For this example case, no special in-service and no depot level inspections will be required to protect the integrity of the lower wing structure shown in Figure 1.3.11.  The implication is that no inspections are desired; however, there are cases in which the flaw size at failure is so small that such a flaw might easily be overlooked during an inspection.  Thus, the in-service non-inspectable category covers those cases where inspections are neither desirable nor practical.

Residual Strength Load, P_xx

From Table 1.3.4, the required level of residual strength P_xx for non-inspectable structure is P_LT.  This is the maximum load that could occur in one lifetime.  Example 1.3.3 describes the method for establishing this load level.

Analysis Guidelines

The slow crack growth and residual strength guidelines for this category are illustrated in Figure 1.3.13.  This figure specifically shows that the initial manufacturing damage is restricted from growing to critical size and causing failure of the structure due to the application of P_LT in two (2) design service lifetimes.  Note that the damage limit is the ultimate failure of the wing.  Engineering judgement may dictate that a more reasonable limit and, perhaps, an easier situation to adhere to, would be to establish the limit at some intermediate point, such as the failure of the primary load path panel ­.  This might be accomplished in design at very little expense to overall weight.

Figure 1.3.13.  Illustration of Damage-Growth and Residual-Strength Guidelines for Example Problem Qualified as Slow Crack Growth Non-Inspectable