On Sunday, March 24, 2019 at 12:30:03 PM UTC-4, soarin wrote:
In regard to the 10/7/2017 low level tow plane crash/fatality, the final
NTSB report has now been published.

The report confirmed that the Pawnee had a Tost reel installed and
that the weak link had not failed as designed, rather the tow rope
itself broke. Additionally the report clarified that the tow planes
guillotine had not been activated and no weak link was recovered.

The NTSB metallurgist report states the following regarding the tow
rope, but no determination as to the rope mfg name or rope strength.

The received section of tow rope was visually and microscopically
examined in the NTSB Materials Laboratory.The as‐received rope was
separated at one end and intentionally cut at the other as show in
attached image 1711Image57402. The ¼ inch diameter rope had a
braided outer layer covering approximately 13 twisted inner strands.
Magnified inspection of the separation found mushroomed fiber ends
and partially fused strands indicative of adiabatic heating resulting
from high strain rate overstress separations of synthetic fiber ropes.
The longest strands were twisted as if they were part of a knot at the
time of separation. A dark band with a metallic sheen was visible on
the braided cover adjacent to the separation, see attached image
1711Image57407. A hand held x‐ray alloy analyzer revealed
significant amounts of aluminum in the darkest part of the band.

The docket did not include any line crew statements in regard to what
weak link may have been used for the tow.

A reasonable question for those World-Wide operations that use Tost
reels, would be. Is it uncommon for tow ropes to break, rather than
a prescribed weak link. If this was a rare anomaly, is there
some logical reason for it happening?

M Eiler

It is very common for a Tost reel rope to break before a white weak link (500daN) breaks. The slug on the glider end is anchored by a single overhand knot in the rope inside the slug, and with the strain from that knot, the rope is slightly weaker than the weak link. In the eight years that I towed with the system, we never broke a weak link, usually broke the rope right in front of the weak link.