Tow Plane Upsets......

Originally Posted by Peter Whitehead

A manual release system will never be suitable to stop the rapid pitch up caused in this situation. It happens too fast for a manual (actually a sensory-brain-muscle-manual) system, however good the manual system on the tug.

Glider CofG winch hooks are not really suitable for aero-towing, are they? Some of these gliders can't be controlled in pitch once the kiting starts. I have seen people leave their shoulder starts loose on areotowing, and that means the pilot can slide backwards - with hand held on the stick, so that moves too - on pitch-up. (Always use tight shoulder straps on launching, please).

I have lost two acquaintances, both gliding "greats" in such tug upsets. I would say banning the use of aero-towing using a CofG winch hook would be the safest bet to save lives, until an automatic system is available.I suspect this is something EASA and the FAA would sort very quickly ( the banning bit, not the automatic system). Sensors to measure pitch, rate rate of the tug, and also tug/rope angle and angular rate,suitably processed and actioned automatically could allow immediate release BEFORE the critical situation is reached - if we feel that we must continue to launch by aero-tow using CofG hooks.

A rule of nose hooks or "compromise" hooks only, meanwhile, would probably reduce the risk towards zero.

Peter,

Both of these incidents were with nose releases not CG hooks. I've never really had a problem with a CG glider but I guess it could happen. Both of these incidents were due to extreme carlessness and or inexperience on the part of the glider pilot. My concern is with the inability to release at the critical moment. It will be my purpose to communicate this condition as broadly and relentlessly as possible. No commercial operation or club should be towing with a Schweizer hook or a release not immediately available to the tow pilot.

Walt

Well, I've only done about 4,700 tows, and I quit towing over a year
ago. My worst upset experience (Pawnee with Schweizer release) was when
an HP-14 kited high and outside of my turn. As I was yelling on the
radio and grabbing the release lever, he regained control (or so I
thought), turned inside of me and dived across my tail. The rope
wrapped around his wing, cutting it to the spar, and breaking the rope.
He went on to fly about 4 hours, discovering the damage after landing.
I vowed never to tow him again.

As I said, it's been over a year but, if I recall correctly, the
Schweizer release is mounted with a single vertical bolt so that it can
pivot left and right. And, at least on the Pawnee I last flew, if the
release was more than a few degrees off center, pulling the cable would
not open the release. It wasn't a matter of force, it just wouldn't
release if the cable was offset much off the center line of the release.

I agree: Replace Schweizer releases on tugs with Tost.

Dan, former tow pilot.

On 4/23/2017 3:09 PM, Walt Connelly wrote:
I have been towing for about two and a half years and have logged over
6,500 tows. Having recently experienced a couple of rather violent
glider kiting incidents, one at about 300 feet AGL I decided to
investigate the experiences of other tow pilots in these circumstances.
I have spent several hours reading posts about gliders kiting and
upsetting the tow plane, many of the posts deal with creating an
automatic release that will recognize the actions of the glider and
release without input from the tow pilot. Nice idea, some really
creative approaches and the bottom line is anything can be done if you
have enough money and can get the FAA to buy into it and give
approval.

Lots of opinions and ideas as to why these things happen. Bottom line
is that they do happen, they have resulted in loss of tow pilot lives
and the causes are well known.

Of my two most recent experiences one was with a 67 year old licensed
private pilot, glider only, no other pilot ratings who at 2000 feet
tried to release, “slipped back in the seat pulling back on the stick
and going vertical.” I took a stab at the release to no avail, the
pressure was too great and I soon found myself at about 75 degrees nose
down at full power. The natural instinct at this point is to pull back
the power and try again to release. He managed to release before I
could try for a second time and I recovered. If this has to happen to
you, 2000 feet is a good place to be. My understanding is that this was
his first tow in more than a year. Something to think about.

My second experience in the same week was with a 15 year old student on
her 3rd solo pattern tow. At about 250 to 300 feet I started a turn
from the downwind drift to the right. I felt a tug on my tail pulling
my nose to the left. After a few seconds I tried again to turn to the
right knowing that she was behind me to the right. Could not see her,
no mirror on the right. (much easier if she was a little to the left)
The second time the pull of my tail to the right and nose to the left
was a bit more violent…..followed by a very hard tug of my tail UP and
to the right. We could not have been much more than 300 feet when I
found myself nose down and trying to release….again the pressure was too
much to overcome and I needed to retard the throttle. Before I could
make a second attempt at release I heard a loud BANG as the rope broke,
I regained control and recovered just above the trees. (glad that I had
made a pit stop before I started towing). She made it back to the field
and didn’t seem at all concerned……I shall withhold my comments at this
point.

I had to straighten out the Schweizer hook which was off to the right
about 30 degrees before I could make the last 8 tows of the day.

