What happened
On May 27, 2015, at 07:15 UTC, a significant electrical failure occurred at the Belgocontrol facility in Steenokkerzeel, Belgium. The incident took place during a routine verification of the facility's standby diesel-powered generators. While the initial power loss was managed by the Uninterruptible Power Supply (UPS) system, a subsequent disruption occurred when non-essential systems were reconnected to the network. This caused the electrical power to the CANAC 2 surveillance room to fail, instantly disabling primary and backup radar displays as well as the main voice communication systems used to contact aircraft.
At the time of the outage, 28 aircraft were under the supervision of the Brussels FIR. Although radar and standard radio links were lost, air traffic controllers utilized the Last Resort (Radio) System, including the emergency frequency 121.5MHz, along with mobile phones and flight tracking websites to maintain safety. Controllers implemented an emergency "clear the sky" procedure, directing inbound flights to land at local towers or divert to neighboring sectors. By 07:30, all aircraft were either safely on the ground or had exited the Brussels FIR, and no loss of separation between aircraft occurred.
The investigation
The AAIU(Be) investigation focused on the sequence of electrical failures and the adequacy of the facility's contingency planning. The investigation established that the disruption was triggered by a defect in an electrical motor within an industrial air conditioning unit. This defect created a significant imbalance in the power distribution network. A critical finding was that the neutral wire of the diesel-powered generators had not been earthed, which allowed the imbalance to cause an overvoltage that damaged vital equipment in the control room.
Findings
- The primary cause of the service disruption was an electrical power failure occurring during a routine check of standby generators.
- A cascade of events began with a faulty motor in an air conditioning unit, leading to a power distribution imbalance.
- The absence of an earthed neutral wire in the generator setup contributed to the overvoltage of critical ATC equipment.
- The existing contingency plans at Belgocontrol did not account for a total loss of the electrical power supply.
- The investigation noted that while the controllers' rapid response prevented an accident, the lack of specific build specifications requiring neutral wire verification allowed the earthing defect to remain undetected.