Actual Rope Break

Hi Bruce

thank you for noting there is extra tension in the aerorow rope due
to the angle of climb. If the climb rate is 3kts (which is typical for a
2-seater at my club) at a towing speed of 60kts, the angle of climb
(theta) = 3/60 radians = 3 degrees, and the extra tension in the
rope is (weight of glider)*(sin theta) = typically 1000*(sin 3
degrees) = 50lbs. This would add to the tension in the rope due to
glider drag, as you say, to make a typical aerotow rope tension of
35+50 = 85lbs.

My estimate of the tension in the aerotow rope during initial
ground roll, assuming acceleration of a 1000lb glider to 60kts in
10secs, is about 315lbs.

Whilst the numbers can be juggled for different sailplanes and
towplanes, I agree with you that the greatest rope tension is likely
to be during initial acceleration i.e. during the ground roll. So
aerotow ropes are 'proof tested' on every ground roll, to a useful
degree. This does not assure that the rope meets the full rated
breaking strain however.

I read Bill's comment of June 2 about the rope perhaps taking
some longer time to actually break. This is a new idea to me, and I
don't know what too make of it. I'd like to hear the evidence for
this effect.

The more important discussion, is whether it is a good idea to train
or teach 200ft turn-backs to our students. Despite it being long
accepted practice, in the USA anyway, I doubt that it is a good idea
in terms of reducing serious accident rates. I note nobody suggests
we teach students to do final turns under 200ft. I wonder why.

I was interested in the suggestion from others that Germany has a
much lower PT3 accident rate, due to their stronger ropes and
weak links, and that this could perhaps be allowed by the FAA.


At 07:04 02 June 2014, Bruce Hoult wrote:
On Monday, June 2, 2014 4:50:10 PM UTC+12, Andrew wrote:
My experience seems logical when one considers that after
liftoff, the tension on the rope should be close to the drag on
the
glider, i.e. about weight divided by L/D, i.e. about 35lbs or less.

No, that's not the case unless you're not climbing.

With a tug flying at 65 knots and climbing at 6 knots (typical for
our
glass two seaters) somewhere around 9% of the weight of the
glider (up to
600 kg or 1300 lb) is being borne by the rope. That's about 120
lbs in
addition to the 35 lbs from drag.

With a 300 kg all up single seater (PW5, Libelle etc) flying a bit
slower
and climbing at over 1000 fpm there is actually even more strain
on the
rope.

I do agree that if it doesn't break on initial acceleration then it
probably won't.

I'm not going to go into the turn back or not question again other
than to
say if you can land safely more or less straight ahead then of
course do
so, but you should also be competent to turn back if that's best.

On Wednesday, June 4, 2014 2:17:46 PM UTC+12, Andrew wrote:
I note nobody suggests we teach students to do final turns
under 200ft. I wonder why.

I believe we train students to try to manage the circuit to aim to make the turn to final at about 300 ft, depending on how close in they are.

If they get it wrong (or hit sink) and find themselves at 200 ft instead, I don't think anyone is going to advise them not to make the turn!


If you were *planning* to make the final turn at 200 ft (which would be perfectly safe) but actually ended up the same 100 ft lower then that's getting very low indeed.


Finding yourself *actually* at 200 ft is not the same thing as planning to be at 200 ft at some point in the future with a chance that you might be higher or lower.

Reasonable questions, Andrew. I'll try to answer some of them to a degree below.

On Tuesday, June 3, 2014 8:17:46 PM UTC-6, Andrew wrote:
Hi Bruce



thank you for noting there is extra tension in the aerorow rope due

to the angle of climb. If the climb rate is 3kts (which is typical for a

2-seater at my club) at a towing speed of 60kts, the angle of climb

(theta) = 3/60 radians = 3 degrees, and the extra tension in the

rope is (weight of glider)*(sin theta) = typically 1000*(sin 3

degrees) = 50lbs. This would add to the tension in the rope due to

glider drag, as you say, to make a typical aerotow rope tension of

35+50 = 85lbs.

