What happened
On 18 January 2010, an Asiana Airlines B76K-300, registration HL7248, was cruising at 34,000 feet en route from Incheon International Airport when the flight crew encountered significant electrical system issues. Approximately 20 nautical miles southwest of Jeju International Airport, the crew received "L GEN DRIVE" and "L GEN OFF" messages, indicating a failure in the left integrated drive generator (IDG).
In accordance with the Quick Reference Handbook (QRH), the crew attempted to stabilize the aircraft by operating the auxiliary power unit (APU). However, roughly nine minutes later, an "APU FAULT" message appeared, signaling that the APU had also ceased functioning. With two primary power sources unavailable, the crew decided to divert to Jeju International Airport. To ensure the aircraft was within safe landing weight limits, the crew performed two turns at the initial approach fix before landing on runway 06. There were no injuries among the 222 passengers and 11 crew members, and the aircraft sustained no damage.
The investigation
The ARAIB examined the maintenance history of the aircraft's electrical components and the specific procedures used during recent servicing. The investigation focused on the integrated drive generator (IDG) and the APU to determine why both systems failed nearly simultaneously. Investigators performed teardown inspections on both the IDG and the APU to identify mechanical or procedural discrepancies.
Findings
- The failure of the left IDG was caused by excessive oil injection during a previous maintenance session. The investigation found that the tools required to accurately measure scavenged oil during the supply procedure were missing from the official equipment list, making it difficult to prevent overfilling. This excess oil caused the internal temperature of the IDG to rise to critical levels.
- The APU failure was attributed to a worn aft turbine carbon seal. The degradation of this seal allowed oil to leak, which subsequently dropped the oil pressure and raised the internal temperature, triggering the system's automatic protection logic to shut down the unit.
- The APU had been in service for significantly longer than its average service time since the last repair, contributing to the seal's wear.
Safety action
Following the investigation, the ARAIB issued several recommendations to Asiana Airlines, including:
- Implementing specific measuring tools for IDG oil supply to prevent overfilling.
- Enhancing preventive maintenance by shortening check cycles for the BPCU and BITE systems.
- Improving APU reliability by replacing the aft turbine carbon seal during every APU overhaul.