The 747 is an exceptional example of engineering for durability in a large airframe. Other U.S. made large-airframe aircraft have not fared so well, in particular the Lockheed C-5 Galaxy whose wing-spar weakness was so profound that the entire fleet was re-winged shortly after going into service but still enjoys a dismal distinction as having the highest operating cost of any US Air Force weapon system.
And then there's the F22 whose MTBF is, on average, less than 60 minutes.
Higher than the B-2? (3million+ per aircraft/month according to some). Maybe the C-5 has highest not-classified operating costs.
For perspective, the Canadian Sea King helos are rumored to take 50+ man-hours for every hour in the air, comparable to the C-5 but on a much much smaller airframe.
"When it was introduced, back in 1970, the C-5 Galaxy was the largest plane in the world. It also has the highest operating cost of any US Air Force weapon system, owing to extremely high maintenance demands as well as poor fuel economy. Worse, availability rates routinely hover near 50%."
As for the Sea King, I have often read of maintenance ratios of 30 man hours maintenance per hour of flight but 50 is not surprising, either. By comparison, the fantastic (Hughes) MD-500E has a ~.42 man-hours/flight-hours ratio for the airframe, excluding the engine, and that's a very well sorted ship that's far simpler than the Sikorsky.
There are lots of things to fail. On most military aircraft it is rare to fly with every tiny system running at 100%. Five or things can break during a flight without any real risk.
Not the engines, but something like a datalink antenna or camera system. If you include little things like tray-tables and seats getting stuck up/down, even the 747 probably has a failure rate measured in hours.
" The Air Force also budgeted $132 million in fiscal years 2007 and 2008 for reliability and maintainability upgrades, $28 million more than the amount budgeted for fiscal years 2006 and 2007. Despite these efforts, the F-22A continues to operate below its expected reliability rates. A key reliability requirement for the F-22A is a 3-hour mean time between maintenance intervals, which is required by the time the program achieves 100,000 operational flying hours, now projected for fiscal year 2010. Mean time between maintenance is defined as the number of operating hours divided by the number of maintenance actions. Currently, the mean time between maintenance is less than 1 hour, or about half of what was expected by the end of system development in December 2005. There has been no significant change reported regarding the current mean time between maintenance since last year’s review"
Wondering about that value and for the sake of being more accurate, always a good thing, I found a "F-22 Assertions and Facts" document used in the US Senate in 2009 to defend the F-22 program. It has substantially different numbers for the F-22.
"The F-22 is required to achieve 12.0 direct maintenance man-hours per flight hour (DMMH/FH) at system maturity, which is defined to be when the F-22 fleet has accumulated 100,000 flight hours. In 2008 the F-22 achieved 18.1 DMMH/FH which then improved to 10.5 DMMH/FH in 2009. It’s important to recognize this metric is to be met at system maturity, which is projected to occur in late 2010. So the F-22 is better than the requirement well before maturity."
About F22 reliability:
"Reliability is measured by Mean Time Between Maintenance(MTBM. One of the F-22 Key Performance Parameters (KPPs) is to have an MTBM of 3.0 hours at system maturity, which is defined to be when the F-22 fleet has accumulated 100,000 flight hours. Through 2008, F-22s averaged 2.0 hours MTBM while the fleet has accumulated 50,000 flight hours. The F-22 is on-track to meet or exceed 3.0 hours of MTBM at system maturity, projected to occur in late 2010, and the latest delivered F-22s, known as Lot 6 jets, are exhibiting an MTBM of 3.2 hours."
And then there's the F22 whose MTBF is, on average, less than 60 minutes.