Static strength testing of aircraft. Aircraft test shop

The creation of any aircraft is a long and complex process, the result of the joint efforts of a huge team, many divisions and departments. The Ilyushin Aviation Complex, along with the experimental design bureau, also includes a large number of structural laboratories necessary for conducting full-fledged tests, including very important tests for the future aircraft - structural strength tests. In the course of static and life tests of full-scale samples of prototype products, the calculated conclusions are confirmed experimentally. The tests confirm the correctness of the design of the structure for the given loads, and the issue of the correctness of determining the loads is solved with the help of flight strength tests, which are already carried out by specialists at the FRI with the participation of specialists from the department. Today we will take a closer look at just such a complex. Vladimir Ivanovich Tkachenko, the deputy head of the laboratory of the complex of strength tests "AK named after S.V. Ilyushin", candidate of technical sciences, met us and gave us a tour...
Vladimir Ivanovich spoke about the types of strength tests, and specifically about the studies that are carried out in this laboratory. There are two independent disciplines of strength calculations - calculation of strength for statics and calculation for a resource. Static tests, in which the load on the elements of the airframe exceeds the operational one by 1.5 times. Wing load during flight exceeds 1000 tons. Create the most approximate conditions of the structure due to the stress-strain state of the structure. The operational load in the calculations is assumed to be 67% (this is out of airworthiness standards). If, for example, we multiply this value by the safety factor (for calculations, the value is taken - 1.5, which takes into account the life of the airframe), then exactly 100% of the calculated load is obtained, although such a load never occurs during the flight ...
The wing, as the most damaged part of the aircraft structure, is tested with design loads up to 120%. The fuselage, although it has various structural cutouts and cavities, and it would seem that it should have less strength, is not subject to such loads in flight as the wing receives. Therefore, testing with 100% load is enough for him ...
This Il-76TD (RA-76751), released in 1988, first flew in Aeroflot and managed to fly 2,500 hours, and in 1994, after landing on the Khodynka field, it came into the possession of the Design Bureau to install and fly around new PS- 90. However, the engines were never installed on this car, and it was decided to leave this board for life testing. For this, a special program was developed. According to a similar program, life tests of the Il-476 are now being carried out ...
IL-76 was originally designed for 20,000 flights. But in order to provide him with such characteristics, it was necessary to carry out the whole complex of strength, and then life tests. And then, to this day, continue to conduct tests already to ensure the extension of the resource ...
It is these studies that are carried out in this laboratory. The landing gear has been removed from the aircraft. The aircraft is suspended under powerful beams on special suspensions, which also include hydraulic cylinders that can create a load of tens of tons on a structural element. The automatic tracking system, developed jointly with TsAGI, allows you to stabilize the suspension and provide the desired flight condition. The forces of these hydraulic cylinders are proportional to their diameters. The load with the help of additional beams and brackets is evenly distributed over the structural elements. Pylons for standard D-30 engines are left on the left half-span of the wing, while pylons and reinforced structural elements and attachment points for PS-90 engines, which are heavier and more powerful than the 30s, are installed on the right half-span ...
On average, to determine the resource, a flight lasting 3-4 hours and a service life of 20-25 years are taken into account. These values are confirmed first. In the future, to increase the resource, they begin to conduct additional tests, which can last for years. Usually, the resource according to the state of the material (corrosion, fatigue, wear) is less than according to flights, and it is more difficult to extend such a resource. Now, according to the results of tests, the service life of flying Il-76 with D-30 engines has been extended to 10,000 hours ...
A program flight usually lasts 20 minutes and is performed with full load on the structural elements of the wing and fuselage (wing loads are performed with a safety factor of 2). The wing is subjected to pressure and various vibrations through the action of hydraulic cylinders. The load for the calculation is summed from all cylinders. At the final stage of the flight, the wing is subjected to pressure loads that simulate a landing. According to the program, which is now being worked out on the 76th, it is necessary to perform 20,000 such flights ...
If damage occurs, the tests are stopped and the assembly is repaired. After that, the testing process is resumed. Damage to the structure and individual elements is detected in various ways, both visual (if large) and instrumental (there are several special methods) that can find even minimal cracks... Usually, the longer the resource is extended, the more restrictions are imposed on the operation of the aircraft. For example, they restrict flights due to weather conditions or transfer from passenger flights to cargo transportation ...
We also looked inside the plane. To create a load on the floor, ring weights are laid in various places ...
The place of the navigator and again the loads on the floor ...
Wire harnesses from sensors of measuring equipment are stretched throughout the cabin ...
View of the wing through the porthole, entangled in a network of beams, brackets and wires...
The study of the insides of the aircraft was not without an attentive local "controller";)
If cracks are found, their fixation or elimination is now possible with the help of adhesive methods, the so-called "stoppers". Such innovations began with the creation and testing of the wing for the Il-86, which, during development, required other, higher, strength characteristics ...
Today, a very large number of IL-76s are still in operation around the world, including those of foreign operators, which in turn requires additional research on the resource. Therefore, these types of strength tests for this machine will continue further ... Below are two new pylons for PS-90 engines, handed over by the plant for installation and strength testing...
The entire wing is hung with various levers and counterweights, combined into one common complex system... From this cab, located a few meters above the floor, the operator controls the test programs. There are two such cabins in the hangar...
Well, then we got acquainted with another amazing aircraft - a wooden full-size model of the Il-96-300, created mainly to solve problems in the layout of the cabin.
Even part of the wing and the engine are recreated on the mock-up...
Not only the interior, but also the external design features are modeled in sufficient detail...
Having climbed on board, the first thing we do is look into the cockpit, because it is also available in this layout...
Inside, at first glance, everything looks like in the real 96th. Differences become noticeable only upon closer examination. For the manufacture, in most cases, wood and plywood were used. Although, in some places, real interior trim elements are also installed ...
There are several salons, as in real Il. There is plenty of free space inside. They say that this layout was also used in the development of the interior equipment of the presidential IL-96 ...
On the frame below - the 103rd machine (five-seater IL-103), which has already passed the entire scope of tests, and is now also in this department, sheltered next to its older sister ...
And finally, another general view of the laboratory...

