Business & GA, Commercial

Safety in Avionics: Surviving a ‘Sucker Trap’

By David Evans | June 1, 2002
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An aphorism exists that rubber, not aluminum, should always touch the ground first. The saying relates directly to controlled flight into terrain (CFIT) accidents. A recent close call ended with rubber touching the ground first, as in the landing gear on the runway. A second case ended in tragedy, as aluminum cut a swath through trees on a hill within sight of the runway. In the first case, the captain later confessed in sweaty-palmed relief that he was saved from falling into a "sucker trap." In the second case, the captain said he felt nothing unusual in the seconds before impact.

What made the difference? The added margin of safety provided by a particular item of avionics: a terrain avoidance warning system (TAWS). One airplane had this system; the other didn’t. The difference was life and death.

On April 15 an Air China B767 on a flight from Beijing crashed into an 1,800-foot high hill some 3.5 nautical miles (nm) short of Runway 18R at Gimhae International Airport in Busan, South Korea. There were just 38 survivors of the 166 passengers and crew aboard. Capt. Wu Xinhu was among the survivors. In the left seat, he was attempting to fly a circling approach in rain and mist. He would be looking across the cockpit to maintain sight of the runway.

In this instance, Wu was tasked to maintain a minimum height (until wings level on final and assured of landing). He was to remain within the stipulated distance for terrain clearance and adjust for a southwesterly wind that would tend to push him in toward the runway–which would require a greater bank angle in the final turn. As one pilot observed, the circumstances were such that Wu was "almost assuredly going to be blown farther downwind and through the Runway 18R centerline with an increasing bank angle–which he was, with disastrous results."

A former head of flight safety for a major U.S. carrier described the challenge with circling approaches: "In general, the difficulty of a circling approach is rooted in the fact that it can require significant maneuvering of the aircraft at low altitude, using visual cues. This is less difficult in ideal weather…but can become quite dicey [with] various combinations of low ceilings, low visibility, wind and/or night conditions. At ‘terrain sensitive’ airports, a circling approach would be considered one of the more challenging maneuvers that a pilot might be required to execute."

The right wing of the Air China jet, dipped down in that final bank, clipped the treetops, and the airplane ploughed a 100-yard (91-meter) long trail of broken timber. The fact that the airplane struck tail first (where fatalities were most concentrated) suggests that Wu may have been attempting a last-second effort to clear the top of that fog-shrouded hill. In any event, if he’d had just 100 more feet of altitude, Wu would have cleared the ridgeline.

Advance warning would have come from the TAWS his airplane, built in 1985, did not have. The B767 was fitted with an early version of ground proximity warning system (GPWS) known as the Mark 3. This obsolete technology measures terrain clearance vertically, which means that if one is flying across level ground straight at a vertical cliff, the system provides no warning. Or, if the airplane were in the landing configuration, as was the case in the crash of Air China Flight 129 at Busan, the system would not provide a warning.

Of the 10 B767 aircraft flown by Air China, none was fitted with the enhanced GPWS (EGPWS–also called TAWS). Of the 480 commercial aircraft in China, only some 156 (about 30 percent of the fleet) are equipped with TAWS/EGPWS. This minority represents the newest aircraft in China’s fleet, indicating that virtually no retrofit of the latest equipment has occurred.

TAWS/EGPWS might have made the difference between catastrophe and recovery. According to a preliminary analysis by Don Bateman, chief engineer of flight safety systems at Honeywell International Inc., a TAWS/EGPWS would have sounded "Caution! Terrain!" some 32 seconds before Flight 129’s projected impact. The system, by the way, continues to function when the airplane is configured for landing. At 30 seconds before projected impact, the system would warn, "Terrain. Terrain. Pull up!" In such a circumstance, 30 seconds was the difference between peoples’ lives, and whether or not Wu would carry the scarring memory of the crash for the rest of his life.

Now consider an incident with a happier, if nerve-racking, outcome. It involved a B737 during an approach to land this past January at Tucson, Ariz. The first officer was the pilot flying. The captain’s account speaks for itself:

"During late January we were cleared for a visual approach to ILS Runway 11 at Tucson approximately 20 nm from the airport. I normally refuse a visual approach clearance, as it is something I do not do unless I am inside the outer marker, but I did. It was a beautiful clear night. No moon. Lots of stars. No traffic and the beautiful lights at Tucson. My copilot was flying us on an intercept to the localizer and started us gradually down. Everything was smooth. He called for flaps one. We did not realize it at the time, but [we] were falling into a ‘sucker trap.’ Our instrument and altitude scan had gone to hell.

"At about 15 nm from touchdown, we suddenly got a EGPWS ‘Caution! Terrain!’ and a big splash of yellow appeared on both EFIS [electronic flight instrument systems], and it scared the hell out of me." Terrain on the EGPWS map-like display will turn a solid yellow color when the crew has about one minute to pull up or turn away from terrain that is at the same altitude or higher than they are flying.

"Looking out, we could see that the stars and ground lights were slowly disappearing behind a vague dark line that was above us. Our altitude was less than 4,000 feet! During our [immediate] climb, the EGPWS ‘Terrain! Terrain! Pull up!’ warning came on with a bigger splash of yellow and red." The color red warns of terrain an aircraft will impact in about 30 seconds. "We immediately increased our climb, the warning stopped, the red shrank to yellow and then to green and we acquired the glideslope. We could not believe our stupidity.

"After we landed, and our passengers disembarked, we both sat in our dark cockpit. My copilot was shaking. I was in a state of shock. We would have all perished, my crew and all my 100 passengers, save for a timely wakeup from our EGPWS. I could not sleep that night. I still cannot believe what almost happened, after all my 18,000 hours and 30 years of flying experience."

If less than a third of China’s commercial air fleet is equipped with TAWS/EGPWS, the situation in the United States offers no grounds for complacency. At a recent safety workshop, Capt. John Cox, executive air safety chairman of the Air Line Pilots Association, groused, "Nearly half the U.S. fleet does not have EGPWS."

All U.S. aircraft down to those carrying as few as 10 passengers must have TAWS/EGPWS installed by 2007. That’s two years later than the date proposed earlier. Given the alacrity with which cockpit doors are being hardened and all baggage is to be screened–basically in a year’s time following the Sept. 11, 2001, terrorist attacks–security obviously is a matter of urgency. How about a similar priority for terrain avoidance safety technology? If there’s any doubt, ask Capt. Wu.

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