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Weekly Review #272: Understanding ACAS and TCAS

Written by Kasia Żmudzińska | December 11 2024
Last week EUROCAE published ER-032 'European Industry Position Report on RTCA SC-147 ACAS sXu'. It brought me back to my first internship at EUROCONTROL, where I worked on validating Europe's new airborne collision avoidance system (ACAS) known as ACAS X. But, what is ACAS? And, how is it different to the transport collision avoidance system (TCAS)? Let's explore.
 
Firstly, we need to go back in time. Several mid-air collisions occurred in the US in 1956, 1986, and 1987, followed by another in Überlingen, Germany, in 2002. These incidents accelerated the development of the system we know as ACAS. The term ACAS refers to the technical standard or concept. TCAS, on the other hand, refers to a current implementation.
 
Generally speaking, ACAS is designed to serve as a last-resort safety net to prevent mid-air collisions. It generates two types of alerts: Traffic Advisories (TAs) and Resolution Advisories (RAs). TAs aim to help pilots spot other aircraft and to alert them to be ready for a potential resolution advisory. In contrast, RAs recommend avoidance manoeuvres to the pilot, such as 'level-off', 'climb', or 'descend'. 
 
One of the common challenges of ACAS is the unnecessary issuing of RAs, often caused by high vertical rates of climb or descent. While high rates can save fuel and time, they can become a problem when aircraft continue to climb/descend with a high vertical rate close to their cleared level. ACAS will issue an RA when it calculates a collision risk based on the two aircraft's closing speed. A high vertical rate before levelling off may cause ACAS to predict a conflict with another aircraft, even when each crew is following the appropriate ATC instructions. This is just one example of system issues that have led to the development and validation of new generations of ACAS. 
 
The ACAS X version of the system appears in different variants: ACAS Xa for general purpose, ACAS Xo for specific operations like parallel approaches, ACAS Xu for unmanned aircraft, and ACAS sXu for small, unmanned aircraft. 
 
This topic is complex. But if you want to know more, Eurocontrol's page on ACAS is a great starting point. In the meantime, check out this week's news selection below. 
 
Kasia

ANSP news

  • DSNA and Aireon Collaborate for Enhanced Air Traffic Surveillance- The partnership between Frances DSNA and Aireon aims to utilize space-based ADS-B data for enhanced air traffic surveillance in the Cayenne FIR region. By integrating Aireon's advanced technology, DSNA seeks to improve overall airspace management and optimize air traffic control operations. This collaboration signifies a significant step towards enhancing aviation safety and efficiency in the specified airspace.
  • Airservices Australia Air Traffic Controllers Ready to Guide Santa This Christmas- Learn how Airservices Australia's air traffic controllers are gearing up to ensure a smooth and safe journey for Santa Claus this Christmas. Despite the busy holiday season, these professionals are ready to guide Santa through the skies to deliver presents around the world. Working diligently behind the scenes, they play a crucial role in managing air traffic during this festive period.
  • NATS signs contract extension for the provision of Air Traffic Controllers to Hong Kong International Airport - Airport Authority Hong Kong (AAHK) has agreed a contract extension with NATS Services to support the on-going secondment of Air Traffic Controllers. The partnership, originally signed in 2018, will see NATS extend its commitment to provide world class controllers for Hong Kong International Airport until 2029.
  • DFS control tower at Hannover Airport celebrates its quarter century anniversary- Twenty-five years ago, DFS, the German air navigation service provider, opened the new control tower at Hannover Airport to be prepared for the increased traffic volumes expected for EXPO 2000. The airport in the capital of Lower Saxony was the first airport in Germany to be equipped with the uniform tower model.
  • Airservices Australia renews airspace management agreement with the Solomon Islands- Airservices Australia has renewed its agreement with the Solomon Islands government to continue providing upper airspace management services for an additional five years. The renewed agreement ensures the provision of area control services in Class A airspace (above 24,500 feet) within the Honiara Flight Information Region, which is managed from Airservices’ Brisbane Air Traffic Services Centre. “Renewing this partnership further strengthens Australia’s relationship with our Pacific neighbours and highlights our commitment to safe and efficient aircraft operations across the region,” said Airservices Interim CEO, Rob Sharp.

