Beech 1900 N31704 accident near San Antonio, Texas

Probable cause

The failure of the left main landing gear upper drag leg arm due to substandard mechanical properties, which resulted in a collapse of the landing gear upon landing.

NTSB analysis

During the cargo flight’s initial climb, the landing gear did not retract when the landing gear handle was raised. The pilot attempted to cycle the landing gear handle once more, which yielded the same result. Following the second unsuccessful gear retraction, the pilot elected to leave the landing gear handle in the DOWN position and continued to the destination.

Prior to landing the pilot completed the before landing checklist and noted three green lights and no red lights, which indicated the landing gear was down, locked, and safe for landing.

The airplane touched down on the runway and the left main landing gear collapsed, the red unsafe light in the landing gear handle illuminated and was followed by an aural horn indicating the landing gear was now unlocked and unsafe for landing. Simultaneously, the airplane began to dip toward the left, causing the left-wing tip to contact the runway. The airplane exited the runway, resulting in substantial damage to the left wing.

A postaccident examination revealed that the left main landing gear upper drag leg arm fractured in bending and twisting overload. There was no evidence of preexisting cracking or corrosion. The dimensions of the part were consistent with the drawing and the chemical composition and microstructure were consistent with 7075 aluminum alloy, as required. The mechanical properties were lower, and the electrical conductivity was higher than the requirements for 7075-T6 aluminum alloy. The measured mechanical properties and electrical conductivity did not conform to any standard temper for 7075 aluminum alloy.

A review of the operator’s maintenance records revealed that several months prior to the accident, the left main landing gear drag brace assembly was replaced on the airplane because it was due for overhaul. The left main landing gear drag brace assembly installed was the unit removed after the accident. The drag brace assembly had been overhauled by the operator. Based on the paperwork, the overhaul work performed did not interfere with the mechanical properties of the upper drag leg arm.

The failure of the left main landing gear upper drag leg arm during the touchdown resulted in a collapse of the gear and the left wing’s subsequent contact with terrain. The failure of the upper drag arm was the result of substandard mechanical properties.

Text from the NTSB. Open the NTSB docket for CEN20LA175 for the full record.