I haven't had time to read all the posts on this thread, but none of those
I have read has given any real information on what actually happened.

I was involved in some testing of tow-plane accident scenarios many years
ago. I'm not suggesting that what happened in Oregon was this sort of
accident but nor do I know that it was not. For what it is worth,
herewith my recollections of those tests:

Whilst I was Chief Instructor at Booker Gliding Club, we conducted two
series of test on the phenomenon variously referred to as “Kiting”, “Winch
Launching behind the Tow-Plane” and “Sling-Shot Accident”, one in 1978 and
one in 1982; my memory of them is quite vivid.

Airplanes used were, for the first series, a Beagle Terrier (a side by
side, two place, high wing, tail-dragger), fitted with an Ottfur Glider
hook for towing (very similar to the Tost hook, dissimilar to the
Schweitzer hook) with a 160 hp Lycoming engine; for the second series of
tests a PA18-180 with a Schweitzer hook was used. Gliders used were a
Schleicher Ka 8b and ASK 13. Tow rope initially used was a heavy (4000 lb
breaking strain) rope with a thinner rope weak link at the glider end
(nominally 900 lb, but a well worn specimen could break at as little as
200 – 300 lbs – laboratory tests, not opinion), the second series of test
used the same heavy duty rope with “Mity” links at each end, 1100 lbs at
the Tow-Plane end and 900 lbs at the Glider end – these links use metal
shear pins, one under load and a second unloaded, which takes over if the
first one fails. This eliminates failure due to fatigue and means that
the links always fail at close to their nominal load even after some time
in service – again laboratory tested, not just subjective opinion. Rope
length was around 180 feet in all cases.

I was the Glider Pilot on all tests; Tow-Plane Pilot was Verdun Luck (then
my deputy Chief Instructor) for the first series of tests and Brian
Spreckley (then Manager of Booker GC) for the second. The object of the
tests was to try to reproduce the “Kiting” under controlled circumstances,
with a view to developing a Tow-Plane release mechanism that would
automatically release the glider if it got dangerously high above the
Tow-Plane. All tests were conducted at about 4000 feet agl.

First test: Terrier Tow-Plane and ASK 13 on nose-hook. At about 4000
feet I took the glider progressively higher above the tow-plane,
eventually reached about 100 feet above tow-plane (i.e. rope angle more
than 45 degrees above horizontal). At about this point, the tow pilot,
who had been using progressively more back stick, ran out of back stick
and the Tow-Plane began to pitch nose down but not excessively violently.
I released at that point. It took a very positive control input on my part
to achieve the displacement, we both felt it was something unlikely to
occur accidentally, even with an inexperienced glider pilot, and there was
plenty of time for either party to release if it did occur.

Second test: Terrier Tow-Plane and ASK 13 on C of G hook. I pitched the
glider about 25 – 30 degrees nose up – the weak link broke immediately!
Tow pilot reported a sharp jerk, but no significant change to flight
path.

Third test: Terrier Tow-Plane, K 8b on C of G hook. I pitched the glider
about 25 degrees nose up. The glider continued to pitch up fairly rapidly
(as at the start of a winch launch) and substantial forward movement of
the stick only slightly slowed the rate of pitch. The glider achieved
about 45 degrees nose up, speed increased rapidly from 55 knots to about
75 knots and the glider was pulled back towards level flight (again as at
the top of a winch launch). I released at that point. The entire
sequence of events occupied a VERY short period of time (subsequently
measured as 2 - 3 seconds). The Tow Pilot reported a marked deceleration
and start of pitching down which he attempted to contain by moving the
stick back; this was followed immediately by a very rapid pitch down
accompanied by significant negative “G”. The tow-plane finished up about
70 degrees nose down and took about 400 feet to recover to level flight.
We both found the experience alarming, even undertaken deliberately at
4000 feet. Our conclusion was that the combination of the initial pitch
down and the upward deflection of the elevator caused the horizontal
stabilizer/elevator combination to stall and the abrupt removal of the
down-force it provided caused the subsequent very rapid pitch-down and
negative “G”.

