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Answered in October 2016Dave Robinson’s answer is complete and excellent, but, in the late 80s and early 90s, catastrophic failures began to affect several old jets, putting several lifecycle assumptions into serious doubt. I lived through that phase of the Boeing 747 fleet, and believe me, it was terrifying.So let us begin at the beginning.The following is from Air & Space Magazine:Why can some planes seemingly keep flying forever, while other, newer ones are already used up?"An aircraft's lifespan is measured not in years but in pressurization cycles. Each time an aircraft is pressurized during flight, its fuselage and wings are stressed. Both are made of large, plate-like parts connected with fasteners and rivets, and over time, cracks develop around the fastener holes due to metal fatigue."Aircraft lifespan is established by the manufacturer," explains the Federal Aviation Administration's John Petrakis, "and is usually based on takeoff and landing cycles. The fuselage is most susceptible to fatigue, but the wings are too, especially on short hauls where an aircraft goes through pressurization cycles every day." Aircraft used on longer flights experience fewer pressurization cycles, and can last more than 20 years. "There are 747s out there that are 25 or 30 years old," says Petrakis.How do airlines determine if metal fatigue has developed in their passenger-liners? Bob Eastin, an FAA specialist on aircraft fatigue, says, "[Airlines] are really relying on the manufacturer's maintenance programs. The manufacturers design the aircraft to be trouble-free for a certain period of time. There are maintenance actions to preclude any catastrophic failures, but that's not to say that the aircraft might not [experience metal fatigue] before those times…. When you get to a certain point [in the aircraft's lifespan], you need to inspect or replace certain parts."Yeah, sure. Sounds good and all wrapped up, isn’t it?Then Aloha happened.How Old is Too Old? The Impact of Ageing Aircraft On Aviation Safety.Widespread Fatigue Damage (WFD) in a structure is characterised by fatigue damage originating cracks at multiple locations of sufficient size and density, to the extent that the structure no longer maintains its required residual strength. Traditional application of damage tolerance analysis from a single or dual crack origin is not sufficient to preclude WFD. Fatigue cracks related to WFD can grow quickly and interact in such a way that an operator cannot inspect the susceptible structures effectively or often enough to ensure detection of the cracks before they lead to a structural failure. A separate WFD assessment and determination of specific maintenance actions is necessary to adequately address WFD. The Aloha Airlines accident highlighted the issues associated with WFD, and in particular Multiple Site Damage (MSD) which is the presence of multiple fatigue cracks in the same structural element. Multiple Element Damage (MED), the other form of WFD, is the presence of fatigue cracks in adjacent structural elements. To preclude WFD, operation should not be allowed beyond a certain point in the life of the airframe, known as the limit of validity of the structural maintenance program me.Limit of Validity (LoV): LoV is not more than the period of time, stated as a number of total accumulated flight cycles or flight hours or both, for which it has been demonstrated that WFD is unlikely to occur in the aircraft structure; and that the inspections and other maintenance actions and procedures resulting from this demonstration and other elements of the fatigue and damage tolerance evaluation are sufficient to prevent catastrophic failure of the aircraft structure. The LoV terminology is usually used in the context of 'Limit of validity of engineering data that supports the structural maintenance programme'. The term 'structural maintenance programme' refers to the structure's part/section of the maintenance programme.The FAA-Drexel Fellowship Research Program on Aging Aircraft▲ It’s a terrible business, maintaining old aircraft in airworthy condition.Soon afterward, alarming structural cracks and corrosion-related failures began to be detected in older airplanes, particularly the large numbers of 747s then in operation.The Queen of the Skies was rapidly turning into the Maintenance Nightmare of the Airlines.Keeping older jet aircraft in an airworthy condition was been found to present special difficulties which had not all been addressed by prescribed maintenance. The serious continuing airworthiness issues which had arisen in many ageing aircraft have often been a direct consequence of the gap between current and former practices required for Aircraft Type Certificate issue and Maintenance Programme approval.The FAA had to put in dozens of Airworthiness Directives in place to ensure continued airworthiness of the 747 fleet, and