What happened
On the evening of 25 October 2000, the Christchurch main trunk air traffic services centre experienced a sudden and total loss of power to its GAREX radio and telephone communications system. The outage lasted approximately five minutes and occurred during non-routine maintenance intended to upgrade the system's battery power packs.
While the outage disrupted all standard radio and telephone communications through the controllers' consoles, the impact was mitigated by the fact that the incident occurred during a period of low traffic. Some aircraft departures were delayed, and several in-flight aircraft were temporarily held until instructions could be re-established. Because the outage was brief, no in-flight difficulties or collisions occurred.
The investigation
An investigation by the Commission revealed that the power loss was the result of an unidentified single failure point within the system's circuitry. During the maintenance process, technicians removed a fuse intended to protect the standby battery power pack. While they had performed a scenario analysis, they did not anticipate that removing this specific fuse would trigger a total system shutdown.
The investigation found a wiring deficiency: the solenoid-actuated switch, which controls the connection to power sources, became de-powered when the fuse was removed and the battery pack was disconnected. This created a latent vulnerability where the system could be inadvertently shut down by actions that appeared safe.
Furthermore, the investigation noted that while some controllers had access to independent battery-powered radios, three area sectors (Taranaki, Kaikoura, and South) lacked this independent backup. Additionally, the investigation found that the emergency section of the Instrument Flight Guide (IFG) did not yet contain instructions for pilots facing a loss of air traffic control communications, as a previous safety recommendation had not yet been implemented due to procedural delays.
Findings
- The power loss was an unanticipated consequence of maintenance caused by a hidden single failure point in the GAREX circuitry.
- The controllers' ability to manage the disruption would have been bolstered if all controllers had immediate access to independent telephones.
- The risk of future disruptions is reduced by the ongoing implementation of independent standby radio systems for all area sectors.
Safety action
Following the incident, the Airways Corporation of New Zealand Limited implemented a recommendation to ensure all controllers have access to a telephone independent of the main GAREX system, using either separate telephone circuits or cellular phones. Additionally, a modification to the GAREX wiring was performed to ensure the solenoid power supply is connected before the battery power pack fuse, effectively eliminating the identified single failure point.