Safety nets are ground-based or airborne system functions that alert controllers or pilots to an increased risk to flight safety (airborne collisions, violations of traffic segregation rules, controlled flight into terrain (CFIT) accidents, etc). Ground-based and airborne safety nets operate independently, often within the same airspace.
The ground-based safety nets Short Term Conflict Alert (STCA) and Area Proximity Warning (APW) work alongside their airborne equivalent Airborne Collision Avoidance System (ACAS), also known as Traffic alert and Collision Avoidance System (TCAS). The Airborne Collision Avoidance System II (ACAS II) has been introduced in order to reduce the risk of mid-air collisions or near mid-air collisions between aircraft. It serves as a last-resort safety net irrespective of any separation standards.
The ground-based safety nets Minimum Safe Altitude Warning (MSAW) and Approach Path Monitor (APM) assist in preventing a controlled flight into terrain accidents, and should work in concert with their airborne equivalent (Enhanced) Ground Proximity Warning System ((E)GPWS), also known as Terrain Awareness and Warning System (TAWS).
The Safety Nets Performance Improvement Network (SPIN) sub-group was created to draft specifications and guidance material for STCA, MSAW, APM and APW, and to propose ECIP objectives for ECAC-wide implementation of those specifications. SPIN is the main stakeholder group for EUROCONTROL's safety nets work. The group meets twice yearly and welcomes new members from ANSPS, regulators and industry.
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