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Calling the Ball

Getting aligned with a runway in visual conditions is no big deal; it's intuitively obvious when your flight path is on the extended centerline of the runway, but vertical alignment along the proper slope is a different story and more complicated than you might imagine.

Getting aligned with a runway in visual conditions is no big deal; it's intuitively obvious when your flight path is on the extended centerline of the runway, but vertical alignment along the proper slope is a different story and more complicated than you might imagine. Sure, you can salvage a shallow approach by adding power, and you can always throttle back, put out 30 or 40 degrees of flaps, or do a forward slip if you're too high. At your home field, you've possibly done all of those things, despite knowing that the best landings usually follow stabilized approaches. After all, we don't exactly have to contend with the heaving deck of an aircraft carrier. But what happens when the runway, as well as the local geography, are less familiar? That's when mere embarrassment can become danger, and when those unfortunates fail to successfully "salvage" an approach they often become acquainted with the insurance industry's definition of that term.

Not Just Duct Tape, A Stick, And A Light Bulb In A Tube...
Arriving at an unfamiliar airport at night over featureless terrain is one good example of when a visual approach system is particularly welcome. Several thousand U.S. runways have some form of visual approach slope indicator (VASI) system. While their cost is only a small fraction of what an airport would pay for an ILS, their cost (somewhere around ten thousand dollars) is not exactly pocket change. You'd think it would be a simple matter to point a couple of narrow colored beams of light off the end of the runway to bracket a three degree angle, but it's a little more complicated than that.

Some History...
VASI's date back only to the 1960s. In 1984, the International Civil Aviation Organization (ICAO) switched over to its successor, the Precision Approach Path Indicator, or PAPI. Both ICAO and FAA testing supported this because unlike the VASI, which provides slope information only, the PAPI provides both slope and trend information. How? Here's how.

The Basics...
First, as you probably know, VASI installations come in two and three-bar form and originates from just two units ... or four, six, 12, and as many as 16, on one or both sides of a runway. (When it's as many as 12 or 16, they're usually on both sides.) The most common is two rows of two lights each, about 50 to 60 feet from the left side of the runway (though if terrain dictates, they go on the right).

The Box...
Each three-legged VASI box (technically called a Lamp Housing Assembly, or LHA) is positioned 16 feet from its neighbors, and may be powered up with the runway edge lights, via microphone clicks or a switch in the control tower. Many are just kept on all the time. The front end has an open slot, through which shine beams of light from three 2000-hour, 200-watt light bulbs, behind which are positioned red and white lenses. (These lights are usually controlled by a photoelectric device, to reduce their intensity at night.) When they're installed, the beam is aimed to within plus or minus one thirtieth of a degree by adjusting the single rear leg of the LHA box. Normally, each individual box projects a beam at an angle three degrees up from the horizontal -- it's not less for the "near one" and more for the "far one" -- and each has a white upper half and a red lower half.

What It Does...
As you can see from my accomplished artwork (obviously exaggerating the slope), this provides a middle zone wherein one sees that familiar "red over white; you're all right". (In the left-hand diagram below, each of the two boxes might actually be a row of more than one LHA.) When you're too high, both beams appear white; when the approach is too shallow, both would appear red. A three-bar VASI simply has two such zones, the upper of which is actually centered on a 3.25 degree slope and is intended for the lofty cockpit of a large aircraft such as a 747.

Figure 1

Limitations And Special Situations...
VASIs aren't supposed to be accountable beyond more than 10 degrees off the centerline, but they do promise an obstacle-free arrival from as far out as four miles from the runway threshold (even though they have an "acquisition range" of up to 20 miles away at night). There are also "T-VASI" configurations where the lights appear in an inverted T when above the proper approach slope, or as a red T formation when well below.

PAPI, The Latest Version...
PAPIs use a single row of the same kind of LHA boxes as a VASI (usually four in number, to the left), but in this installation, when you're on the money -- right at three degrees -- you should see two red outer lights, and then two white lights closest to the runway. The closest box sits 50 to 60 feet from the runway, and the boxes are between 20 to 30 feet apart. Go above the proper path, and you'll see more white -- at 3.2 degrees, it's three white and one red; and they'll all be white, once you're more than half a degree high. Get a half-degree low (a 2.5 degree slope) and they'll all be red. It's that gradual shift over that row of lights, which provides the "trend" information.

VASI To PAPI Differences
The difference from VASI installations (aside from the fact that PAPI lights are less likely to be left always on) is of course that each LHA is set at a slightly different angle. (In a "four box" PAPI starting with the unit closest to the runway, the standard aiming angles are half a degree high, one-sixth of a degree high, one-sixth of a degree low, and one-half degree low, respectively.)

Safety: PAPI boxes each have a "tilt" switch that will cause it to shut itself down if the angle decreases by more than one half of a degree (or increases by more than one degree). Many have the capability for remote maintenance monitoring -- a technician can check the system remotely, via modem.

Inside Information: VASIs and PAPIs are not supposed to be much good beyond four miles, so it's a good number to remember. You'll see many references say four nautical miles and ... that's wrong. According to the FAA Advisory Circular on the subject, 150/5345-28D, it's really four statute miles. Also, some VASI installations have glide slopes that are well over four degrees where high terrain dictates the need. And according to the Advisory Circular, PAPIs also have obstacle clearance only within four miles inside an arc 10-degrees to either side of the runway centerline, with an obstacle clearance surface angle only one-degree below the lowest on-course aiming angle, within that wedge of airspace.

Oddballs...
There are two other less common types of lighted approach path installations (only a few dozen examples of each reside in the entire US). Those types are:

Tri-color visual approach slope indicators. These are a single unit projecting three colors. If you're too high, you'll see amber; in the groove, it's green; and too low, red (of course), although during the transition from green to red, the color can appear as a dark amber. It's daytime range isn't much more than one mile, perhaps five at night.

Pulsating visual approach slope indicators. Also a single unit, the pilot sees a pulsating white light if high, a steady white light when on glide path, steady red when slightly low, and pulsating red when too low. Its range is four miles in daylight and 10 at night. According to the Aeronautical Information Manual though, one might mistake its blinking lights for other aircraft or ground vehicles, after dark.

Then There's The One With Duct Tape...
The so-called "alignment of elements systems" are the poor man's VASI. There are a hundred or so of these throughout the US. It usually doesn't involve anything more than three plywood panels, normally painted either black and white or fluorescent orange, placed in two "rows", with the center one behind and between the other two. (Some may be illuminated for nighttime use.) Their useful range is only about three-quarters of a mile. With these, the pilot simply positions the aircraft so that the elements are in alignment.

Figure 2

BOTTOM LINE: You might be able to make a squeaker at home when you know what's where, but it's still a good idea to use what's out there. On some dark night somewhere, you might be glad you did.

Ref: the Aeronautical Information Manual, Chapter 2, Section 1, Paragraph 2-1-2.

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