Aircraft maintenance checks refer to the periodic inspections that every commercial and civil aircraft must go through after a certain number of flying hours or time of use. Military aircraft may or may not have the same types of checks as commercial, but they must have their own maintenance programs as well. The aviation industry is extremely regulated, and commercial operators must comply with the inspection programs of authorities like the Federal Aviation Administration, Transport Canada, and the European Aviation Safety Agency.

Every operator must establish a Continuous Airworthiness Maintenance Program (CAMP) in its operations, including routine and detailed inspections of their air assets. The FAA institutes a series of inspections known as checks, classified as A, B, C, and D. A and B are relatively minor checks, while C and D are more exhaustive. Aircraft operators can conduct lighter checks in their own facilities, but must perform heavier checks at the site of a certified maintenance, repair, and overhaul (MRO) company.

A-checks are performed every 400-600 flight hours or 200-300 cycles (a cycle is one takeoff and landing). This inspection takes roughly 50-70 man hours, and requires an aircraft to remain on the ground for about ten hours, depending on its condition.

B-checks are conducted every six to eight months, and requires 160 to 180 man hours, depending on the aircraft’s type and condition. B-checks typically take one to three days.

C-checks are performed every 20 to 24 months, or after a specific number of manufacturer-set flight hours. They are more expensive than A and B-checks, involving inspections of large numbers of aircraft components, and require an aircraft to stay at an MRO site for at least two weeks and up to 6,000 man hours of work.

3C is an intermediate layover, and refers to light checks for corrosion or deterioration of specific parts of an airframe. Operators also take the 3C check as a chance to perform cabin upgrades (like new avionics, carpeting, etc), or incorporate the 3C into their D-checks.

The D-check is the most intensive type of check for aircraft. Also called a heavy maintenance visit, the D-check is performed every 6 to 10 years, and can see the entire aircraft disassembled for inspection and repair. The aircraft must be stationed at a spacious maintenance base, can take up to two months, requires 50,000 man-hours of work, and can cost about 1 million dollars to complete. Most operators choose to retire their aircraft at this point, as the cost of repairs exceeds the value of the aircraft.

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Spacecraft and airplanes are intricate machines that are designed and built to precise specifications, including which alloying element works best for individual components. Aluminum, copper, and nickel are most commonly used because of their ability to resist wear and tear, their heat resistance,  and their magnetic properties. New applications have been found to utilize these metals and vastly improve existing designs.

Aluminum has been used in aircraft construction for decades. It provides excellent strength as well as an economically friendly weight to cost ratio. It is estimated that almost 80% of the materials in modern aircraft is aluminum. The aircraft fuselage, wings, and supporting structures of commercial aircraft are all constructed of this metal alloy; aluminum can withstand a high level of UV rays as well. New technologies are on the forefront of aluminum as new casting technologies offer lower manufacturing costs, the ability to form complex shapes, and the flexibility to incorporate innovative design concepts.

Copper-based alloys are commonly used where construction requires materials that have high strength, resistance to corrosion, and excellent ductility. These parts are often safety critical and require long term operation such as electrical components, copper wire, generators, and data transfer systems. Copper is also a non-magnetic metal which means it won’t interfere with any electrical applications, making it practical to use in these applications. It is easily malleable and highly conductive. Its reliability and wide array of uses makes it an important alloy in the construction of a n aircraft. The same applies to nickel.

Nickel alloys are used in gas turbine engines, combustion chambers, engine exhaust valves, spacecraft, and many more applications. It has magnetic properties, excellent resistance to wear and tear, and can sustain extreme temperatures. In gas turbine engines, nickel can be found in the combustion chamber of an engine. The continuous stream of pressurized gas, as well as the constant flame, makes nickel the best metal alloy for this function. It is also found in the exhaust valves of aircraft. When nickel is mixed with tungsten or molybdenum, it allows it to withstand even higher temperatures. On spacecraft, nickel can be found on the outer parts of the vessel, where it protects against UV rays as well as tiny meteoroids. Without this heat resistant alloy, we may not have been able to walk on the moon.

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Every aircraft is built and maintained with a specific job in mind. It isn't possible for one aircraft to cover every characteristic a plane can have. For example, a fighter aircraft is made for defense and attack functionality. Thus, it has an extremely strong structure and is extremely powerful. Each characteristic is selected to coincide with one another, so the aircraft is built into a cohesive machine.

A fixed-wing aircraft is used for commercial purposes, one of its most important characteristics is to be comfortable for the passengers as well as the pilot. There are nine components to a fixed-wing, or conventional, aircraft: the fuselage, engine mount, nacelle, wings, stabilizers, flight control surfaces, landing gear, arresting gear, and catapult equipment. In this article we will be focusing on the fuselage.

