THE AIRCRAFT: DESIGN, MANUFACTURING AND MAINTENANCE

The engines:

A fact that designers take into account when choosing the engines for a new model, is the total weight that the plane will have at the moment of takeoff-own weight + passengers + cargo + fuel-, since these must be able to generate, at least, a force - tons of thrust - equivalent between a third and a quarter of the total weight of the airplane to get it moving, accelerate, counteract the resistance to the advance of the air and reach enough speed for the wings to generate the necessary lift to take off. For example, in a commercial airplane with a capacity of 100 passengers and 50 tons of weight, each of its two engines needs to generate eight tons of force. While in the case of the A380, each of its four engines has to generate about 35,000 kilos of thrust x 4 = 140,000 kilos to move an approximate weight of 560,000 kilos, a quarter of the weight.

On the other hand, without the development and evolution that the jet engine has experienced, the advance of air transport itself would not have been possible. An example is found in the DC-3 piston engine, which needed a review every 500 hours of flight, while the engine of a Boeing 767 should be reviewed every 30,000, which gives an idea of ​​the reliability of one and the other. Reliability that is due, among other things, to the fact that the materials with which it is built can withstand thousands of hours of operation at enormous temperatures without deteriorating.

Another aspect that has also evolved has been the increase in its diameter, the immediate consequence of which is a drastic reduction in the noise generated due to the reduction in the speed of the exhaust gases.

We are all surprised by the apparent ease with which large commercial airplanes gain speed on the runway and, in less than a minute, raise their several hundred tons in the air. In those moments, each one of the engines, for example of a Boeing 747, develops a thrust of almost 30 tons. Push, whose greater part of the effort is supported by the great rotor of almost three meters of diameter that is appreciated when observing the engine in front. It is known as a fan-fan.

The development and production of these engines is a very specialized task on the part of the manufacturing companies. The competition is tough and all aim to design the most efficient engine that provides the greatest thrust with minimum fuel consumption, as this decisively affects the operating costs of airlines. The key aspect is the development of materials more resistant to stress and fatigue, lighter and that tolerate high temperatures without losing their qualities. A turbofan engine, such as the one developed for the Airbus A380, absorbs 1.2 tons of air in every second during its flight operation thanks to the fan. 87% of this air is driven directly backward, as would a propeller, while the remaining 13% is compressed and mixed with the fuel to produce, after passing through several stages, enough energy to move at high speed the fan.

And the greater the diameter of the fan, the compression ratio of the air and the temperature in the combustion chamber, the more efficient the engine will be. The problem is to have materials that resist these efforts and temperatures.

The fans are made of a titanium alloy. Their blades are not solid, but hollow and stuffed by a lattice of supports like honeycomb. It is striking that they are of one piece, without rivets or welds, and that they are attached to the central disc also without welding. The reason is due to a physical property of certain alloys known as superplastics.

Superplasticity is a curious phenomenon that occurs even in the hardest and most resistant alloys known. Take for example titanium, a light metal but very hard and resistant even at very high temperatures. However, alloying it with 6% aluminum and 4% vanadium becomes extremely ductile between 900 and 950 degrees Celsius, well below its 1,600 degree melting point. Temperature fork in which the alloy is left to mold as if it were plastic. Without losing its hardness, the alloy can be stretched to more than fifteen times the original size and different pieces can be joined by simple pressure.

The scientific explanation of this curious property is not simple, because it is born from the complex dynamics of the atoms that make up the alloy. At present, half of an airplane engine is made up of superplastic alloys, and the tendency is to build it entirely with this type of material. A curiosity, which I thought it appropriate to tell you in case you want to show off to your friends.

1. From manufacturing to flight online:

All aeronautical aircraft, parts and equipment that are manufactured in the member countries of the European Union, must be built according to the same technical standards, and validated following a single certification system defined by the European Aviation Safety Agency, EASA. In this way, homogeneity is guaranteed in all aircraft manufactured in Europe. There are just over two dozen countries whose industry has the capacity to design and manufacture aircraft. The rest, usually recognizes the type certificates [17] issued by the North American aeronautical authority -FAA- and the European Aviation Safety Agency already mentioned.

