09-21-2014: Hobb Tach
When enroute to Crater Lake, I made a fuel stop at Roseburg, OR (KRBG) airport. Parked and tied down the aircraft for a quick lunch. When taxing out, at very slow speed, I felt a pull, and stopped immediately. Got out of the airplane, and realized I forgot to take the tiedown chain off the tiedown ring. The right side tiedown ring was bent. No visible damage was found. The tiedown ring is made of forged steel, with major diameter of 3/8". The moment arm is measured to be about 1.5". After a quick calculation, assuming 100ksi yield stress (most forged steels have yield stress of about 50-60ksi, so this estimate is very conservative). The force required to bend the ring is less than 400 lbs. Due to the angle of the tiedown chain, most of the force will be in wing span direction. Quick calculation showed that the 400 lbs in any direction shouldn't create any damage to the wing spar or wing skin. Flew the airplane to Crater lake and back to PAE, took the inspection cover off, and closely inspected the wing spar area around the tiedown bracket. No visible damage was found either. I just need to order a new tiedown ring.
