Winch Launch Safety Study

On Mar 17, 7:45*pm, wrote:
In the recent Aerokurier, there is a very good 2 part article on winch
safety, which I think everyone should read who is interested in winch
launches. I read it in the hard cover version in German, so I don't
know if an online version is available.

The main takeaways for me are the following statistics:

- Winch launches are the safest launch type
- Tension controlled winches are far safer than speed controlled ones,
such as the new electric, hydro and diesel designs. They recommend
modifying them to tension controlled ones, as the data is clear.
- initial tension at he beginning of the launch should be about
between 0.5g and 1g - depending on type.
- At the beginning of the launch, there is a danger that if the
acceleration is too strong, it could exceed the elevator authority of
gliders to stay in a safe angle of attack. A high speed (or in the
case of a rope break) low speed stall could occur - which are almost
always fatal in this phase. Plenty of evidence of this is features in
the article, including the fact that GROB models are never involved in
this type of accident because of their large elevators and low mounted
wings, generally limiting pitch up forces.
- Permissible tension forces, tow speeds the weak links should be
changed to allow for faster tows (though not in the beginning phase) -
Especially in the case of GROB models as the weak link used for GROBs
are too weak as seen in those often breaking and those models often
involved in accidents involving broken weak links. Stronger ones from
1.6-1.9g should be used for GROBS.

- The biggest causes of accidents during winch launches are :

1. Winch malfunctions
2. Rope Breaks

These two points can be addressed by proper care and maintenance of
the equipment.

And again, speed controlled winch designs, like the newer ones, are
far more accident prone, than tension controlled ones. What is needed
is tension, not constant speed, so the metric being measured is the
wrong one and it is more difficult for the winch operator to adjust
from one model to the next based just on speed.

Also speed controlled winches give the opposite response to pilot
inputs compared to tension controlled winches. On a Speed controlled
winch, pulling will increase airspeed and pushing will decrease it. On
a Tension controlled one, pulling will reduce airspeed, while pushing
will increase it. They both produce opposite results per pilot input.

In the beginning phase of the launch, the tension controlled one has
obvious clear advantages and is far safer statistically. Having said
all this, winch launches are safer than aerotows.

Someone please ask commercial operators stateside to offer winch
launches, please.........like at Llano, Cal City, Minden,
etc.........you can pick up a used Tost winch starting around 7k Euros
- a lot cheaper than rebuilding, insuring and operating a Pawnee.
Tom Vallarino

Tension controlled winches are certainly safer and provide higher
launches than speed controlled ones - any winch with an automatic
transmission is speed controlled since that's what the automotive
drive train was designed to do.

I'm not sure I follow the rest of your post. Hopefully an English
translation will be available.

Bill Daniels

The article says that the older gasoline powered winches with
automatic transmissions are tension controlled types, or at least
allow themselves to be controlled that way. I'm not familiar with
transmissions to know either way, just citing what the article says.

They go on to recommend that the existing electric ones and the
Diesels commonly used at least install a tension meter. Further, the
older Hercules winches should be abandoned as they are not up to par,
break down often and are not of the highest standard. These are mainly
from eastern Europe.

To take away, besides maintaining the equipment, is to realize that an
avoidable danger is in accelerating too fast in the first few seconds,
because this can (emphasis on "can") cause an unavoidable pitch up
movement that can not be compensated for with the available elevator
authority - resulting in either a high speed and nose high stall at
low altitude due to an excessive angle of attack or a rope break (weak
link). the former is almost always fatal. The latter not, but can be
if the pilot does not react properly and can contribute to
accidents.

On Mar 17, 11:12*pm, wrote:
The article says that the older gasoline powered winches with
automatic transmissions are tension controlled types, or at least
allow themselves to be controlled that way. I'm not familiar with
transmissions to know either way, just citing what the article says.

They go on to recommend that the existing electric ones and the
Diesels commonly used at least install a tension meter. Further, the
older Hercules winches should be abandoned as they are not up to par,
break down often and are not of the highest standard. These are mainly
from eastern Europe.

To take away, besides maintaining the equipment, is to realize that an
avoidable danger is in accelerating too fast in the first few seconds,
because this can (emphasis on "can") cause an unavoidable pitch up
movement that can not be compensated for with the available elevator
authority - resulting in either a high speed and nose high stall at
low altitude due to an excessive angle of attack or a rope break (weak
link). the former is almost always fatal. The latter not, but can be
if the pilot does not react properly and can contribute to
accidents.

The pleasing part is that the new German statistics show winch launch
actually safer than air tow. I've always felt that this should be the
case if the equipment was well maintained and the pilots were trained
to the same level as is common with air tow.

However there seems to be a lot of technical confusion in the article.

"Tension Controlled" means that rope tension is continuously monitored
and controlled through an automatic feedback loop. There are very few
winches that do this. Two American built super winches are both
tension controlled with feedback systems.

