Explained: Traffic Alert and Collision Avoidance System

‍DALLAS — A Traffic Alert and Collision Avoidance System (TCAS), or an Airborne Collision Avoidance System (ACAS), is an anti-collision system that prevents mid-air collisions (MACs) between airplanes. 

The collision avoidance system is independent of air traffic control (ATC). It senses nearby aircraft with an active transponder and issues pilot alerts and avoidance commands. TCAS talks to transponder-bearing aircraft to create a three-dimensional airspace model, range, bearing, and altitude. 

The International Civil Aviation Organization (ICAO) requires TCAS for aircraft with a maximum takeoff mass (MTOM) greater than 5,700 kg or those permitted to carry more than 19 passengers. 

The following analysis stems from a detailed look into various transponders and their response times. 

Historical Development of TCAS

We can trace the history of TCAS back to in-air collisions that induced aviation safety reforms. The most notable incident occurred in 1956. 

That year, a United Airlines (UA) Douglas DC-7 crashed into a Trans World Airlines (TWA) Lockheed L-1049 Super Constellation over the Grand Canyon. The 128 casualties revealed ATC shortfalls, resulting in more advanced air traffic surveillance systems.

The development of radar technology and transponder-based communication systems over the years contributed to the advancement of TCAS. The early versions of TCAS were primitive and offered minimal collision detection. 

Contemporary TCAS, however, is an essential part of aviation safety. It provides highly accurate threat assessment and resolution advisories. TCAS has become mandatory for commercial aviation today, allowing aircraft to detect and respond to potential threats independently.

How TCAS Works

TCAS operates by an active interrogation-and-response mechanism. The essential elements are:

• TCAS unit: Consisting of a computer, antennas, and software algorithms.
• Mode S transponder: Necessary for transmitting and receiving aircraft position information.
• Cockpit display system: Gives pilots visual and aural warnings.

Interrogation, Response Mechanism

The TCAS-equipped aircraft transmits interrogation signals at 1030 MHz, which causes nearby aircraft transponders to reply at 1090 MHz. This communication happens several times per second, enabling TCAS to compute the range and altitude of nearby aircraft. 

The system continually updates relative positions, examining potential collision threats by trajectory and closure rate.

Closest Point of Approach (CPA) and Tau Calculation

To assess the risk of a collision, TCAS calculates the Closest Point of Approach (CPA)—the point at which two aircraft will be closest to one another. It also determines Tau, the time left before arriving at the CPA.

• Horizontal Tau: Calculated as: 

= [Range] / [Speed (Own Aircraft) + Speed (Intruder)] x 3600

• Vertical Tau: Calculated as: 

= [Altitude Separation] / [Vertical Closing Speed] x 60

For clarification, “Range” is in nautical miles, “speed” in knots (with multiplication by 3600 converting hours to seconds), and “Altitude Separation” (in feet) divided by “Vertical Closing Speed” (in feet per minute) multiplied by 60 yields seconds.

When Tau drops below a critical value, TCAS provides a Traffic Advisory (TA) or a Resolution Advisory (RA).

TCAS Alert Levels, Pilot Response

There are various TCAS alert levels, depending on proximity and closure rate:

1. Other Intruder: Not an immediate threat, presented as a white hollow diamond.
2. Proximate Traffic: An aircraft within a particular range but not immediately threatening, as indicated by a solid white diamond.
3. Traffic Advisory (TA): Given when there is a possible collision within a prescribed period. Pilots hear an aural warning: "TRAFFIC, TRAFFIC."
4. Resolution Advisory (RA): Given when a collision is imminent, urging pilots to initiate evasive maneuvers. Example command: "DESCEND, DESCEND.”

In newer glass cockpit aircraft, pilots see RAs on vertical speed tape, with recommended maneuvers displayed in green and red regions. In older planes, digital vertical speed displays serve this purpose. 

When the aircraft is out of conflict, TCAS displays a "CLEAR OF CONFLICT" message, and pilots notify ATC to this effect.

Sensitivity Levels, TCAS Envelopes

TCAS sensitivity levels (SL) vary the system's warning thresholds to accommodate safety versus operationally feasible safety. Higher SL settings enhance protection but could result in unnecessary warnings. Sensitivity levels vary by altitude:

• SL 1: TCAS in standby mode (no warnings provided).
• SL 2: Only Traffic Advisories (TA).
• SL 3–7: Increased levels of advisory protection, with RAs triggered at higher altitudes.

