[aus-soaring] Weak Links

[Roger Druce]

A note of caution re use of rope as a weak link material.  If you have rope as a weak link material then initially you may in fact achieve the weak link strength being sought based on the rope meeting its rated strength.  One may test batches of rope to verify strength.   In service the rope will deteriorate.  Members handling ropes (tuggies & others) are usually used to checking for the obvious source of deterioration namely due to surface abrasion of the rope.  A rope weak link is usually put at the tug end and should be kept short enough to keep it always off the ground and therefore avoid abrasion damage.   However there is one source of rope deterioration which is serious and is not generally recognised or acknowledged.  And that is degradation due to ultra violet exposure.  This can be devastating.  We broke two 'new' 6 mm rope straws many years ago on successive launch attempts, as the tug took out the tow rope slack and went to full power, at Mount Beauty Airstrip (1100 metres, with trees in the surrounds).  The failures came because these rope weak links had been brought from home where they had been hanging for months as spares in our field bus in the sunlight.   Subsequently we have gone up in weak link rope size, such that, yes, they will be overstength when new.  But then at least there will be a some margin against strength reduction due to abrasion (inspectable and usually minimal) and UV degradation (not checkable by inspection).  And, yes, I do not oblige any longer by leaving rope weak link material (made up links or raw uncut lengths) around in the field bus or otherwise "on the field".   On the separate matter of emergency parachutes, (Peter Stephenson 20 April email), I suggest that the repack period was increased by the Parachute Federation of Australia from 4 monthly to 6 monthly some years ago.   Roger Druce