Section 220.127.116.11. Fastener Policy
In practice, the growth of flaws from fastener holes can be
retarded by the use of interference fit fasteners, special hole preparation
such as cold work, and to some degree, by joint assembly procedures like
friction due to joint clamp-up. Because
these procedures delay flaw growth, the slow crack growth lives (or intervals)
can be significantly longer than those obtained from structure containing
conventional low torque clearance fasteners
Experience has shown that to achieve the beneficial effects of
these techniques consistently, exceptionally high quality process control is
required during manufacture. However,
this is not always obtained. As a result,
it is thought unwise to consider all interference or hole preparation systems
effective in retarding crack growth.
As stated in JSSG-2006 paragraph A3.12.1.g, to maximize safety
of flight and to minimize the impact of manufacturing errors, the damage
tolerance guidelines should be met without considering the beneficial effects
of specific joint design and assembly procedures such as interference fit
fasteners, cold expanded holes, and joint clamp-up.
Exceptions to this policy can be considered. The limits of the beneficial effects used in
design should be no more than derived from assuming a 0.005 inch corner flaw as
initial damage in an as-manufactured, non-expanded hole containing a neat fit
fastener in a non-clamp-up joint.