The fuselage is the heart of the aircraft and where every other structure connects to. Usually composed of all metal, most fuselages are modified in a monocoque design. This design is structured in a way that relies on the strength of the shell (exterior) to carry various loads. Shell or skin thickness depends on what the aircraft's purpose is and what stresses it will have to encounter. To distribute these forces a cross sectional shape is composed and made up of bulkheads, station webs, and rings. Longitudinal members, such as longerons, formers, and stringers, take the brunt force of bending tension.

There is also a semimonocoque fuselage made up of aluminum alloy or graphite epoxy. In this design, longerons are used in conjunction with stringers, which are lighter and used more commonly. Vertical members, known as bulkheads, frames, and formers, are used to support concentrated loads and attach parts such as wings, engines, and control stabilizers. This design is more streamlined and sturdier as compared to the monocoque design since all structural components aid the structure. This configuration is most likely used in fighter jets to protect the structure from the damage it may contract.

Fuselages may be constructed in as little as two sections to as many as six sections for larger aircrafts. Each aircraft will have differing maintenance and inspection manuals depending on where the access doors and inspection panels are located.

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When you’re in flight, the only thing separating you from the thin air outside is an airplane window. On one side, there’s a warm, pressurized cabin where you can work, watch movies, sleep — and on the other, air that is not suitable to breathe. Between the two, incredibly sturdy windows. Aircraft cabin windows and windshields are designed to withstand high pressure environments that normal windows couldn’t function in. 

A cabin window consists of three panes: an outer pane that is flush with the outside fuselage, an inner pane which has a pressurization hole in it, and a thinner, non-structural plastic pane called a scratch pane. Passengers can’t touch the inner pane or the outer pane for safety reasons; instead, passengers can rest their weary heads against the scratch pane. The scratch pane isn’t actually part of the window assembly itself but installed separately.

As your aircraft gains altitude, the pressure acting on the outside of the plane drops; the air is much less dense the higher your plane climbs. Because aircraft cabins are pressurized to about 6,000 feet for passenger comfort, there is more pressure inside the plane than acting on it from the outside. That pressure is bearing on the fuselage and the cabin windows. The little hole on the inner panel allows some of the cabin air to escape into the pocket between the inner and outer panes and equalize. This forces the outer pane to take all of the load, albeit slowly. The small hole is designed to function so that as the plane ascends the pressure slowly equalizes.

The inner and outer pane thickness is specific to each type of aircraft. Inner panes are generally thinner at approximately 0.2” thick and are only present as a fail-safe if the outer pane fails. The outer panes are thicker—at approximately 0.4” thick—and carry the pressure loads for the life of the window. The increased thickness is meant to allow for engagement with the airframe structure while maintaining the required strength. The air gap is approximately 0.25” and also varies for each aircraft.

Aircraft cabin windows are not made of glass but with a material referred to as stretched acrylic. It’s a lightweight material manufactured by a few global suppliers for the various aircraft flying today. One such supplier is UK-based GKN. The largest manufacturer of cabin windows worldwide, GKN makes cabin windows for the Boeing 737 and the Boeing 787, and most other aircraft. Stretched acrylic is produced by stretching the base material of as-cast acrylic. It provides better resistance to cracks, reduced crack propagation, and improved impact resistance.

Another type of window that exists on aircraft is the windshield/cockpit window. It consists of a toughened glass pane, a heating/deicing element, a vinyl layer, surrounded by another layer of reinforced glass. Airliners utilize acrylic as well due to its versatility. The cockpit windows are thicker and stronger as they have to withstand bird strikes—which aren't an issue on the sides of the fuselage where the cabin windows are. Jet windows are also made of stretched acrylic but are a single layer in a far more complex curved form.

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Shaft couplings are different from some other types of connectors because instead of just joining two parts, they actually transmit power from one end to the other on rotating shafts— this is their primary function. Several other features that they have are that they accommodate misalignment and create mechanical flexibility, they reduce transmission shock, and they protect against overload. Shaft couplings join two pieces of rotating equipment such as motors, pumps, compressors, and generators. Shaft couplings generally do not disconnect during operation, but there are torque limits; if exceeded, the coupling may slip or disconnect. There are numerous types of couplings styles; they support particular torque or power transmission and afford various misalignments.

Different couplings supply support for different types of misalignment: parallel, angular, etc. Beam couplings are used for applications in which torque does not exceed 100 inch-lbs. Bellows couplings do not accommodate as much parallel and angular alignment as a beam coupling, but they are great for positioning applications because they provide high torsional stiffness. Oldham couplings can handle high levels of parallel misalignment. Schmidt couplings are useful for shafts that are offset. Although clamping couplings aren’t good at handling misalignment, they are inexpensive and are zero-backlash devices. Disc couplings transmit power, have high torque transmitting abilities, and accommodate angular misalignment, but are not useful for managing parallel misalignment.

