M ANAGEMENT S YSTEM
1.2 The ATM System
The ATM system is a complex, highly interactive engineering sys-tem that involves many organizations and a large number of subsys-tems and components onboard and in the ground. According to the International Civil Aviation Organization (ICAO), the purpose of
THE AIR TRAFFIC MANAGEMENT SYSTEM 5
the ATM system is the provision of a set of airborne and ground functions to ensure the safe, orderly and expeditious movement of air-craft in all operational phases.
The three main functions of the ATM system include (ICAO 2001, 2007a):
1. Air traffic service: ATS is the primary functional component of the ATM system that relies on Flight Information Services (FIS), alerting services, air traffic advisory services and air traffic control (ATC) services (i.e., area control, approach control, and airport tower services).
2. Airspace management: ASM refers to airspace utilization strategies and policies including, management activities for achieving the most efficient use of airspace while avoiding airspace segregation.
3. Air Traffic Flow Management: ATFM enables the safe, orderly and expeditious flow of air traffic by ensuring that ATC capacity is effectively utilized and traffic volume is compat-ible with the capacities declared by the appropriate authori-ties. ATFM flow controllers are employing efficient airspace management tactics and policies by directly interacting with the ATS units.
Figure 1.1 shows a high-level functional representation of the ATM system, where the airborne-based and ground-based parts interact to attain the main system objectives. The airborne-based ATM system includes many onboard systems that provide communication, naviga-tion, and surveillance (CNS) capabilities as well as alerting services (e.g., traffic alert and collision avoidance system [TCAS]). In case that the separation distances between aircraft fall below certain criti-cal values, TCAS generates traffic advisories (TAs) and resolution advisories (RAs) – i.e., collision avoidance advisories in the vertical plane. In complying with a TCAS RA, for example, a pilot may devi-ate from ATC instructions and assume responsibility for traffic sepa-rations since the controllers are no longer responsible for this event.
A vertical profile of flight phases and associated ATC services is depicted in Figure 1.2. In the following sections, the main duties and challenges faced by controllers and pilots will be described with refer-ence to the different ATC units and flight phases.
6 COGNITIVE ENGINEERING AND SAFETY ORGANIZATION
To meet the goals of safe, orderly, and expeditious traffic, the ATM system relies on the smooth interaction between adequately trained and licensed practitioners, highly automated systems, and interna-tional regulations and procedures. The practitioners at the sharp end,
Tower control
Starting up engine Taxiing Taking off
Approach control
Initial climbing
Area control
En-route climbing
Area control Cruising
Area control
En-route descending
Approach control Descending Approaching
Tower control Landing Taxiing Parking
Figure 1.2 Vertical profile of flight phases and Air Traffic Control Services.
Air traffic management (ATM)
Ground-based ATM
Air traffic services
Airspace management Air traffic flow
management
Air traffic control Flight information
services
Air traffic reporting office
Approach control
Aerodrome control Area control
Airborne-based ATM
Figure 1.1 A high-level functional representation of the air traffic management (ATM) system.
THE AIR TRAFFIC MANAGEMENT SYSTEM 7
the technology, and the regulations that compose the ATM system can be assigned into six discrete control elements:
1. Procedures and regulations: They refer to the national and international legislation (ICAO, European Union, EASA, and State legislation) according to which the ATM system operates.
2. Air traffic controllers (ATCOs): The properly trained and licensed practitioners responsible for the provision of ATM services.
3. Automation systems: The computers, displays, Controllers’
Working Positions (CWPs), and the special-purpose soft-ware that provides information related to the status, position, and separation of aircraft.
4. Communication systems: Air–ground, ground–ground, and air–
air voice communications as well as data exchange systems.
5. Navigation systems: They provide real-time 3D positional information to aircraft in order to support navigation through the airspace and movement on the airport.
6. Surveillance systems: They provide near real-time positional and other information to controllers for tracking aircraft and monitoring hazardous weather conditions.