But before a type certificate is issued, and even before the production of the aircraft begins, the aeronautical authority must validate the design from the point of view of compliance with the airworthiness standards [18] established by those bodies.

The process for type certification involves an extensive program of tests, among which are:

• Structural and fatigue tests of material.

• Flight tests.

• Evacuation tests of the passenger cabin in case of emergency

The objective of this process is to verify that the design of the airplane conforms to the regulations in force, which complies with the flight characteristics included in the design, as well as the requirements regarding the resistance of materials, the structure, engines or on-board equipment.

The testing program:

A test program requires the construction of several prototypes. Each of them will be dedicated to cover specific sections of the program. If a prototype - or several - were not available and the series aircraft were built directly, it is very likely that there were problems, some unpredictable, which would lead to numerous modifications of the series aircraft, an increase in the cost of the project and risks for passengers. For this reason, it is necessary to carry out several thousand flight hours of prototype tests or prototypes to verify and adjust the operating data calculated during the design process, as well as the modifications that are introduced in the model. After this phase, the final design of the series aircraft, the production process and the certification tests required by the aeronautical authority are reached.

A part of the tests is performed on the ground without the need to take off, such as the bending of the wings and fatigue of the material.

Structural fatigue tests:

During the structural and fatigue tests to which an airplane is subjected, studies are carried out to verify, among others, the maximum loads that it will be able to withstand during take-off, in flight and during landing, as well as the forces and deformations that the structure is capable of supporting. The design philosophy regarding structural fatigue must ensure that during the operational life of the aircraft and any circumstance that it can find, the possible cracks that may arise do not reach a critical size without being detected. So, during the certification tests special attention is paid to structural behavior subjecting the aircraft to fatigue conditions.

The fatigue tests to which the plane's wing is subjected are very striking due to their spectacular nature, which mimic the flight cycles to which they are going to be exposed - detachment, flight and landing, as well as other possible circumstances such as turbulence or forced landings - to ensure that any cracks that are created do not reach a critical size.

In this test, mechanical actuators subject the wing to loading and unloading successively during a high number of cycles, which reaches a value close to that expected during its operational life.

Eventually, cracks occur on purpose and he continues to flex up and down to study his behavior. The speed of propagation is also verified in order to establish the appropriate inspection intervals, which guarantee their detection during the operational life of the aircraft.
With the results of the tests, manufacturers develop crack detection mechanisms that ensure that they never jeopardize the safety of the aircraft.

The test flight:

The test flight is intended to verify the general qualities of aircraft handling, their operational characteristics and the operation of the systems, both in normal operation and in case of failures and extreme conditions. This type of flight is carried out by expert test pilots.

When an airplane leaves the hangar where it has been assembled to make its first flight, it will be the first of a series within the test program that will be combined with other activities of experimentation and analysis on the ground. As already stated, these tests must show that the aircraft complies with the regulations in order to be certified and declared fit for the operation. Throughout the process, the data that is obtained is transferred to the design department to make the necessary corrections and improvements.

Test conditions:

The following tests are carried out on the airplane, among others: It is exposed to adverse weather conditions, which leads to extreme temperatures that can range from 45 degrees below zero to 50 degrees above zero, during which it is verified the correct operation of essential systems such as: motors, brakes and navigation, among others. And to fly through storms to check how the airplane behaves under conditions of strong turbulence, negative forces "g", and the operation of anti-ice systems.

It is also subjected to operations on tracks at high altitude, such as the one at La Paz Airport, in Bolivia, at an altitude of 4,000 meters.

Another test is to make the plane land on a wet runway to check its braking capacity. And another with strong crosswind, in which its maneuverability and stability is verified. The high-capacity aircraft that are manufactured today are capable of landing in crosswind conditions of up to 100 km / h. And they must do so with a maximum deviation from the runway axis of just over five meters. Because of the danger to the engines of ingesting water, the plane is subjected to landings in flooded runways to verify that the aerodynamic design of the airplane , as well as that of the landing gear, prevent the entry of water into the engines.

