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

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

After my first reply and thinking mo
It seems that the problem is a bit beyond the end of the fin where it plugs into the tail post. If the brace wires are not doing their job(tension set correctly), the tail post could be seeing a lot of bending it was never intended to see.
Certainly another thing to look at.
UH

There are a number of ADs for the Pawnee, one specifically for corrosion
issues in the rear fuselage structure. You might just be seeing these
problems coming to light.

On Wednesday, January 14, 2015 at 1:45:04 PM UTC-8, John Ferguson wrote:
There are a number of ADs for the Pawnee, one specifically for corrosion
issues in the rear fuselage structure. You might just be seeing these
problems coming to light.

Adding to what Fred said re corrosive and hygroscopic chemicals in their former life, many Pawnees live outside where they are often subject to large temperature and humidity changes. Any condensation within the tubing runs downhill. The tail post cluster is a likely first victim.

The horizontal stab spars have an AD too.

It's hard to believe that boxing the wake and slack line recoveries damaged the Pawnee. I fly both ends of the rope, tow and CFIG. The tubes holding the Schweizer hook and bolts would show damage before the lower tube weld cracks. I'd suspect hard tailwheel first landings on pavement, or pilots that land short in gravel and roll up onto a paved runway "over a lip" would cause more damage and flat tail wheels.

Our Pawnee saw crop duster service for 20+ yrs before converting to glider tow. The records show the entire lower fuselage frame has been rebuilt from corrosion over the years. It has been in a dry climate for the last 15 yrs..

Why do we box the wake? Why do we teach our teenage drivers how to parralle park. Because it shows you know where the corners of the car are and that you can "put it where you want it". Same with boxing the wake, same with boom limits when air refueling, you know where the limits are, you can control your aircraft throughout the entire envelope.

BillT

Corrosion. Boxing the wake is not the problem. The energy is not transferred in that way to crack them.

Boxing the wake causing cracking of frame structure in a Pawnee is just nuts. The impact of the tailwheel on the pavement during landings has to be much more significant.
So...we are to illiminate a basic flight competency, boxing the wake, because the tow planes are so underengineered that they are getting ruined by the soft lateral loads imposed by boxing the wake?
Why not. Spins are obviously dangerous and very stressful on the airframe so we have to elliminate them.
Loops involve 3.5-4 Gs which is obviously a great stress on an airframe so those must be illegal.
My G meter has registered 5 Gs from flying through rough air so flying through rough air must be made illegal.
My G meter has registered 10 Gs from trailering so trailering a glider is obviously stressful to an airframe so that must be made illegal.
How do we survive?

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