Air traffic controllers calling in sick on March 30, 2012, left two ATC sectors–Kimberley and Cable–too short of personnel to keep them operational, according to a recent report from the Australian Transport Safety Bureau (ATSB). Airservices Australia, the air navigation service provider, erected a temporary restricted area (TRA) designed to prevent aircraft from entering because there was no one to provide necessary ATC service. Despite the TRA, two Airbus A330s somehow entered and flew through the unmonitored airspace.

Airservices Australia said there was no loss of separation in the incident, but it did register the events as “losses of assurance of separation.” In one case, ATC first became aware that one of the A330s (flying from Denpasar, Indonesia, to Melbourne in Australia) was on frequency only when the crew called for instructions as they approached the Melbourne FIR boundary. The second A330 (flying from Denpasar to Sydney) went unnoticed until it too approached the same FIR boundary.

The ATSB identified four safety issues, including the failure of the Airservices Australia process for notifying other air traffic services providers of TRAs, as well as three more that related directly to line and management processes for guaranteeing staffing levels met operational requirements.