80 Years of Aviation Safety

This article is the third in a series on landmark events in aviation since Flight Safety Foundation began in 1945.

An airliner flies a holding pattern at 3,000 ft, with a solid overcast above. Below, mist limits visibility. The aircraft’s gray paint scheme blends with what little horizon is discernible.

Meanwhile, a Cessna 172 on a visual flight rules (VFR) flight plan approaches the fix where the airliner is holding. The decade-old Cessna’s white paint has dulled and darkened with age. The Cessna presents even less outline than the gray jet. An alert air traffic controller sees the potential conflict and calls out a warning to the airliner.

But the call gets blocked by another aircraft checking in on the frequency. Through no fault of his own, the controller does not realize no one heard the warning. The airliner and the Cessna are now on a collision course. The airliner’s pilots cannot see the Cessna. The Cessna pilot is looking down at a chart.

An aural annunciation sounds in the airliner’s flight deck: “DESCEND, DESCEND.” The first officer, acting as the pilot flying, thumbs his touch-control steering (TCS) button to override the autopilot. He pitches down, guided by a green rectangle that appears on his primary flight display (PFD). The captain advises the controller that the aircraft is responding to a TCAS resolution advisory (RA).

The Cessna passes directly over the airliner. A few seconds later, an aural annunciation calls “CLEAR OF CONFLICT.”

This lifesaving technology came about after a number of deadly midair collisions going back to the 1940s. Among them was the June 30, 1956, collision of a Trans World Airlines Lockheed Super Constellation with a United Airlines Douglas DC-7 over the Grand Canyon in Arizona, U.S. The accident took 128 lives. An investigation noted several factors: clouds in the area reduced the time available for each flight crew to see the other airplane, the crews were preoccupied with providing passengers with a scenic view of the Grand Canyon, and air traffic advisories were inadequate.

On Sept. 10, 1976, a British Airways Hawker Siddeley HS-121 Trident collided with an Inex-Adria Aviopromet DC-9 over Croatia, killing 176 people. An investigation blamed improper air traffic control (ATC) operations and inadequate lookout and radio monitoring by the flight crews. 

On Sept. 25, 1978, a Pacific Southwest Airlines (PSA) Boeing 727 collided with a Cessna 172 near San Diego [California, U.S.] International Airport. That accident took 144 lives. The U.S. National Transportation Safety Board (NTSB) cited the failure of the PSA crew to comply with a maintain-visual-separation clearance, including the requirement to advise ATC when visual contact was lost. The NTSB also blamed ATC procedures that authorized controllers to use visual separation in a terminal environment when the capability was available to provide vertical or lateral separation. The report also cited contributing factors, including the Cessna pilot’s failure to maintain an assigned heading. 

Accidents like these made clear the need for technology to back up controllers and pilots.

A forerunner of present-day traffic-alert and collision avoidance systems (TCAS), the Beacon Collision Avoidance System (BCAS), was developed during the 1970s. BCAS relied on data from ATC radar beacon system transponders — which at the time were installed in all airliners and military aircraft and in many general aviation aircraft — to determine an aircraft’s altitude and distance from other aircraft.

Beginning in the early 1980s, TCAS systems were developed, using the basic BCAS concept with added capabilities. Piedmont Airlines flew about 2,000 hours of operational evaluation flights with TCAS during the 1980s before more widespread evaluations were conducted by regulatory authorities in the United States, Europe, Japan, and elsewhere. and in 1987, the U.S. Congress passed legislation requiring TCAS on all airliners by the end of 1993. 

In 1995, the European Air Traffic Control Harmonisation and Integration Programme Project Board established a policy for mandatory collision avoidance systems — which are also known as airborne collision avoidance systems (ACAS). 

TCAS works independently of controllers or equipment on the ground. The system uses transponder signals to detect potential conflicts but provides no protection against aircraft without a working transponder. 

