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

On Wednesday, January 14, 2015 at 9:16:25 AM UTC-7, wrote:

Boxing the wake might teach essential skills but if it's doing damage to a tow plane, why on earth should we continue the practice.

Because it teaches ESSENTIAL skills.

On Wednesday, January 14, 2015 at 10:15:02 AM UTC-7, Andrew wrote:

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

No doubt. These are VERY old airplanes and maintenance requirements tend to rise exponentially with age. All steel tube airframes suffer rust, corrosion and fatigue with the lowest areas suffering most. If your AI doesn't know to check this, get another AI.

I once saw an old Piper (PA-20, I IIRC) siting in the weeds with its nose pointing higher than it should. It appeared the tubes forward of the tail wheel had failed from the weight of the parked airplane.

On Wednesday, January 14, 2015 at 12:36:01 PM UTC-5, 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

I highly doubt that boxing the wake has anything to do with the problem you are describing. The rope loads are not that high as to cause additional load.
I'm not sure why one would ever bang the rudder hard against the stops. Seems like a poor technique.
The most likely cause, in my experience would be the beating the structure takes doing 7-8 landings an hour.
Rather than repair, it may pay to replace the rudder post with new.
Boxing the wake is a useful training exercise and should not be reduced or eliminated because of a tail post issue.
42 years instructor and tow pilot.
UH

I agree, I can't see very high loads from "boxing the wake" as you will turn the towplane 1st.
I CAN see the abuse from the tail hitting the ground (on landing), rough surface banging the tailwheel, tight "lock" on the tailwheel putting bending/twisting loads when turning on the ground, etc.

As to whether or not to teach "boxing the wake", I also think it's worthwhile to do. Better in "training" than "learn as you go" if you get out of position.

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

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)





--

Dan Marotta

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?