|Quoting Ozair (Reply 41):|
Except money, time, risk and incentive. A 50 year old airframe tarted up is still a 50 year old airframe and the experience integrating the AGM-142 onto the F-111 clearly shows how difficult this process actually is.
Correct. Repeating myself on DMSMS, but here's some tidbits in regards to why one shouldn't keep old aircraft kicking around:
US Bomber Contract Awarded Oct 27th (by metalinyoni Oct 26 2015 in Military Aviation & Space Flight)
|Quoting ThePointblank (Reply 59):|
The aircraft itself may be perfectly fine, but the vendors that supply components that go into the aircraft don't make the components anymore, or no longer exist. This is a big issue in defence procurement as the usable lifetime of an system may far exceed the availability of the components used to produce that system. Depots and boneyards can only do so much before the maintenance and overhaul costs will exceed the cost to just buy a brand new system that doesn't have DMSMS issues from the start.
There are a number of obvious signs that you have DMSMS issues:
1. A vendor notifies you that a part they are producing is about to go out of production;
2. A particular system uses a certain unique part that can only be produced by a single manufacturer;
3. Parts for a system are dwindling, but no clear replacement for that system over time;
4. A system doesn't have easy up-gradable sub components that one can quickly upgrade and replace as parts become obsolete;
5. A system goes into production with sub components already totally obsolete and near EOL
Military hardware is especially susceptible to DMSMS because of the low volume of production, and the fact that the systems are intended for use for a long period of time. This means that over time, vendors will move on to newer technologies and products, and they will discontinue older components because of a lack of profitability.
Also, the F-111 has some unique and specific depot requirements... an explanation:
Back when the F-111 was being introduced, the aircraft was having a devil of an issue with the wing pivot system. Specifically, the wing pivot system used forged D6AC ultra-high strength steel. The steel was extremely strong, but small cracks could grow very fast and result in failures. It could be described as "brittle", since it behaved something like glass - very strong, but intolerant of cracks.
This lead to a loss of a F-111A in December 1969, where a wing of a F-111A failed catastrophically. The F-111 was immediately grounded after that incident, and a USAF
committee investigating the accident recommended the use of fracture mechanics, already in use with the USAF
's missile systems to study the problem of the F-111's wing pivot system. Coupled with how inaccessible some areas of the F-111's wing pivot system was, a new structural integrity test, called the Cold Proof Test which was meant to address the issues with the F-111's high strength steel construction.
The Cold Proof Test extensively used fracture mechanics theory. Fracture mechanics has the capability to predict the sizes of flaws necessary to cause component failure when exposed to a given level of stress. As stress increases, the flaw size that would cause failure (called the critical size) will decrease.
The proof test philosophy, then, is to apply a high as stress as possible, without creating other problems such as yielding or secondary structural damage, which, if the structure survives, will allow quite a reliable determination of the largest sizes of flaws that could exist. When these "proof test" flaw sizes are less than the critical sizes required to cause failure during normal operational loads, then the fracture mechanics analysis procedures can predict a period of safe operations before another inspection, or proof test, will be required. Also, for steel material, the critical flaw size will decrease as the temperature decreases. The material essentially becomes more brittle: or more technically, the cold temperature reduces its fracture toughness.
As such, with the F-111, the aircraft was placed in a large structural steel test fixture, to hold the aircraft firmly and react the test loads which are applied to the aircraft through hydraulic rams. This structure was placed in a highly insulated test facility (more on why later). The hydraulic load system is computer controlled and incorporates sophisticated control and safety systems to prevent overload of the airframe during failures or system malfunctions. The system has the ability to "dump" up to 130,000 psi of force almost instantaneously as a failure is detected by a load cell feed back loop.
To achieve the cold part of the test, they used about 6000 gallons of liquid nitrogen per test that is vaporized to form gaseous nitrogen after being poured into the test chamber plenium ducts. Fans are used to circulate the cold nitrogen around the airframe, cooling the surrounding air to negative 65°F. Using thermocouples attached deep within the critical components of the airframe, the chamber and structure temperatures are eventually stabilised at between minus 43°F and minus 47°F.
Once the temperature has been stabilised, loads are applied to the structure at four different pivot settings using the computer controlled hydraulic load system, while being monitored by computers and test personnel. Loading takes about 2 to 3 hours, and the total proof test procedure can be carried out in a single day.
After the test is complete, the acoustic emission data is analysed to determine if any significant sounds were generated during the test. If any sounds are considered to be indicative of system or structural failures, the position of the sound source are identified by the computer, with an inspection carried out in that area by maintenance personnel. The test fixtures and other test equipment is then disassembled and removed, while the aircraft is returned to the maintenance facility for continued preparations and procedures required prior to return to the operating squadron.
In all, ever since the cold proof test was implemented with the F-111 fleet, there hasn't been a single crash related to in-flight structural failures of the wing pivot system, and only 11 failure events between the USAF
were ever identified throughout the entire service life of the F-111 fleet.