Note: Because aviators worldwide specify altitude and separation requirements in nautical miles and feet, those units have been retained in this article rather than converted to their metric equivalents, as is IEEE Spectrum's policy.

By the year 2015, if the U.S. air transportation system does not change in any significant way, there could be a major aviation accident every seven to 10 days.

This projection, reported by Neil Planzer, director of Air Traffic System Requirements Service for the Federal Aviation Administration (FAA), Washington, D.C., is based on the anticipated growth of air traffic, combined with an accident rate that has been statistically flat for the past 15 years and, without much effort, is expected to remain at that level [Fig. 1]. Planzer made the projection in May at the "Communicating for Safety" Conference in Chandler, Ariz.

Because weekly accidents are well beyond what a traveling public is willing to tolerate; and because the current air traffic control system [see "Air traffic control upgrades around the world "], on which the safety of air travel depends, has been rapidly losing reliability due to aging equipment and accompanying maintenance problems; and because air traffic-controlled­caused delays cost the airline industry an estimated US $5.5 billion annually, the FAA has for the past two decades been scrambling to replace, modernize, and improve it.

So far, success has been minimal.

Watching today's skies

The U.S. air traffic control system is organized around three types of facilities [Fig. 2] and a bevy of acronyms: airport towers, which monitor aircraft on the ground and give take-off and landing clearances; terminal radar approach control (Tracon) facilities, which handle aircraft ascending and descending to and from airports; and en route centers, which handle aircraft flyingbetween airports at the higher altitudes.

Defining Terms

Signals from the radar that scans the skies for aircraft are processed at the Tracons and the en route centers, where controller displays and the computers that feed them information form the heart of the air traffic control system. At the Tracons, the computer system used is the automated radar terminal system (ARTS), and its displays are data entry and display subsystems (DEDSs) at most facilities or the newer full digital ARTS displays (FDADs). At the en routecenters, the computers, called the Hosts, send information to the computer display channels or display channel complex rehosts, which are other large computers that drive plan view displays (PVDs).

Both the PVDs and the DEDSs are 1960s-designed displays. The FDAD, in use at some of the busiest Tracons, is a '80s' microprocessor-based system. All these displays look basically the same—a round tube of about 0.56 meter in diameter, with a dark background and green text [Fig. 3, top]. The PVDs and DEDs (but not the newer FDADs) confront the same reliability problems.

Much of this equipment had been expected to be on a scrap heap by now. The PVDs installed by Raytheon Co., Lexington, Mass., in the early '70s had an anticipated lifetime of 10­15 years; those in the centers today are now at least 10 years past this estimate.

These displays fail regularly—according to controllers and technicians. At each en routecenter, which may have 30 to 60 PVDs in operation, it is not unusual to replacetwo to four of these units a day. When a PVD goes dark, the controller at that station rushes to another screen and urges the controller there to alter his or her display to include aircraft previously tracked on the failed display.

PVDs slipping out of adjustment also cause the size and clarity of the alphanumeric type they display to vary—fuzzy type makes controllers confuse 3s and 8s, which can lead to errors, an Indianapolis controller told IEEE Spectrum. And the units themselvesare unstable. Their aging ceramic connectors are brittle and falling apart. Insulation on the wires is brittle, too. The vibration caused in moving a display, as is necessary when a replacement must be brought in, often disables it when fragile connections are broken.

Meanwhile, the Host and ARTS computers that drive the displays are problematically obsolete as well. The Host computer computes radar tracks, maintains a database of flight plans, and issues safety warnings—such as aconflict alert, when two craft are in danger of violating separation standards, and aminimum safe altitude warning, when an aircraft is at risk of hitting terrain. It contains half a million lines of Jovial code and assembly language that was first installed in 1972 and ported from IBM 9020 onto IBM 3083 computers, starting in 1985.

But Host has at most only 16MB of RAM, a serious limitation. And it badly needs replacing. (The ARTS computers in the Tracons are also severely limited in memory, but those are scheduled for replacement.) "The Host software is our biggest problem," a controller from Chicago told Spectrum. "There are so many patches, no one knows how it works. We can't change anything; no one dares touch it, because if we break it, we're gone."

In the mid-'80s, a multibillion dollar effort was started to update both the en route centers and the Tracons by replacing their displays and computers with networked workstations. (Airport towers use feeds from Tracon computers for radar tracking of airborne craft; they use separate surface-monitoring equipment for aircraft on the pavement.)That 10-year effort failed and has, for the most part, been abandoned. Called the Advanced Automation System, the program was sunk by unrealistic specifications and human factors difficulties, among other problems. New efforts to help controllers and pilots are under way, but have yet to make an impact on the present system.

As to what the main features of an air traffic control system for the '90s should be, system developers, controllers, and some FAA officials are agreed. It should have controller workstations with high-resolution bit-mapped displays that can distinguish information by color. It should not drop planes and vital traffic control information from displays (as happens today when computer capacity is exceeded). And it should not go dark on a regular basis. What's more, it should be based on commercial off-the-shelf (COTS) hardware, making it upgradable and expandable, so that when controller tools intended to increase safety and efficiency, presently in the prototype stage, are completed, they can easily be ported to this new system—a transfer that is out of the question with current hardware.

This system is still merely a dream for most of the 14 500 U.S. controllers employed at the more than 200 en route centers and Tracons. But at one, just one, FAA control facility, it is a reality.