Aerotow ropes: short or long, breakable or unbreakable?

On Thursday, November 14, 2013 12:36:01 PM UTC-5, son_of_flubber wrote:
On Thursday, November 14, 2013 11:35:41 AM UTC-5, Evan Ludeman wrote: So... is this operation experiencing rope failure without overload? That does not tell us that their current procedure has a reasonable safety margin. In 27 years in the sport, I've never seen this. If I did see it I'd change my procedures. Wait for a failure before evaluating procedures?? At some of my favorite places to fly, a PTT at 50 - 300' has some significant risks, so we take launch prep fairly seriously, the tow plane pre-launch checkout includes a warmup flight, etc. We don't break ropes, we inspect them visually and service them before they get ugly. We have no need for an ASTM approved test program.... The collective wisdom and experience of many people have developed the status quo, and I'm sure that it makes sense on balance, but does anyone know how close to failure ropes get before they are retired? And there is the matter of how our uncertainty about the rope affects training and flight reviews. I watched a glider enter a spin after a "simulated rope break" during a biennial flight review (glider totaled, no injuries, pilot retired). Maybe "that should not have happened", but I'm left with the impression that PTT simulation is risky. That the probability of a PTT is currently high enough to justify the risk of PTT simulation is part of my motivation. Can the probability of a PTT be lowered to the point that the risk of PTT simulation is no longer justified? How far can better rope material choice and better (practical) inspection go towards eliminating the possibility of PTT?

12000 glider flights, about 7500 tows given, and the only rope breaks have been during initial acceleration. Maybe 1/2 dozen or so of those. And a couple on the tug end from hanging the rope in a tree.
I've done hundreds of PTT and do not see why they should create an increased risk that would justify not doing that training. I want to KNOW(as well as possible) that the person I'm teaching or reviewing will respond correctly in the event that a failure happens. That failure could be many reasons, broken rope being one.
Real rope breaks are, thankfully, pretty rare. From this I conclude most operations are taking suitable care.
FWIW
UH

Well... if you had a winch, that 100-300 ft rope break would occur while you still have 2000 ft of runway ahead of you - I like that option a whole lot better than the trees off the end of the runway whilst on aerotow.

Sorry, it's November and I just couldn't resist throwing the winch wrench into the discussion

On Thursday, November 14, 2013 6:54:40 PM UTC-5, Dave Springford wrote:
Well... if you had a winch, that 100-300 ft rope break would occur while you still have 2000 ft of runway ahead of you - I like that option a whole lot better than the trees off the end of the runway whilst on aerotow.



Sorry, it's November and I just couldn't resist throwing the winch wrench into the discussion

I wish we had the space for winch launch! The undeniable advantage of a good tow plane is being able to operate out of a small airport.

T8

What is small? We have 3000 ft fence to fence and use about 2500 for the launches. Typical heights 800-900 AGL in the summer, 1100-1200 in the winter. Best launch was 1600 AGL.

satellite image of the airport:
http://goo.gl/maps/ngz44

Our ropes are 200 ft long so the jig would either have to be very large or
the rope would need to mounted and dismounted numerous times.

I've had three rope breaks in the past three years while flying the tug at
Moriarty - never before in the previous 20+ years. Two were the fault of
the glider pilot horribly mishandling the glider and one of those resulted
in the rope being wrapped around the wing of the HP-14. The wing was cut
back to the spar before the rope broke. The third rope break had one of
our instructors flying his Libelle and the rope broke at about 300' AGL on
tow. He and I were both surprised and he handled the emergency perfectly.
The rope broke about 10 feet in front of the glider, probably in an area of
high wear.

We inspect our ropes daily in the morning and during operations throughout
the day. If sufficient wear is noted, the rope is replaced. More often
it's the weak link which gets replaced due to abrasion with the pavement.
We use the top half of drinking water bottles, slipped over the weak link,
and wrapped heavily with duct tape. These work well, but, if the tug lands
such that the protector hits the end of the runway, it's ripped off. We
then replace the weak link.

(Whew!)


"son_of_flubber" wrote in message
...
On Thursday, November 14, 2013 10:50:46 AM UTC-5, Dan Marotta wrote:
Are we gonna fly today or will the eight hour test and evaluation session

use up all available daylight?

Good point. Time is limited.

If an elongation test would detect weakening ropes, you would only need to
do it once a week or so because many forms of degradation is gradual. If
you had a permanent jig for testing elongation, it would take 10 minutes.
Or you could test the ropes at the beginning of wave season when you know
they are gonna get stressed (or even better, you could replace your ropes at
the beginning of wave season).

Ever have a rope break at Moriarty? I know that you have a lot of grit.

On Thursday, November 14, 2013 7:44:29 PM UTC-5, Dave Springford wrote:
What is small? We have 3000 ft fence to fence and use about 2500 for the launches. Typical heights 800-900 AGL in the summer, 1100-1200 in the winter. Best launch was 1600 AGL.

http://www.airnav.com/airport/2B9

Evan Ludeman / T8

That is small!

Is there a difference between a Winch launch Weak Link, and an Aerotow Weak
Link?

I would think the Winch launch requires a much stronger link because of the
higher angle of attack.


At 00:46 15 November 2013, Dan Marotta wrote:
Our ropes are 200 ft long so the jig would either have to be very large or

the rope would need to mounted and dismounted numerous times.

I've had three rope breaks in the past three years while flying the tug at

Moriarty - never before in the previous 20+ years. Two were the fault of

the glider pilot horribly mishandling the glider and one of those resulted

in the rope being wrapped around the wing of the HP-14. The wing was cut

back to the spar before the rope broke. The third rope break had one of

our instructors flying his Libelle and the rope broke at about 300' AGL on

tow. He and I were both surprised and he handled the emergency perfectly.

The rope broke about 10 feet in front of the glider, probably in an area
of

high wear.

We inspect our ropes daily in the morning and during operations throughout

the day. If sufficient wear is noted, the rope is replaced. More often
it's the weak link which gets replaced due to abrasion with the pavement.

We use the top half of drinking water bottles, slipped over the weak link,

and wrapped heavily with duct tape. These work well, but, if the tug
lands

such that the protector hits the end of the runway, it's ripped off. We
then replace the weak link.

(Whew!)

On Friday, November 15, 2013 7:04:22 AM UTC-5, Peter Higgs wrote:
Is there a difference between a Winch launch Weak Link, and an Aerotow Weak

Link?



I would think the Winch launch requires a much stronger link because of the

higher angle of attack.



Peter - you seem to confuse angle of attack with deck angle. The angle of attack of the glider during a winch launch is no different than if the glider was in free flight at the same speed! During a winch launch, the glider is flying up a trajectory with a deck angle of up to 45degr.
And yes, the weak link strength spelled out in the plane's POH typically call for a stronger one for the CG hook vs. the nose hook.
With our winch, we measured the line forces and the results show an interesting force distribution - not at all what everybody expected.

Uli Neumann
'GM'

At 12:39 15 November 2013, GM wrote:
... The angle of attack of the glider during a winch launch is no
different than if the glider was in free flight at the same speed!

Actually, it is significantly different: the wing has to deal with the
tension in the cable (including the cable self-weight) as well as
supporting the weight of the glider. This could easily double the
AoA required in steady flight & smoth conditions towards the end of
a winch launch.