The TCAS envelope establishes zones about an aircraft:

• Outer Zone: Intercepts remote aircraft (Other Intruder).
• Middle Zone: Classifies aircraft as Proximate Traffic.
• Inner Zone: Produces Traffic Advisories (TA).
• Critical Zone: Initiates Resolution Advisories (RA).

Coordinated TCAS Maneuvering

When two TCAS-equipped aircraft sense a collision threat, they coordinate resolution advisories with Mode S transponders. The initial aircraft to calculate an RA transmits its suggestion to the second aircraft so that complementary maneuvers are performed. 

For example, if an airplane receives the CLIMB RA, the other aircraft will receive a DESC RA. No two aircraft in the situation will receive the same commands. 

The above-computerized procedure ensures the two aircraft avoid each other at all costs. For legacy TCAS logic, if both aircraft generate identical RAs independently, the aircraft with a higher ICAO 24-bit Mode S address reverses its RA. However, some modern systems may incorporate more sophisticated coordination algorithms. 

TCAS Inhibitions

To prevent unnecessary alerts, TCAS automatically inhibits certain advisories during specific flight phases:

• Below 1,000 feet AGL: RAs to avoid terrain conflicts.
• Below 500 feet AGL: TAs to minimize distractions during takeoff and landing.
• During rapid descents: Descend advisories may be suppressed.

It is important to note that inhibition altitudes can vary with system design and regulatory requirements. Current TCAS implementations inhibit advisories below 500 feet AGL (especially RAs) to avoid ground-related false alerts. 

TCAS, Air Traffic Control

Pilots are required to report to controllers when acting on RAs. During an RA event:

• Pilot Flying (PF): Disconnects autopilot and acts on RA instructions.
• Pilot Monitoring (PM): Reports to ATC: "TCAS RA."
• Post-Event: After clearing the conflict, the PM announces, "Clear of conflict." ATC may provide new instructions if needed.

TCAS always has priority over ATC instructions since avoiding a collision is the priority.

Incidents

A series of crashes and accidents have occurred due to system weaknesses, pilot misinterpretation, and handling of TCAS advisories rather than air traffic control (ATC) commands. These accidents have highlighted the need for ongoing technical advancements, governmental regulations, and pilot training.

Below are some of the most significant TCAS-related accidents that comment on its effectiveness, limitations, and the need to comply with Resolution Advisories (RAs)to the letter:

1996 Charkhi Dadri Mid-Air Collision (India)

• Aircraft Involved: Saudi Arabian Airlines Flight 763 (Boeing 747-100) & Kazakhstan
• Airlines - Flight 1907 (Ilyushin Il-76)
• Fatalities: 349 (all on board)
• Cause: Disobedience of ATC altitude commands and insufficient situational awareness

The deadliest mid-air collision in aviation history happened on November 12, 1996, when a Kazakhstan Airlines Ilyushin Il-76 flew below its cleared altitude and hit a Saudi Arabian Airlines Boeing 747 off Charkhi Dadri, India. 

The tragedy was also caused by the Kazakhstani crew's poor English proficiency and the absence of TCAS onboard. Though the Saudi 747 had TCAS on board, it never received an RA to prevent the collision.

This accident led India to make TCAS-II mandatory on all commercial flights within Indian airspace.

1999 Lambourne Near-Collision (United Kingdom)

• Aircraft Involved: Boeing 737-300 & Gulfstream IV
• Location: Lambourne Holding Stack, near London Heathrow
• Cause: Opposite flight paths resulting in a TCAS-RA

This accident testified to the success of TCAS in collision avoidance in busy airspaces. The Lambourne Holding Stack, a congested holding area for Heathrow arrivals, witnessed two aircraft coming in from opposite sides, risking a head-on collision.

TCAS automatically issued a Traffic Advisory (TA) and a Resolution Advisory (RA) seconds later. With the estimated time of collision under 25 seconds, both planes responded to their respective RAs and barely avoided collision. The near-miss emphasized pilot response to TCAS advisories over ATC clearances in the event of a crash.

2001 Japan Airlines Mid-Air Incident

• Aircraft Involved: Japan Airlines Flight 907 (Boeing 747-400) & JAL Flight 958 (McDonnell Douglas DC-10)
• Fatalities: None, but reported injuries
• Cause: ATC clearance in conflict with TCAS Resolution Advisory (RA)

On January 31, 2001, Japan Airlines Flight 907, an aircraft flying from Tokyo Haneda to Naha, Okinawa, narrowly avoided colliding with JAL Flight 958, an aircraft flying from Busan, South Korea, to Narita. 

