What happened
On 25 April 2011, a Ryanair Boeing 737-800, registration EI-DAD, was performing a scheduled commercial flight from Skavsta Airport to Paris Beauvais. Shortly after takeoff, the crew received a 'Source Off' warning, indicating that one of the aircraft's two generators had disconnected. While the aircraft's automated systems initially redistributed power to maintain both electrical systems, the situation escalated during the crew's attempt to rectify the fault.
As the pilots followed the Quick Reference Handbook (QRH) to reconnect the generator, a secondary failure occurred. The electrical connection between the two main buses was severed, causing the second transfer bus (X-Bus2) to lose all electrical power. This resulted in a cascade of system failures, including the loss of flight instrument displays on the first officer's side, the disengagement of the autopilot, and the loss of critical functions such as pitot heating, flap indications, and transponder altitude reporting. The crew subsequently decided to return to Skavsta Airport.
The investigation
The Swedish Accident Investigation Authority (SHK) examined the aircraft's electrical architecture, specifically the Generator Control Unit (GCU) and the Bus Power Control Unit (BPCU). The investigation focused on why the attempt to reconnect the generator led to a total loss of power on one bus and why the Auxiliary Power Unit (APU) could not be used as a backup power source.
Technical analysis and simulations revealed that an intermittent short circuit between phases in the feeder lines from the Integrated Drive Generator (IDG2) had initially triggered the generator disconnection. However, the investigation established that the subsequent total loss of power on the bus was not caused by the short circuit itself, but by how the aircraft's control logic responded to the hardware state.
Findings
- The primary trigger for the incident was an intermittent short circuit between phases in the feeder lines from IDG2.
- The investigation identified that the system logic for the GCU and BPCU allowed an erroneous status signal from the Generator Control Breaker (GCB) to cause the transfer bus to lose power.
- Specifically, a faulty or loose auxiliary contact in the GCB provided incorrect position information to the control units, preventing the necessary bus interconnection.
- The crew's attempt to use the APU as a power source was also unsuccessful due to this same logic error.
Safety action
Following the investigation, the SHK issued recommendations to the FAA and EASA, urging Boeing to implement measures ensuring that the electrical system logic prevents an X-bus from losing power due to erroneous signals from the GCB. Additionally, a review of the QRH procedures for reconnecting the IDG was recommended to address potential discrepancies in pilot procedures.