On June 1, 1996, about 1430 Alaska daylight time, a tundra tire equipped Cessna 206, N9608G, sustained a broken nose wheel strut during landing on a remote beach, about 74 miles southwest of Kodiak, Alaska. The airplane was being operated as a visual flight rules (VFR) cross-country positioning flight under Title 14 CFR Part 91 when the incident occurred. The airplane, registered to Redemption Inc., Kodiak, Alaska, and operated by Island Air Service, sustained minor damage. The certificated airline transport pilot, the sole occupant, was not injured. Visual meteorological conditions prevailed. VFR company flight following procedures were in effect. The flight originated at the Kodiak airport about 1350. The pilot reported that he was landing on a beach near the Ayakulik River to pick up fishermen. During the landing roll-out, the pilot slowed the airplane to about 5 knots and was beginning to turn the airplane around. The nose wheel encountered an area of soft sand during the turn and the nose wheel strut fractured. The airplane received damage to the engine cowling and propeller. The airplane is equipped with Bushmaster Management Company, 10.5 X 28.5 X 15 (Model SWS-1100-A) tundra tires, wheels, and hubs that were installed on the main landing gear under a Federal Aviation Administration (FAA) Supplemental Type Certificate (STC) SA3611NM. The STC requires that the airplane must be equipped with an FAA approved nose wheel/tire installation. The airplane did have an 8.50 tire and tube installed that was field approved by an FAA airworthiness inspector from the Anchorage Flight Standards District Office (FSDO) on 5/11/89. A review of the FAA's Airworthiness Inspectors Handbook, order 8300.10, revealed that in Volume 2, Chapter 1, Perform Field Approval of Major Repairs and Major Alterations, Paragraph 7, Required Engineering Approval, states in part: "...A. Many alterations that are commonly called major alterations are actually major design changes and will require a Supplemental Type Certificate. Previously unapproved major changes to structural strength, reliability, and operational characteristics affect the airworthiness of the product and therefore require engineering approval. Typical major alterations in the category include the following:...(2) Installation or relocation of equipment and systems or changes that may adversely affect structural integrity, flight, or ground handling characteristics of the aircraft...(5) Changes in basic dimensions or external configurations of the aircraft, such as wing and tail platform or incidence angles, canopy, cowlings, contour or radii, or location of wing and tail fairings...(6) Changes to landing gear, such as internal parts of chock struts, length, geometry of members, or brakes and brake systems...(8) Changes to primary structure that may adversely affect strength or flutter and vibration characteristics."