The accident occurred in Shizuoka, Japan, when the Tokyo Area Control Center air traffic controller incorrectly cleared Flight 907 to descend, contradicting the TCAS climbing instruction. The Boeing 747 pilot followed the ATC clearance over TCAS, leading it onto a collision path with the descending DC-10. 

The DC-10's TCAS correctly instructed it to descend, but due to human reaction time, the aircraft was 100 meters (328 feet) apart at a relative closing speed of 1,200 km/h (750 mph). Investigations following the incident led to increased TCAS training seminars in Japan, reaffirming that pilots should always follow TCAS RAs over ATC clearances in the event of a collision. 

The overworked, inexperienced air traffic controller resigned following the incident.

2002 Überlingen Mid-Air Collision (Germany)

• Aircraft Involved: Bashkirian Airlines Flight 2937 (Tupolev Tu-154) & DHL Flight 611 (Boeing 757-200F)
• Fatalities: 71 (all occupants of both aircraft)
• Cause: TCAS RA conflict with ATC instructions

On July 1, 2002, a Russian Tupolev Tu-154 carrying schoolchildren en route to Spain crashed into a DHL Boeing 757-200F near Überlingen, Germany. The Tupolev pilots followed ATC instructions rather than TCAS, while the DHL pilots followed their TCAS RA. 

The understaffed Zurich ATC had instructed the Tu-154 to descend without knowing that TCAS had already instructed its pilots to "CLIMB" RA. This led to a head-on collision at 34,900 feet.

2006 Mid-Air Collision: Gol Transportes Aéreos Flight 1907 (Brazil)

• Aircraft Involved: Gol Flight 1907 (Boeing 737-800) & Embraer Legacy 600
• Fatalities: 154 (everyone on board the 737)
• Cause: Inoperative transponder inhibiting TCAS function

On September 29, 2006, a brand-new Embraer Legacy 600 corporate jet crashed into a Gol Transportes Aéreos Boeing 737-800 over the Amazon rainforest, killing all aboard Brazil's worst aviation tragedy ever. The primary issue was that the Embraer's transponder had been turned off accidentally, which meant:

• TCAS could not work on the Legacy 600.
• The Gol Flight 1907 TCAS could not detect the Embraer, making it invisible to the collision avoidance system.
• The 737 crew was never notified of the oncoming plane, and the Embraer pilots were not provided with an RA to start evasive action.

The left wingtip of the Legacy 600 hit the 737's fuselage, which ruptured open during flight. The Embraer, which lost part of its wing, landed safely. 

This accident demonstrated the importance of transponders always being in working and active mode. According to regulatory bodies, flight crews must certify transponder operability before and during flights.

Fribourg Near-Collision (Switzerland)

• Aircraft Involved: Hahn Air Flight 201 (Raytheon Premier I) and Germanwings Flight 2529 (Airbus A319)
• Cause: ATC error resulting in conflicting altitude clearances

Germanwings Flight 2529 descended into Zurich Airport on July 10, 2011, and Hahn Air Flight 201 was climbing in the same sector. Geneva ATC cleared the Airbus to descend to FL250, and Zurich ATC cleared the Hahn Airplane to climb to FL270. TCAS presented an RA to descend the Airbus and climb the Raytheon, with which the two crews complied. 

Nine seconds later, according to TCAS, ATC incorrectly commanded the Raytheon to descend while still climbing. This created a hazardous reversal situation, in which the planes came almost head-on to a collision of at least 100 feet (30 meters). 

A mid-air crash catastrophe could have resulted if each crew had followed ATC's incorrect commands instead of TCAS. This accident demonstrated that TCAS has absolute overriding authority over ATC commands in conflict situations.

Conclusion

TCAS is a monumental technology in the aviation industry that has evolved for years and is only getting safer. By constantly monitoring airspace, alerts, and RAs, a safety standard can be set to avoid mid-air collisions. 

In the upcoming years, TCAS will advance even further to ACAS X, advancing predictive algorithms, mitigating false warnings, and facilitating horizontal maneuvering advisories. It will also further increase collision avoidance, making air transportation safer than it already is.

REFERENCES: NBAA, GOV.UK, SUST.ADMIN.CH

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