The last three elements are collectively referred to as communica-tion navigacommunica-tion and surveillance (CNS) systems. All control elements of the ATM system are briefly presented in the following sections.
1.2.1 Procedures and Regulations
The operation of the ATM system is described in detail through the annexes, documents, and other guidance material published by ICAO. The overriding goal of ICAO legislation is the harmoniza-tion of internaharmoniza-tional procedures and regulaharmoniza-tions in order to provide a smooth integration of national and international ATM systems.
Hence, many efforts have been made for national differences to be kept to a minimum and to be communicated adequately to airspace users.
The ATC system comprises three interconnected levels supported by the following units:
8 COGNITIVE ENGINEERING AND SAFETY ORGANIZATION
1. Airport control tower (TWR) that provides control services to departing and arriving aircraft.
2. Approach control unit (APP) that provides services to aircraft approaching a terminal maneuvering area (TMA).
3. Area control center (ACC) that provides services to overfly-ing aircraft.
All ATC units incorporate standardized control positions, areas of responsibility (AoRs), CNS systems, standard operating procedures (SOPs), operations manuals, contingency plans, unit training plans (UTP), unit competency schemes (UCS), and letters of agreement (LoAs); they also apply specific separation minima that are deter-mined by the class of the airspace that they control. These elements are elaborated in the aeronautical information publication (AIP) issued by or with the authority of a State which contains aeronautical information essential to air navigation (ICAO 2007a).
The fundamental operating characteristics of the three ATC units are shown in Table 1.1 (ICAO 2001, 2005a, 2007a).
The ATC units provide air traffic services for instrument flight rules (IFR) flights conducted in accordance with instrument flight rules in instrument meteorological conditions (IMC) and
Table 1.1 Fundamental Characteristics of the ATC Units UNIT
CHARACTERISTICS
AIRPORT CONTROL (TWR)
APPROACH CONTROL (APP)
AREA CONTROL (ACC) Control positions Airport controller Executive/tactical
controller
Executive/tactical controller Ground controller Coordinating/
planner controller
Coordinating/planner controller
Delivery controller Area of responsibility Airport traffic zone
(ATZ)
Terminal maneuvering area (TMA)
Control sector
Airspace classification Class D,E Class C,D Class A,B,C Applicable legislation
and procedures
ICAO, European commission, EASA, national legislation, local SOPs, LoAs, operation manual, contingency plan.
Applicable separation minima
Visual Radar Radar
Wake turbulence 2.5–5 Nm horizontally
5–10 Nm horizontally
Time based 1000 ft vertical 1000–2000 ft vertical
THE AIR TRAFFIC MANAGEMENT SYSTEM 9
visual flight rules (VFR) flights in accordance with visual flight rules in visual meteorological conditions (VMC). In IFR flights, controllers are responsible for safe separation from obstacles and other aircraft by providing appropriate services in accordance with the ATS type, available CNS systems, and airspace classification.
In VFR flights, flight crews are responsible for visually separating their aircraft from obstacles by remaining outside clouds.
1.2.2 Air Traffic Controllers (ATCOs)
Controllers remain the cornerstone of the ATM system as they manage to adapt the operation of the system to many irregularities that have not been foreseen in the initial design. Controllers work at the sharp end of the system to ensure safe, efficient, and expedi-tious traffic. The training of controllers is extensive and meticu-lously structured in accordance with international standards. All European Union ATC units usually base their training regimes on two documents:
1. The unit competency scheme (UCS), which indicates the methods by which the units maintain the competence of all licensed controllers
2. The unit training plan (UTP), which details the processes and time frames that allow the unit procedures to be applied under the supervision of an on-the-job training (OJT) instructor.
In general, controller training is divided into four phases that corre-spond to the progression of student controllers to licensed controllers and to special roles (e.g., on-the-job instructor (OJTI), unit assessor, and supervisor). By successfully completing the first two phases, the student becomes a licensed controller. European commission regula-tion 2015/340 provides a detailed framework of technical requirements and administrative procedures relating to air traffic controller licenses and certificates. Table 1.2 briefly explains the four phases of controller training.
