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

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

Ueli,

slipping is not part of the instruction syllabus in Switzerland, except for FCL aerobatics.
However, it is still a very useful skill and could have saved the odd DuoDiscus during outlandings.

On Wednesday, 14 January 2015 15:58:40 UTC, 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

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.

Reply from Ian Strachan, UK glider and power pilot.

I was a glider and power (civil and military) instructor for 40 years, now retired from instructing but still flying gliders and tow planes at Lasham in the UK. In my gliding career I was Chief Instructor at two UK clubs and in the military instructed on single, twin and four engined types.

When I started instructing in the 1960s there was no such thing as "boxing the wake". Before sending someone solo on aerotowing you ensured that the guy could get back to the central tow position from being out of position left and right, up and down.

This was done by the instructor flying gradually and smoothly to the out-of-position position and handing over to the student to recover to the "central" tow position. The emphasis was on smooth, gradual and safe use of the controls, not rough and unneccessary control applications. The dangers of overcontrol leading to overshooting the central two position and the risk of oscillation were discussed and demonstrated. In the UK the standard tow position is "high tow" above the wake and part of pre-solo instruction was to show the wake and make sure that the pupil could get back into high tow position from a wake encounter. We did not deliberately teach the "low tow" position itself, at least until a guy had already been solo on aero tow.

When the fashion started for so-called "boxing the wake", I was horrified because many instructors took it as a licence to be rough with the controls and IMHO took it too far. As a tug pilot I looked at what was happening behind me as a demonstration of poor airmanship and probably off-putting to many pupils. A long-term instructor often forgets that some pupils are quite nervous. Pilots who have been instructors for many years need to be reminded of the basic principles such as gradual and sympathetic demonstration and then student practice of the various skills, without taking things too far because the instructor has lost the ability to put him/herself in the place of the student.

When instructing, I refused to Box the Wake but continued as I had done before. I don't think my aero tow students had any problem when they went solo on tow.

Clearly C1's claim that boxing is "essential", is not right. Unless you regard what I describe above as "gentle boxing".

I guess it all turns on what you mean by "boxing".

The problem was that once it started, there was far to much "aggressive boxing" by instructors who maybe were trying to show how clever they were (and annoying tow pilots like me) and had IMHO fogotten one of the basic tenets of instruction which is "not to do the advanced course before the basic", or not to demonstrate running before the pupil can walk, if you see what I mean. I have seen instructors take it too far, for instance breaking the rope/weak link or even arriving just off the tug wing tip with a huge loop in the rope.

I would hope that we could agree that "aggressive boxing" during pre-solo aero tow instruction is unneccessary.

IMHO it is a poor instructional technique which is not necessary before safely sending a student solo on tow. Even on post-solo check rides, I see no need for it and suggest that a less "agricultural" approach should be used.. An aero tow is not an aerobatic and controls should be used gradually, not over-used when it is not necessary.

Ian Strachan
Lasham Gliding Centre, UK

I have been instructing gliders for about 27 years, and for me the boying the wake excercise is essential during pre-solo training.
Part of it is to show the student the limits of possible positioning behind the tug (and on the lateral positions of the box, the tug pilot will have to use quite some rudder to keep going straight), and especially to show the point beyond which immediate release is mandatory on high positions.
Low position (well below the wake) is something the student needs to learn for descent and approach on tow anyways.

Having said that, there is no aggressive maneuvring involved at all, with me talking the students through the excercise. Changing positions is done gradually all the way through the excercise, with constant tension on the rope.

It usually is an excercise which is flown once with the student, and at some point when he is at a level where I don't have to interfere anymore on a normal tow.

Bert TW

At 09:43 21 January 2015, Ian Strachan wrote:
On Wednesday, 14 January 2015 15:58:40 UTC, 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.
T=
he crack is between the lowest and the middle hinges.
=20
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
sa=
id that, contrary to some opinions in my club, I don't believe aggressive
b=
oxing the wake is the only contributing factor in our costly maintenance
is=
sues. FYI, our Pawnee has been hangared at least in the past 20 years!
=20
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
break=
s (weak link about 1250 lb).=20
Many Thanks,
C1
=20
Boxing the wake is essential to=20
learning how properly control a glider on tow. =20
I box the wake on virtually every tow, just to keep my skills sharp.=20

Reply from Ian Strachan, UK glider and power pilot.

I was a glider and power (civil and military) instructor for 40 years,
now
=
retired from instructing but still flying gliders and tow planes at
Lasham
=
in the UK. In my gliding career I was Chief Instructor at two UK clubs
and
=
in the military instructed on single, twin and four engined types.

