S282
I totally agree with 737eng. My company specializes in this particular work and that is one of the biggest problems we have with customers. They do not seem to understand why they cant always get a great inspection interval (usually a C or D check) and get it quickly and cheaply.
Just to give you a couple examples. If the repair is a simple patch within one frame or stringer bay and is in an area where fuselage pressure loads only apply, then the analysis can be developed fairly quickly assuming there are no ADs located in the area. If instead lets say its a patch on the corner of an entry or cargo door, much more work is involved in developing loads and stresses. Similarly, if the repair or modification is located over the top of the fuselage where fuselage bending loads are predominant, then external VMTs must be developed in order to create the full fatigue spectrum.
Another example where repairs are more difficult is anything on a wing. Wings have fairly sizeable fatigue stresses which are due to wing shears, moments and torsions. In order to perform the analysis, one must develop a representative set of wing fatigue loads. These must then be turned into internal loads either thru a Unit Beam method or FEM before even doing a DTA.
Most literature on DTA focuses on the stress calculations, stress intensities, material data and crack growth equations. However, one of the MOST deciding factors in developing a representative inspection interval is developing ACCURATE fatigue spectra. Without the expertise in developing fatigue loads and spectra, one will always end up with having to make overly conservative assumptions. We spent the first two years at our company developing the expertise and tools to be able to develop fatigue loads from scratch for most commercial aircraft and went thru the FAA approval of them. This investment in up front methods development can be a bit costly in money and time but it has allowed us to be much more capable of developing reasonable inspection intervals for our customers. On a couple programs, we have even developed complete airframe external and internal loads to support a full DTA of the airplane without the customer going to the original manufacturer. Regardless, the key is that whatever company you select to support your repair efforts, make sure you chose one that has the expertise to do the work.
As a final note, here is one method which has been tossed around by the regulatory agencies in presentations as acceptable if loads are not available for fuselage structure where pressure and bending are concerned because of its conservatism and in my opinion it is one of the reasons why so many inspections are being developed too conservatively. The method basically is to take the material Ftu divide by 1.5, subtract PR/2t and divide the remainder by a limit maneuver load factor of 2.5. This is the 1g stress which then has to be used with an Nz spectrum. I would challenge anyone to come up with a decent inspection interval with this method especially when you realize limit load is set at Fty! Also, it assumes limit load is due to maneuver and not gust, which is not always true for all structure.
Anyways, did not mean to go so long but this is a major issue with our industry in understanding how the inspection interval, schedule and price is directly related to the amount of technical work involved in DTA and the capability of developing accurate fatigue loads.
Good luck
