The safety nets available to civil aviation
A "safety net" is any technical device that comes into operation if aircraft fail to maintain the minimum separation distances required. These systems may be installed aboard the aircraft (as with ACAS/TCAS and GPWS), or they may be installed on the ground (such as STCA and MSAW) as part of the air traffic management system. The on-board and ground systems are not interconnected.
ACAS/TCAS (Airborne Collision Avoidance System/ Traffic Collision Avoidance System)
is an on-board system that warns the pilots of a possibly dangerous situation arising from the approach of another aircraft. The way these systems work is explained at length in various documents, such as those that can be found on Eurocontrol’s www.eurocontrol.int/acas/ webpages.
ACAS/TCAS gives the pilots different types of warnings, of which Resolution Advisories (RAs) are the most important. An RA will tell the pilot to climb or descend to protect their aircraft. The air traffic controller on the ground will only learn that an RA has been issued by ACAS/TCAS if they are informed of this by the pilot
STCA (Short Term Conflict Alert)
is a ground-based air traffic management system that alerts an air traffic controller via visual and/or acoustic warnings that a potential conflict is emerging and they need to intervene to resolve it. Pilots have no possibility of receiving an STCA warning.
MSAW (Minimum Safe Altitude Warning)
is a ground-based safety system that is part of the radar system. The MSAW will issue an alarm if an aircraft deviates from its prescribed course in a way that brings it into possibly dangerous proximity to the terrain. The visual and acoustic warnings will only be seen and heard by the controller.
GPWS (Ground Proximity Warning System)
is an on-board system that warns the pilot when the aircraft gets close to the ground.
More info:
- STCA-ACAS: Near-simultaneous operation of pilot and air traffic controller collision warning systems in the event of a conflict require better interaction coordination to improve overall safety, identified a recent report by the Civil Aviation Safety Office and Eurocontrol (media release of June 18, 2007 by the Department of the Environment, Transport, Energy and Communications)