Through the 1990s, the operating costs of the U.S. Navy's aging fleet of P-3 antisubmarine and maritime recon aircraft have gone up. All the increase (from $1,500 an hour to $2,400 an hour) were a result of additional depot level maintenance. This went from $610 an hour to $1,250 an hour. The problem is age, the P-3 airframes are a 50 year old design and many of the aircraft are nearly as old. The depot level maintenance is when key items on aging aircraft are replaced. As aircraft get older, more stuff have to be replaced. In theory, an aircraft can be flown forever if you keep replacing worn out parts. But this gets extremely expensive when aircraft get over thirty years old. The navy is considering replacing the four prop engine P-3 with a two jet aircraft (using a militarized version of the Boeing 737.) There have been several unsuccessful attempts to come up with a P-3 replacement over the last two decades. Something will have to be done soon, for in the next 5-10 years the P-3 airframes will become so old that they will be too expensive to maintain (it will become cheaper to buy new aircraft than rebuild the old ones.)