Our first conclusion was that, in the event of this sequence occurring
accidentally as a result of an inadvertent pitch up by the glider pilot,
there was effectively no chance that either the glider pilot or tow-pilot
would recognise the problem and pull the release in the available time.

Attempts to produce a tow-plane hook that would release automatically were
unsuccessful for reasons that became apparent later.

These tests were repeated a few years later with a PA18 – 180 as the
tow-plane, Brian Spreckley flying it. The third test described above was
repeated and photographed from a chase plane using a 35 mm motor drive
camera on automatic (this took a frame every half second – video
camcorders of small size were not readily available then). The photo
sequence started with the glider in a slightly low normal tow position and
starting to pitch up, the second frame has the glider about 30 degrees nose
up and about 20 feet higher than previously in the third frame it is about
45 degrees nose up and has gained another 30 feet or so, the tow-plane is
already starting to pitch down, in the fourth frame the glider is about
100 feet higher than its original position and the climb is starting to
shallow, the tow-plane is about 50 degrees nose down, the final frame
shows the tow-plane about 70 degrees nose down and the glider almost back
in level flight , almost directly above it (that was about the point that
I pulled the release).

Sufficiently alarmed by events, Brian Spreckley had been trying to pull
the release in the tow-plane earlier and found that it would not operate
until my releasing at the glider end removed the tension from the rope.
Subsequent tests on the ground showed that the Schweitzer hook fitted to
the tow-plane, whilst perfectly satisfactory under normal loads, was
jammed solid by the frictional loads when subject to a pull of around 700
lbs with a slight upwards component – not something that a normal
pre-flight check would reveal.

We solved that problem on our tow-planes by replacing the bolt that the
hook latches onto with a small roller bearing. So far as I know no one in
the UK has tested the Schweitzer hook as fitted to a glider, but I would
not be surprised if it exhibited the same characteristics at high loads.

The photo sequence also showed that at no time was the glider at an angle
greater than 30 degrees above the tow-plane’s centre-line. However, of
course once the glider has pitched up, the wings generate considerable
extra lift and that extra lift provides extra load on the rope. With a
large, heavy glider it is easy to exceed weak link breaking strains and
with a lightweight machine the tension can easily rise to 700 lbs or so.
With that much load on the rope, quite a small upward angle provides
enough of a vertical component to produce the results described.

That of course is the reason that attempts to produce a hook that released
if a certain angle was exceeded were unsuccessful. The, quite small, angle
between the rope and the fuselage centreline needed to trigger the “Kiting”
when the glider is pitched significantly nose-up is not much greater than
the amount of out of position commonly experienced in turbulent
conditions. We did build an experimental hook and tried it, but, set to
an angle that prevented “Kiting” it occasionally dumped an innocent glider
in turbulence, and set to an angle that prevented that, it didn’t prevent
the “Kiting”. What was needed was a hook that responded to the vertical
component of the load, not the angle at which it was applied, and that
problem we decided was beyond us (at least in a form robust and fool-proof
enough to be attached to the rear end of a tow-plane).

Our conclusions for preventing “Kiting” we

Don’t aerotow gliders, especially lightweight, low wing-loading gliders,
on C of G hooks intended for winch launching (I think the JAR 22
requirement for nose hooks to be fitted to new gliders for aerotowing was
at least in part a result of these tests).

Don’t use short ropes. The speed at which things happen varies directly
with the length of the rope.

Don’t let inexperienced pilots fly at anywhere near aft C of G.

Don’t let inexperienced pilots fly solo in turbulent conditions.

Replace or modify all Schweitzer hooks fitted to tow planes. (So far as I
know there are none on gliders in the UK, so that question never arose).

We did also modify our PA18’s so that instead of the release cable ending
at a floor-mounted lever, it went round a pulley where that lever used to
be, and then all the way up the side of the cockpit, anchored at the roof.
This meant that grabbing any point on the wire and pulling it in any
direction could operate the release; considerably easier than finding a
floor mounted lever when being subject to about minus two “G”. We never
regarded this modification as being likely to prevent a worst-case
scenario, because, as stated earlier, it was the opinion of all involved,
that in a real “Kiting” incident, there was no realistic hope that either
pilot would respond in time.