1.2.3 Automation Systems
The improved reliability and computational power of modern digi-tal computer systems and their networking capabilities allowed a
10 COGNITIVE ENGINEERING AND SAFETY ORGANIZATION
large scale introduction of automated features in the ATM system.
The uniqueness of digital computers over other machines stems from the fact that practitioners end up having a powerful special purpose machine (Leveson 2012). For instance, a TCAS is a special purpose machine built on a set of algorithmic instructions to accomplish an advisory service. The same applies to a vast array of automation sys-tems built under the simple principle of writing appropriate software for digital computers.
The automated systems in the ATM domain can be divided into two broad systems:
1. The controller’s working positions (CWPs), which constitute the working environment and the tools through which control-lers practice their profession. A CWP consists of a range of standard voice and data input/output (I/O) devices (e.g., key-boards, displays, mouse, VHF headsets, and telephones) and special-purpose software that enable controllers to perform the following tasks:
a. Communicate with aircraft b. Communicate with other units
c. Monitor the functionality status of CNS systems d. Monitor meteorological data
e. Manage flight progress and other type of information f. Manage CWP displays and data presentation
Table 1.2 The Four Phases of Controller Training
TRAINING PHASE DESCRIPTION
Initial training Training on technical subjects, ATC theory and simulator. The objective is to prepare Ab-Initio students for training at ATC units.
Unit training Transitional training between pre-on-the-job (OJT) and OJT training, leading students to obtain an ATCO license, with appropriate rating and unit endorsements.
Continuation training Training for licensed controllers in order to augment their knowledge and skills. It includes refresher training in abnormal situations and conversion training that provides knowledge and skills appropriate to changes in the operational environment.
Development training Training to provide additional knowledge and skills for specific job profiles (e.g., OJTI, Unit Assessor, Unit Supervisor, Team Resource Management, Safety Occurrence Investigation, Safety Assessment and Safety Surveys).
THE AIR TRAFFIC MANAGEMENT SYSTEM 11
g. Operate aeronautical ground lighting systems (e.g., precision approach path indicators, taxiways, stop bars, runway lighting)
2. The decision support systems (DSS), which support decision-making in managing air traffic. DSS can be classified into three main categories:
a. Sequencing managers: Automation systems designed to provide controllers with suggestions about the optimal management of departure and arrival traffic flows under normal conditions. For instance, the departure manager (DMAN) provides information on a calculated departure sequence of aircraft to the runway while the arrival man-ager (AMAN) provides an arrival plan that is monitored and updated regularly by the system.
b. Monitoring aids (MONA): Automation systems that assist controllers in track monitoring and routine clear-ance tasks. Examples are: the route adherence monitor-ing (RAM) that verifies whether aircraft are adhermonitor-ing to their routes and the cleared level adherence monitoring (CLAM) system that verifies whether aircraft are adher-ing to their cleared flight levels.
c. Air traffic flow and capacity management (ATFCM) aids:
These decision support systems are available to flow control-lers and include: enhanced tactical flow management system (ETFMS), integrated initial flight plan processing system (IFPS), and central airspace and capacity database (CACD).
Safety nets are important affordances in the provision of ATM services and work closely together in a control loop that is shown in Figure 1.3. The loop starts when a controller issues an instruction to the flight crew (e.g., a flight level change) using the communica-tion systems. The crew acknowledges the instruccommunica-tion and makes an appropriate input into the autopilot system. The aircraft initiates the commanded change (e.g., a level change) which is captured by ATC surveillance sensors (e.g., the radar). The data on the radar screen are processed in combination with other relevant data (i.e., flight level changes of aircraft in the vicinity) through special computer algo-rithms. The resulted information is depicted on the CWP screens
12 COGNITIVE ENGINEERING AND SAFETY ORGANIZATION
(e.g., visual and/or aural warnings) when the prescribed horizontal and/or vertical distance may be infringed in a certain period. Finally, the controller detects and acknowledges the warnings and intervenes in order to resolve aircraft conflicts.
