On October 22, 2006, at 1430 central daylight time, a Cessna R182, N5274S, operated by Hap's Air Inc., received substantial damage on impact with terrain when the nose landing gear collapsed during landing at Ames Municipal Airport (AMW), Ames, Iowa. Visual meteorological conditions prevailed at the time of the accident. The 14 CFR Part 91 instructional flight was not operating on a flight plan. The commercial pilot and certified flight instructor (CFI) were uninjured. The local flight originated from AMW at 1400. The CFI reported that he was practicing takeoffs and landings with the commercial pilot during a dual instructional flight. During the final landing attempt, the pilots did not receive "a green light indicating gear down and locked." The pilots could visually see that the main gear was extended; however, they could not determine whether the nose gear was locked down. The pilots reported that they cycled the landing gear several times but could not get a down and locked indication. The CFI stated that during the landing, he attempted to keep "the nose wheel off of the ground as long as possible." As the airplane slowed "to nearly taxi speed," the nose gear collapsed. Inspection of the landing gear system revealed that one of the two landing gear down lock actuator pins (part number 1280209-1) was partially extended out of the nose gear actuator assembly bearing end (part number 1280600-5). The extended pin was loose and the other pin could not be moved until company maintenance personnel had to "get rough with it" and at that time looseness of the pin was noted. The pins were cylindrical steel dowels with grooves machined into the outer diameter for a transverse roll pin in the assembly. The groove was cut nearer to one end of the locking pin and oriented to the outer edges of the bearing end when assembled. The two pins and actuator bearing end were sent to the National Transportation Safety Board (NTSB) Materials Laboratory for a metallurgical examination. The pin fracture surfaces were relatively smooth, flat fracture surfaces with feature typical of fatigue progression in both pins. In both pins, the fatigue initiated at the bottom of the groove on the side nearest the longer portion of the pin. Both pins also displayed multiple fatigue origins around the groove diameter. The total fatigue area was about 75 percent of the groove cross section. Cessna issued SB SEB95-20 on December 29, 1995, recommending a reoccurring 200-hour inspection of the down lock pins for looseness. SEB95-20 stated, in part: E. Visually inspect [down lock] actuator pins while physically attempting to rotate and/or move actuator pins about all three axes in the bearing end. No movement is allowed, as pins are a press fit. F. If any looseness or movement is observed (1) Install SK210-155 prior to further flight Cessna Aircraft Company has designed and manufactured updated down lock actuator pins that have been available through SK210-155 since December 29, 1995. The incident fractured pins were of the older design, and the actuator bearing end had not been modified for installation of the newer pins. The diameters of the locking pin holes in the bearing end were measured at the outer and inner edges. At the outer edges, the hole diameters measured 0.377 inch for the left side hole and 0.376 inch for the right side hole. The diameters at the inner edges of the holes were larger, measuring 0.387 inch and 0.383 inch for the left and right holes respectively. As defined in the instructions for Cessna Service Kit SK210-155, the maximum allowable hole diameter is 0.3760 inch. The airplane was last inspected during an annual inspection on August 1, 2006, and accumulated a total time of 6,406.2 hours at the time of the accident. The company director of maintenance stated that the landing gear actuator pins were inspected within the 200-hour time frame cited in SEB95-20. There was no logbook entry pertaining to the inspection of the pins. The director of maintenance also reported that the airplane propeller (McCauley B2D34C218-13) accumulated a total time of 3,423.6 hours since last overhaul. McCauley Propeller Systems service bulletin (SB) 137W, states in part the overhaul frequency of B2D34C218 model propellers to be 2,000 hours or 72 calendar months, whichever occurs first. The NTSB investigated a fatigue failure of a landing gear actuator pin on a Cessna R182 under IAD05IA066.