What happened
On 12 April 2005, a Lockheed L3/82G Hercules, registered as C-GHPW, was performing a scheduled flight from High Lake, Nunavut, to Yellowknife, Northwest Territories. The aircraft, operated by Bradley Air Services Ltd. (First Air), was climbing through 18,000 feet when the crew heard a loud bang originating from the cargo area. Upon inspection, the crew identified air escaping from the left side of the compartment and discovered a crack, approximately 24 inches long, in the left wing-to-fuselage attach angle.
In response to the structural failure, the crew depressurized the cabin and leveled the aircraft at flight level 230. To manage the situation, the crew donned oxygen masks, reduced the airspeed to 180 KIAS, and declared an emergency. The aircraft maintained a level altitude for approximately 35 minutes before descending to 10,000 feet to ensure a sufficient oxygen supply. The crew chose to continue to Yellowknife rather than returning to High Lake, as the latter's ice strip was unsuitable for a zero-flap landing and nearby alternates lacked necessary emergency services. The aircraft landed safely at 1312 MDAS with no injuries reported.
The investigation
Investigators examined the aircraft's structural components and maintenance history. Post-flight inspections revealed that the crack had expanded to 33 inches in length. The component in question, which secures the fuselage sidewall to the center wing box, had accumulated over 35,000 hours in service and 31,205 pressure cycles.
Further analysis showed that the crack had been actively propagating for several years. While a previous repair had been performed on the attach angle at a different location (FS 497), the investigation found that this repair lacked a corresponding maintenance program to detect cracks at the FS 577 station. Additionally, the investigation noted that the electronic scale system used to weigh the aircraft was unsuitable for this aircraft type, leading to an inaccurate basic empty weight recorded for approximately 75 flight hours.
Findings
- The primary cause of the failure was fatigue crack initiation and propagation within the bend radius of the left attach angle.
- A previous repair at FS 497 effectively extended the component's service life but left the area at FS 577 without a viable method for crack detection.
- The use of a DER-approved repair overrode existing service bulletin recommendations, which increased the risk of undetected structural degradation.
- The weighing system used for the aircraft provided imprecise data, contributing to an error in the aircraft's recorded weight.