While I have concerns with both of these pilots my major concern is that
the difficulty of actuating the release when the glider kites is well
documented. To make matters worse, the release in the tug I was flying,
although a bit more manually accessible than the other two I fly
requires that the pilot pull it back and push down to fully actuate the
release. Not an easy thing to do under the circumstances. The other
two release handles are down on the floor to the left and for someone of
my height, sitting on a cushion and trying to grab the release is very
difficult. It would seem to me that an easy fix would be to simply make
a longer handle curved up so one could both see and feel for it in an
emergency. A longer handle would also provide a bit more mechanical
advantage although having read many of the posts I am not sure this
would overcome the pressure on the Schweizer hook. Of course I hear all
the noise about needing to apply for a 337 and get FAA approval….next I
will need approval to put a cushion on the seat or to have an inspection
after washing the airplane.

So, the fixes appear to be going to a Tost hook or inverting the
Schweizer hook which I understand can be done with an available STC.
In my case I would also like to see a more accessible release
handle…would (or should) an STC be required for this? This would be
the short term fix I would like to see. If the handle was made from the
same stock as the current device, just a foot or so longer it would seem
that this should be able to be done without a long drawn out approval
process.

I love flying, enjoy the gliderport environment and people but I am NOT
willing to give up my life. When the glider kites things happen very
fast. Unless your hand is on the handle (not feasible in my
circumstances) you will not be able to release before the pressure gets
too great. If it happens much lower than I experience it will be lights
out. So remember my glider pilot friends….fly well….the life you save
might be your tow pilot.

Walt Connelly





--
Dan, 5J

Ban CG hooks on air tow? How many gliders would be immediately
grounded? Six of my seven gliders had a CG hooks (only) and I never
kited. How about better training? And, BTW, shoulder straps, no matter
how loose (or not even worn), will not cause a pilot to slip AFT. The
seat back will prevent that. Of course the seat back could fail, or the
seat BELT could be too loose allowing the pilot to slide under the BELT
and that might cause him to pull back on the stick.

On 4/24/2017 4:06 AM, Peter Whitehead wrote:
A manual release system will never be suitable to stop the rapid pitch up caused in this situation. It happens too fast for a manual (actually a sensory-brain-muscle-manual) system, however good the manual system on the tug.

Glider CofG winch hooks are not really suitable for aero-towing, are they? Some of these gliders can't be controlled in pitch once the kiting starts. I have seen people leave their shoulder starts loose on areotowing, and that means the pilot can slide backwards - with hand held on the stick, so that moves too - on pitch-up. (Always use tight shoulder straps on launching, please).

I have lost two acquaintances, both gliding "greats" in such tug upsets. I would say banning the use of aero-towing using a CofG winch hook would be the safest bet to save lives, until an automatic system is available.I suspect this is something EASA and the FAA would sort very quickly ( the banning bit, not the automatic system). Sensors to measure pitch, rate rate of the tug, and also tug/rope angle and angular rate,suitably processed and actioned automatically could allow immediate release BEFORE the critical situation is reached - if we feel that we must continue to launch by aero-tow using CofG hooks.

A rule of nose hooks or "compromise" hooks only, meanwhile, would probably reduce the risk towards zero.

--
Dan, 5J

Good testing and great write-up!

While not intending to excuse the short comings of the Schweizer release
as a tow hook, I recall seeing one mounted to a ground launch vehicle
via a very robust hinge (something you'd see on a farm field gate). In
a kiting incident, the release would rotate upwards keeping it aligned
with the tow rope and making release possible. Of course, the final
mounting location of the release cable would have to be such that the
cable could still exert sufficient pull distance to effect the release.

On 4/24/2017 4:39 AM, Chris Rollings wrote:
I have posted what follows on a previous occasion but, given the posts in
this thread, I think it bears repeating.

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.
Â
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 Schweizer
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.
Â
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.
Â
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.
Â
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.
Â
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�.
Â
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.
Â
Attempts to produce a tow-plane hook that would release automatically were
unsuccessful for reasons that became apparent later.
Â
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).
Â
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.
Â
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 Schweizer hook as fitted to a glider, but I would not
be surprised if it exhibited the same characteristics at high loads.
Â
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.
Â
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).
Â
Our conclusions for preventing “Kiting� we
Â
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).
Â
Don’t use short ropes. The speed at which things happen varies directly
with the length of the rope.
Â
Don’t let inexperienced pilots fly at anywhere near aft C of G.
Â
Don’t let inexperienced pilots fly solo in turbulent conditions.
Â
Replace or modify all Schweizer hooks fitted to tow planes. (So far as I
know there are none on gliders in the UK, so that question never arose).
Â
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.


