When flying the globe in the Lockheed C-5A Galaxy(vintage 1969 models), I found the LIMFAC was almost always Cabin Altitude. We would flight plan our oceanic crossings at FL310, as that would usually represent Cabin Alt of 9,500’. The AF Flt Planners, i.e. “dispatchers”, would pushback when we called for FL310 fuel burn, as they wanted us at FL350 or FL390. “No can do, the plane leaks like a sieve.”
During eight years of high-tempo flying, I reached FL410 exactly twice. The first was at the end of a nine-hour flight from Korea to Hawai’i, with only a single Apache (AH-64) in the cargo box. I was so thrilled that I took photos of the steam gages to verify the achievement. Of course, descent out of FL410 was a bear, as the cabin wanted to rush above 9,500 as soon as the throttles were retarded...
I understand the C-5M Super Galaxy does not have this problem.