"In 2005, the FAA announced its intention to mandate inerting systems, which pump nitrogen into the fuel tanks to reduce their flammability, on board every new airliner and on any existing airliner built since 1991. The final rule was issued in 2008, and every applicable airliner in the United States — which by now is virtually all of them — had a nitrogen inerting system installed by 2018."And, not coincidentally, United and Delta retired their passenger 747s just before the deadline. It was never mentioned in any of the press stories about the last flights, those tended to be more nostalgic.
https://www.cnn.com/travel/article/delta-boeing-747-retireme...
https://www.prnewswire.com/news-releases/farewell-your-majes...
The fact that the two airlines chose to continue to fly unmodified fuel tanks for a decade after the ruling is somewhat frightening.
