The magnitude of the required residual strength load depends upon the exposure time in service because the longer the exposure time, the greater the probability of encountering a high load.  Accordingly, the value of required P_xx load increases with an increase in the inspection interval or period of unrepaired service usage (allowable crack growth period).  For the short service exposure times between inspections for the In-Flight Evident, Ground Evident and Walk Around Visual categories, the probability of encountering limit load conditions is low and thus the required P_xx may be significantly below design limit load.  For the longer exposure times between depot or base level inspections, the probability of encountering limit load is much higher, and therefore for Depot Level and Non-Inspectable categories, the minimum required P_xx must be at least limit load, but P_xx need not be greater than 1.2 times the maximum load in one lifetime.

The value of P_xx is established from load spectra data derived from a mission analysis of the particular aircraft considering average usage within each mission segment.  Unless otherwise stated, MIL-A-8866 is the basic source of load factor data for the various classes of aircraft.  Since safe operation depends upon the residual strength capability and since any individual aircraft may encounter loads in excess of the average expected during the particular exposure time, the P_xx load required is larger than the average derived value. 

One way to determine the level of P_xx required is to hypothetically increase the service exposure time for the aircraft between inspections by a factor of M.  This is the method used in JSSG-2006.  The values of M are specified in JSSG-2006 Table X, and summarized in Table 1.3.4.  For example, under the ground-evident level inspectability category, the P_xx load is the maximum load expected to occur once in 100 flights (M x inspection interval = one flight x 100).

Table 1.3.4.  Inspection Interval Magnification Factors from JSSG-2006 Table X

* P_xx = Minimum average interval member load that will occur once in M times the inspection interval.  Where P_DM or P_LT is determined to be less than the design limit load, the design limit load shall be the required residual strength load level. P_xx need not be greater than 1.2 times the maximum load in one lifetime if greater than design limit load.

The basis for the specified M values is somewhat arbitrary although it is felt that the loads derived by this method are not unreasonably conservative.  The basis for M = 100 is exceedance data for transport type aircraft, where it has been observed that shifting exceedances by approximately two decades (i.e., M = 100) magnifies the value of load factor (or stress) by approximately 1.5 (Figure 1.3.10).  It was recognized that for fighter data, exceedances approaching or exceeding design limit values are probable but that extrapolation of the basic exceedances curve very far beyond limit load factor (n_z) is often meaningless and unwarranted due to physical limitations of the vehicle and crew.  Furthermore, in most cases actual service data is somewhat sparse for this region of the curve.  Therefore, (1) an upper limit was required on P_xx for fighter aircraft and (2) the value of M should be less for longer inspection intervals in order that unreasonable factors would not be imposed should the actual derived P_xx be less than the specified upper limit.  The values of M equal to 20 and 50 are arbitrary but probably not unreasonable.  Where the derived P_xx is larger that that associated with the design limit conditions, P_xx can be taken as 1.2 times the maximum load expected to occur in one design lifetime.

Figure 1.3.10.  Illustration of Procedure to Derive M Factor to Apply to Exceedance Curve