It was a night tailor-made for flying—smooth air, barely a cloud in the sky, and miles of visibility. The center controller had handed the crew off to approach control with a friendly, “G’night,” and within a few minutes the pilots were cleared for a visual approach to the active runway about 15 miles straight ahead. From their position, the crew could easily see the airport, enveloped by the sodium-vapor shimmer of the city’s vast downtown. Just another routine flight, to be followed shortly by a van ride to the hotel and some well-deserved shuteye.

Everything appeared normal until at some point during the descent the calm in the cockpit suddenly was broken by a stern male voice calling out, “Caution, terrain!” For reasons neither pilot could immediately grasp, the Enhanced Ground Proximity Warning System had triggered an alert. The pilots did not understand why. The airport was right there, less than 10 miles ahead.

Perhaps the city lights were a little too mesmerizing, but for whatever reason the pilot at the controls had descended too hastily. The airplane was several hundred feet lower than it should have been and dropping fast.

Contours of red flashed on the cockpit displays as the city lights ahead of the windshield began to blink out. It was as if somebody had switched off a power grid. In an instant, the crewmembers realized their mistake. A mountain lurking at the southern edge of the city was obscuring the view of the lights. Perhaps not fully understanding how they’d gotten so low, but heeding the EGPWS alert nonetheless, the captain shoved the throttles full forward and pulled hard on the yoke.

In seconds the airplane had transitioned from a steep descent to a maximum-performance climb. The caution lights and warning sounds subsided. Having regained sufficient altitude, the crew leveled off and proceeded to make an otherwise normal landing.

The Value of Terrain Warning Systems

This recounting of an actual incident is just one of dozens of reports from crews who have found themselves in a potentially dangerous CFIT (controlled flight into terrain) scenario, according to EGPWS maker Honeywell. Thankfully, all had happy endings, but they might not have had it not been for the intervention of the onboard terrain warning system. All told, Honeywell has recorded some 40 incidents where flight crews reported that the EGPWS played a role in preventing a CFIT accident or at least of warning the crew of uncomfortably close proximity to terrain.

The truly amazing statistic with regard to EGPWS and other enhanced terrain awareness and warning systems (TAWS) is that in the nearly 10 years since the technology has been in use, not a single TAWS-equipped airplane has been involved in a CFIT crash. Even a tertiary examination of accident statistics in the years before the introduction of TAWS (when the non-enhanced, radar-altimeter version of the ground proximity warning system was all a crew had to rely on) leads observers to no other conclusion than the technology is responsible for saving lives.

How many lives is impossible to say, but the safety benefits of TAWS were clear enough that the FAA and Eurocontrol earlier this year decided to make TAWS equipment mandatory for most turbine-powered airplanes.

The FAA TAWS mandate stipulates the following regarding two classes of TAWS equipment: all Part 121 and 135 operations with aircraft configured for 10 or more passenger seats must carry class-A systems. Class-A equipment includes expanded terrain warning system functions with a visual display and other safety features. Part 91 aircraft with six or more passenger seats and Part 135 aircraft with six to nine passenger seats must have at least a class-B system. Class B is defined as equipment with the basic functions available in terrain awareness equipment and does not require a display.

Today there are a wide range of choices for TAWS gear. Besides Honeywell, which by far has sold the lion’s share of systems to the airlines and business aircraft operators, manufacturers of TAWS equipment include ACSS, Universal Avionics, Sandel Avionics, L-3 Avionics Systems, Garmin, and Chelton.

AIN recently polled readers about their experiences with TAWS since the mandate went into effect. Pilots reported that the systems appear to be performing their intended job, although none related a story about the TAWS actually stepping in to prevent a possible accident in the making. One pilot, who flies with a Universal class-A TAWS, said the system has provided valuable situational-awareness information on frequent trips he’s made to Colorado.

According to Greg Francois, Honeywell director of surveillance products, the company has sold 37,000 EGPWS units since late 1996, which have flown somewhere north of 500 million flight hours and 250 million flight segments. As far as the world’s business aircraft fleet is concerned, Francois estimated that some 7,500 to 8,000 carry EGPWS. Most of these systems were sold and installed before the TAWS mandate when there was a big rush among operators to equip.

“Sales have definitely tailed off,” Francois said of the period between last March 29, the date of the FAA’s TAWS mandate, and today. A portion of revenue is still coming from the retrofit side of the business, Francois said, “but clearly it’s not the big push we had right before the mandate.” Last year was the highest in terms of revenue from EGPWS sales, with brisk sales continuing through this year’s first quarter as smaller business jet operators waited until the final weeks and days before the mandate, he said.

Francois added that nuisance alerts are much less frequent now than they were in the early days of EGPWS. Honeywell has released 40 database updates to correct errors in the EGPWS database and continues to hone the information all the time.

He said he and his engineering team have their fingers crossed that another space shuttle mapping mission will be flown at some point to fill in gaps in the data in remote parts of the world. While many people just assume that world terrain data is complete, Francois said there are areas where the data is far from perfect. It’s those areas that should truly worry TAWS designers, he said, because even though the technology has a perfect safety record thus far, that record can be shattered in an instant.