This is the second in a series on landmark events in aviation since Flight Safety Foundation began in 1945.
An aircraft descends through darkness in mist and low clouds. The captain, perhaps because he misreads a chart, perhaps because he gets distracted, loses situational awareness. The first officer, busy with radio calls, does not catch the navigational error. In the cabin behind them, 150 passengers drowse, oblivious to danger. There is nothing wrong with the aircraft, and it remains under the captain’s full control. But aircraft is about to descend into a mountain.
An aural warning sounds in the flight deck: “TERRAIN, TERRAIN.”
The captain’s simulator training kicks in. “Terrain, max thrust,” he calls. The captain punches off the autopilot, shoves the thrust levers, and pitches up 20 degrees. A terrain display appears automatically on his multifunction display (MFD), showing a ridge ahead in red. The first officer calls out altitudes: “One thousand, climbing. Fifteen hundred, climbing.” As the aircraft gains altitude, the color-coded terrain display changes from ominous red, to a less threatening amber, to a safe green.
“Clear of terrain,” the first officer calls. Thanks to technology and training, everyone goes home to their families.
The scenario might have ended differently, especially in years past. According to the International Air Transport Association (IATA), controlled flight into terrain (CFIT) is the second-highest cause of fatal aviation accidents. (Loss of control–in flight is the first.)
IATA said that the most recent batch of 10-year statistics, from 2008 through 2017, shows that aircraft with a maximum takeoff weight of 12,540 lb (5,688 kg) or more were involved in 42 fatal CFIT accidents with 892 lives lost.
“Although few in number, CFIT accidents are almost always catastrophic,” the IATA report said.
That same period was included in a larger Airbus analysis that found a significant decline in CFIT accidents from 1997 to 2017; during that period, the rate of CFIT accidents fell by a factor of seven.
That’s largely due to glass cockpit technology, which improved navigation performance, coupled with advances such as ground-proximity warning systems (GPWS).
The CFIT problem is many decades old. Flight Safety Foundation’s Aviation Safety Network lists 895 accidents in the CFIT-Mountain category alone, going back to 1935.
One of the most infamous CFIT accidents took place on Dec. 29, 1972. Eastern Airlines Flight 401, a Lockheed L-1011, was on approach to Miami [Florida, U.S.] International Airport. According to the U.S. National Transportation Safety Board, the flight crew did not notice an inadvertent autopilot disconnect while they were troubleshooting an unsafe landing gear indication. The aircraft began an uncommanded descent into the Everglades. The resulting crash killed 112 of the 163 people on board.
That accident and others made clear the need for solutions. Airlines began voluntarily installing GPWS in their flight decks, and in 1974, the U.S. Federal Aviation Administration mandated GPWS for U.S. airlines.
The earliest form of GPWS depended on the radar altimeter to measure proximity to the ground. This “basic” GPWS helped reduce CFIT accidents, but it offered little or no look-ahead capability, and, thus, little warning of steeply rising terrain. In the 1990s, Honeywell developed an enhanced ground-proximity warning system (EGPWS) that compared aircraft position with a nearly worldwide terrain and obstacle database. The system “knew” when a mountain loomed ahead. This advanced system, known generically as the terrain awareness and warning system (TAWS), provided comprehensive alerting for a variety of potentially dangerous situations.
Honeywell engineer Don Bateman led the development of GPWS and EGPWS. Bateman held 40 U.S. patents and 80 patents in other countries for aviation safety systems. When Bateman died in 2023, Flight Safety Foundation President and CEO Dr. Hassan Shahidi said, “Don was responsible for saving more lives than anyone else in aviation history. He was a pioneer and innovator who solved one of the highest risks in aviation history, controlled flight into terrain, by his invention of GPWS.” In 2011, Bateman received the U.S. National Medal of Technology and Innovation, the highest U.S. honor for technological achievement.
Flight Safety Foundation helped the industry make the most of Bateman’s invention. In 1992, the Foundation created an International CFIT Task Force that included more than 150 representatives from airlines, equipment manufacturers, aircraft makers, and others. The task force set a five-year goal of reducing CFIT accidents by 50 percent. As the statistics cited earlier in this article suggest, the effort paid off.
