Civil Aviation Authority of New Zealand
16 April 2012


For further information contact:

Senior Communications Adviser, Emma Peel
Tel: 04 560 9646, or 027 272 3545


Understanding Traffic Alert and Collision Avoidance systems

The CAA has today classified as minor an event last week in which an Air New Zealand Airbus aircraft descending to land at Auckland suddenly climbed 1000 feet after receiving a cockpit warning.

The flight was never at risk.

Airline aircraft are fitted with equipment that detects the position of other aircraft via radio transmissions. The equipment is designed as an added safety feature to ensure aircraft are kept separate. The system is known as Traffic Alert and Collision Avoidance System (TCAS), and alerts pilots of the presence of other aircraft in adequate time for them to take action. For example if it detects another aircraft as close as 6 miles horizontally, or 1200 feet vertically, the system will alert the pilot that there is another aircraft nearby. If it senses another aircraft 25 to 45 seconds away, it will audibly warn “traffic, traffic”. If the aircraft comes within 20 to 30 seconds, the system will also give the pilot audible advice such as "descend, descend" or "climb, climb".

The system is a valuable adjunct to safety, however because it does not know what airspace it is in, it can give unnecessary alerts. This happened on Friday when the system detected another aircraft in the uncontrolled airspace below the controlled airspace in which the Airbus was flying. The pilots were correct to follow the TCAS advice.

TCAS alerts are infrequent, but a normal part of aviation life worldwide. On average, the CAA receives reports of about 100 to 150 alerts a year, and of these only a handful require further investigation.

How the civil aviation system works in New Zealand (PDF)