Airport news

Market news

  • Contract sign for a Ground Simulator Training Device - Aerodata AG- Armed Forces of Malta have signed a contract for their own ground simulator training device with a complete simulation of sensor technology and generation of typical scenarios. This training station, contains a cockpit simulation including an AeroMission® Workstation whereby the entire crew, the cockpit and the cabin, can thus be trained on the ground with the help of various training tasks. This allows the operator to learn how to use the mission system and also train targeted communication between the pilot and the operator.
  • Jotron Ricochet to record Belgian air traffic- Jotron will provide fully redundant recorder systems with the capability to record and synchronously replay over 2000 audio channels. The contract encompasses installation at 6 sites throughout Belgium, including the country’s main airport in Brussels. All sites will possess an independent recording and replay system. Jotron will also supply a centralized replay system at skeyes’ headquarters at CANAC Air Control Center in Brussels.

Context information

  • QCAA Organizes Training Program for Egyptian Air Traffic Controller- As part of the ongoing collaboration between the State of Qatar and the Arab Republic of Egypt in the field of civil aviation, and their shared commitment to enhancing bilateral cooperation and exchanging expertise, Qatar Civil Aviation Authority organized a training program for the third group of Egyptian air traffic controllers.
  • Madrid hosts the Eighth Conference on Safety and Just Culture to improve the operational safety of air transport- The legal and aviation sectors are coming together at the Eighth Conference on Safety and Just Culture, being held in Madrid at the Palacio de Zurbano. The aim of the conference is to share the various needs regarding the processing of information and the protection of air transport professionals in aviation incidents, in order to promote the continuous improvement of safety.
  • Revolutionizing Aviation Training: The Ahlex Initiative- Discover the latest innovation in aviation training with Ahlex. This new standard is set to revolutionize the industry by providing state-of-the-art training methods and cutting-edge technology. Find out how Ahlex is shaping the future of aviation education and helping prepare the next generation of skilled professionals.
  • The CANSO Latin America and Caribbean Conference 2024 is focusing on the next generation of air traffic professionals- As the conference begins, delegates engage in discussions and workshops focused on addressing key challenges in air traffic management. Topics include improving safety, enhancing efficiency, leveraging innovative technologies, and preparing the next generation of air traffic professionals. The event also highlights the importance of collaboration and sustainability in shaping the future of aviation in the region.
  • Single European Sky Regulation enters into force, aiming for a more efficient and sustainable airspace - European Commission- Yesterday the new Single European Sky (SES2+) Regulation entered into force, in a step forward towards a more efficient, and more sustainable European airspace. SES2+ creates rules and incentives for monopoly air navigation service providers to pursue more efficiency gains and adopt modern technologies, with the aim to alleviate congestion in Europe’s airspace and to provide better quality services, but also foster innovation and the development of new services in the sector. These aspects will help mitigate situations such as this summer, when air travel was severely disrupted, with nearly every other flight experiencing delays due mostly to overwhelming congestion in Europe’s airspace.

Research and innovation

  • Frequentis commits to transition to a new service delivery model in air traffic management - SESAR JU- On 6 December, Frequentis officially joined a growing list of industry leaders committed to transitioning to a new service delivery model for air traffic management in Europe. By signing the joint statement, Frequentis reinforces its dedication to advancing the modernisation and digitalisation of European airspace, as envisioned in the European ATM Master Plan.
  • UK CAA announces new AI strategy for aviation sector- The UK CAA has unveiled a groundbreaking AI strategy aimed at transforming the aviation sector. This new initiative promises to revolutionize the way airlines operate, enhancing efficiency and safety through the use of artificial intelligence.
  • Collaborating to increase flexibility of controller validations - NATS Blog- As part of our contribution towards the SESAR programme, we have been working on the next phase of the Increased Flexibility in Air Traffic Controller Validations project. The IFAV project intends to bring in tools for controllers to reduce the sector-specific knowledge currently needed for the licences, aiming to allow controllers to work more flexibly over a wider area. As part of this goal, we are also exploring whether increasing automation in the systems the controllers work on could potentially lead to more generic controller endorsements in future.

UAV and UTM
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