But a C-5 can carry about twice as much as one C-17 and not have to refuel on transatlantic trips. With the RERP program to reengine and extend the life of exiting C-5's the MTOW will be increased giving them even more range and payload capabilities. Even though the C-5A and C-5B fleets now have almost the same flight times, the C-5A's have inferior alloys and more corrosion than the C-5B's. There is a question of whether to spend the money refurbing all or part of the C-5B fleet. The one C-5A that has been converted to a C-5M had lots of additional expense compared to the conversion of two C-5B's. Supposedly the C-5A fleet has about 70% of its structural life left. The current cost of the RERP program is almost $85 million per plane, but that's still only about half the cost of building one new C-17. For the cost of building one C-17, the USAF could refurbish two C-5's and get the equivalent capacity of more than four C-17's.
Correction. The air force is committed to putting all 49 surviving C-5B's and 2 C-5C's through the RERP program. The question is whether to put the the 60 C-5A's that are still being operated in the RERP program. A further complication is that all surviving C-5A's were rewinged in the 1980s. A few years ago the first eleven C-5A's and three other C-5A's were sent to the boneyard to supply spares for the remaining fleet.