There is also the aborted takeoff test. This test consists of checking the response of the aircraft with the maximum weight at takeoff, given the possibility of having to perform an emergency braking in case the takeoff is aborted just before reaching the decision speed V1. For this, the prototype is installed a brake system manipulated so that its operating conditions correspond to those that would have 90% of the maximum allowed wear and the plane is subjected to maximum braking.

The aim of the test is to verify that, after maximum braking, the braking system does not catch fire, nor does it cause serious damage to the aircraft.

Evacuation tests in emergency:

Another important test block includes those related to the evacuation of the passenger cabin in case of emergency. To overcome this test, the aircraft must be completely evacuated by passengers and crew in less than 90 seconds - time set in the rules - in the dark of night, with half of the exits blocked, and with luggage and other obstacles placed in the corridors and escape routes. The interior of the aircraft can only be illuminated with emergency lighting [19].

Route evaluation flight:

Another test to which the airplane is subjected in order to verify the functionality and reliability of the conditions for the passenger, is the one that simulates a line flight with passage -generally employees of the manufacturer- representative of the commercial characteristics objective and with a time of flight similar to what will then be done in a normal operation. In it, the correct functioning of all the systems of the plane is checked, paying special attention to the aspects of the passenger cabin, such as ventilation systems, temperature, noise levels, the operation of the toilets, etc. This gives the opportunity to correct possible deficiencies and make improvements.

Infrastructure compatibility:

Finally, it should be noted that in the event that the new aircraft is somehow an innovation, as was the Boeing 747 and currently the Airbus 380, tests are carried out to verify its compatibility with airport infrastructures. During these tests, the new airplanes carry out visits to a series of airports around the world, which verify their compatibility with the services offered by airports such as: handling, refueling, fingers, etc.

All these tests can come to assume, in the case of the latest generation long-haul aircraft, more than 2,400 flight hours over more than 700 flights.

On the other hand, once the aircraft is already operating with an airline, if it detects any deficiency, it immediately informs the manufacturing company, which will be in charge of notifying the rest of the airlines that have aircraft in their fleet. model. At the same time, an analysis process will begin to find out the cause of the deficiency, find a solution and, finally, disseminate it to all of its clients for correction.

We take care of your plane, we take care of you:

A modern passenger transport plane is a very complex and sophisticated machine that requires careful manufacture and an exquisite maintenance to guarantee its perfect operation. These are the reasons that have led to the high levels of security achieved by current aviation and its subsequent success. Therefore, as there are many things that can fail, make no mistake, before an aircraft is considered available to perform its mission, a large group of specialized professionals must have worked on land accumulating in it many hours of work in order to guarantee its correct functioning [20].

This work is based on the maintenance plans that each manufacturer makes for the models of aircraft that they manufacture, which are incorporated into the technical manuals, which specify in detail how maintenance operations should be carried out by the airlines. Along with the maintenance program, manufacturers also provide airlines with a Minimum Equipment List (MEL), which includes, among other things, the minimum conditions that an aircraft must meet in order to operate. In addition, each plane has a "deferred book" in which the maximum time a repair can take is delayed when the anomaly is not considered important. Then, airlines can introduce more restrictions to these lists, but never less. Maintenance plan, technical manual, MEL and deferred book must be approved by the aeronautical authority of each country, which in the Spanish case is the General Directorate of Civil Aviation.

There are two types of maintenance: scheduled and unscheduled.

The purpose of scheduled maintenance is to maintain the aircraft in optimum flight conditions based on a preventive programming that must conform to the specifications given by the manufacturer, to use the information provided by other airlines that use the same aircraft and for the technical circulars and safety directives issued by the State Aviation Safety Agency. It is divided into three categories, which cover inspections whose intervals and tasks are progressively longer: on-line maintenance, minor maintenance and major maintenance.

Online maintenance:

It is usually carried out in the same parking lot and is composed of three inspections: daily, transit and revision.

Daily:

Before the departure of the first flight of the day, a maintenance technician carries out a review in the same parking lot to check the general condition of the aircraft: possible fuselage and wing damage that may have occurred during the time it has been parked, as well as of indicators, registers, connectors, etc., of all the services that have been connected to the plane since arriving at the airport. State of wheels and brakes, extension of the dampers of the landing gear, check of oil levels, hydraulic, oxygen pressure of the auxiliary system and review of the on-board emergency equipment.

