N403LC

On December 18, 2024, about 1600 Alaska standard time (AKST), Lynden Air Cargo LLC flight LYC141, a Lockheed L-382C (Hercules) airplane, N403LC, experienced a rapid decompression while climbing through FL220 (22,000 ft) after departure from the St. Mary’s airport (KSM), St. Mary’s, Alaska. There were no injuries to the four occupants onboard the airplane. A postaccident inspection revealed that the airplane sustained substantial damage. The regularly scheduled cargo flight was operating under the provisions of Title 14 Code of Federal Regulations Part 121 from KSM to the Ted Stevens Anchorage International Airport (ANC), Anchorage, Alaska.

History of Flight

The flight crew consisted of the captain who was the pilot monitoring, the first officer (FO) who was the pilot flying and the flight engineer. In addition, a load master was present on the flight. The flight crew reported that after departure from KSM and while climbing through about FL220 they heard a loud bang followed by a loss of pressurization. They donned their oxygen masks and initiated a descent to 13,000 ft mean sea level (msl).

With no visible sign of damage, the flight crew suspected a bleed air problem. The quick reference handbook (QRH) was reviewed and all bleed air systems turned off. The flight crew elected to continue to their destination. About 30 minutes before arrival to ANC, the number 3 engine’s torque began to fluctuate. Concerned that the torque fluctuations were related to the bleed air systems they decided on a precautionary shutdown of the number three engine in accordance with the QRH and Lynden Air Cargo’s procedures. An emergency was declared with air traffic control and the flight continued to ANC and landed without further incident.

A post flight inspection by maintenance personnel revealed a crack in the forward pressure bulkhead, see figure 1. The fracture was harvested by maintenance personnel and shipped to the NTSB Materials Laboratory for further examination.

Figure 1. Location of the crack found in forward pressure bulkhead is shown within the red circle. (Source: Lynden Air Cargo LLC)

As part of the investigative process, the NTSB invited the qualified parties to participate in the investigation. These included the Federal Aviation Administration (FAA), and Lynden Air Cargo LLC.

The parties were formed into specialized investigative groups led by NTSB group chairs in the areas of structures and digital flight data recorder (DVDR).

The investigation continues.