My estimate of the tension in the aerotow rope during initial

ground roll, assuming acceleration of a 1000lb glider to 60kts in

10secs, is about 315lbs.
-----------------------------
The European certification standards under CS-22 mentions the aero tow rope tension designers should expect. I seem to recall that number is 150daN. That would be close to your estimate.
-----------------------------

Whilst the numbers can be juggled for different sailplanes and

towplanes, I agree with you that the greatest rope tension is likely

to be during initial acceleration i.e. during the ground roll. So

aerotow ropes are 'proof tested' on every ground roll, to a useful

degree. This does not assure that the rope meets the full rated

breaking strain however.

-------------------
I suspect the loads encountered in slack recovery could be the greatest.
-------------------


I read Bill's comment of June 2 about the rope perhaps taking

some longer time to actually break. This is a new idea to me, and I

don't know what too make of it. I'd like to hear the evidence for

this effect.
------------------
There are lots of engineering papers on rope testing and failure modes available on the Internet. The basic idea is a rope is a large bundle of twisted fibers. Not all those individual fibers are equal in strength nor are they loaded equally. When it gets overloaded some of those fibers reach their breaking point before others and the rope starts to unravel.

The unraveling process is usually spotted when the rope is inspected but sometimes it progresses fast enough for the rope to fail before it gets inspected. In almost all cases there is a time interval between fiber breakage and catastrophic failure, otherwise rope inspections wouldn't work.

All but a couple of the rope breaks I know of happened after lift off and below about 1500 feet with normal tension on the rope.
------------------

The more important discussion, is whether it is a good idea to train

or teach 200ft turn-backs to our students. Despite it being long

accepted practice, in the USA anyway, I doubt that it is a good idea

in terms of reducing serious accident rates. I note nobody suggests

we teach students to do final turns under 200ft. I wonder why.

----------------
This one is easy. A turn to final is a normal operating procedure not an emergency like a rope break recovery. There's no justification for practicing low turns to final. A rope break is a true emergency so the rule book can be disregarded as far as necessary to deal with it safely.

I think there are enough stories, some related here, to say lives have been saved and injuries avoided when pilots returned to the runway after a rope break. I can't recall any training accidents practicing the return-to-runway maneuver but there may be a few. I've done hundreds and don't recall sweating out one of them.
----------------

I was interested in the suggestion from others that Germany has a

much lower PT3 accident rate, due to their stronger ropes and

weak links, and that this could perhaps be allowed by the FAA.

----------------------
This is the ultimate solution - just eliminate rope breaks altogether by using a rope so strong it's hard to imagine it breaking. However, weak links could still break so I would continue the training.

It's not so much the US government "allows" stronger weak links and rope - the Federal Air Regulations and Occupational Safety and Health Administration rules require it. It's just that we haven't been following those rules.

This one is easy. A turn to final is a normal operating procedure not an emergency like a rope break recovery. There's no justification for practicing low turns to final.

Here I think I would disagree with you. In a number of land out situations (small fields) turning final at lower than 200ft would be preferable as it would increase the chance of safely landing in the field.

Andrew,

thank you for your post. It helps to understand "why the rules are the way there are".

On Monday, June 2, 2014 6:50:10 AM UTC+2, Andrew wrote:
Hi Kevin



Congratulations on managing this emergency safely. A low tow

termination of the tow is a true emergency, that some people have

not managed successfully, as discussed on this website.



I've been gliding for a long time, and my experience is that

unplanned tow terminations are very rare. I have only had two

unplanned releases, the lowest at 400ft at Lasham (Euroglide 73),

the other at 1000ft at Portmoak. On the one at Lasham, I was very

aware that the entire competition grid were watching me. Both were

wave-offs, due to towplane engine problems (neither serious as it

turned out later, but the tow pilots were understandably concerned at

the time). My experience seems logical when one considers that after

liftoff, the tension on the rope should be close to the drag on the

glider, i.e. about weight divided by L/D, i.e. about 35lbs or less. Apart

from shocks from slack lines tightening, if the rope doesn't break

during the first few seconds when the glider is being dragged over

the ground, a 'pure rope break' is unlikely. The rope is essentially

'proof tested' in the first 30 seconds of every tow. I haven't had any

ring or tow-mechanism malfunctions. So from my experience, the

most likely (but rare) problem is a tow plane engine problem.



