BazzLow wrote:It's probably worth inspecting every 10-20 years. My issue is the time between inspections. This is a big job and you need to strip paint to properly inspect.
The way we perform damage tolerance analysis and how inspection intervals are typical set are through DT and crack growth analysis. In the FAA world we grow a crack using a load spectrum from critical fasteners assuming there was one there during production. We continually check the residual strength of the structure with the crack until it grows large enough to shows zero residual strength. Then we take the amount of time it took to grow the crack, divide it in half and that's the inspection interval.
Now what's better than analysis is statsical data over time. Has there ever been an in flight structural failure of a tail? My guess no. That means the youngest airplanes have been flying with adequate residual strength for 40 years. The oldest for 67. Assuming the above logic and being ultra conservative the tail should be inspected every 20 years. We also define inspection methods (visual vs various ndi) based on the size of the crack that would cause zero residual strength.
I think the tail inspection is worth doing but every 5 years is wrong. Either the FAA published the proposal at 5 year intervals to look like they are being reasonable when they "compromise" to 10 year intervals or some engineer at the FAA pulled 5 years out of thin air. It doesn't line up with their aircraft certification regs.
Are you going to file a comment? My first thoughts were also that 5 years is too often. I don't understand why there is chronological time based part to it. Should be flight hours based only.
