N3UR

On March 10, 2022, about 1430 Alaska daylight time, a Cessna 310R airplane, N3UR, was substantially damaged when it was involved in an accident near McGrath, Alaska. The pilot and three passengers were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

According to the pilot, he back-taxied along runway 16 towards the departure end of the runway at McGrath Airport (PAMC). When he arrived at the end of the runway, he started to turn to the airplane to align it with the reciprocal runway. However, as he began the turn, the right main landing gear collapsed and the wing and propeller both contacted the runway. The right elevator also contacted the runway and was substantially damaged.

A postaccident examination of the right main landing gear well by the pilot found fractures at the bellcrank lower arm fork (item “1” in Figure 1), bellcrank pivot bolt (item “2 in Figure 1), and the trunnion bellcrank lug (item “3” in Figure 2). An NTSB materials laboratory review of photographs of the fracture surfaces revealed fracture characteristics that were all consistent with overstress. No evidence of fatigue was observed on any of the fracture faces; however, the quality of the photographs precluded the observation of any small regions that may have exhibited fatigue cracking. According to the airplane manufacturer, an overstress failure of the bell crank assembly at the trunnion is commonly an indication that the down lock side brace was not in the down and locked position.

Figure 1: Bellcrank assembly fracture points

Figure 2: Bellcrank bolt fracture at trunnion

A logbook entry from the airplane’s most recent annual inspection indicated that the right main landing gear door and gear strut were removed and replaced with new bolts. The landing gear was subsequently re-rigged in accordance with the maintenance manual before the airplane was returned to service. The accident pilot was also the certified airframe and powerplant mechanic responsible for completing the last annual inspection on the accident airplane. According to his recount, after he replaced the landing gear strut after a previous right main landing gear failure, he checked the down lock tension and discovered that it was “too tight.” However, when he subsequently made an adjustment to reduce the tension, he found that it was out of tolerance. He made one final adjustment that placed the down lock tension within tolerance before he returned the airplane to service, 22 flight hours prior to the accident.

The accident airplane maintenance manual provided specific instructions for adjusting main landing gear down lock tension (Chapter 32, Section 32-30-00).

“(1) Inspection the main gear down lock tension for proper adjustment

(a) Position landing gear in down position

(b) Check main gear down lock for proper engagement and tension (40 to 60 pounds)

NOTE: When checking down lock tension always place a finger on the down lock switch, pull scale at a right angle to the lock link and read the scale at the point when switch actuation can be felt.

(2) Adjustment

(a) If the down lock tension is not 40 to 60 pound, adjust push-pull tube in one half turn increments until proper tension is obtained. If the down lock tension is in excess of 60 pounds, the down lock tension should be rigged to 40 to 50 pounds. Lengthening the push-pull tube decreases and shortening the push-pull tube increases tension.”

The pilot recalled using a spring scale to measure the down lock tension and the push-pull tube to adjust the tension. He added that he was particular about following each step in the Cessna maintenance manual during this inspection as he was new to maintaining the accident airplane make and model.