You say you are pleased that you practiced for this. I assume you

mean that you are glad you practiced 180 turns from 200ft. I wish to

say that my personal opinion is that a verbal briefing (to go straight

ahead) would have been much safer for your instructors to teach

you, with a verbal briefing that any other alternatives must be

delayed until high enough for 'some maneuvering'. I'd put that at

300ft minimum, when a 90 turn to look back, and do some thinking,

would be be ok, but even then, a turn away to the safest area should

be made, even if off-field. Otherwise, go more-or-less straight ahead,

and let the insurance company worry about their glider. I'd

recommend that instructors should teach that a low rope break is an

emergency, and the only responsibility on the pilot is to get himself

and his passenger down without harm. Damage to the glider should

not be considered. I obviously don't know for sure, but I think its

arguable that this teaching approach might produce more minor

damage to gliders, but fewer fatalities.



It's not that a typical glider isn't capable, aerodynamically-speaking,

of performing a 180 at 200ft. They obviously can. Its that an early

solo pilot may not be able to, and experienced pilots under that

stress may not be able to either.



Safety is a tricky concept. My view is that, to be safe, one should

'stop before it becomes unsafe'. That sounds obvious, but then

consider that this logically means that we should 'stop while we are

still safe'. Ie..... we should stop when we could have safely gone a bit

further. The price for safety, is to stop too early. I can remember

stopping flying (for weather) knowing people were thinking we could

have safely gone on a bit longer. And they were completely right.

Straining this logic, its arguably safer to teach to go straight ahead,

even if an excellent pilot could do a 180.



I stand by my remark made earlier, that a site where a straight-

ahead landing is likely to produce more than minor damage, is not a

safe site.





At 23:20 01 June 2014, Kevin Christner wrote:

I had my first "rope break" ever today. I was approximately 200-

250ft

abov=

e the ground. Emergency procedures were not a problem.



The Tost released for some reason. Its a nose hook so the back

release

mec=

hanism could not have been the culprit. Further testing with the

wheel

bre=

ak on the ground revealed no problems so I took another tow and

had no

prob=

lems. The ring may not have been engaged properly (doubtful) or

the slack

=

I got in the rope was just too much when it tightened back up

(probable

but=

the release still should have held).



In any case has anyone else ever experienced an actual emergency

unplanned

=

release? In 14 years of flying I have never heard of one. Good

thing we

d=

o practice this. =20



2C

You say you are pleased that you practiced for this. I assume you

mean that you are glad you practiced 180 turns from 200ft. I wish to

say that my personal opinion is that a verbal briefing (to go straight

ahead) would have been much safer for your instructors to teach

you, with a verbal briefing that any other alternatives must be

delayed until high enough for 'some maneuvering'. I'd put that at

300ft minimum, when a 90 turn to look back, and do some thinking,

would be be ok, but even then, a turn away to the safest area should

be made, even if off-field. Otherwise, go more-or-less straight ahead,

and let the insurance company worry about their glider. I'd

recommend that instructors should teach that a low rope break is an

emergency, and the only responsibility on the pilot is to get himself

and his passenger down without harm. Damage to the glider should

not be considered. I obviously don't know for sure, but I think its

arguable that this teaching approach might produce more minor

damage to gliders, but fewer fatalities.


Hi Andrew,

Here is the US the standard instruction is that you make a 180 deg turn at or above 200ft or a straight in below 200 feet. This is what the FAA Glider Flying Handbook says.

Our site is a little tricky. The runway is parallel to a 400 foot high ridge or so. You can only make a turn to the north. Generally you should always make your turn into the wind so that the wind will cause you to drift back towards the runway. In this case I was turning away from the wind. Luckily the runway parallels a large field on the other side so if you get blown away from the runway its still possible to land in the field.

2C

On Sunday, June 1, 2014 10:50:10 PM UTC-6, Andrew wrote:
Hi Kevin



Congratulations on managing this emergency safely. A low tow

termination of the tow is a true emergency, that some people have

not managed successfully, as discussed on this website.



