In late 1992 the Bureau of Air Safety Investigation (BASI) noted a large increase in violations of controlled airspace (VCAs). Whereas in most of 1991, there had been around 25 such incidents per month, in December 1991 there were 42 of these incidents and throughout 1992 BASI received an increasing number of VCA notifications. The increase in VCAs was of particular concern as it may have reflected difficulties with the implementation of the Airspace Management and Air Traffic Services ( AMATS ) changes to Australian airspace. A VCA is a serious incident because of the risk of a collision between the intruder and an aircraft under air traffic control (ATC).