Chain couplings can transmit hundreds of horsepower and can handle small amounts of misalignment. Diaphragm couplings are used in high power transmission applications, like turbomachinery, and have the capacity to handle high torque transmission and high-speed orientation. Gear couplings have the ability to transmit high levels of torque, but diaphragm couplings have an advantage as they do not require lubrication. Grid couplings are capable of high torque transmission, have shock absorption, torsional vibration dampening, operate without lubricant, and accommodate various misalignments. Jaw couplings are used for motion control and light power transmission.

Particular couplings may be beneficial for different applications. It all depends on the direction of misalignment and its force, torque load, and power transmission requirements. While some couplings may offer benefits to all areas, they are not always necessary. Some applications only have one or two requirements and sometimes the costs associated with each type, or the material they are composed of, needs to be taken into consideration.

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Threaded fasteners play an integral role in the operations of an aircraft. Though small in size, these fasteners are just as highly specified as any other aircraft component. There are various classifications that categorize types of fasteners and where they can be used on an aircraft.

Threaded aircraft bolts, screws, and nuts are classified by their thread series or threads per inch. The classifications include the American National Coarse (NC), American National Fine (NF), American Standard Unified Coarse (UNC), and American Standard Unified Fine (UNF).

The UNF thread series is commonly seen in the aerospace industry and designates a fastener that has better load carrying capacities and better torque locking than UNCs. They have a fit that is specific to tighter fastening and finer tension adjustments. Threaded fasteners are also categorized by “Class of Fit”— which designates the tolerance allowed during manufacturing of a threaded fastener. Aircraft bolts are usually a Class 3 (medium fit) and aircraft screws are usually a Class 2 (free fit).

The dual series— National and Standard Unified, differ in one important way when it comes to fine thread fasteners. NFs have a 1-inch diameter size, and thread specifications of 14 threads per inch. It’s labeling looks like this: (1-14 NF). UNFs specify 12 threads per inch, and its labeling is as follows: (1-12 UNF). Both are also categorized by the number of times the threads rotate around 1 inch of a given diameter; 4-48 thread means a ¼ inch diameter bolt with 48 threads in 1 inch of its length.

Overall, aircraft threaded fasteners must be able to hold aircraft parts under high loads without being riveted or welded. This requirement is due to the need for efficient airflow, and redundancy of aircraft parts. A fastener that has been too tightly secured to an airframe or is the incorrect fastener for the applied load and receiving hole, can cause severe breakdowns and weakening of materials over time. Selecting the appropriate threaded fasteners can streamline dismantling and replacement of aircraft parts that are frequently reassembled or replaced. As such, proper threaded fastener selection is a very important aspect of aircraft manufacturing and maintenance.

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In frigid temperatures of 32 degrees and below, the components of an airplane can reach damaging temperatures that reduce their longevity. When an airplane is started up before it is evenly heated, it has to work much harder to get going. The extraneous efforts can cause wear and tear on mechanics that can reflect 100 to 500 hours of operation. Uneven heating can detrimentally affect pistons, cylinders, camshafts and other components. In a cold weather predicament, it is important to have an aircraft preheating method.

Preheaters are often used for aircraft cockpit and engine heating. These devices are designed to heat airplane components during cold weather and can come in a multitude of configurations. Preheating devices that are the most commonly used in avionics are a forced-air preheater, an electric heater, or a heated hangar.      

Forced-air preheating mechanisms can be provided by an FBO (fixed base operator). In which case, a forced-air cart will likely be used. Typically, these tools will have capability to simultaneously vent exhaust and pump hot air through their heating tube and are safe to use on the engine and cockpit. Downfalls of using a forced-cart include uneven heating, and high demand if you are docked at an airport.

Electrical heaters are a more practical option in a bind, but they need to be installed on the aircraft. FAA regulations allow the installation of heating components as long as they are made under a parts manufacturing approval (PMA) and are inspected by an airframe and powerplant mechanic (A&P). Electric preheaters operate through heating elements that can be attached to engine components. This device is handy in a bind but can take up to 6 hours to heat an engine evenly.

Another preheating option to consider is a heated hangar. Using this technology allows comprehensive heating of the aircraft. Heated hangars take around 8 - 12 hours to fully heat an airplane, depending on size and entering temperature. However, an evenly heated aircraft will save time and money in the long run.

Additions that may aid in faster preheating is an insulated cowl attachment, and a thermostat. Insulation provided by the attachments can help keep heat trapped in the engine compartment or cockpit, which is integral to time efficient preheating.

