What happened
On 8 September 2024, a Textron B200, registration TF-NLA, was conducting a contracted passenger flight from Station Nord (BGNO) to Mesetersvig (BGMV) in Greenland. While climbing through FL250, the crew heard a loud bang followed by the sound of rushing wind in the aft cabin. The aircraft's cabin altitude gauges rose rapidly, indicating a loss of pressure.
The pilot in command immediately initiated an emergency descent, reducing power and configuring the aircraft for approach. The crew declared a Mayday via SATCOM and diverted back to Station Nord. During the descent, the cabin altitude briefly reached approximately 14,500 feet, triggering an altitude warning and the automatic deployment of oxygen masks. The crew did not don the masks immediately, as the commander prioritized maintaining clear communication with the first officer and managing the descent.
The aircraft leveled off at FL100 and landed safely at BGNO. Upon landing, it was discovered that the forward upper cabin entry door locking pin had fractured.
The investigation
The RNSA investigation focused on the failure of the forward upper cabin entry door hook (P/N 101-430029-1). Maintenance records showed the component had accumulated 8,585 flight cycles, reaching approximately 86% of its 10,000-cycle airworthiness life limit.
Detailed metallurgical analysis, involving representatives from the manufacturer, NTSB, and FAA, revealed that the fracture was likely caused by intergranular corrosion originating at the threaded portion of the hook. The presence of sodium and chlorine at the fracture site suggested environmental contributors to the corrosion process. The investigation confirmed the part was within its service life and met the required material strength specifications.
Findings
- The primary cause of the depressurization was the failure of the forward upper cabin entry door hook.
- The failure was driven by intergranular corrosion cracking that propagated through the part's threads.
- The component had not yet reached its mandated 10,000-cycle airworthiness limit at the time of failure.
- The crew's decision to prioritize communication over donning oxygen masks prevented immediate mask usage, though the cabin altitude remained below critical levels for a short duration.