it was a nightmare for the airlines to comply with.All that time, Boeing kept saying, effectively, “There is no real life limit to an airplane, as long as it is maintained according to our guidelines.”It turns out those guidelines were simply not adequate!Approximately 63 percent of the 10,500 Boeing commercial airplanes in service in 1999 were built according to type designs that are more than 20 years old. These airplanes may not have been more than 20 years old, but all Boeing DC-8, DC-9, DC-10, 707, 727, 737-100/-200, and 747-100/-200/-300 airplanes were designed before 1979 and had accumulated 67 percent of the 403 million total hours flown.The problem was so serious, the White House got involved.The U.S. White House Commission on Aviation Safety and Security recommended that the FAA work with operators and original equipment manufacturers (OEM) to expand the aging aircraft program to include nonstructural components. The plan released by the FAA later outlines seven initiatives to address aging airplane systems:Establish an aging transport systems oversight committee to coordinate the various aging systems initiatives within the FAA.Conduct an in-depth review of the aging transport fleet and make model-specific safety recommendations related to airplane systems.Enhance airplane maintenance to better address aging airplane systems.Add aging systems tasks to the aging airplane research program.Improve reporting of accident, incident, and maintenance actions involving wiring system components.Evaluate the need for additional maintenance of transport airplane fuel system wiring and address any potentially unsafe conditions.Improve wiring installation drawings and instructions for continuing airworthiness.Later, the FAA formed a committee to propose revisions to applicable Federal Aviation Regulations and associated guidance material. The goal of the committee was to revise the materials as necessary to ensure the continued airworthiness of nonstructural systems on aging transport airplanes.The Aging Aircraft Safety Act of 1991 passed by the US Congress in 1991 — and later codified by the FAA as the AASR — requires airlines to ensure that repairs or modifications made to their airplanes are damage-tolerant. As part of the requirement, airlines must have a damage-tolerance-based maintenance program in place by December 20, 2010. This includes the development of an FAA-approved Operator Implementation Plan that contains the processes and timelines the operator will use for obtaining and incorporating maintenance actions to address the effects of repairs and alterations.By December 20, 2010, airlines that operate airplanes under Title 14 of the Code of Federal Regulations (CFR) 121 or 129 had to revise their U.S. Federal Aviation Administration (FAA)-approved structural maintenance program to comply with the FAA’s Aging Airplane Safety Rule (AASR). This revised maintenance program must include damage-tolerance-based inspections; a means to address the effects that repairs, alterations, and modifications may have on fatigue-critical structure and these inspections; and a means by which all changes to the maintenance program receives FAA approval.Naturally, every other civil aviation regulator followed.Until quite recently, some significant issues arising from aircraft age had not been recognised and addressed until after fatal accidents had occurred. More recently though, the general principles of system deterioration, which affect all older aircraft, are receiving renewed attention.The United States, which has seen most examples of accidents attributed to aging aircraft problems, has for some years now had a joint civil-military organisation called the Joint Council on Aging Aircraft (JCAA) to co-ordinate the development of risk management solutions for the various types of aging aircraft problem, especially structures. Awareness of these safety issues in the other leading airworthiness jurisdictions of design, production and maintenance regulation is now similarly high and preventive interventions are being developed.The maintenance issues which have particularly arisen with aging aircraft structural failure have generally been seen as arising from fatigue or corrosion, with corrosion sometimes initiating fatigue effects.Here are the latest FAA requirements on ageing aircraft:Transport Airplanes Aging Aircraft ProgramMoral: The effect of age on aircraft is unpredictable. Several corrective steps have been put into place, but it is for the operator to decide whether to continue with the increasingly costly groundings and inspections, or to retire the airplane. The operator will not get any help from the manufacturer, who will never admit his product is now unsafe. And, as we have seen, the regulatory authorities often miss a lot. It is for the operator alone, from his own experience and the experience of others, to decide whether to keep his old aircraft in service.

