The thermal link has been designed to activate at
  the standard temperature of 72°C in the event of
  an elevated temperature within the duct.

  The link is fitted within a holder, which is screwed
  into the case to lock the blade into the desired
  position. When set, the locking nut is
  'hand-tightened' to secure the cartridge and the
  blade.

  In the event of the thermal link being activated,
  the integral indicator will extend from the thermal
  link cartridge to indicate to the maintenance
  engineer that the thermal link has been activated,
  with inspection of the system required if deemed
  safe to do so.