How Does Vasi Work?

Mon Apr 24, 2000 12:06 pm

Can anyone here explain how the VASI indication system is able to judge an aircraft's actual approach altitude relative to the proper altitude? Is it connected to the tower's radar system, does ATC personnel manually control it, or does it have some other method of judging altitude/distance from the runway?

Also, what happens when more than one aircraft are simultaneously approaching a single runway? Which plane would the VASI correspond to?

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RE: How Does Vasi Work?

Mon Apr 24, 2000 12:16 pm

The VASI lights have no idea where any aircraft are and it doesn't matter. Each set of lights is focused in such away so that if the aircraft is above a certain angle with respect to the lights, the pilot will see the white set. If the aircraft is below that certain angle the pilot sees the red set of lights. At most runways, a 3 degree descent angle will make the red set appear on the set farther away from the aircraft and the white set appear on the set closer to the aircraft.
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RE: How Does Vasi Work?

Tue Apr 25, 2000 2:16 am

Good explanation, Ralgha. I'd like to add that if more than one plane is approaching the same runway, the VASI corresponds to all aircraft who can see it. Your angle of descent determines what color light you see.
John Hancock

RE: How Does Vasi Work?

Tue Apr 25, 2000 3:24 am

There are usually 2 or 3 pairs of VASI Boxes either side of the runway. As has been explanined, 2 rows of white lights means you are too high, 2 rows of reg lights and you are too low. How is this achieved?

Inside each VASI box there are very intense lights, and coloured filters are placed over these - top half red, bottom half white. At the other end of each box is a slit, through which you see the lights - the geometry of the boxes is determined by the glideslope angle. Now just think about looking through the slit, if you are lower down, you see the top half of the filter (red) If you are higher up you see the lower, white section.

Other approach guidance systems are;
"T" type-VASI
Non-Standard VASI - French system (trust the French!)
PAPI - Precision Approach Path Indicator - Common in UK
PLASI - Pulse Light Approach Slope Indicator
CHAPI - "Chopper" system

I'm not going to explain all these because I don't have a spare 5 hours!

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