARE NO DIFFERENT to tuboprops! :)

As you know, as you get closer to the PAPIs the cone of operation reduces. ie The sensitivity of the lights increases. Perhaps he means this?? It is not as critical to get a turboprop down at the same point as a jet (on a short runway!) However if you are this close to the PAPIs you shouldnt be looking at them, but looking down the runway. :D

Eff Oh :cool:

[fly_boy]

Thanks all for the information, chaps.
Yes, I would agree that the onslope indication will clear you from the known obstacles as this coincides with the ILS on glideslope indications. Why the suggestion to fly "one light low" will be a mystery still.

[MrWoollie]

The statement may be valid depending on what type of approach lighting is in place.
There are significant differences between a T-VASIS and a PAPI.
If flying a T-VASIS it is important to look at what Runway Point of Intercept (RPI) and threshold crossing height is provided. For example at Amberley Australia RWY 15 has 1006ft and 51ft while RWY 33 has 626ft and 30ft. On a T-VASIS changing the fly-up/down indication changes the the TCH and RPI. Therefore, if flying a heavy onto RWY 33 you should fly a high indication or your TCH may be too low.
This is not true on a PAPI. ON a PAPI changing the fly-up/down changes your glideslope but maintains the same RPI and can be dangerous if the same techniques are applied.
There is a good diagram of the differences in the latest Australian Flight Information Handbook Australia (FIHA)page 271 with a warning on the next page ... "Since the meaning of a four white PAPI indication is significantly different to the same T-VASIS indication, pilots should verify which system is installed prior to conducting an approach."

The diagrams also contain a summary

PAPI
Aim points - SAME
TCH - Small differences

T-VASIS
Aim Points - DIFFERENT
TCH - Large differences possible (50 ft difference between 2 white fly up and 2 white fly down)

Hope this helps.

[Fuzzywozyone]

my dear fly_boy
I think your dear capt.has confused himself between the papi's and the T-VASI's as before 2 reds and 2 whites are the correct indication whatever the angle of your f/deck or a/craft 3deg (concorde/74/73/or a cessna)is still 3 deg,and if he refers you to the indication on your istrument,hes also mistaken because of some tx are fitted in different places,best of luck,keeping 2 whites and 2 reds

[Gaza]

At EGPH if I ignore the PAPI's screaming red's I can make the first turn-off.

Might have something to do with the fact I'm in a PA28 ;)

[Fuzzywozyone]

GOOD ON YOU GAZA
keep that up and someone will pull you out of a DITCH one day

[hvassk]

PAPI's have 4 lights in a horizontal row. VASI's have two (sometimes three) lights seen vertically. In a larger aircraft they'll fly the "upper" glideslope on a VASI if there are three lights. That means the smaller a/c (like you are flying) will use the lower glideslope; aka 2 red and 1 white.

[High Volt]

PAPI's and the Glideslope beam are set (to the best of my knowledge) to enable an aircraft with a very large eye-height to gear ie 747 to cross the threshold at 50' gear height. All things considered, touchdown then occours at the 1000' point. If the performance is geared up to get a fat plane down at the touchdown markers then you have plenty of grace in a smaller craft.

In my opinion, once visual he has a point in "shaving a bit off" because in a turbo-prop you are not likely to leave your gear behind in a field before the runway if you mess it up. Technically though, he is wrong.

Can't help you on precise location of said wisdom but for ultimate source perhaps try ICAO annexes (or Jepp text). Boeing fat plane pilot manuals give detailed advice on visual aim points and PAPI interpretation.

Top Tip: At night four pinks mean that you're in the slot.

[ 17 November 2001: Message edited by: High Volt ]

[Lucifer]

Gaza: light aircraft should not follow the PAPIs for a normal approach, and certainly Gaza if you see more than 2 reds, for a light a/c you are way too low. If you glance at them flying a normal light a/c approach, they would be about 3 whites and a red, however they can be followed for an instrument approach when the aircraft will be in a different configuration from normal approach anyway (less flap etc).

[NextLeftAndCallGround]

Fuzzy,

At EGPH I think you'd have to go off the side to end up in the ditch!! ;)

[DownIn3Green]

If you can see 'em, you're obvivously looking out of the window.

Fly the airplane (whatever the type), aim for the touchdown zone and put it there.

Admitedly, I've never flown a "Wide Body", which must take a different technique as all contractors seem to require at least "500" hrs in type.

[scanscanscan]

Christ!.... do not let 411A see this lot, he will have a bloody heart attack.
IMHO Rumour is....
The normal vasi is a different system and layout from the T vasi.
Both are different from the papi system.
Each is designed to do the same job for heavy and light aircraft types.
Each of these three seperate systems has to be interpreted differently by the pilot for heavy or light aircraft types.
Each system is shown differently on the charts. The reason for this is..So you know what, for which, you are looking, like for, on the visual approach!

Full details of which "you are looking, like, for," are in the Jepperson.

Maybe the UK Caa should teach this stuff as it is important and can kill you.