What happened
On November 26, 2010, a Bombardier CL-601 (registration HL7577), operated by the MLTM Flight Inspection Center, departed Gimpo International Airport to perform a signal range inspection of mobile TACAN facilities. While cruising at 30,000 feet, the aircraft proceeded with its mission along the airway between Gimpo and Jeju.
Approximately ten minutes after takeoff, while the aircraft was maintaining 4,000 feet at 250 knots, the crew observed the red left engine JET PIPE/PYLON overheat (OVHT) light illuminate. Following established abnormal procedures, the flight crew attempted to monitor the light while reducing thrust on the left engine; however, the warning remained active. Consequently, the crew shut down the left engine and initiated an emergency return to Gimpo Airport. After declaring an emergency and dumping 700 lbs of fuel over the Yellow Sea, the aircraft successfully performed a single-engine approach and landed on runway 32R without further incident.
The investigation
The ARAIB examined the aircraft's maintenance history and the functionality of the fire and overheat detection system. The investigation focused on the sensing elements of the engine's fire protection system and the maintenance protocols managed by the operator's maintenance provider. Investigators also reviewed several manufacturer-issued Service Bulletins (SB) intended to improve the reliability of the fire detection system.
Findings
- The illumination of the overheat light was caused by a malfunction in the engine sensing element, specifically where resistance values in the middle section of the element were found to be higher than normal.
- The aircraft's maintenance program included visual inspections of the fire sensing cables every 300 hours, but the investigation noted that malfunctions related to the fire protection system had been occurring more than once per year.
- While the manufacturer had issued four Service Bulletins between 2006 and 2007 to improve the fire detection system—including recommendations for rewiring sensing elements and improving harness routing—the operator had previously decided not to adopt three of these bulletins.
- The investigation found that the aircraft's operating environment and age had changed since those decisions were made, and the frequency of fire protection-related malfunctions had increased.