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And why do military fighters/bomber require so many maintenance hours compared to commercial airlines, which seems like just kicking the tires? What do they actually do?
How does maintenance differ for fighters vs. bombers?
As many of the comments and answers have illuminated, "different missions drive different design priorities." Examples of differences in specific maintenance activities would be greatly appreciated.
I think a few things need to be examined here:
1) The type of flying. A commercial aircraft is closer to a military C-17 than a B-52 or F-16 in how it is used. It has several stops throughout the day for loading and unloading. The maintenance in between stops does not need to be very extensive - because it has just landed, presumably working fine, the things that need to be checked primarily involve what has the most chance of decaying during a flight (tires, oil, general wear and tear, etc). Fighters/bombers don't generally fly several legs in a day. Fighters might double/triple turn, and in between each of these flights the maintenance inspection is a much lower requirement than between each day of flying.
2) Component Requirements. Supersonic jet engines need maintenance more often than high bypass commercial engines. They are stressed much more, and work on a much tighter margin of error, thus fighter/bomber aircraft need maintenance more often.
3) Age. Military fighter/bomber aircraft are on average much older than the average commercial aircraft. B-52's are from the 50's, and B-1's are from the 80's (along with most of the fighter fleet). Older aircraft require more maintenance, and there is no avoiding it.
Between fighters and bombers, they each have different flying styles. Fighter engines work a lot harder than bomber engines (except the B-1, which essentially has the same engine as an F-16), and thus require maintenance more often. Bombers are much larger, and have more systems that can break.
Hope this helps.
Update: Incorporating a suggestion from the comments below, it is probably useful to specifically mention a few more specific maintenance items unique to combat aircraft
1) Ejection Seats: Most military combat aircraft carry ejection seats, which have a very specific maintenance routine. This may include repacking the associated parachute, and checking to ensure that all components are functional/in good repair.
2) Weapons: The obvious difference between fighters/bombers and commercial airliners is that they carry weapons. This means that there is a software package that needs to be maintained (that controls the weapons), the aircraft hardware needs to be maintained (ie bomb bays, missile racks, hard points, etc), and the weapon itself requires building/deconstruction.
3) Flight Computer: Combat aircraft often have tactically specific functions in their flight computer that a commercial airliner would not. There are routines to help calculate bomb trajectory, to link with other combat aircraft and many other military specific tasks that a commercial airliner would not need.
I would agree with an earlier statement stating that performance has a slightly higher emphasis than reliability in military aircraft than in commercial aircraft. It's easy to see how the requirement to carry bombs/missiles increases aircraft gross weight, which lowers fuel efficiency. Every internal bomb bay could potentially be a fuel tank extending range. Hard points, even when bombs aren't installed, cause drag lowering performance. In the end, designers have to determine which capability is more important - endurance, payload, power, etc. You may want to install a triply redundant INS in a fighter aircraft, but you're adding weight that decreases performance, and taking space which could be devoted to other avionics (like radar).
Military aircraft are designed to maximize performance, while commercial airliners are designed to maximize reliability.
That's not to say that reliability is unimportant for the military aircraft or that performance is unimportant for the airliners, but when almost everything is done and the last tradeoff is considered, the military jet can accept a little more maintenance in return for a little better performance, while the airliner will probably sell better with better reliability & the ability to go longer between maintenance down-time, even at the cost of the last 0.2% better performance/weight/economy/etc.
The assumption for military aircraft, fighters & bombers in particular, is that they'll finish the day back at their base, and they CAN be worked on for several hours before they're launched again. (Generality here -- yes higher tempo ops are possible, but it's the exception not the rule.) Airliners, on the other hand, typically will fly for days between maintenance checks (beyond adding oil, checking tire pressure, and maybe swapping a particular broke part) -- and that's what's desirable because that's how the airplane makes the most money. So that ability to go for so long between maintenance checks is built in because that's what the customer (the airlines) demands.
Could you get more performance out of an airliner if you were willing to accept more maintenance costs? Sure, but it wouldn't be worth it to the airlines. At the extreme, the Concorde, for all its cool factor, was an economic disaster, even though its performance was superlative.
The military aircraft have redundant systems in order to survive -- limp back to base. Commercial aircraft have redundant systems in order to be able to be dispatched again -- stay in service until repairs can be performed. (Obviously, not everything can be deferred; some things ground the airplane & have to be fixed before it flies again... but a LOT of stuff in the airliner can be deferred & you still have redundant capability to land safety even if another system goes down.) As an example, the F-16 was (may still be) single-INS + single-FMS. The system works or you don't launch. Modern airliners have two or three INS's and FMC's -- dispatch with one INS inop is possible (probably day VMC only), and dispatch with one FMC inop has minimal limitations (no long overwater legs, essentially).
Different missions drive different design priorities, and that in turn shows up as differing levels of maintenance that's required.
