On Friday, March 8, 2019 at 1:05:48 PM UTC-5, wrote:
On Friday, March 8, 2019 at 9:39:11 AM UTC-6, Paul Kaye wrote:
On Friday, 8 March 2019 15:26:18 UTC, Paul Kaye wrote:
Many years ago the late Chris Rollins, while CFI at Booker GC in the UK, did some controlled testing of glider "kiting" behind tugs. He posted something about it on URAS he

https://groups.google.com/forum/#!ms...g/Uz0Ga95XVCsJ

It makes interesting and sobering reading.

Some excerpts:

"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”. "

"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 Schweizer 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. "

Thanks Paul, very instructive. Makes me wonder why that Schweizer tow-hook is still legal and widely used.

I do not wonder why, I know why. It is largely a "we've always done it this way" attitude and a financial consideration. The cost of retrofitting a tow plane with a Tost system and a release handle up where the tow pilot can reach it in the seated, upright position WITH adequate mechanical advantage to effect a release is a few thousand dollars, not something some are willing to spend.

The FAA in their own Advisory Circular, date 3/3/08 AC No 43.13-2b, Page 76, Par 808 states as follows:

“When the glider under tow operates above a certain angle to the tow plane, the ring may slide upwards on the hook, causing excessive load on the hook and difficulty in releasing the tow rope ring.”

The Soaring Society of America thru their Soaring Safety Foundation, Tow Pilot Training course additionally acknowledges IN RED as follows:

“If at any time the nose of the tow plane is pulled to a dangerously high or low pitch attitude, - PULL THE RELEASE!”

It goes on to say:

“Depending on the installation of the tow hitch, it may be possible for the release mechanism to become jammed due to the excessively high position of the glider, (American style hook).”

What they mean by “depending on installation of the tow hitch,” is that if you install it upright instead of inverted it may be possible for the release mechanism to become jammed due to the excessively high position of the glider.

The SSA and the FAA are well aware of this situation and have been for many, many years and yet the system remains approved. I can assure you that not all kiting incidents are slowly developing situations. I've been there at just above 300 feet when in the blink of an eye you find yourself nose down before you could even think of releasing. The gentleman who died in this tow plane incident was probably 100 feet below the level at which mine started. In both cases our tow ropes broke, the glider pilot didn't release. I had barely enough room to recover, he did not.


Walt Connelly
Former tow pilot
7000 tows