If you're assigned to a base in the U.S. or overseas in a non-combat location, when flying, pilots tend to train or perform routine, recurring missions.Shorter-range aircraft (fighters, helicopters and trainers) tend to stay close to home or fly out-and-back missions, most often to military airfields or civilian airports where they have fuel purchasing arrangements.Airlift aircraft and bombers might fly long-duration training missions, sometimes involving simulated or actual bomb, cargo or personnel drops. Airlifters also might be tasked against so-called "channel" missions or other scheduled or unscheduled operational requirements (see https://doctrine.af.mil/download.jsp?filename=3-17-D40-Appendix-1-MSN-Types.pdf ) as needed.Tanker aircraft might fly long missions supporting the training of other aircraft (fighters, bombers or airlifters); alternately, they might wind up flying to and hanging around designated air refueling tracks to support intercontinental or transcontinental one-way missions for aircraft re-positioning, resupply, or other purposes.If you aren't flying, you might be catching up on required ground training, or perhaps doing one of the additional duties necessary to help the unit maintain readiness, such as:Standardization/Evaluation Officer: the check ride-givers. There's always paperwork to accomplish, whether examinees pass or fail.Safety Officer: the people who try to help prevent smoking holes from forming, and who dig through the smoking holes when they do. You monitor trends associated with your unit's safety performance and things happening with other users of your unit's aircraft, and try to identify and correct hazards before they can bite somebody.Mobility/Plans Officer: the "readiness" folks. There's a lot of ongoing study associated with pre-developed plans, and different kinds of potential taskings require different materiel and documentation to be ready for deployment on very short notice.Weapons/Tactics Officer: any combat-coded unit involves either shooting, delivering ordnance, supporting the people who do those things, or trying to prevent becoming a casualty of the folks on the other side of the battlefield who want to do the same to you and yours. Weapons and tactics officers refine the training associated with the exercise of those combat specialties, and frequently are graduates of graduate-level programs in the art and science of their craft.Scheduling/Training Officer: usually more than one person is required to handle this set of responsibilities, which relate to ensure that all flying and ground training events -- self-paced and classroom -- are accomplished at least at the minimum frequency called for in Air Force instructions. There's plenty of computerized help in flagging people coming due for various requirements; the tricky part can be getting them slotted to fly on sorties where they can get the specific activities done. (Air refueling can be a particular headache if the unit doesn't have a regular need to do so, since it involves coordinating support from a tanker unit that might be juggling its own currency and real-world tasking issues.)Life Support Officer: the "flight equipment" job. For pilots, it can be kind of dull work sometimes, but it's critical. Life support equipment includes everything from helmets to headsets to oxygen masks to parachutes and chemical warfare flying ensembles. All of it has to be kept in good repair, and all of it has regular servicing and inspection cycles associated with it. Life support specialists are the go-to guys, and most units have enough of them that they'll get both a non-commissioned officer in charge and an additional duty officer to serve as an approver and manager.Some of the above involve supervising other officers or enlisted personnel; others are primarily subject matter experts with certain resource management responsibilities. At various times in my career I had the opportunity to serve in the first four positions, and I learned an awful lot in every one of them.

At the moment, not very. The issue is that it’s speed falls within the capabilities of defensive armaments currently deployed as well as planned upgrades. The issue is that a missile going 10k mph will be going against weapons that are designed for systems going 18k mph with fusing systems with enough range and processing power to initiate a “cone of destruction” in the Zircon’s path. But the actual fielding of such a system would mean that the Navy would have to evaluate and change procedures as the biggest drawback the Navy has is that it would need a wider radar net in order to detect the Zircons further out as waiting till they get within the present range would leave only a short time for engagement.When we get to a full Mach 20 attack speed at lower altitudes, then there will be a major issue of not being able to defend as the missile will hit the edge of even an extended radar net and be on top of the CG before it could defend itself. But by that time, other forms of detection will be available that capitalizes on the multiple defects that hyper sonic flight brings to the table.For one, hyper sonic flight produces a clear signal in the EM range that can be detected and tracked at long range even if the Zircom is out of radar range and the signal can even be detected from space.The second issue is the heat generated. The air around a hyper sonic missile gets super heated. Not enough to glow visibly unless at lower altitude, but is easily clear to IR sensors both in the air and from space.Three, the EM field generated by the incoming missile plus the plasma has a major league problem of interference with radio signals unless at close range. You remember all the spaceflights where spacecraft re-enter the atmosphere and ground stations lose contact for a time? The spacecraft entered into the atmosphere at hyper sonic speed and created the same conditions a hyper sonic missile goes through. During that time, no radio or data contact can be made until the spacecraft drops below hyper sonic speed in the lower atmosphere (comparatively.) And the issue continues today. Now, some people are trying to say that they can transmit instructions in flight to the Zircon? If they had the technology, they would be getting rich already off of it as it would revolutionize several different fields where electrical interference interferes with all voice and data communications.And I’m not even going to start on the “long range radar” as it has proven easy to spoof since the 1960’s. You send up an aircraft and match the radar wave and send out your own signal that will create false targets depending on how you pulse the signal as you don’t need accuracy when spoofing as you do when you’re the operators trying to gather targeting data. You don’t even need to match the original power as you’re not relying on a return. One aircraft set to spoof can change one CG operating over 100 miles with a half dozen ships to a fleet covering hundreds of miles with hundreds of ships just by modulating the signal and timing for return to the transmitter. That is why the US backed away from it during the 1960’s and 1970’s as it had long range but was extremely unreliable when faced with a half way competent enemy equipped with ECM gear so it wasn’t worth the cost vs the return. But, you’re going to have to protect the aircraft that is spoofing as it will be one of the first targets the Russians would want to take off even though almost all aircraft on a CG can be equipped with pods that could mimic the signal. That leaves satellite intelligence to target the CG (unless they want to sent recon flights of vulnerable aircraft) and those satellites are within range of anti-satellite missiles as the current crop of Russian satellites is well within the flight envelope.