Although the technology for automatic interventions already exists, controllers are kept in the loop because automation is not allowed to intervene in an autonomous fashion. This is not the case however in aviation, where airlines may allow automation to inter-vene autonomously in order to keep aircraft within a safe flight envelope (e.g., avoiding to exceed a certain bank angle or speed, preventing a stall, etc.). In many cases, following flight automa-tion advisories may be regulatory mandatory. For instance, the TCAS system generates resolution advisories that flight crews are obliged to follow by regulation, irrespective of ATC clearances (ICAO 2007a).
Safety nets are subject to false alarms and technical problems that reduce their reliability. Although reliability engineers may recog-nize these failures, other subtle problems can escape their attention, especially when the interactions between the safety nets are hid-den or implemented in unexpected ways. A case in point concerns the interaction between the short term conflict alert (STCA) and TCAS systems. In the ATC domain, the STCA is designed to warn
Aircraft systems
Flight deck
Flight crews Communication
systems Controllers
Controllers working position Alerting logic Surveillance sensors
Data
Data Data
Data
Command
inputs Instructions
clearances information
Instructions clearances information
Indications advisories
alerts
Aircraft Operations room
Figure 1.3 Safety nets loop in the ATM system.
THE AIR TRAFFIC MANAGEMENT SYSTEM 13
controllers of imminent separation minima infringements. Normally, controllers are warned by STCAs before the activation of TCAS in the aircraft cockpit. However, in some rare cases, the flight crew may get a TCAS TA or RA in the cockpit before the identification of the conflict by the STCA in the radar screen of controllers. In certain conflict geometries, the information update rate of the TCAS system can be faster than the STCA update rate. This implies that control-lers may unexpectedly have to manage a TCAS RA while they were certain that their traffic planning was appropriate. The subsequent vertical movement of the aircraft that responds to the TCAS RA may cause a significant disruption in traffic management as well as second-ary activations of TCAS on other aircrafts.
1.2.4 Communication Systems
Controllers communicate with flight crews, directly using voice com-munications, or indirectly using data links. Air–ground communica-tions include very high frequency (VHF) systems as well as data links for information exchanges. Every ATC unit is assigned a set of fre-quencies that enable controllers to communicate verbally with aircraft using standard radio telephony (RTF) procedures. Communications are vital to the safe and expeditious operation of aircraft while many incidents occurred due to the use of nonstandard procedures and phraseology (ICAO 2007b). The crew–controller communication loop constitutes a confirmation-correction process and includes some degree of redundancy, as illustrated in Figure 1.4. Controllers also communicate with other ATC units or services via land lines. For this purpose, ground voice and data communication networks are installed that enable them to communicate virtually with any other ATM facility in the world using the aeronautical fixed telecommuni-cations network (AFTN).
In VHF RTF communications, crews and controllers cannot use the same frequency simultaneously because when one is transmitting, the other is receiving and vice versa; hence, controllers and crews can-not transmit and receive simultaneously. Even though this technical shortcoming is well known and properly documented, it remains a causal factor in a large number of incidents.
14 COGNITIVE ENGINEERING AND SAFETY ORGANIZATION
1.2.5 Navigation Systems
Navigation systems (i.e., commonly termed navaids) refer to a group of land and space based systems that enable pilots to know their exact position in the airspace or, in the vicinity of an airport. The en-route navigation depends on airways that essentially form a network of
“highways” in the sky. An airway is a control area that forms a sort of corridor in the airspace (ICAO 2007a). In the vicinity of airports, navigation depends on creating funnels for approach and landing with the routes that connect the airport with the surrounding airways. For instance, a Standard Instrument Departure (SID) is created when an IFR departure route links a runway with a specified significant point normally in an airway. With the aid of appropriate flight instrumenta-tion systems, the flight crew can make use of an Instrument Approach Procedure (IAP) in order to maneuver from an initial approach fix, or the beginning of an arrival route, to a point for landing in the runway.
