TCAS Presentation

Mid Air Collisions (MAC)

Mid air collisions involve two aircraft making physical contact with each other whilst in flight (Skybrary, 2019).

It is considered to be one of the most sudden and catastrophic accidents imaginable.

• Usually involves tens/hundreds of people
• Almost certain fatality rate
• Risk of damage to life/property from falling debris
• Involves vehicles traveling at high speeds

TU-154 and DHL 757 Collision over Überlingen 2002 (Fighter Jets World, 2019).

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Beginnings of Collision Avoidance Systems

Dates back to the mid-1950s when a mid air collision happened between two U.S. air carrier aircraft over the Grand Canyon.

Different approaches were taken in making and using Avoidance Systems.

In 1974 the Federal Aviation Administration (FAA) began focusing on the Beacon Collision Avoidance System (BCAS).

(Kuchar and Drumm, 2007)

1956 Grand Canyon Disaster (Reimann, 2017).

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The Birth of TCAS

A second mid air collision over San Diego 1978 between an Air Carrier and General-Aviation aircraft.

1981 BCAS name changed to Traffic Alert and Collision Avoidance System (TCAS)

A third collision in 1986 near Cerritos lead Congress in 1987 to implement legislation requiring the FAA to have an airborne collision system at the end of 1992 (extended to 1993).

Applied to all turbine aircraft carrying over 30 passengers.

(Kuchar and Drumm, 2007)

San Diego Flight 182 crash (Derner, 2015).

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Impact zone of the 1986 Cerritos plane crash (Grobaty, 2016).

International TCAS - ACAS

Changes to TCAS.

Resulting in 'Version 7' or Airborne Collision Avoidance System (ACAS).

Began in 2003 and now is a mandate for aircraft with:

• Over 19 passenger seats
• Maximum take-off weight of 5700kg+

Equipped to over 25,000 aircraft world wide

(Kuchar and Drumm, 2007)

Integrated TCAS Solution (Rockwell Collins).

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TCAS II

Updates from 7.0 to 7.1 improving the safest option (Skybrary, 2019).

TCAS II (version 7.1)

Developed by 'EUROCONTROL' but was undertaken by both RTCA (Radio Technical Commission for Aeronautics) and EUROCAE (European Organisation for Civil Aviation Equipment.

Commands to the pilot such as "Level off, level off" to avoid collisions.

(Skybrary, 2019)

Mandated by FAA for 30+ passengers or 33,000lbs.

(FAA, 2011)

Updates from 7.0 to 7.1 allowing for Reversal Logic (Skybrary, 2019).

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Surveillance

Diagram showing an aircraft interrogating another (Romli).

'Surveillance' is the first line of action for TCAS.

No need for any ground inputs.

Antennae on the aircraft transmit a 1030MHz signal to interrogate other aircraft each second at closer ranges (5 seconds for longer).

Nearby aircraft picks up signal with transponder and replies with a 1090MHz signal.

Reliable range of 14nmi in traffic densities of up to 0.3 aircraft per nmi.

(Federal Aviation Administration, 2011)

Directional Transmission of an aircraft (Federal Aviation Administration, 2011).

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Transponders

Two types of transponders are used:

• ATCRBS (Air Traffic Control Radar Beacon System)
• Mode S

TCAS can track up to 30 transponders within 30nmi.

Mode S Surveillance.

Address known as ICAO 24-bit.

Used to find the range, bearing and altitude of the aircraft.

(Federal Aviation Administration, 2011)

Graph showing 'Whisper-Shout' Interrogation(Federal Aviation Administration, 2011).

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Threat Detection and Display

Algorithms are used to decode the range, bearing and altitude signal.

Trajectory Extrapolation

Linear Extrapolation using current velocity

Gives a path that the aircraft is currently following

Threat Detection

Looks at the trajectory and compares with its current path

Tau - Time until the closest point between aircraft

Traffic Display

Display in cockpit shows nearby aircraft giving their:

• Range
• Bearing
• Relative Altitude

And shows if they are climbing or descending by displaying an arrow.

'Non-threatening' aircraft further away show as hollow diamonds.

(Kuchar and Drumm, 2007)

TCAS Traffic Display as shown in cockpit (Aerosavvy, 2015).

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Traffic Advisory (TA)

TA shown when collision point is within 3.3nmi or 40s.

Seen to be an intruder but not critical.

Shown on the TCAS display as an intruder directly from the threat detection process.

(Skybrary, 2019)

Intruder regions for TA and RA (Skybrary, 2019).

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Traffic Advisory (TA)

TCAS Traffic Display as shown in cockpit (Aerosavvy, 2015).

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Threat Resolution Resolution Advisory (RA)

When within 2.1nmi or 25s from collision a TA becomes an RA

This is shown on the Display as a different icon

(Skybrary, 2019)

Intruder regions for TA and RA (Skybrary, 2019).

TCAS Traffic Display as shown in cockpit (Aerosavvy, 2015).

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Threat Resolution Resolution Advisory (RA)

When RA criteria is met a threat-resolution algorithm decides what the pilot should do.

It generates this selection in two stages:

• Firstly - If the aircraft needs to climb or descend
• Secondly - how rapidly the altitude needs to change

TCAS only recommends changing altitude as the bearing accuracy is not considered accurate enough.

A delay of 5 seconds is assumed.

Then a 0.25g of vertical acceleration before reaching 1500ft/min.

(Kuchar and Drumm, 2007)

TCAS algorithm provides largest separation to the other aircraft (Kuchar and Drumm, 2007)

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TCAS algorithm deciding how fast it should maneuver (Kuchar and Drumm, 2007)

TCAS II (Version 7.1) Updates

Due to some issues with TCAS some changes were made to simplify the system.

Updates from 7.0 to 7.1 improving the safest option (Skybrary, 2019).

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Updates from 7.0 to 7.1 allowing for Reversal Logic (Skybrary, 2019).

TCAS II - Advising the Pilot

Different advisories made by the TCAS II system (Skybrary, 2019).

These commands are played to the pilot in the cockpit through speakers to ensure they notice them.

The pilot is required to comply and if the A/C deviates from its ATC clearance the pilot is not responsible.

(Skybrary, 2019)

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