The Foundation offered CFIT-reduction aids, including an FSF CFIT Checklist, which helped pilots and aircraft operators assess risks for specific flights. The checklist was published in English, Arabic, Chinese, French, Russian, and Spanish. The Foundation also created a CFIT Education and Training Aid, a two-volume package that addressed issues ranging from CFIT causal factors and avoidance to aircraft-specific CFIT escape maneuvers.
In 1996, the Foundation published an analysis of CFIT accidents in commercial operations from 1988 through 1994. The Netherlands National Aviation and Aerospace Laboratory — NLR (now known as the Royal Netherlands Aerospace Centre) produced the study, which focused on 156 accidents. The study found that the descent and approach phase of flight accounted for about 70 percent of the accidents, and that 75 percent of the accident aircraft did not have GPWS equipment.
The analysis pointed up the catastrophic nature of CFIT accidents. The study noted that in 97 percent of the 139 accidents where full data was available, the aircraft was destroyed. The fatality rate of those accidents was 91 percent.
The study’s recommendations urged all operators to comply with current and future requirements for installing GPWS. Other recommendations included instrument approach charts with colored contours to depict terrain, and visual cockpit terrain displays. The report also suggested radar altitude callouts to improve crew awareness, and better international data-sharing on CFIT accidents.
Airline crews and passengers now benefit from these recommendations. Modern TAWS equipment is classified Class A or Class B, depending on the system’s sophistication. Class A systems provide indications for the following:
- Excessive descent rate;
- Excessive closure rate to terrain;
- Negative climb rate or altitude loss after takeoff;
- Flight near terrain when not in landing configuration;
- Excessive downward deviation from an instrument approach glidepath; and,
- Aural callout of “500” when the aircraft descends to 500 ft above ground level.
Class B systems provide indications for the following:
- Excessive descent rate;
- Negative climb rate or altitude loss after takeoff; and,
- Aural callout of “500” when the aircraft descends to 500 ft above the nearest runway elevation.
Aural alerts include callouts such as “SINK RATE,” “TERRAIN, TERRAIN,” “DON’T SINK,” “TOO LOW–GEAR,” “TOO LOW–FLAPS,” “TOO LOW–TERRAIN,” and “GLIDESLOPE.”
In addition, manufacturers may provide other functions. Common examples include callouts of “100” and “50” as the aircraft descends through 100 ft and 50 ft for landing.
Flight Safety Foundation’s work to reduce CFIT accidents continued into the 21st century. A 2002 report warned against erroneous instrument landing system (ILS) indications that present a CFIT hazard. The report described how the International Civil Aviation Organization (ICAO) had noted a number of incidents caused by ILS signals radiated during testing and maintenance. The Foundation made several recommendations to guard against the problem, including:
- Be aware of potential erroneous ILS indications, with or without ILS warning flags.
- Check notices to air missions (NOTAMs) to determine the status of ILS components.
- Ensure that reported discrepancies with ILS receivers and/or indicators have been addressed.
- Ten minutes before beginning descent, conduct a thorough briefing that includes automatic flight control system modes, terrain, and typical vertical speed at the expected final approach groundspeed.
- Conduct a stabilized approach.
- Be especially alert when conducting an ILS approach to an uncontrolled airport, where ILS-critical areas are not protected by air traffic control (ATC).
- Ensure ILS receivers are properly tuned with the signal properly identified.
- Use the radar altimeter to enhance terrain awareness.
- Cross-check aircraft altitude with published glideslope intercept altitude.
- Disregard any ILS indications from components identified as inoperative, regardless of apparent validity.
- When in doubt about the status of an ILS component, query ATC.
- Cross-check groundspeed and descent rate.
- Operators should equip aircraft with TAWS.
- Crews should remain go-around–prepared and go-around–minded.
Future technology promises to continue making the skies safer. For example, military fighter aircraft have used automatic ground collision avoidance (Auto GCAS) technology for more than a decade. If a pilot becomes incapacitated, Auto GCAS can automatically pull up an aircraft before it impacts the ground. The U.S. Air Force’s Research Laboratory credits the system with saving at least 11 aircraft and 12 pilots. Although the system was designed for tactical aircraft such as the General Dynamics F-16 and the Lockheed Martin F-35, similar technology could make its way into commercial aviation, especially if future airliners operate with reduced crews.
Image: Denis Belitsky / shutterstock.com
Thomas W. Young is a retired airline captain and a former instructor flight engineer with the West Virginia Air National Guard. Young has logged nearly 12,000 hours of pilot and flight engineer time.