At 10:06 24 April 2017, Peter Whitehead wrote:
A manual release system will never be suitable to stop the rapid pitch up
c=
aused in this situation. It happens too fast for a manual (actually a
senso=
ry-brain-muscle-manual) system, however good the manual system on the
tug.
Glider CofG winch hooks are not really suitable for aero-towing, are
they?
=
Some of these gliders can't be controlled in pitch once the kiting
starts.
=
I have seen people leave their shoulder starts loose on areotowing, and
tha=
t means the pilot can slide backwards - with hand held on the stick, so
tha=
t moves too - on pitch-up. (Always use tight shoulder straps on
launching,
=
please).=20

I have lost two acquaintances, both gliding "greats" in such tug upsets.
I
=
would say banning the use of aero-towing using a CofG winch hook would be
t=
he safest bet to save lives, until an automatic system is available.I
suspe=
ct this is something EASA and the FAA would sort very quickly ( the
banning=
bit, not the automatic system). Sensors to measure pitch, rate rate of
the=
tug, and also tug/rope angle and angular rate,suitably processed and
actio=
ned automatically could allow immediate release BEFORE the critical
situati=
on is reached - if we feel that we must continue to launch by aero-tow
usin=
g CofG hooks.=20

A rule of nose hooks or "compromise" hooks only, meanwhile, would
probably
=
reduce the risk towards zero.


--
Dan, 5J

[quote='Chris Rollings[_2_];944055']I have posted what follows on a previous occasion but, given the posts in
this thread, I think it bears repeating.

Chris,

Yours was one of the posts I paid the most attention to. I appreciate the approach you took to the problem and agree with your assessments although I think it's somewhat not feasible to eliminate aero tows to CG hook gliders. The vast majority of the high performance models I tow are CG but overall they are also being flown by the best pilots, never had big problem with one of them.

Short ropes, of course, bad idea but interestingly the rope on the student who kited low was a new rope and we make them longer to allow for repairs. This was not a problem in this instance.

These were largely inexperienced pilots, one student and one older gentleman who had not done an aerotow in quite some time. As I have noted earlier I will reserve the right to not tow someone for whatever reason. Having observed this older gentleman and had I known he hadn't done an aerotow in quite some time I would have insisted on some dual with an instructor.

Turbulence.....whlle there was some cross wind and we always have some thermals off the end of the runway it was not excessive, she flew two tows prior to the upset, she was not great but it was acceptable overall.

I agree with replacing all Schweizer hooks and this will be accomplished post haste with no argument or I will be gone.....

Release handle position....NO Question. These are things which I think should be on the FAA radar and under an AD note.

Thank you for your input, it greatly supports my position.

Walt

I just looked. I don't think McFarlane sells that improved Schweitzer tow hook anymore. The Tost tow release is only $2,000. Well worth it in my opinion.

The Pawnees at Turf Soaring had a 337 mod to invert the Schweizer releases. In theory that would work on a kiting situation, since the pull force would be on the fixed arm so the release arm would be free to move.

But that begs the question of the glider end - if a 1-26 or 2-33 kites, now HIS release is under tension, so HE can't release! A truly poor design, IMO.

Probably the best solution is the TOST internal reel with the guillotine - we checked ours on the ground (at rope replacement time) and even with relatively little tension on the rope (compared to a kiting glider), the guillotine (lever right in front of the throttle in the Pawnee) cut the rope like a piece of spaghetti. Actually surprised us how easy it was! And enough to convince us to install the same system on our other Pawnee, replacing the POS Schweizer release.

Kirk
66

I recently had a discussion with a CAP CFIG from Alaska that stated they actually tested the schewizer release on there tow plane at 45 degree angle up at 4000 pounds the release pressure on the handle was less than 25 lbs. Now I didn't get specifics as to method or condition of release. But it was his belief that it, as mentioned earlier in this thread, was more of a reaction time issue than ability. YRMV.

I have had the pleasure of taking tows behind Walt. He is best in show and a class act. I only have a few hundred tows in our club Pawnees and about 800tt in power and 50 in sailplanes so I'm no expert. What I do know is this: sailplane pilots are usually type-A and do a great job of rationalizing risk factors to themselves and others. Tost hook be damned, Walt- the variability in sailplane pilots at any club/commercial op will be a risk factor no modified release will solve for. It's a question of whether or not you are comfortable blocking out that idea or not. If I recall correctly, your past career is heavily analytical and clinical- I don't think this question is solvable that way.

The older, taller gliders were more problematic with CG hooks because the
hook was well below the actual CG. Some have a tendency to pitch up
excessively even on a winch.

I've taken aerotows on a CG hook in Pilatus, LS-4, ASW-20 and 27. None
harder than a nose hook in a Blanik.

The new designs are easier as long as the tow pilot gives you the
manufacturer recommended aerotow speed. One towpilot shorted me 10 kts
once too many times. I don't take tows from him any more as I got high
behind him twice and yanked.

The 27 with CG hook is lovely on aerotow, even through rotor. YMMV with
water and different CGs.

A nose hook is more work as you have to compensate for yaw and pitch
couples.

My question is which gliders with CG hooks are giving towpilots trouble?

Or is it a pilot problem?