Any point on an aircraft at which single mal-assembly could lead to catastrophe, i.e. result in loss of aircraft and/or in fatalities.
Certain parts in an aircraft’s structure or system (including controls and control systems) which are vital to the safety of the aircraft, are not only designed to achieve the appropriate high integrity but are also dependent upon specified maintenance actions to safeguard their integrity throughout the life of the aircraft. For such parts normal inspection procedures and techniques may not provide verification with a sufficiently high degree of confidence, and it will be necessary for two independent (duplicate) inspections to be carried out after initial assembly, or re-assembly
following disconnection or adjustment