Navaids can be used either in the vicinity of an airport for the pur-pose of approach and landing or for en-route navigation. The main characteristics of mainstream ground navaids are described in the Table 1.3 (ICAO 2006a, b, c).
The quality of the required navigation information may differ in each phase of the flight. For the approach and landing phases, the requirements for signal accuracy are the most stringent due to the close proximity of the ground and the limited maneuvering potential of the aircraft. Moreover, the requirements for availability, reliability, and integrity are higher in the approach and landing phases than the en-route phase.
Aircraft cockpit Operations room
Listen/transmit
Controller’s hearback Pilot’s readback
ATC clearance
Acknowledge/correct Transmit/listen
Figure 1.4 Flight crew–controller communications loop.
THE AIR TRAFFIC MANAGEMENT SYSTEM 15
1.2.6 Surveillance Systems
Surveillance is the function that provides controllers with aircraft information about range, bearing, and altitude. ICAO (2007a) pro-vides detailed guidance for the surveillance function which can be accomplished as follows:
• Pilot reporting: Using voice communications, flight crews can report aircraft position in reference to certain navaids.
• Responses of primary surveillance radar: A PSR is a sort of radar system that utilizes a rotating antenna in a ground station that emits electromagnetic pulses that are reflected by the metallic exterior of the aircraft and returned back to the antenna. This is a noncooperative form of surveillance because it does not require the cooperation of the aircraft (carriage of a transpon-der device). PSR can also provide significant weather data such as storm cells positions and areas of precipitation. The PSR is useful in cases of detecting noncooperative aircraft that affect traffic planning (e.g., military traffic, aircraft with nonfunctioning transponders).
• Returns from secondary surveillance radar: The SSR uses a rotat-ing antenna in a ground station that emits interrogation mes-sages in the form of electromagnetic train pulses that trigger automatic responses from the transponder of the aircraft and are subsequently received by the antenna.
Table 1.3 Characteristics of Mainstream Ground-Based Navaids
NAME
ILS
(INSTRUMENT LANDING SYSTEM)
VOR/DME(VERY HIGH FREQUENCY OMNI DIRECTIONAL RANGE/
DISTANCE MEASURING EQUIPMENT) Operational
description
Enables vertical and lateral guidance during approach and landing
Enables short range en-route navigation and approach
Guidance Vertical and lateral Lateral (range and bearing) Three levels of precision
approaches
En-route navigation
(ILS category I – II – IIIa, IIIb, IIIc) (VOR-VOR) Operational use Approach and landing En-route & approach Instrument
procedures
Instrument approach procedures (IAPs)
IAPs, SIDs, STARs, holdings
16 COGNITIVE ENGINEERING AND SAFETY ORGANIZATION
• Automatic dependent surveillance: ADS is a data-link that peri-odically broadcasts the state vector of the aircraft and other flight information (e.g., estimated time over the next way-points, weather data, and navigation data). The ADS-B system improves the use of airspace, reduces ceiling/visibility restric-tions, improves surface surveillance, and enhances conflict management.
Most of the previous systems represent the legacy surveillance function currently used in ATM systems while the ADS represents the near future.
Powerful radar data processors (RDPs) transform raw data from PSR returns and SSR responses received via radar antennas into digitized aircrafts tracks on radar displays. The complex progression of signal reception and processing is referred to as surveillance processing chain.
Most radar systems provide controllers with the following infor-mation for all aircraft carrying a transponder:
• Identification derived from SSR Mode A
• Callsign of the aircraft derived from SSR Mode A and flight plan correlation
• Altitude derived from SSR Mode C
• Velocity Leader derived from RDP Processing
• Ground Speed derived from RDP Processing
• Attitude Indicator derived from SSR Mode C and RDP processing
Figure 1.5 shows an example of a correlated track (e.g., a SSR return coupled with flight plan details) depicted on the radar screen of controllers. The surveillance system displays an integrated picture related to aircraft position and other information known as correlated track. In essence, the track is a digital representation of the aircraft state vector information.