Boxing the wake cracks Pawnee tail tubings or long term fatigue?

teach them to hold the stick 6-8" below the top, and explain to them what "milking the mouse" means

On Wednesday, January 14, 2015 at 12:32:47 PM UTC-5, Dan Marotta wrote:
Aggressive or sloppy?* Some students are barely noticeable when
boxing the wake while others yank the tail around so hard they
exceed the tug's rudder (and elevator) authority.* Of course,
they're the same ones who swing high and wide on every turn.* What
CFIs need to teach is that minor corrections will get the glider
into position much more quickly than using large control inputs.






On 1/14/2015 10:15 AM, Andrew wrote:



On Wednesday, January 14, 2015 at 9:35:43 AM UTC-5, 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


Thanks for sharing this info. I imagine these cracks are present on many other Pawnee glider tugs without the knowledge of the operators...

I'm a little confused about the location of one of the cracks. For the crack that's between the bottom and middle rudder hinges, is the crack next to the bottom hinge weld or directly in the middle of the tube between the two hinges? Note, I'm assuming it's in the vertical stab tube that the hinges attach to, correct me if I'm wrong...

Also, is the crack opening facing aft, port, starboard, etc?

How did you guys discover the crack? I'm hoping it's easy to spot even with the covering on.

Where are some of the other cracks too? I'm getting at the ones not near the rudder hinges.

We don't have these cracks as far as we know. Although, we've seen failure in tailwheel bolts. One of them being the bolt that goes through the leaf spring. Another was nearer to where the leaf spring attaches to the fuselage. I'm having a hard time picturing the exact bolts at the moment...

P.S. I think it's unlikely that loads on the tow hook are contributing to the crack considering how the hook attaches to the fuselage and how small tow loads are when compared to other loads, like you've mentioned. Unless the wake boxing is done very aggressively... Worst case scenario is maxing out the weak link to 2500lb with a 30deg rope angle causes a 1250lb side load at the hook which should not happen during normal boxing... Most of that is quickly absorbed by rotational inertia and the sideslip angle quickly reducing from that maximum value. For a more reasonable steady state case of a 15/1 L/D and 1250lb glider banking 15deg and a 20deg rope angle would give about 354lbs.
1250/15*tan(20deg) +1250*sin(15deg)





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Dan Marotta