On the morning of June 18, 1994, a Learjet 25D carrying 10 passengers and two pilots crashed less than a mile from the threshold of Runway 1R on approach to Dulles International Airport.

The NTSB investigation that followed the accident culminated in a landmark recommendation calling for the mandatory installation of terrain awareness and warning systems (TAWS) in nearly all U.S.-registered turbine-powered airplanes. More than 10 years after the crash–a span of time that would see many more accidents happen under similar circumstances–the FAA, on March 29, enacted its TAWS mandate requiring in-service turbine-powered airplanes with six or more passenger seats to carry these advanced terrain-alerting systems.

The causes of the Dulles crash were ultimately traced to poor piloting skills. The 27-year-old Learjet captain, who had upgraded from copilot only two months before the accident and reported just 87 hours of Learjet PIC time in his logbook, had declared a missed approach on his first attempt to land.

The ATIS at the time of the crash was reporting the weather as “indefinite ceiling 600 feet, sky obscured, visibility one-half mile fog, temperature 71, dew point 71, wind one-four-zero at four.” The crew of United Flight 186 Heavy, a DC-10, reported a missed approach shortly after the Learjet did and immediately asked for vectors to their alternate. The Learjet captain, perhaps fatigued from a long flight from Mexico City after a prolonged customs stop in New Orleans, requested clearance for another ILS approach to Dulles.

On the second approach, the captain’s localizer tracking was better, but the airplane descended at an average rate of 1,300 fpm between 1,300 feet and about 500 feet msl. The airplane then climbed at 1,300 fpm to 600 feet and, five seconds later, began a 3,000-fpm descent until impact with terrain.

Enhanced Ground Prox Warnings
In its final report the NTSB wrote that had the airplane been equipped with a ground proximity warning system (GPWS), the crew would have been alerted to their rapid descent rate long before the crash. Introduced in the 1970s, GPWS was commonplace in airliners but had been installed on only the largest business jets.

It was around the same time that investigators were piecing together what went wrong during the Dulles Learjet accident that AlliedSignal introduced the enhanced version of GPWS, called EGPWS, which used a sophisticated worldwide database of terrain and GPS position to pinpoint an airplane’s proximity to the ground instead of using the radar altimeter to look straight down at terrain. With the introduction of EGPWS, pilots could be warned of a potentially dangerous terrain conflict much farther in advance than they could before.

After a well publicized accident near Cali, Colombia, in which a GPWS-equipped American Airlines Boeing 757 strayed off course and flew into a cliff face, the FAA decided to adopt the NTSB’s recommendation and require the installation of EGPWS or similar avionics in a wide range of turbine airplanes, conceding the limitations of the original GPWS.

Once the FAA issued the mandate, a number of competitors entered the market for TAWS avionics over the next several years. Today, no fewer than seven avionics manufacturers offer TAWS equipment that meets the specifications of the mandate. And in spite of a patent-infringement lawsuit filed by EGPWS maker Honeywell (which merged with AlliedSignal in 1999), none of the producers has been barred from selling or marketing TAWS units, thanks to court rulings and licensing agreements.

The rules for TAWS equipment are split into two classes. For operators of Part 91 airplanes with six or more passenger seats and Part 135 airplanes with six to nine seats, a class-B device is all that’s needed. Such systems are required to include only aural caution and warning messages, but many have optional terrain display capability. Meanwhile, Part 135 airplanes with 10 or more passenger seats must be equipped with the more expensive class-A TAWS unit, which includes aural and visual cues.

In addition to Honeywell’s EGPWS family of TAWS avionics, similar systems are available from Universal Avionics, Sandel, Aviation Communication and Surveillance Systems (ACSS), L-3 Avionics Systems, Chelton Flight Systems and Garmin.

Although there have been a number of controlled flight into terrain (CFIT) crashes since the 1994 Learjet accident at Dulles, remarkably there has not been a single CFIT accident of an airplane equipped with TAWS since the introduction of EGPWS in 1996. And, according to Honeywell, EGPWS is credited with helping to prevent at least 30 crashes that might have occurred since.