Part 8. Collision Avoidance
• An airborne collision avoidance system (ACAS) is an aircraft system that
operates independently of ground-based equipment and air traffic control in
warning pilots of the presence of other aircraft that may present a threat of
• A traffic collision avoidance system or traffic alert and collision avoidance
system (both abbreviated as TCAS) is an aircraft collision avoidance system
designed to reduce the incidence of mid-air collisions between aircraft. It
monitors the airspace around an aircraft for other aircraft equipped with a
corresponding active transponder, independent of air traffic control, and
warns pilots of the presence of other transponder-equipped aircraft which
may present a threat of mid-air collision (MAC). It is a type of airborne
collision avoidance system mandated by the ICAO to be fitted to all aircraft
with a maximum take-off mass (MTOM) of over 5700 kg (12,586 lbs) or
authorized to carry more than 19 passengers.
• ACAS / TCAS is an aircraft system based on secondary surveillance radar (SSR)
transponder signals, which operates independently of ground-based
equipment to provide advice to the pilot on potential conflicting aircraft that
are equipped with SSR transponders.
• A ground proximity warning system (GPWS) is a system
designed to alert pilots if their aircraft is in immediate danger
of flying into the ground or an obstacle. The United States
Federal Aviation Administration defines GPWS as a type of
terrain awareness warning system (TAWS). More advanced
systems are known as enhanced ground proximity warning
systems (EGPWS)
Medium-Term Conflict Detection (MTCD) is a flight data processing
system added functionality designed to warn the controller of potential
conflict between flights in his area of responsibility in a time horizon
extending up to 20 minutes ahead.
Short Term Conflict Alert (STCA) is an automated warning system for air traffic
controllers (ATCO). It is a ground-based safety net intended to assist the controller in
preventing collision between aircraft by generating, in a timely manner, an alert of a
potential or actual infringement of separation minima.
Safety Nets
In extend to STCA other ground based safety net functions are typically implemented
to support the ATCO. These in include:
• Minimum Safe Altitude Warning (MSAW); and
• Area Proximity Warning (APW)
The equivalent system on board an aircraft is TCAS

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