First-generation TCAS provided traffic advisories (TAs) on the position and course of nearby aircraft, and pilots had to decide how to respond. The second generation, TCAS II, provided specific instructions, or resolution advisories (RAs) in addition to TAs. For example, if two aircraft are on a collision course, one gets an advisory to descend, while the other receives a climb advisory. 

In addition to aural advisories, the system also provides visual information on traffic displays through the use of standard symbology. Nearby traffic, or proximate traffic, is depicted as a blue diamond. Closer, or intruding traffic, is depicted as a yellow circle. Threat traffic is depicted as a red square. The symbols are accompanied by relative altitude data. For example, a yellow circle accompanied by a downward arrow and the numbers “-02” indicates intruding traffic that is 200 ft below the reference aircraft and descending. A red square accompanied by an upward arrow and the numbers “+01” indicates threat traffic that is 100 ft above the reference aircraft and climbing.

Current systems provide various types of vertical guidance for collision avoidance. Depending on the circumstances, the aural annunciations, accompanied by visual PFD guidance, include “CLIMB, CLIMB”; “INCREASE CLIMB”; “LEVEL OFF”; “DESCEND, DESCEND NOW”; and “MONITOR VERTICAL SPEED.” 

These advisories are calculated by measuring time to the closest point of approach (CPA) rather than distance. An RA can be preventive or corrective. Preventive RAs provide restrictions on vertical speeds, and corrective RAs require a change in vertical speeds. In the event of multiple threat traffic, TCAS II systems give RAs that provide adequate vertical separation from all the threat aircraft.

To calculate these advisories, the system uses slant range between aircraft, divided by the closure rate. According to an FAA publication, this concept is credited to John S. Morrell of Bendix. 

When responding to RAs, it’s important that pilots precisely follow TCAS guidance. The FAA booklet notes: “Pilots sometimes deviate significantly further from their original clearance than required or desired while complying with an RA.” The documents adds: “While over-reactions to TCAS RAs are not common, they can lead to loss of separation with other aircraft that were not originally involved in the encounter.” Through simulator training, however, pilots can become accustomed to cockpit indications during RAs and learn to pitch up or down in accordance with TCAS guidance.

On most modern commercial aircraft, pilots make TCAS selections through transponder settings via the multifunction control and display unit (MCDU). On some aircraft, selections can be made on a dedicated control unit for the transponder. Options include standby (STBY), TA-ONLY, and TA/RA. TA/RA is normally used in flight, but pilots can select TA-ONLY if aircraft performance becomes limited by an engine failure or other malfunction. In TA-ONLY, pilots will not receive resolution advisories and must judge how to avoid a traffic conflict based on their knowledge of the aircraft’s degraded capability.

TCAS is considered a last-safety net. Pilots are trained to follow resolution advisories regardless of any instructions from ATC. The primacy of TCAS resolutions became standard after a July 1, 2002, collision involving a passenger aircraft and a cargo aircraft over Überlingen, Germany. The accident killed 71 people. TCAS systems on both the Bashkirian Airlines Tupolev Tu-154M and the DHL Boeing 757 alerted the crews to conflicting traffic. An investigation found the Tupolev crew followed an ATC instruction to descend even though TCAS advised them to climb.

As the technology matures, TCAS/ACAS systems will provide even more flexibility for collision avoidance. The next generation, currently under development, will provide resolution advisories with both vertical and horizontal guidance. 

It is difficult to quantify how many lives have been saved by TCAS, but some estimates place the figure in the thousands. 

No system is perfect, however, and TCAS/ACAS does not relieve pilots and controllers of basic responsibilities. The FAA’s TCAS booklet notes: “It must be stressed . . . that TCAS II cannot resolve every near-midair collision and may induce a near-midair collision if certain combinations of events occur. Consequently, it is essential that ATC procedures are designed to ensure flight safety without reliance upon the use of TCAS II and that both pilots and controllers are well versed in the operational capabilities and limitations of TCAS II.” 

Image: Veltman34 / shutterstock

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.