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Although air travel is considered to be the safest mode of travel based on statistical data, it is not so in reality. Air travel is the riskiest mode of travel and requires extreme precision in the implementation and use of its technology to ensure safe journeys. Air accidents or crashes can be loosely classified based on their cause as follows:

  • Mid-air collisions
  • Engine failures
  • Cabin depressurization
  • Mechanical defects
  • Fuel leaks
  • Aircraft Cooling System failures
  • Bad weather conditions
  • Pilot fatigue
  • Autopilot malfunction
  • Airplane stalls
  • Landing gear failure
  • Communication failure with ATC

While European nations had already adopted new technology for the avoidance of mid-air collisions, India was lagging. The government didn’t realize the importance of these systems until the Charkhi-Dadri mid-air collision took place in 1996. Before 1996, the ATC tower of Delhi airport only gave information regarding the position of incoming and outgoing aircrafts in airway corridors (called primary radars) but did not give any indication as to the aircraft’s altitude or identity (secondary surveillance radar). This proved to be devastating as they failed to avoid the deadly mid-air collision that took place over Charkhi-Dadri.

Moreover, at that time, all incoming and outgoing aircrafts used the same air corridor as most of the airspace around Delhi airport was taken up by Indian Air Force. Most airports worldwide used separate corridors for departures and arrivals by that time. All crew members and passengers of both the planes died in the collision. Taking into consideration the precarious nature of air travel, the government introduced amendments to the air-traffic management systems of all Indian airports.

Modern Traffic Alert and Collision Avoidance System (TCAS), which is currently indispensable for international and domestic flights in the majority of countries’ airspace, must meet several ICAO (International Civil Aviation Organization) standards. TCAS communicates with a conventional airborne mode S transponder to get altitude information. The Mode S transponder is a component of the secondary radar beacon ATC system, which was created many years before the TCAS system.

Modern aircrafts implement several collision avoidance systems while they are airborne. This article describes one of these systems in brief.

Traffic Collision Avoidance System (TCAS)

A transponder is a device that responds to a signal by emitting a separate signal. In the context of aviation, these transponders receive a signal in the form of a radio-frequency interrogation. Radar is a detecting device that employs radio waves to identify an object’s distance, angle, or velocity. Air Traffic Control (ATC) uses two types of radars to retrieve information about aircrafts:

1.Primary radar : It is a standard radar sensor that uses an electromagnetic wave to light a broad area of space and then receives reflected waves from targets inside that area. As a result, the phrase refers to a radar system that detects and locates potentially hostile objects.

2.Secondary Surveillance Radar (SSR) : Unlike primary radar systems, which use detected radio signal reflections to determine a target’s bearing and distance, secondary surveillance radar relies on targets equipped with a radar transponder, which respond to each interrogation signal by transmitting encoded data such as an identity code, the aircraft’s altitude, and other data depending on the mode selected. The transponder on the aircraft consists of two parts:

  • A radio receiver : Receives on 1030 MHz frequency
  • A radio transmitter : Transmits on 1090 MHz frequency

Transponder codes

Transponder codes are four-digit numbers sent by an aircraft’s transponder in response to a secondary surveillance radar interrogation signal to help air traffic controllers separate traffic. Air traffic controllers give a discrete transponder code (also known as a squawk code) to each aircraft in a flight information zone to identify it individually (FIR). This enables airplanes to be easily identified on radar. The dials of a transponder read from zero to seven inclusively, and codes are made up of four octal (numbers from 0-7 expressed as a power of eight) numbers. Transponders with four octal digits may represent up to 4096 distinct codes, which is why they’re frequently referred to as “4096 code transponders.” When giving a transponder code to an aircraft, air traffic control units use the word “squawk,” for example, “Squawk 7321.” As a result, “squawking 7321” can be translated as “choose transponder code” or “I have picked transponder code 7321.”

Classic modes of function of transponders

These transponders function in different modes and the aviation industry uses specific modes for the retrieval of specific data related to the aircraft. These modes are:

Mode A : The equipment of this mode transmits a transponder code only.

Mode C : The ATC can view the aircraft’s height or flying level automatically with Mode C equipment.