Transit:

Before each flight and as close as possible to the exit, an external inspection must be carried out in order to check again the general condition of the aircraft: possible damage to the fuselage, landing gear and wings, which may have occurred during the flight or during the stopover. It is also verified that the registers and connectors of all the services that have been connected to the airplane on the scale, such as fuel, wastewater, electricity, etc., are correct for the flight.

Review:

Every hundred hours of flight, all aspects related to safety are checked, possible anomalies are corrected and a complete set-up is made to the aircraft.

Minor maintenance:

It consists of an inspection around the plane, the review of some specific elements and the correction of those who need it. It consists of three inspections:

Revision A: includes a general inspection of systems, components and structure, both from inside and outside, to verify its status.

Revision B: of greater importance than the previous one, in it the safety of systems, components and structure is checked, the set-up is made to the airplane and the elements that require it are corrected.

Revision C: it is a complete and extensive inspection, by areas, of the interior and exterior of the aircraft, including the systems, the facilities and the visible structure.

Major maintenance:

This is Review D, also known in professional slang as "Great stop" and, since it is the most complete and meticulous that can be done to an aircraft that can be used for several months, in it They include works such as the complete pickling of exterior paint [21], the disassembly and revision of engines, landing gear, flight control surfaces such as rudders, ailerons or flaps, ramps, seats, hydraulic systems, window panes and all interior cladding panels, both on the walls and the floor, not forgetting hundreds of kilometers of cable. Then, everything is assembled and painted. And before the plane returns to transport passengers, a test flight will be conducted by specialized pilots and technicians, who exhaustively and according to the manufacturer's standards, verify that the aircraft meets the required specifications. It's as if the plane had just left the factory with zero hours of flight. Like new

This type of maintenance in the case of an Airbus 340 is carried out every 10 years, being able to be in this phase for several months during which around sixty thousand hours of work are used. The cost of the process can be between 4 and 5 million euros, of which 15 to 20 percent are used in the purchase of spare parts [22].

It could be said, in short, that in the history of an airplane there should be more hours of maintenance than of flight.

To conclude, in Spain, the State Aviation Safety Agency is the body responsible for supervising the maintenance program and conducting periodic audits to verify that there are no deviations from the established maintenance procedures. Also, once it has been verified that they have been carried out After all the established revisions, it is the person who must renew the airworthiness certificate of each plane every year.

FOOTNOTES:

[16] The weight, is the measure of the force of attraction that exerts the terrestrial gravity on a body. If the force of attraction is made with an acceleration of 9.8 m / s2, it is said that the force is 1g and will be equal to the weight of the body.
If the acceleration were double, the force would be 2g and its weight would be double and so on.
Examples of force g:

• When detecting an acceleration equivalent to 3g, the airbags of the cars are activated.
• A fighter in a turn can produce 7g.
• A Formula 1 can produce in a braking 5g and 3g laterals in the curves.

[17] The type certification of a new design is a responsibility that corresponds to the aeronautical authority of the country where the company that designs and manufactures the aircraft is located. The aeronautical authorities of those countries where such aircraft are to be registered, can choose to accept the type certificate issued, or perform a technical verification or validation process, after which they issue their own type certificate.

[18] Airworthiness is defined as the situation of an aircraft that denotes the best technical condition to interact in the air environment safely.

[19] If any of the doors is inoperative, the airplane's capacity for passage must be reduced.

[20] All personnel performing maintenance tasks must be accredited as a Technical Specialist in Aircraft Maintenance, TMA, and all work must be supervised by a certifier, who offers the guarantee that the work has been done in accordance with the standards established.

[21] The reason for stripping the plane of the paint is to check the panels of the fuselage and the rivets that join them. Once the fuselage, the wings and the control surfaces have been checked, they are inspected with X-rays to check if cracks exist, the necessary elements are replaced, everything is assembled again and repainted. The exterior of an Airbus 340 requires about one ton of paint.

[22] It must be verified that damaged parts and those that have completed their useful life cycle do not re-enter the aircraft repair circuit. This is achieved with a rigorous control that can even verify the physical destruction of the replaced parts.

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