What happened
On 24 August 2002, an Airbus A310-300, registered P4ABU, was performing a private VIP flight en route from Jeddah to Washington with a technical stop at Shannon Airport. While established on the localiser for Runway 24, the aircraft broke through the cloud layer at 1,800 feet, at which point the crew experienced a simultaneous failure of both engine-driven generators. This loss of power deactivated AC Bus 1, AC Bus 2, and the Normal DC Bus, leaving the aircraft reliant on the standby generator.
Faced with limited instrumentation and the risk of entering instrument meteorological conditions (IMC) without primary electrical power, the captain elected to proceed with an immediate landing rather than initiating a missed approach. During the landing, the aircraft suffered a burst on the number 8 main tyre and subsequent brake damage. Despite the technical failures, there were no injuries among the 10 crew members on board. The aircraft was able to taxi to the apron under its own power.
The investigation
The investigation focused on the electrical system failure and the condition of the Integrated Drive Units (IDGs). Analysis of the Ground Power Control Unit (GPCU) revealed that both generators had experienced a Shorted Rotating Diode (SRD) event. Data extracted from the Generator Control Units (GCU) confirmed that the right IDG failed first, followed by the left IDG approximately 2.5 seconds later.
Laboratory examinations of the right IDG revealed that the unit contained approximately 20% Jet A fuel within the oil, likely due to an internal leak from the oil cooler. This fuel contamination, combined with evidence of oil degradation (coking) due to high temperatures, likely contributed to the failure. Furthermore, the investigation found that the left IDG had also experienced an SRD fault during a previous flight, which had been reset in the cockpit without being recorded in the aircraft's technical log.
Findings
- The primary cause of the dual failure was that both generators entered the flight with existing defects.
- The right generator tripped first due to a shorted diode condition, exacerbated by jet fuel contamination in the oil.
- The resulting increased electrical load demand caused the already compromised left generator to trip almost immediately after the first.
- The crew's decision to land immediately was driven by the need to avoid IMC conditions while operating on limited emergency power.
Safety action
Following the investigation, it was suggested that the aircraft manufacturer review the incident and consider upgrading the manufacturer's Service Bulletin regarding the 100-ohm special resistor assembly from an optional to a recommended modification to improve IDG reliability.