A thermostat, on the other hand, can help to automatically regulate temperature control in your engine compartment and cockpit. Well-designed thermostats will be able to detect outside temperatures, and jumpstart preheating. Thermostats can also be equipped with overheat protection systems and low to high watt power settings to give more protection to the aircraft components and more control over the preheating process. 

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How do Electromechanical Relays Work?

To put it simply, an electromechanical relay is essentially just a switch. However, relays are controlled by electromagnets, whereas switches are manually actuated. They are used in a control circuit when a low power signal is required, or when several circuits need to be controlled. Inside the relay, a small current is used to create a magnetic field within the coil. This magnetic field is used to switch or control a much larger current. Relays come in a variety of sizes and can use different technology, but they operate with the same basic concept that makes them an electromagnetic relay.

It should be noted that like many other circuit diagram symbols, the symbols for relays can vary quite a bit. The most common way to see a relay symbol shows the relay coil as a box with protruding contacts shown as open or closed. Older circuit diagrams might show the replay as an actual coil. Regardless, it is a good idea when working with relays to know how they may or may not appear.

Although solid state switches are currently more popular due to their higher efficacy, relays are still ideal for many applications due to their unique properties. When a current flows through the coil box an electromagnetic field is created, pushing contact ends together, completing a full circuit. Relays have several advantages and disadvantages just as any other type of technology. On one hand, relays provide physical isolation between circuits and can withstand extremely high voltages. On the other, they are slower than semiconductor switches and have a limited lifetime.

Where are Electromechanical Relays Used?

Relay switches can be used in automotive applications, industrial applications, aerospace applications, large power operations, etc. Each relay is specific to the application in which it is to be applied and there are many to choose from on the market. So, it is important to weigh out the pros and cons when selecting any type of electronic component.

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Also known as a “gas turbine” or a “combustion turbine”, the turbine engine is a type of internal combustion engine categorized by its use of a special type of oxidizer mixed with fuel and combusted in a carefully designed combustion chamber. Turbine engines convert energy into a type of mechanical-based motion to power aircraft, trains, ships, electrical generators, pumps, gas compressors, and tanks.

There are three main components of a turbine engine: the upstream rotating gas compressor, the downstream turbine, and the combustion chamber. Energy is created and added a stream of gas in the combustion chamber. Air and fuel mix and then ignited, increasing the pressure, which in turn causes the fuel to experience higher levels of combustion and an overall increase in temperature. Then, the mixture is sent to the turbine where the gas flow starts to move in high volumes and at a very fast rate. Afterward, it moves to a specially designed nozzle that emits the fluid mixture over the blades on the engine which spin and power the compressor. Eventually, the emitted pressure and overall temperature dissipate.

Compared to standard engines that use pistons, turbine engines are significantly simpler in operation but are more powerful in energy output. Turbine engines are considered simpler than standard engines because out of all the parts, there is only one main component that is considered a moving part. On the other hand, piston engines have dozens of individually moving parts. Turbine engines are also more optimal than standard engines. They’re designed to run optimally in lower pressures and higher altitudes, operate at higher velocities, have better internal lubrication, and support more weight while still providing more power than standard engines. As a result, turbine engines are considered the more popular and optimal choice amount manufacturers in many industries.

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The pyramids of ancient Egypt, barstools, gearsets in car transmissions, and the earthquake-proof San Francisco International Airport all have one unlikely thing in common: bearings. While relatively simple to understand, the difference that using a bearing makes can be massive. And the reason is simple: according to basic physics, things that roll meet less frictional resistance than things that slide. Aptly named, bearings “bear” the load by using smooth metal for the balls or rollers, and the inner and outer surfaces they roll against to minimize the contact area and allow for less friction and smoother movement.

There are many different types of bearings which can be classified into three categories defined by their type of loading: radial, thrust, or a combination of both. While radial loads act at the right angles of the shaft, thrust loads act parallel to the axis of rotation. Radial loads require bearings such as roller bearings which use cylinders to hold heavy loads, or their thinner counterparts, the needle bearing. On the other hand, thrust loads can be handled with roller thrust bearings or ball thrust bearings which are commonly found in gearsets and barstools respectively. And finally, combination loads can use ball bearings for smaller loads and tapered roller bearings for larger loads like car hubs require.

Of course, beyond the examples already listed, many other technologies require bearings of all kinds to function, notably aviation technology. From the common ball bearing to the slightly less common tapered bearing and needle bearing, aviation technology has its uses for bearings of all sorts. With the diverse options in bearings on the market today, finding the right bearing to suit your aviation needs may seem daunting.

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