Mode S : This is an advanced version of Mode C and is mandatory in all civil aviation procedures. The SSR system, TCAS receivers onboard aircraft, and the ADS-B SSR system all receive data from Mode S transponders. ADS-B stands for Automatic Dependent Surveillance-Broadcast in which an airplane detects its location using satellite navigation or other sensors and transmits it regularly, allowing it to be tracked. Mode S transponders have reverse compatibility, meaning that an aircraft equipped with one may still respond to Mode A or C interrogations. Mode S transponders not only provide information about the altitude but also permit data exchange.

The Mode S transponder is usually a distinct piece of airborne avionics equipment that functions with its antennae. TCAS employs extra antennas and works in the same radio range.

Combined TCAS-Transponder S Assembly

Most modern aircrafts employ a combined multifunctional TCAS- Transponder S system which has all the necessary equipment in one assembly. The TCAS-Mode S Transponder system is meant to ensure a safe separation between aircraft whose predicted trajectories suggest a collision potential, while also minimizing trajectories’ departure or drift from flying parameters mandated by ATC.

Modes of operation of the TCAS-Transponder S system

Although there are many modes of operation of the system only three basic modes are discussed here :

TA (Traffic Advisories Only) is a mode in which only traffic advisories (TA) are generated and no collision avoidance recommendations are supplied. The transponder has three modes of operation: A, C, and S. (the replies contain altitude data).

TA/RA is a mode that provides traffic advisories as well as collision avoidance suggestions; in other words, it is a mode that provides resolution advisories (RA). The transponder has three modes of operation: A, C, and S (the responses carry altitude data).

MODE S (Mode S transponder) is a mode in which the whole system solely functions as a mode S transponder; the collision warning feature is switched to STBY. The transponder has three modes of operation: A, C, and S (the responses carry altitude data).

Classification of alerts produced by TCAS

Classification of alerts produced by the TCAS

1.Traffic Advisories (TAs) : The purpose of traffic advisories (TAs) is to assist pilots in the visual acquisition of intruder aircraft and to warn them of the possibility of a resolution advisory.

2.Resolution Advisories (RAs) : The pilot is advised to use resolution advisories (RAs), which are avoidance maneuvers. An RA will inform the pilot of the vertical rate range within which the aircraft should be flown to avoid a dangerous aircraft. All aircraft having an altitude reporting transponder (Mode S or Mode A/C) may generate a RA. TCAS isn’t required for intruder aircraft. When an intruder aircraft is also equipped with a TCAS system, both systems use the Mode S data connection to coordinate their RAs and choose complimentary resolution recommendations. TCAS cannot identify aircrafts that are not equipped with a transponder or have a transponder that is not working.

ICAO Standards

The International Civil Aviation Organization has devised certain standards for aircrafts in general and aircrafts using TCAS. These standards are different for commercial and non-commercial airliners :

A)For non-commercial aircraft, the International Civil Aviation Organization (ICAO) Standards and Recommended Practices (SARPS) state:

-All turbine-engined airplanes of a maximum certificated take-off mass over 15 000 kg or authorized to carry more than 30 passengers, for which the individual airworthiness certificate is first issued after 24 November 2005, should be equipped with an airborne collision avoidance system (ACAS II).

-All turbine-engined airplanes of a maximum certificated take-off mass over 15 000 kg or authorized to carry more than 30 passengers, for which the individual airworthiness certificate is first issued after 1 January 2007, shall be equipped with an airborne collision avoidance system (ACAS II).

-All turbine-engined airplanes of a maximum certificated take-off mass over 5700 kg but not exceeding 15 000 kg, or authorized to carry more than 19 passengers, for which the individual airworthiness certificate is first issued after 1 January 2008, should be equipped with an airborne collision avoidance system (ACAS II).

B)B) For commercial aircraft the SARPS states :

-From 1 January 2003, all turbine-engined airplanes of a maximum certificated take-off mass over 15 000 kg or authorized to carry more than 30 passengers shall be equipped with an airborne collision avoidance system (ACAS II).

-From 1 January 2005, all turbine-engined airplanes of a maximum certificated take-off mass over 5700 kg or authorized to carry more than 19 passengers shall be equipped with an airborne collision avoidance system (ACAS II).

-All airplanes should be equipped with an airborne collision avoidance system (ACAS II).

All fixed-wing turbine-powered aircraft with a maximum takeoff weight of more than 5,700 kg (12,566 lbs) or more than 19 passenger seats must have ACAS II (essentially TCAS II, version 7.1), according to the European Aviation Safety Agency (EASA). All flights in European Union airspace must comply with this regulation.

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