Hot answers tagged

25

How do the engineers, maintenance team, etc know if an aircraft has metal fatigue? First of all, is not an "aircraft" that has fatigue, but a component. To the naked eye, there is no way to detect metal fatigue until it starts being too late: you can only see cracks that are already forming and/or propagating. There are tools available to allow for early ...


16

It's not a stress concentrator; it's just the opposite. What you're missing is that the floor itself at the pinched part forms a tension bridge that allows a more or less 'ovalized' circle while still maintaining tension loading on the skins and frames as if it was a pure circle. If I had a rubber balloon filled with air and was able to run a string ...


14

Engineers have a variety of ways to detect metal fatigue in aircraft: Visual inspection: sometimes cracks are visible to the naked eye, or can be seen under magnification Auditory inspection: sometimes a thump sounding wrong can give a clue that there's an issue. It's not a primary means of checking for fatigue, but something that may clue in an astute ...


11

The problem isn't the relief of stress from dumping of loads. If it was, you would have the same failures happening because of the act of landing, where the wing goes from being heavily loaded to unloaded in about the same amount of time. And it's once per flight in any case and is something that all airplanes have to cope with. Fatigue is all about the ...


7

Yes they do, and here is why. Airplanes are made mostly of aluminum rather than steel. It is possible to design a steel part in such a way that it will never succumb to fatigue failure (crack propagation at a stress significantly below the yield stress) but this is not true of aluminum and its alloys. For this reason it is necessary to periodically tear ...


7

Metal fatigue and fatigue corrosion are and have been a problem since the type's introduction. They always fatigue and corrode, they always undergo expensive repairs due to corrosion. DIEGME degradation of fuel tanks has been a particular culprit, and it has been reduced with new coatings. Bombers aren't pressurized, so the whole fuselage doesn't fatigue ...


5

To add to John's answer, why did they do it? To make more room in the passenger area. It is important to remember that, opposite a submarine, pressure is higher on the inside of the aircraft in flight, rather than lower. So structural design favors a cross brace to hold it together. A submarine would be strongest if it were perfectly spherical. Also, ...


5

When you subject aluminum to cyclic loadings, there is a relationship of cycles it takes to crack/ vs material thickness, or structural meat you might say. If the structure is sufficiently meaty, the number of load cycles to generate cracks become effectively infinite. Modern airliners have structures carefully designed by analysis to last a certain number ...


5

Most (large) aircraft life times are measured in cycles. "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 ...


5

This is called the "double bubble". The 737 has a similar but less pronounced design. Both aircraft had a primary requirement to seat a specific number of passengers in each row: 5 in the DC-9 and its successors, 6 in the 737. Seat-bottoms are the widest parts of the seat, so they made the fuselage the widest there. But continuing arc downward from there ...


4

It wasn't the square-ish windows (they still had round corners, just not as rounded as modern square windows). They knew square windows were less than ideal stress wise and built the frame structure to take the expected loads. They just screwed up the calcs and didn't quite make them heavy enough. The problem was stress concentrations at the rivet holes ...


4

Earlier B-52s did not carry all their fuel in the fuselage. They also carried their fuel in the wings using rubber bladder-type wing tanks. The new B52G “wet wing” was a weight savings measure which also increased the fuel capacity over the bladder-type in-wing tanks. The metal fatigue of the new wing was due to a poor structural design which was supposed ...


4

The decrease in fatigue is not a property that's automatically conferred to the plane by the introduction of wet wing - it's a product of smart design. Usually the decision to use a wet wing is made during the initial design of a plane, and the support structure of the wing is built accordingly, so that it can take the dynamic strains that occur upon ...


3

Back in the day the BUF (correct acronym for "D" model) airframe usage was calculated using variable aspects of actual flight that were outside the assumptions that the engineers used to predict "ordinary" useful hours of the airframe. If for example the plane had a 20,000 hour useful life before major overhaul or permanent grounding, ...


