What happened
The pilot was performing a landing on a beach located near a remote river for the purpose of picking up fishermen. During the landing roll-out, the pilot reduced the speed of the aircraft to approximately 5 knots and began a turn. While executing this turn, the nose wheel moved into an area of soft sand, which caused the nose wheel strut to fracture.
The investigation
The investigation focused on the landing gear configuration of the aircraft. The main landing gear was equipped with 10.5 X 28.5 X 15 tundra tires, wheels, and hubs installed under an FAA Supplemental Type Certificate (STC). This STC mandates that the aircraft must also be equipped with an FAA-approved nose wheel and tire installation. However, the aircraft was fitted with an oversized 8.50 nose wheel tire and tube. While this specific nose wheel/tire setup had been approved by an FAA Flight Standards District Office (FSDO) inspector via a field approval, the FAA's handbook for airworthiness inspectors specifies that modifications to landing gear are considered major alterations and require an STC.