MX304 wrote:
Here's an example of the corrosion problems they are fighting in FiFi. This is a fuselage stringer, but I am not sure exactly where on the aircraft it was found.

Hi,
Thanks for posting this on the progress on FiFi. I must be honest about this photo. Having worked on a great many Boeing aircraft in my career from a "structures" perspective. Seeing this photo, does not surprise me. Boeing builds a helluva strong aircraft. From 707's right up the line to the 747. To do this they make use of 7075 and 7079 aluminum in the predominent amount of the airframe. But, in doing so they trade strength for longevity. By that I mean that 70 series aluminum is exceptionally strong, but also very prone to corrosion as you see in the photo. Called either exfoliation or intergranular corrosion it is one of the "worst" kinds of corrosion to be found on an airframe from a structural repair perspective. Because you never trully know, unless you fully change out the part affected, if you removed all of the corrosion. And the repairs to this type of corrosion are very extensive. Wing spars are especially suceptible to this in Boeing aircraft due to moisture trapping between surfaces or just plain age.
Douglas on the other hand makes use of 2024, while not as strong per se', it is has excellent corrosion resistive properties over the long term. For proof...no one needs to look further than the venerable DC-3. 50 plus years and going strong.
Please, before this begins an aurgument, I am wishing the folks who are putting FiFi through a heavy maintenance check only the best. It is taking even Boeing and their volunteer group in Witchita how long to put Doc back together ? I just hope that FiFi gets the level of care she deserves during her long stay on the ground. And I trully hope that they don't find this type of corrosion in too many more places on FiFi. Believe me, it is a real bear to fix and fix it right.
Respectfully,
Paul