Downhill from here

The key to a smooth landing, we're so often reminded, is a stable approach. This involves correct pitch and power, which means not allowing airspeed to decay or letting the aircraft sink below the approach path. We also have to keep the airplane aligned with the runway centerline and maintain directional control. Unless we're careless, distracted, or oblivious, it's usually obvious when our flight path is not tracking the extended runway centerline.

Vertical alignment along the proper slope, however, isn't always so apparent...the average approach angle is a very small number. For most visual approaches, as well as the glideslope on a typical instrument approach, it's only about 3 degrees. That's roughly the angle your eyes would make if you walked to the edge of a 100-foot-wide runway and looked over to the other side. Why 3 degrees? Although there are exceptions, generally the risk of a rougher landing increases with steeper approaches. And the risks soar for shallower ones.

When the time comes for a landing at an airport that is new to you, you'll no longer have the comfort of familiar references. (According to the AOPA Air Safety Foundation's 2002 Nall Report, more than a third of all accidents happen during the approach and landing.) That is why you'll be glad for amenities like the visual approach slope indicator (VASI) or precision approach path indicator (PAPI).

At your home field, you might salvage a shallow approach by adding power, or salvage an overly steep one by throttling back, adding more flaps, or performing a forward slip. But when the runway and the local geography are totally unfamiliar-and especially at night or over featureless terrain-additional help comes in handy.

VASIs

Several thousand U.S. runways have some form of VASI system. While their cost isn't negligible (somewhere around $10,000) and is only a small fraction of what an airport would pay to install an instrument landing system (ILS), that's still 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 3-degree angle, but it's a little more complicated than putting some light bulbs in tubes and connecting them to sticks with duct tape.

The principle of the VASI involves two things, the first of which is obvious: color differentiation between red and white. Each light unit projects a beam of light having a white segment in the upper part of the beam and a red segment in the lower part of the beam. VASI installations come in two- and three-bar form. A VASI can be just two units, or four, six, 12, or as many as 16, on one or both sides of a runway. The most common is two rows of two lights each, about 50 to 60 feet from the left side of the runway. (If local terrain requires it, however, they can go on the right.)

Each three-legged VASI box (called a lamp housing assembly) is positioned 16 feet from its neighbors, and can 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 containing red and white lenses, through which shine beams of light from three 2,000-hour, 200-watt light bulbs. These lights are usually controlled by a photoelectric device, reducing their intensity at night.

The beam is aimed to within plus or minus one-thirtieth of a degree by adjusting the height of the back of the box. Normally, each box projects a beam at an angle 3 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. This provides a middle zone wherein one sees that familiar "red over white; pilot's delight."

That brings in the second functional factor behind this and any other means of visual vertical guidance-the "power of parallax." When you're too high, both beams appear white; when the approach is too shallow, both would appear red. When you are on the correct path you see red over white.

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 Boeing 747. (Note that these light beams are not laser-like; they appear to be "beams" simply because the light sources are recessed inside a box, the openings of which have a precisely defined orientation.)

Although the results are similar, a VASI differs from the way that an ILS glideslope works. With a glideslope, the only visual presentation is a cockpit instrument's depiction of the aircraft's position relative to an invisible radio beam. The full-scale deflection of the glideslope needle on the instrument panel is just seven-tenths of a degree above or below the typical 3-degree angle. With a visual approach such as the VASI or PAPI, you simply see with your own eyes whether you're too high, too low, or right down the middle. VASIs aren't supposed to be usable beyond more than 10 degrees laterally, either side of the centerline, but they do promise an obstacle-free arrival from as far out as four miles from the runway threshold (even though they can be seen from as far as 20 miles away at night).

Some runways have "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. It is important to remember that the VASI system is certified to only four miles from the runway threshold. Using the VASI when you are farther from the runway does not necessarily assure terrain and obstacle clearance on the approach.

PAPIs

Unlike the VASI, which provides slope information only, the PAPI provides both slope and trend information. PAPIs use a single row of the same kind of lamp housing assembly boxes as a VASI (usually four, to the left of the runway), but in this installation, when you're on the money-right at 3 degrees-you should see two white outer lights and then two red lights closest to the runway.

Go above this path, and you'll see more white. At 3.2 degrees, it's three white and one red; they'll all be white once you're more than half a degree high. Drift a half-degree low (a 2.5-degree slope) and they'll all be red. It's that gradual shift across that row of lights which provides the trend information.

The difference between PAPI and VASI installations is that each of a PAPI's LHAs 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.) Also, each PAPI box has 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 1 degree), and many have the capability for remote maintenance monitoring-this allows a technician to check the system via modem.

Again, don't bet the farm on VASIs and PAPIs beyond four miles. (You'll see many references that say four nautical miles, but according to FAA Advisory Circular 150/5345-28D, it's really four statute, miles.) Also, where high terrain dictates the need, you may find VASI installations with glideslopes that are well over 4 degrees. According to the AC, PAPIs also provide obstacle clearance only within four miles inside an arc 10 degrees to either side of the runway centerline, with an obstacle clearance surface angle within that wedge of airspace only 1 degree below the lowest on-course aiming angle. (A similar "obstacle identification surface" exists for an ILS, at 1 degree below the vertical navigation path between decision height and the touchdown zone, bounded by rays splaying 10 degrees to either side and originating from a point 1,000 feet beyond the runway threshold.)

Oddball installations

There are two other less common types of lighted approach path installations (only a few dozen of each in the entire United States). The first is the tri-color visual approach slope indicator, which is a single unit projecting three colors. If you're too high, you'll see amber; at the proper angle, you see green; and if you're too low, red (of course). During the transition from green to red, the color can appear as dark amber. Its daytime range isn't much more than one mile, perhaps five at night.

The other is known as a pulsating visual approach slope indicator. On final approach, 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. It is a single unit as well, and its range is four miles in daylight and 10 at night. According to the Aeronautical Information Manual, though, after dark one might mistake its blinking lights for other aircraft or ground vehicles.

And finally, there is a visual indicator that employs plywood and, perhaps, duct tape. This is the so-called alignment of elements system that really is the poor man's VASI. There are a hundred or so of these throughout the country. It usually doesn't involve anything more than three plywood panels, normally painted either black and white or fluorescent orange, 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.

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 whatever tools are available to you. On some dark night somewhere, you might be glad you did.

Jeff Pardo is an aviation writer in Maryland with a commercial private pilot certificate for airplanes, and instrument, helicopter, and glider ratings. He has logged more than 1,200 hours in 12 years of flying. An Angel Flight mission pilot, Pardo has also flown for the Civil Air Patrol.

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