TOST Release Failure - Two Uncommanded Release's

Weirdest thing today. On the grid, ready to takeoff. I had a highly experienced contest pilot hookup the small ring into my TOST hookup. When the towplane started rolling, after about 20 feet, it released uncommanded.

I thought that it must have been the way it was hooked up, so we tried it again. Before the second attempt we did a very focused test and it seemed normal. Then on my second takeoff the same thing happened and I rolled to a stop again.

We took the towrope and hooked up the ring just like we always do for a third time. Towplane engine shut off at this point. This time I pulled the rope with both hands as two guys were holding back on the sailplane and it disconnected just with my force.


I now have the seat pan out, the instrument panel out and looking it over very thoroughly with a couple other pilots and see nothing. The cable looks normal, the sleeve looks normal, pulley system looks normal, the clamps at each end of the cable look fine. Essentially, everything associated with the release system looks brand new.

The plan is to purchase a new TOST hook from Tim Mara and be done with it.

I'm just writing this on here to inquire information regarding this from others. I am puzzled as to why this would have happened.

On Thu, 11 Oct 2012 17:03:04 -0700, scottandrewalexander wrote:

I now have the seat pan out, the instrument panel out and looking it
over very thoroughly with a couple other pilots and see nothing. The
cable looks normal, the sleeve looks normal, pulley system looks normal,
the clamps at each end of the cable look fine. Essentially, everything
associated with the release system looks brand new.

The one thing you didn't mention is the state of the springs in the hook.
Were any of them broken?


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

On Thursday, October 11, 2012 8:03:04 PM UTC-4, (unknown) wrote:
Weirdest thing today. On the grid, ready to takeoff. I had a highly experienced contest pilot hookup the small ring into my TOST hookup. When the towplane started rolling, after about 20 feet, it released uncommanded. I thought that it must have been the way it was hooked up, so we tried it again. Before the second attempt we did a very focused test and it seemed normal. Then on my second takeoff the same thing happened and I rolled to a stop again. We took the towrope and hooked up the ring just like we always do for a third time. Towplane engine shut off at this point. This time I pulled the rope with both hands as two guys were holding back on the sailplane and it disconnected just with my force. I now have the seat pan out, the instrument panel out and looking it over very thoroughly with a couple other pilots and see nothing. The cable looks normal, the sleeve looks normal, pulley system looks normal, the clamps at each end of the cable look fine. Essentially, everything associated with the release system looks brand new. The plan is to purchase a new TOST hook from Tim Mara and be done with it. I'm just writing this on here to inquire information regarding this from others. I am puzzled as to why this would have happened.

The last time I saw what you describe it turned out that the release cable was rusted inside the guide and just prevented the release from completely closing.
Pee in the belly is a bad thing.
Another case from the past was in 19's and 20's, guys would put the guide that supports the release and rudder pedal adjustment on the back of the bulhead they mount to and remove all the slack- Boinggggg!
Another couple experience points
UH

I agree with Martin. It could be the springs.
Juan Carlos

On Thursday, October 11, 2012 8:03:04 PM UTC-4, (unknown) wrote:
Weirdest thing today. On the grid, ready to takeoff. I had a highly experienced contest pilot hookup the small ring into my TOST hookup. When the towplane started rolling, after about 20 feet, it released uncommanded. I thought that it must have been the way it was hooked up, so we tried it again. Before the second attempt we did a very focused test and it seemed normal. Then on my second takeoff the same thing happened and I rolled to a stop again. We took the towrope and hooked up the ring just like we always do for a third time. Towplane engine shut off at this point. This time I pulled the rope with both hands as two guys were holding back on the sailplane and it disconnected just with my force. I now have the seat pan out, the instrument panel out and looking it over very thoroughly with a couple other pilots and see nothing. The cable looks normal, the sleeve looks normal, pulley system looks normal, the clamps at each end of the cable look fine. Essentially, everything associated with the release system looks brand new. The plan is to purchase a new TOST hook from Tim Mara and be done with it. I'm just writing this on here to inquire information regarding this from others. I am puzzled as to why this would have happened.

Saw exactly that a few years ago at the contest in Perry, SC in a '27. Turned out that the pilot, who was a bit on the 'portly' side, had bowed the seat-pan to the point that it was slightly pinching the release cable between it and bottom of the fuselage. That created enough resistance so the spring of the TOST release could not fully close the over-center mechanism. Replacing the cable and re-routing it solved the issue. There was nothing wrong with the release itself.
'GM'

On Thursday, October 11, 2012 6:31:00 PM UTC-6, Martin Gregorie wrote:
On Thu, 11 Oct 2012 17:03:04 -0700, scottandrewalexander wrote:



I now have the seat pan out, the instrument panel out and looking it

over very thoroughly with a couple other pilots and see nothing. The

cable looks normal, the sleeve looks normal, pulley system looks normal,

the clamps at each end of the cable look fine. Essentially, everything

associated with the release system looks brand new.



The one thing you didn't mention is the state of the springs in the hook.

Were any of them broken?





--

martin@ | Martin Gregorie

gregorie. | Essex, UK

org |

Happened a few years ago on our G103 Twin II on the CG hook during two winch launch attempts. We quit after the second attempt. Broken spring on one side of the TOST release. Glider didn't move much either time.

Frank Whiteley

As others note, that is the classic symptom of the overcenter spring
being broken on one side. The spring in question is a two-sided
torsion spring with anchors on both sides of the overcenter arm. The
result of this failure is that the hook will seem to operate normally,
but it will self-release under very low loads.

It could also be the other failures suggested in this thread, but my
money is on the spring being broken on one side.

Thanks, Bob K.

Am 12.10.2012 07:29, Bob Kuykendall wrote:
As others note, that is the classic symptom of the overcenter spring
being broken on one side. The spring in question is a two-sided
torsion spring with anchors on both sides of the overcenter arm. The
result of this failure is that the hook will seem to operate normally,
but it will self-release under very low loads.

It could also be the other failures suggested in this thread, but my
money is on the spring being broken on one side.

Thanks, Bob K.


This is the reason why the TOST release has a nominal TBO of 2,000 tows
(i.e. 10,000 cycles to the spring). Send it in to be services and be
happy for another 2.000 tows...
--
Peter Scholz
ASW24 JE

On Thu, 11 Oct 2012 17:03:04 -0700 (PDT),
wrote:

Weirdest thing today. On the grid, ready to takeoff. I had a highly experienced contest pilot hookup the small ring into my TOST hookup. When the towplane started rolling, after about 20 feet, it released uncommanded.

Hi Scott,

three questions:

- What kind of glider is it?
- How many launches are on the TOST hook?
- Am I correct that it is a CG hook?


Andreas

This is the reason why the TOST release has a nominal TBO of 2,000 tows
(i.e. 10,000 cycles to the spring). Send it in to be services and be
happy for another 2.000 tows...
--
Peter Scholz
ASW24 JE

Well, this TOST hookup was replaced about 100 tows ago.

Looks like there was dirt that got into the sleeve that housed the
release cable. Where do you order a new one of these? Time for a new
one.