When I started instructing in the 1960s there was no such thing as
"boxing
=
the wake". Before sending someone solo on aerotowing you ensured that
the
=
guy could get back to the central tow position from being out of position
l=
eft and right, up and down.

This was done by the instructor flying gradually and smoothly to the
out-of=
-position position and handing over to the student to recover to the
"centr=
al" tow position. The emphasis was on smooth, gradual and safe use of
the
=
controls, not rough and unneccessary control applications. The dangers of
o=
vercontrol leading to overshooting the central two position and the risk
of=
oscillation were discussed and demonstrated. In the UK the standard tow
p=
osition is "high tow" above the wake and part of pre-solo instruction was
t=
o show the wake and make sure that the pupil could get back into high tow
p=
osition from a wake encounter. We did not deliberately teach the "low
tow"=
position itself, at least until a guy had already been solo on aero
tow.

When the fashion started for so-called "boxing the wake", I was horrified
b=
ecause many instructors took it as a licence to be rough with the
controls
=
and IMHO took it too far. As a tug pilot I looked at what was happening
be=
hind me as a demonstration of poor airmanship and probably off-putting to
m=
any pupils. A long-term instructor often forgets that some pupils are
quite=
nervous. Pilots who have been instructors for many years need to be
remin=
ded of the basic principles such as gradual and sympathetic demonstration
a=
nd then student practice of the various skills, without taking things too
f=
ar because the instructor has lost the ability to put him/herself in the
pl=
ace of the student.

When instructing, I refused to Box the Wake but continued as I had done
bef=
ore. I don't think my aero tow students had any problem when they went
sol=
o on tow.

Clearly C1's claim that boxing is "essential", is not right. Unless you
re=
gard what I describe above as "gentle boxing".

I guess it all turns on what you mean by "boxing". =20

The problem was that once it started, there was far to much "aggressive
box=
ing" by instructors who maybe were trying to show how clever they were
(and=
annoying tow pilots like me) and had IMHO fogotten one of the basic
tenets=
of instruction which is "not to do the advanced course before the
basic",
=
or not to demonstrate running before the pupil can walk, if you see what
I
=
mean. I have seen instructors take it too far, for instance breaking the
r=
ope/weak link or even arriving just off the tug wing tip with a huge loop
i=
n the rope.

I would hope that we could agree that "aggressive boxing" during pre-solo
a=
ero tow instruction is unneccessary.=20

IMHO it is a poor instructional technique which is not necessary before
saf=
ely sending a student solo on tow. Even on post-solo check rides, I see
no=
need for it and suggest that a less "agricultural" approach should be
used=
.. An aero tow is not an aerobatic and controls should be used
gradually,
n=
ot over-used when it is not necessary.

Ian Strachan
Lasham Gliding Centre, UK

I agree completely with Ian, over my 50 years of instructing I have seen
some crazy ideas introduced and "boxing" has to be one of them. The
procedure is not required to teach recovery from out of position. I have
never yet flown with a student who did not get out of position naturally
giving the opportunity to learn the skill of recovery.
We are now starting to use motor gliders as tugs and my club uses a Rotax
Falke. The Rotax Falke is a standard Falke airframe fitted with a more
powerful engine and a "proper" undercarriage. It was never ever designed to
be used to tow other aircraft, which is of course the case for most of the
aircraft used as tugs. Who knows what stresses and strains out of position
flying puts on the rear fuselage of a Falke, I suspect we will only find
out when a failure occurs, which will be of more than academic interest to
the tug pilot.

I don't understand the logic of boxing the wake as a training exercise. Why teach students how to move from one undesirable out of position scenario directly to all the other possible out of position scenarios ? When is that going to be useful in real flying? It makes no sense to me.

John Galloway

On Wednesday, January 21, 2015 at 3:49:48 AM UTC-8, wrote:
Why teach students how to move from one undesirable out of position scenario
directly to all the other possible out of position scenarios ? When is that going
to be useful in real flying? It makes no sense to me.

The 4 corners of the box are where one will be to initiate a steering turn in high and low tow.

Flying a smooth box with a pause at each corner allows the student to practice and demonstrate mastery of flying on tow.

Drop into the wake then move to the side until the ride smooths out. Turns out it's not very far and puts minimal strain on the tow pilot's foot.

Again, boxing the wake is to be a demonstration of precise control of the sailplane on tow, it's not something to be rushed.

-Tom