I've been gliding for a long time, and my experience is that

unplanned tow terminations are very rare. I have only had two

unplanned releases, the lowest at 400ft at Lasham (Euroglide 73),

the other at 1000ft at Portmoak. On the one at Lasham, I was very

aware that the entire competition grid were watching me. Both were

wave-offs, due to towplane engine problems (neither serious as it

turned out later, but the tow pilots were understandably concerned at

the time). My experience seems logical when one considers that after

liftoff, the tension on the rope should be close to the drag on the

glider, i.e. about weight divided by L/D, i.e. about 35lbs or less. Apart

from shocks from slack lines tightening, if the rope doesn't break

during the first few seconds when the glider is being dragged over

the ground, a 'pure rope break' is unlikely. The rope is essentially

'proof tested' in the first 30 seconds of every tow. I haven't had any

ring or tow-mechanism malfunctions. So from my experience, the

most likely (but rare) problem is a tow plane engine problem.



You say you are pleased that you practiced for this. I assume you

mean that you are glad you practiced 180 turns from 200ft. I wish to

say that my personal opinion is that a verbal briefing (to go straight

ahead) would have been much safer for your instructors to teach

you, with a verbal briefing that any other alternatives must be

delayed until high enough for 'some maneuvering'. I'd put that at

300ft minimum, when a 90 turn to look back, and do some thinking,

would be be ok, but even then, a turn away to the safest area should

be made, even if off-field. Otherwise, go more-or-less straight ahead,

and let the insurance company worry about their glider. I'd

recommend that instructors should teach that a low rope break is an

emergency, and the only responsibility on the pilot is to get himself

and his passenger down without harm. Damage to the glider should

not be considered. I obviously don't know for sure, but I think its

arguable that this teaching approach might produce more minor

damage to gliders, but fewer fatalities.



It's not that a typical glider isn't capable, aerodynamically-speaking,

of performing a 180 at 200ft. They obviously can. Its that an early

solo pilot may not be able to, and experienced pilots under that

stress may not be able to either.



Safety is a tricky concept. My view is that, to be safe, one should

'stop before it becomes unsafe'. That sounds obvious, but then

consider that this logically means that we should 'stop while we are

still safe'. Ie..... we should stop when we could have safely gone a bit

further. The price for safety, is to stop too early. I can remember

stopping flying (for weather) knowing people were thinking we could

have safely gone on a bit longer. And they were completely right.

Straining this logic, its arguably safer to teach to go straight ahead,

even if an excellent pilot could do a 180.



I stand by my remark made earlier, that a site where a straight-

ahead landing is likely to produce more than minor damage, is not a

safe site.





At 23:20 01 June 2014, Kevin Christner wrote:

I had my first "rope break" ever today. I was approximately 200-

250ft

abov=

e the ground. Emergency procedures were not a problem.



The Tost released for some reason. Its a nose hook so the back

release

mec=

hanism could not have been the culprit. Further testing with the

wheel

bre=

ak on the ground revealed no problems so I took another tow and

had no

prob=

lems. The ring may not have been engaged properly (doubtful) or

the slack

=

I got in the rope was just too much when it tightened back up

(probable

but=

the release still should have held).



In any case has anyone else ever experienced an actual emergency

unplanned

=

release? In 14 years of flying I have never heard of one. Good

thing we

d=

o practice this. =20



2C



The rope tension is more than the glider's weight divided by the L/D which would only be the case in level flight. Aero tow is lifting the weight of the glider into the sky which requires more tension.

The record shows premature termination of tow is far more common than you suggest. Only one incident I know of involved a rope break in the takeoff roll. The rest were rope breaks during the early airborne part of the tow.

The reason is ropes don't simply snap when overloaded. Fibers break then the rope unravels which takes a little time. If the rope was overstressed in the takeoff roll, expect it to part at few hundred feet AGL.

The 200' AGL 180 turn back to the runway is the MINIMUM STANDARD in the US. Either a pilot learns to confidently demonstrate the maneuver with grace and precision or his flying career stops right there. All pilots are expected to retain that level of ability throughout their careers and can expect to be asked to prove it in every Flight Review or Check Ride.

