If you're operating one of the 52 Boeing 707s still on the US register that the FAA has found then you need to do some work. This is to stop the thing blowing up in mid-air (or on the ground for that matter), which after 30 or 40 years would be a sad way for any of these to end their lives. The FAA reckons there are about 185 of the beasts around the world by the way.
It's all about the issue of unwanted ignition sources in the fuel tank. There are some small mods that you'll need to make, and Boeing and the FAA also want you to photograph assorted parts of the tanks and airframe and send your snaps to them so they can keep on top of the situation.
I'm still not sure where all those aircraft actually are, and I'm not at all sure that a lot of them will be going anywhere anytime soon. Leave a comment if you know more.
And I'm glad that there are people at the FAA and Boeing who are still worrying about this stuff.