On Wednesday, January 14, 2015 at 7:35:43 AM UTC-7, mt wrote:
Our 235 hp Pawnee has again some cracks in the tail tubing. One of them is on the bottom of the vertical member where rudder hinges are welded. The crack is between the lowest and the middle hinges.

As a glider instructor and tow pilot (over 35 years) I don't see why we should keep "boxing the wake", as part of glider pilot training. Having said that, contrary to some opinions in my club, I don't believe aggressive boxing the wake is the only contributing factor in our costly maintenance issues. FYI, our Pawnee has been hangared at least in the past 20 years!

What are your experiences:
1- with Pawnee tail fatigues
2- cause of repeated cracks, method of usage
3- methods of fixing them better next time
4- what contributtes to more stress on the tail tubing: hitting rudder stops in everyday operations, boxing the wake, hard landings, or rope breaks (weak link about 1250 lb).
Many Thanks,
C1

Metal fatigue is caused by the REPEATED application and release of stress. All stress is not the same, so certain types of stress will result in faster fatigue than other kinds. I think it is relevant to note that Pawnees were never designed to be towplanes, so their structure wasn't engineered for that type of duty. To blame one particular task as the cause is pretty much a fool's errand barring a detailed engineering structural analysis. Even putting in a poorly analyzed "fix" to strengthen the structure might just make matters worse by increasing loads somewhere else. Just repair the damage and move on.

Tom