Automatic transmissions are designed to help a vehicle driver maintain
highway speed on varying road grades - that's speed control and the
exact opposite of tension control. There is no torque regulation in
an automatic transmission and there is no way to modify one to provide
it. To refer to them as "tension controlled is plain wrong.
Automatic transmissions taken from road vehicles simply don't belong
in glider winches.

Electric winches could easily be tension controlled by just monitoring
the current flowing to the motor as could hydrostatic winches by
monitoring hydraulic pressure although a running line tensiometer
would be a better solution. My understanding is that the ESW-2B is
tension controlled. I know for a fact that both American super
winches under development ARE tension controlled.

Specific attacks on electric winches show a complete lack of
understanding of electric power. The innate characteristics of
electric motors make them ideal for torque control.

"Older gasoline powered winches" most likely mean underpowered winches
that are actually "tension limited" such that the glider pilot can
control airspeed with pitch. To refer to these as tension controlled
is unwarranted and will confuse the issue. These are not "tension
controlled".

The hysterics about initial acceleration needs hard data not theory or
speculation to back it up. The experiments I have done with "pitch
strings" used as an angle of attack indicator suggest that the AOA is
nowhere near stall regardless of initial acceleration. In fact,
greater acceleration actually reduces peak AOA.

The preceding paragraph needs to be qualified by saying there are a
small number of types with well deserved reputations for
uncontrollable pitch-up under hard acceleration. There are relatively
few of these and they are .

Two points Bill,

1) Why do you think that automatic transmissions in vehicles (or winches)
try to maintain a constant speed? If you come to a hill and keep the
throttle setting the same, the vehicle will slow down! As a glider rotates
into the full climb and demands more power, the winch cable will also slow
down if you maintain the same throttle setting. Are you sure you are not
getting confused with cruise control?

2) Obviously you have never watched a dangerously steep, overpowered,
over-rotation on a winch launch. I have!

Generally these involve lighter, high wing gliders such as K6s, K7s and
K8s. Usually they go into a near vertical climb, often despite the pilot
holding the stick on the forward stop, followed by a weak link failure. On
one occasion a K8 got into such a steep climb that the belly hook just back
released, due to the extreme angle between the fuselage and the cable.
These events normally leave the glider pointing almost vertically at the
sky at about 100ft with no further means of propulsion! Fortunately at our
club, none of these glider flicked, and were being flown by youngish,
switched on pilots who managed to get them down safety. I dread to think
what would have happened if a numpty had been flying them.

I know a lady who was almost knocked unconcious during at the start of a
very overpowered winch launch, when she banged her head very hard on the
canopy frame of her K6. Also there have been a couple of fatal flick spin
accidents in the UK where it is believed that the pilots slipped back in
their seats and pulled the stick hard back as a result. These were both in
glass single seaters with rather reclining seatpans.

There is no case for exceeding about 0.8g acceleration during the ground
run. This gets you up to flying speed in about 3 to 4 seconds. Most pilots
find this enough!

Derek Copeland



At 15:16 18 March 2009, bildan wrote:
.

The pleasing part is that the new German statistics show winch launch
actually safer than air tow. I've always felt that this should be the
case if the equipment was well maintained and the pilots were trained
to the same level as is common with air tow.

However there seems to be a lot of technical confusion in the article.

"Tension Controlled" means that rope tension is continuously monitored
and controlled through an automatic feedback loop. There are very few
winches that do this. Two American built super winches are both
tension controlled with feedback systems.

Automatic transmissions are designed to help a vehicle driver maintain
highway speed on varying road grades - that's speed control and the
exact opposite of tension control. There is no torque regulation in
an automatic transmission and there is no way to modify one to provide
it. To refer to them as "tension controlled is plain wrong.
Automatic transmissions taken from road vehicles simply don't belong
in glider winches.

Electric winches could easily be tension controlled by just monitoring
the current flowing to the motor as could hydrostatic winches by
monitoring hydraulic pressure although a running line tensiometer
would be a better solution. My understanding is that the ESW-2B is
tension controlled. I know for a fact that both American super
winches under development ARE tension controlled.

Specific attacks on electric winches show a complete lack of
understanding of electric power. The innate characteristics of
electric motors make them ideal for torque control.

"Older gasoline powered winches" most likely mean underpowered winches
that are actually "tension limited" such that the glider pilot can
control airspeed with pitch. To refer to these as tension controlled
is unwarranted and will confuse the issue. These are not "tension
controlled".

The hysterics about initial acceleration needs hard data not theory or
speculation to back it up. The experiments I have done with "pitch
strings" used as an angle of attack indicator suggest that the AOA is
nowhere near stall regardless of initial acceleration. In fact,
greater acceleration actually reduces peak AOA.