However, demonstrating an ability to perform the maneuver doesn't mean it's required or even the preferred action in a real emergency. It's simply an additional option in the pilots repertoire. A pilot is expected to exercise good judgment in selecting the best option for the situation.

That said, if a pilot damages a glider in a risky off field landing when the glider was in a position for a safe return to the runway, he'll likely be grounded pending a check ride.

At 15:18 02 June 2014, Bill D wrote:
On Sunday, June 1, 2014 10:50:10 PM UTC-6, Andrew wrote:
Hi Kevin
=20
=20
=20
Congratulations on managing this emergency safely. A low tow=20
=20
termination of the tow is a true emergency, that some people have=20
=20
not managed successfully, as discussed on this website.
=20
=20
=20
I've been gliding for a long time, and my experience is that=20
=20
unplanned tow terminations are very rare. I have only had two=20
=20
unplanned releases, the lowest at 400ft at Lasham (Euroglide 73),=20
=20
the other at 1000ft at Portmoak. On the one at Lasham, I was very=20
=20
aware that the entire competition grid were watching me. Both were=20
=20
wave-offs, due to towplane engine problems (neither serious as it=20
=20
turned out later, but the tow pilots were understandably concerned
at=20
=20
the time). My experience seems logical when one considers that after=20
=20
liftoff, the tension on the rope should be close to the drag on the=20
=20
glider, i.e. about weight divided by L/D, i.e. about 35lbs or less.
Apart=
=20
=20
from shocks from slack lines tightening, if the rope doesn't break=20
=20
during the first few seconds when the glider is being dragged over=20
=20
the ground, a 'pure rope break' is unlikely. The rope is essentially=20
=20
'proof tested' in the first 30 seconds of every tow. I haven't had
any=20
=20
ring or tow-mechanism malfunctions. So from my experience, the=20
=20
most likely (but rare) problem is a tow plane engine problem.
=20
=20
=20
You say you are pleased that you practiced for this. I assume you=20
=20
mean that you are glad you practiced 180 turns from 200ft. I wish to=20
=20
say that my personal opinion is that a verbal briefing (to go
straight=20
=20
ahead) would have been much safer for your instructors to teach=20
=20
you, with a verbal briefing that any other alternatives must be=20
=20
delayed until high enough for 'some maneuvering'. I'd put that at=20
=20
300ft minimum, when a 90 turn to look back, and do some thinking,=20
=20
would be be ok, but even then, a turn away to the safest area should=20
=20
be made, even if off-field. Otherwise, go more-or-less straight
ahead,=20
=20
and let the insurance company worry about their glider. I'd=20
=20
recommend that instructors should teach that a low rope break is an=20
=20
emergency, and the only responsibility on the pilot is to get
himself=20
=20
and his passenger down without harm. Damage to the glider should=20
=20
not be considered. I obviously don't know for sure, but I think its=20
=20
arguable that this teaching approach might produce more minor=20
=20
damage to gliders, but fewer fatalities.
=20
=20
=20
It's not that a typical glider isn't capable,
aerodynamically-speaking,=
=20
=20
of performing a 180 at 200ft. They obviously can. Its that an early=20
=20
solo pilot may not be able to, and experienced pilots under that=20
=20
stress may not be able to either.=20
=20
=20
=20
Safety is a tricky concept. My view is that, to be safe, one should=20
=20
'stop before it becomes unsafe'. That sounds obvious, but then=20
=20
consider that this logically means that we should 'stop while we are=20
=20
still safe'. Ie..... we should stop when we could have safely gone a
bit=
=20
=20
further. The price for safety, is to stop too early. I can remember=20
=20
stopping flying (for weather) knowing people were thinking we could=20
=20
have safely gone on a bit longer. And they were completely right.=20
=20
Straining this logic, its arguably safer to teach to go straight
ahead,=
=20
=20
even if an excellent pilot could do a 180.
=20
=20
=20
I stand by my remark made earlier, that a site where a straight-
=20
ahead landing is likely to produce more than minor damage, is not a=20
=20
safe site.
=20
=20
=20
=20
=20
At 23:20 01 June 2014, Kevin Christner wrote:
=20
I had my first "rope break" ever today. I was approximately 200-
=20
250ft
=20
abov=3D
=20
e the ground. Emergency procedures were not a problem.
=20