3

Some metal fatigue can be spotted visually as an visible crack or as a collection of tiny cracks that can give the metal a 'frosty' look but most is done via non-visual non destructive testing techniques such as radiography (basically X-raying) or Ultrasonic testing to search for microfractures before they reach the problem stage. Visual checks are no use ...


2

You can actually see the same effect happening with soap bubbles when they touch and form a flat film connecting them. You can actually still see this type of fuselage in use in the Boeing 717 (Douglas/McDonnell Douglas DC-9). A lot of other narrowbody airliners also have it, but the outer panels fair over the intersection between the two fuselages.


2

The answer is soap bubbles: They are filled with slightly higher pressure air, and when they're attached to each other, they have a planar "reinforcement-like" part between them. (see the picture) This is similar to your "attached circles with reinforcement between them"-style structure. (DC fuselage)


2

Because the effects you describe are too small to place enough bending stress on critical locations to be a significant problem, and to the extent that is a problem, you simply add extra meat as required. If the fuselage grows say 50 thousandths in diameter at max differential and the floor beam stretches by say 20 thousandths, the change in the geometry of ...


2

Lifetime limit depends on the design and on usage. Many aircraft have a lifetime limit given in cycles (i.e. number of takeoffs and subsequent pressurization cycles). For the B-52, flight hours are used instead (because the fuselage isn't pressurized). Modern aircraft are designed to a specific limit, and modern CAD software allows you to predict this pretty ...


1

With some combination of regular Non Destructive Test inspections, patches and reinforcements, and in some critical areas replacement of primary structural fittings, panels and skins, and you can run an airplane forever if you can keep it safe from oxidation corrosion. It only comes down to inspection frequency, and when and what structural parts to ...


1

How and why isn’t this a problem in practice? The answer is in the question: in practice. There will always be stresses in the construction, including bending stresses. The double bubble just has much lower internal stress than a comparable oval shape A fuselage cross-section under pressure will want to acquire a circular shape - if it starts out in a ...


1

Too complex for me to give a specific answer, and this answer is assuming only airline transport not private flights or business charters. Duty time limits are different for every country and may involve several pages of tables and definitions, airlines hire people just to track and comply with this. Time of day relative to the pilots home timezone. If ...


1

It is difficult to estimate how long a particular airframe might last. Also, different aircraft might last different amounts of time for esoteric reasons. For example, mercury amalgamates with aluminum and can weaken it. Even a small amount of mercury can significantly weaken an aluminum structural member. Therefore, you might have one plane with some ...


1

All military aircraft have airframe time limits, and they are mostly related to fatigue. It should be easy to figure out what portions of the airframe are stressed the most -- think of what sticks out the most from the center, and what's connected to that, or what is subject to stress every time the aircraft flies even if it just goes around the patch and ...


1

The safe/fatigue life of a structure is the number of events during which there is a low probability that the strength will degrade below its design ultimate value due to fatigue cracking. The events may be flights, flight hours, landings, pressure cycles or engine cycles. Safe life may be determined by using a similar structure (usually called a fatigue ...


1

It is not the stress concentrating type of sharp corner. That is the natural shape that allows the parts to be in a nice smooth tension at lowest energy. If you made an elastic rubber balloon with a divider membrane creating two chambers and inflated it you would get a similar load distributing shape.


1

Older planes like the DC3 undergo 100 hour and/or annual inspections when used for hire just like other planes, and Airworthiness Directives for maintenance issues are sent out to the fleet as things crop up as well. If the inspections and required maintenance do not show anything, they are signed off to go until the next inspection period. There are also ...


1

The US Federal Aviation Administration does not regulate aircraft owned and operated by the military services. I suspect that is the same situation with Australia's CASA. Parameters for aircraft service life for military aircraft are defined by the manufacturer. The aircraft is designed to meet the requirements specified by the agency requiring the ...


1

The pilot listens and notify the technicians that the plane is giving out strange noises. And now seriously: For each plane type there are cards which parts need to be replaced or inspected after certain flight hours, start count or time. Together with required quality control at production and maintenance it should prevent metal fatigue incidents as much ...


Only top voted, non community-wiki answers of a minimum length are eligible