TOST Weak Links
TOST weak links are required to protect the sailplane against overloading during launch operations. The strength of the spar of an aircraft is calculated on the basis of a given maximum load. The type and rating of the weak link used during the launch must meet the requirements of the sailplane as described in the Type Certificate Data Sheet (TCDS).
Weak links are fitted to aerotow ropes for the protection of both the sailplane and the tug. Apart from the Piper Pawnee 235 (which has a maximum weak link strength of 750kgs), the maximum weak link strength for other tow planes is limited to 450gs. Weak link requirement will be found in the Aircraft Flight Manual (AFM) towing supplement. For sailplanes, this information is found in the TCDS, the sailplane AFM, and the sailplane placards in the cockpit.
If the specified weak link strength for a heavy glider is greater than the specified strength for the tug, the operator is stuck with the weaker of the two values. Remember, sailplane weak link strengths are the maximum allowable to prevent structural overload of the sailplane; a lower strength weak link strength merely provides a higher safety margin. If the specified strength for a light glider is less than that specified for the tug, a separate weak link of the correct glider strength must be inserted at the glider end in addition to the one already in place at the tug end.
If only one weak link is fitted to an aerotow rope, it must be fitted at the tug end. This retains the protection in the event of the tug inadvertently snagging the rope on an obstacle on the final approach. Exception: a tug fitted with a TOST tow rope retraction system will have the weak link fitted at the sailplane end.
It is essential the tow pilot inspects the weak link and tow rope for serviceability before the day’s operation and also after being subjected to a high jolt or load. It is also required that when that weak link fails, the reserve link must also be replaced.
The TOST weak link system is an engineered and approved design which prevents aircraft overloading during winch, autotow and aerotow launching. By using this system, the operator is assured of maintaining the manufacturer’s airworthiness requirements assuring protection to both tow plane and sailplane.
TOST weak links are color coded and are available for loads from 80 to 1000daN with a tolerance of 10%. For conversion purposes, one decanewton is the equivalent of 1 kilogram force.
The GFA recommends clubs and operators use the TOST reserve insert and sleeved weak link system. This uses two weak links in parallel protected by a steel sleeve. Both weak links have attachment holes at each end and are 8 mm in length. The reserve has oval attachment holes and carries no load in normal operations. If the load exceeds the rating, the weak link will fail and the reserve link will take up the load. If the load is more than a momentary jolt both weak links will fail. Source: The Gliding Federation of Australia Inc.
TOST weak links in optimized format combine the following improvements:
• Longer service life.
• Correct marking with load group and manufacturer’s name.
• Made of high-quality certified aircraft steel.
• Clear differentiation from inferior copies.
• Manufacturing tolerance 5% Like our type-approved equipment, also our weak links are manufactured according to the EASA-approved production methods. Each batch is tested on computerized test equipment and the results are documented. This guarantees consistent high quality and traceability.
• Weak links protect your aircraft against overloading.
• Use only the weak links stipulated in your aircraft TCDS or aircraft manual.
• Checking the cable preamble is mandatory according to SBO (German Gliding Operation Regulations); this includes the inspection of weak links.
• Replace the weak link immediately in the case of visible damage.
• We recommend that the weak link insert are replaced after 200 starts: an insert exchanged in time is always safer and cheaper than an aborted launch.
• Always use the protective steel sleeve.
• Use only the correct shackles: they prevent the weak link and the steel sleeve from twisting, leading to an increase of the breaking load.
• Never use two equal inserts, eg, both with round holes, in a reserve system weak link, since this would double the breaking load.
Custom Weak Link
Using combinations of weak link inserts from different manufacturers can double the breaking load because of the elongation of inferior inserts. Refer to our Weak Link Application List for choosing the right weak link.
We manufacture to your requirements custom made weak links with defined breaking load in the load range of 10 daN to 50 kN. Testing on computerized test equipment. Material: plastic, stainless steel, steel. Application examples: bracing of antennas, retaining of flagpoles, Off-Shore-operation, testing of parachutes, test racks, food industry.