The preceding paragraph needs to be qualified by saying there are a
small number of types with well deserved reputations for
uncontrollable pitch-up under hard acceleration. There are relatively
few of these and they are .

On the other end of the scale, only 8.7% of the accidents with winch
launches involves a wing drop to the ground.

On Mar 18, 12:30*pm, Derek Copeland wrote:

2) Obviously you have never watched a dangerously steep, overpowered,
over-rotation on a winch launch. I have!

No, I haven't seen any of the weird things you describe. They all
seem to happen in your immediate vicinity. I wonder why.

At 23:53 18 March 2009, bildan wrote:
On Mar 18, 12:30=A0pm, Derek Copeland wrote:

2) Obviously you have never watched a dangerously steep, overpowered,
over-rotation on a winch launch. I have!

No, I haven't seen any of the weird things you describe. They all
seem to happen in your immediate vicinity. I wonder why.

Probably because I'm a gliding instructor at a large and very busy club
that does a lot of winch launching. I also visit other clubs that winch
launch.

I should add that probably 99% of winch launches go to plan (including
deliberate practice launch failures by instructors) without any
difficulties. About 1% suffer cable or weak link breaks, or other
technical failures of one sort or another. Given proper training these
should be just an inconvenience, rather than an emergency situation.

Once it was recognised that over powered winch launches were a problem for
lighter gliders, we reduced the power settings used for these types and
opened the winch engine throttles a bit more slowly. I haven't seen a
'rocket launch' for quite a few years now.

Derek Copeland

In message , Derek Copeland
writes
At 23:53 18 March 2009, bildan wrote:
On Mar 18, 12:30=A0pm, Derek Copeland wrote:

2) Obviously you have never watched a dangerously steep, overpowered,
over-rotation on a winch launch. I have!

No, I haven't seen any of the weird things you describe. They all
seem to happen in your immediate vicinity. I wonder why.

Probably because I'm a gliding instructor at a large and very busy club
that does a lot of winch launching. I also visit other clubs that winch
launch.

I should add that probably 99% of winch launches go to plan (including
deliberate practice launch failures by instructors) without any
difficulties. About 1% suffer cable or weak link breaks, or other
technical failures of one sort or another. Given proper training these
should be just an inconvenience, rather than an emergency situation.

I have over 300 winch launches - not many by some people's standards but
still a fair number. I have had only 1 genuine launch failure (caused
by the club member driving the winch, not by me or by equipment
failure), so either I'm due a run of cable breaks and the like, or the
percentage of failures is better for some pilots/ glider types than
others... (or a bit of both of course)

snip

--
Surfer!
Email to: ramwater at uk2 dot net

bildan wrote:

On Mar 18, 12:30 pm, Derek Copeland wrote:

2) Obviously you have never watched a dangerously steep, overpowered,
over-rotation on a winch launch. I have!

No, I haven't seen any of the weird things you describe. They all
seem to happen in your immediate vicinity. I wonder why.
Hi Bill

Derek is not alone here.

As additional example - Std Cirrus can also be coaxed into too steep a
climb with too much power too fast.

In this case it is often a combination of
- poor seatbelt location, which allows the pilot to slide back on launch.
- full flying elevator - which is possible to stall if one holds the
stick full forward. (Low speed + big angular deflection)
- aft CG with a light pilot

Maybe if you only ever see launches of modern glass, or heavy two
seaters, with experienced winch drivers who know the different types,
you will not see this. I have yet to meet the perfect winch driver. At a
busy club, or where the winch driver is inexperienced with the glider
type or just inexperienced it is easy on a powerful winch to apply too
much initial power. And a little too fast on the throttle at the start
is very dangerous for a range of gliders.

Basically excessive acceleration can use most or all of the available
control authority. One more thing, like a gust / thermal / cable break
and you exceed the authority and have an accident. Best to avoid. Best
way to avoid is to train winch driving technique correctly. Denial gets
people hurt.

Power limited winches are no fun either...

Bruce

At 03:26 18 March 2009, bildan wrote:


Tension controlled winches are certainly safer and provide higher
launches than speed controlled ones - any winch with an automatic
transmission is speed controlled since that's what the automotive
drive train was designed to do.

I'm not sure I follow the rest of your post. Hopefully an English
translation will be available.

Short of fitting a speed controlling governor, I don't think that any
current winches are 'speed controlled'. Some earlier diesel powered
winches did have this arrangement, but it was found not to work. During a
properly controlled winch launch the motor and drum speeds reach a maximum
as the glider lifts off, and then decrease during the rest of the launch,
due to vector effects and wind gradients.

Winches with automatic transmissions , such as the Tost and the Skylaunch,
will give perfect good constant pull/tension launches as long as you use an
appropriate throttle or power setting. The Tost depends on the skill of the
driver to do this, but the Skylaunch has an adjustable throttle stop that
you can pre-program according to the glider type and headwind.

Derek Copeland