THE AEROMEXICO DISASTER : Planes Fly Complex Web of Southland Airways

The air space above Southern California contains a network of airways and control zones even more complex than the freeway web that marks the ground.

And it has the same purpose: The dominant feature--the Los Angeles Terminal Control Area, known as the TCA--is intended to allow air traffic to move with maximum efficiency, while preventing in-flight collisions of aircraft taking off or landing at Los Angeles International Airport.

Two aircraft, a jetliner and a single-engine private airplane, collided Sunday apparently in the Los Angeles TCA. The experts are trying to discover how such a thing could have happened.

Pilots are taught to visualize the Los Angeles TCA as an “inverted wedding cake,” with its smallest layer at ground level around the airport. Above this, the other layers are stacked in wider and wider ovals, representing higher and higher altitudes.

An airliner making a landing approach flies along a clearly defined line descending through those layers.

Because of this, it is considered safe for other aircraft either to fly above the incoming airliner’s highest authorized altitude or below the lowest.

A government aeronautical chart, updated every six months, depicts the various altitude layers, and the parts of the Los Angeles Basin that they cover.

It shows that Los Angeles International’s TCA “wedding cake” is not really round but elongated east and west because the airport has only east-west runways.

It is huge--52 miles long; 24 miles wide--and separated into 12 slices or zones, each with its own specific lower and upper altitude limits called the “floor” and “ceiling.”

There are no signposts in the sky to tell pilots where they are or where the various zones begin and end. But charts used by pilots make it relatively easy to locate them and navigate through them, either by visual reference to such ground landmarks as freeways, rivers, reservoirs and airports or electronically through use of on-board navigational systems.

Rules for using the Los Angeles TCA are simple and ironclad.

There is a speed limit: No airplane (jet airliners included) is permitted to go more than 230 m.p.h. (200 knots) in the TCA.

And you need the right equipment: That means a two-way radio that can be tuned to air traffic control frequencies, another radio able to receive navigation signals called VOR and a transponder equipped with an altitude encoder that enables the ground controller to spot your position and how high you are flying.

But you do not need all that equipment to fly in the Los Angeles Basin. As long as a pilot keeps out of the TCA (by staying above or below it) and out of other various control areas in the vicinity, he does not need to talk to ground controllers except when making a takeoff or landing, and even that can be avoided if he uses one of the basin’s non-tower-controlled airports.

The pilot of the ill-fated Piper PA-28 Archer that began its final flight at Torrance Airport Sunday needed radio to get off the ground, however. Torrance is a tower-controlled airport, and the pilot needed the ground controller’s permission to taxi to the runway.

Then he had to switch frequencies in order to get the tower controller’s permission to move onto the runway and to take off and to get the controller’s approval for the direction in which he would leave the vicinity of the airport.

Torrance is under the Los Angeles TCA. But the floor of that particular slice of the TCA is 5,000 feet, so the Archer’s movements at any altitude below that would not have interested TCA controllers.

The little plane would have been required only to maintain contact with the Torrance tower--and that only while it remained within five miles of the airport and less than 3,000 feet above its elevation. After that, it would have been perfectly legal and usual for the pilot to switch frequencies or even turn down his radio.

Making a normal takeoff (to the northwest) and turning east, the Archer would have been about eight miles from Torrance Airport before it could reach an altitude of 5,000 feet. But by that time, it would have been just west of the Long Beach Freeway, flying beneath a gap in the TCA (a small bite out of the cake to accommodate Long Beach Municipal Airport) and still not required to talk to anyone on the ground unless it strayed below 3,000 feet.

The TCA begins again about a mile west of the San Gabriel Freeway (Interstate 605), and here the floor is 6,000 feet and the ceiling is 7,000.

It was apparently within this wedge-shaped zone of air space that the two aircraft collided.

Theoretically, it should have been easy to stay out of the TCA. The pilot has only to remain below 6,000 feet until he is four miles past Fullerton Airport, a landmark clearly visible from the air, and be careful not to go more than a mile north of the Artesia Freeway, lest he enter TCA zones that reach all the way down to 2,500 feet in some places.

These boundaries also could have been electronically determined by tuning the navigation receivers to the two VORs in the vicinity.

The Archer could also have climbed faster to be above 7,000 feet, which would have put it above the TCA. But that would have been less desirable, because the nose-up attitude of the airplane would have decreased the pilot’s range of vision and because it increases fuel consumption.

Initial reports, however, indicate that neither thing happened.

They indicate that the Archer somehow entered the TCA without permission.

National Transportation Safety Board investigators will be trying to find out how that happened--and also whether both aircraft were visible on the ground controllers’ radar at the time of the collision.

Most aircraft are tracked on radar through use of the on-board electronic transponder, which indicates a plane’s position and (if equipped with an encoder) its altitude.

There is no rule requiring that a small private airplane have a transponder on board. Radar can usually spot an all-metal airplane like the Archer whether it has a transponder or not.

But in practice, most airplanes have them and most pilots keep them turned on.

In fact, the Archer was equipped with a transponder and did have it on and tuned to the standard code number, 1200, that identifies an aircraft flying under visual flight rules, according to the NTSB.

High-flying aircraft (such as the Aeromexico DC-9) are required to operate under instrument flight rules, and to keep their transponders turned on. What is more, the jetliner’s pilot would have been in constant contact with air controllers all the way up the coast, beginning with the ones at San Diego and then passing along to Los Angeles Air Route Traffic Control Center at Palmdale and then to Coast Terminal Radar Approach Control (TRACON), which would direct his course and descent as he let down toward Los Angeles International Airport.

Nearing Los Alamitos, just before entering the Los Angeles TCA, Coast Approach would hand him off to Los Angeles Terminal Radar Approach Control, where a controller would be waiting to guide him to a safe landing.

Most of the time this system works well.

Most of the time it works safely.

On Sunday, it failed and now everyone wants to know why, so they can try to keep it from happening again.