|Quoting Abqwildcat (Reply 7):|
The best explanation I've ever gotten on these finger doublers is that if one of the "fingers" cracks in service, the total strength of the doubler isn't reduced by much and the crack won't propagate across the entire doubler "unzipping" from hole to hole. I've never seen or designed a doubler with fingers for planes in production, but it's certainly possible there are cases when they'd be appropriate. I also can't remember seeing any finger doublers in the SRM for 747, 767, or 777.
This is partly true. The Douglas concept of finger doublers do two things. They are thin fatigue doublers that allow for a better transition of load across a joint by introducing a lower initial load than the primary doubler(s) and spreads the load across the primary doubler(s) more evenly. They also act as a crack indicator and thus a higher life without supplemental inspections or access from the back side to inspect. For example if a crack starts, it would be first detected in the finger since it is not covered externally. Also, if I am not mistaken, they are not considered part of the overall static strength of a repair, but an extra feature.
Finger doublers are actually quite a novel idea and one of the the best joint designs for long fatigue life with few, if any, supplemental inspections. Unfortunately, they are difficult to install because of this. An airline has to stock this pre-made material or make it which is time-consuming. The DC-10 SRM does allow for not using one in some circumstances, but it does significantly reduce its fatigue life.
One of my theories as to why McDonnell-Douglas ultimately failed was this design. While Douglas airframes are sturdy and utilized some great structural engineering, they were complicated structurally, more expensive to build, and perhaps built a bit too well. What I mean by that is they last too long and thus did not have enough obsolescence built-in like a Boeing aircraft which used and still uses simpler lap splices while not as optimal structurally, do very well for their intention and are easy to assemble.
Actually Lockheed used a fatigue doubler in many of their L-1011 fuselage repairs which actually did some of what Douglas finger doublers did. They aided the structural life of the repair but where not considered integral to its static strength.
35 years of American Trans Air/ATA Airlines, 1973-2008. A great little airline that will not be soon forgotten.