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

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

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

Boxing the wake is essential to learning how properly control a glider on tow. I box the wake on virtually every tow, just to keep my skills sharp. If your tow plane can't handle this stress, it needs some structural reinforcement.

Boxing the wake is essential to learning how properly control a glider on tow. I box the wake on virtually every tow, just to keep my skills sharp. If your tow plane can't handle this stress, it needs some structural reinforcement.

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. Develop other training methods to teach that skill set. It's similar to the requirement in the Powered Aircraft training which requires a commercial pilot applicant to perform a "Lazy Eight". I'm a CFI and I can tell you that a lot of money is wasted teaching that maneuver for the commercial check ride. I'd bet that darn few of those commercial students can perform that maneuver 6 months after the check-ride. It's nothing more than a CFI meal ticket.

Do you know the history of your Pawnee? Was it ever used for aerial application of agricultural chemicals? I am told many of them are hygroscopic (attract water) and so contribute heavily to rusting of the tubular frame members. Of course if yours was built and used exclusively for banner towing, that is not the problem. But do check to see what its history was.

On Wednesday, January 14, 2015 at 11:16:25 AM UTC-5, 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. Develop other training methods to teach that skill set.

Students ask me "Why do we practice the box the wake maneuver?"
It takes all of my effort as an instructor to suppress the smart-alec answer:
"Because the FAA requires us to."
Which inevitably leads to the next question of: "Why does the FAA require us to?"
"Because the FAA requires us to."

Does it hone the skill of precise flying while on tow? Yes.
Does it teach people how to move around behind the tow plane? Yes.
Does it teach people "how much" they can get away with w.r.t. positioning behind the towplane? Yes.

When I went to Switzerland, I found that boxing the wake is not a required item for their flight instruction. It was more done as a demonstration to understand the limits of how far a glider can get out of position.

They also didn't practice forward slips on final to lose altitude. They saw this practice as out-of-touch for use with modern gliders. "Sure it's a useful skill... if you're flying old museum pieces like you do in the United States!" I think that might be a jab against Schweizers.

How many other countries, besides the US, require the box-the-wake maneuver?

Piet, I don't know where you got your information, but forward slip and also boxing the wake (called "extreme Schleppfluglagen") are both required maneuvers in glider training in Switzerland.

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)

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

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

On Wednesday, January 14, 2015 at 12:03:10 PM UTC-5, wrote:
Piet, I don't know where you got your information, but forward slip and also boxing the wake (called "extreme Schleppfluglagen") are both required maneuvers in glider training in Switzerland.

Granted, I wasn't going for a rating there, so I didn't go through the whole training program. Maybe the instructor was just de-emphasizing the concepts.