=20
The Tost released for some reason. Its a nose hook so the back=20
=20
release
=20
mec=3D
=20
hanism could not have been the culprit. Further testing with the=20
=20
wheel
=20
bre=3D
=20
ak on the ground revealed no problems so I took another tow and=20
=20
had no
=20
prob=3D
=20
lems. The ring may not have been engaged properly (doubtful) or=20
=20
the slack
=20
=3D
=20
I got in the rope was just too much when it tightened back up=20
=20
(probable
=20
but=3D
=20
the release still should have held).
=20

=20
In any case has anyone else ever experienced an actual emergency=20
=20
unplanned
=20
=3D
=20
release? In 14 years of flying I have never heard of one. Good=20
=20
thing we
=20
d=3D
=20
o practice this. =3D20
=20

=20
2C
=20


The rope tension is more than the glider's weight divided by the L/D
which
=
would only be the case in level flight. Aero tow is lifting the weight
of
=
the glider into the sky which requires more tension.

The record shows premature termination of tow is far more common than you
s=
uggest. Only one incident I know of involved a rope break in the takeoff
ro=
ll. The rest were rope breaks during the early airborne part of the tow.
=
=20

The reason is ropes don't simply snap when overloaded. Fibers break then
t=
he rope unravels which takes a little time. If the rope was overstressed
i=
n the takeoff roll, expect it to part at few hundred feet AGL.

The 200' AGL 180 turn back to the runway is the MINIMUM STANDARD in the
US.=
Either a pilot learns to confidently demonstrate the maneuver with
grace
=
and precision or his flying career stops right there. All pilots are
expec=
ted to retain that level of ability throughout their careers and can
expect=
to be asked to prove it in every Flight Review or Check Ride.

However, demonstrating an ability to perform the maneuver doesn't mean
it's=
required or even the preferred action in a real emergency. It's simply
an=
additional option in the pilots repertoire. A pilot is expected to
exerci=
se good judgment in selecting the best option for the situation.

That said, if a pilot damages a glider in a risky off field landing when
th=
e glider was in a position for a safe return to the runway, he'll likely
be=
grounded pending a check ride.

Bill, you need to accept that this forum is worldwide. Advice and practice
is different outside the USA. While turn backs may be de rigueur at low
height in the USA they are positively discouraged in the UK and never ever
practiced. People who make low turns receive intensive counseling and/or
remedial instruction.
I would never ever criticise a pilot, or take punitive action for choosing
the option which gives the best chance of survival. Insurance can pay for
damaged gliders, as far as I am aware all the money in the world cannot
resurrect dead people.
You have made it very clear that you feel we are lacking in some way and of
course we think the FAA is barmy, such is life.

Hmm I think the BGA may want to take a hard look at this. My cockpit being crushed by trees would not have been an appealing option.

Bill, you need to accept that this forum is worldwide. Advice and practice

is different outside the USA. While turn backs may be de rigueur at low

height in the USA they are positively discouraged in the UK and never ever

practiced. People who make low turns receive intensive counseling and/or

remedial instruction.

I would never ever criticise a pilot, or take punitive action for choosing

the option which gives the best chance of survival. Insurance can pay for

damaged gliders, as far as I am aware all the money in the world cannot

resurrect dead people.

You have made it very clear that you feel we are lacking in some way and of

course we think the FAA is barmy, such is life.

On Monday, June 2, 2014 10:38:34 AM UTC-6, Don Johnstone wrote:

"Bill, you need to accept that this forum is worldwide. Advice and practice
is different outside the USA."

Don, you need to accept that it's different everywhere outside the UK.

"While turn backs may be de rigueur at low height in the USA."

Wrong. Turns are optional. TRAINING is de regueur.

"You have made it very clear that you feel we are lacking in some way."

In very specific ways.

"We think the FAA is barmy."

Thanks for pointing that out. I've made sure they got the message.