In the course of static and life tests of full-scale samples of prototype products, the calculated conclusions are confirmed experimentally. The tests confirm the correctness of the design of the structure for the given loads, and the issue of the correctness of determining the loads is solved with the help of flight strength tests, which are already carried out by specialists at the FRI with the participation of specialists from the department. Today we will take a closer look at just such a complex.

We were met and given a tour by the deputy head of the laboratory of the complex of strength tests "AK named after S.V. Ilyushin", candidate of technical sciences - Vladimir Ivanovich Tkachenko ...
2.

Vladimir Ivanovich spoke about the types of strength tests, and specifically about the studies that are carried out in this laboratory.

There are two independent disciplines of strength calculations - calculation of strength for statics and calculation for a resource. Static tests, in which the load on the elements of the airframe exceeds the operational one by 1.5 times. Wing load during flight exceeds 1000 tons. Create the most approximate conditions of the structure due to the stress-strain state of the structure.

The operational load in the calculations is assumed to be 67% (this is out of airworthiness standards). If, for example, we multiply this value by the safety factor (for calculations, the value of 1.5 is taken, which takes into account the life of the airframe), then exactly 100% of the calculated load is obtained, although such a load never occurs during the flight ...
3.

The wing, as the most damaged part of the aircraft structure, is tested with design loads up to 120%. The fuselage, although it has various structural cutouts and cavities, and it would seem that it should have less strength, is not subject to such loads in flight as the wing receives. Therefore, testing with 100% load is enough for him ...
4.

This Il-76TD (RA-76751), released in 1988, first flew in Aeroflot and managed to fly 2,500 hours, and in 1994, after landing on the Khodynka field, it came into the possession of the design bureau for the installation and flight of new PS- 90.
However, the engines were never installed on this car, and it was decided to leave this board for life testing. For this, a special program was developed. According to a similar program, life tests of the Il-476 are now being carried out ...
5.

IL-76 was originally designed for 20,000 flights. But in order to provide him with such characteristics, it was necessary to carry out the whole complex of strength, and then life tests. And then, to this day, continue to conduct tests already to ensure the extension of the resource ...
6.

It is these studies that are carried out in this laboratory. The landing gear has been removed from the aircraft. The aircraft is suspended under powerful beams on special suspensions, which also include hydraulic cylinders that can create a load of tens of tons on a structural element. The automatic tracking system, developed jointly with TsAGI, allows you to stabilize the suspension and provide the desired flight condition.

The forces of these hydraulic cylinders are proportional to their diameters. The load with the help of additional beams and brackets is evenly distributed over the structural elements. Pylons for standard D-30 engines were left on the left half-span of the wing, while pylons and reinforced structural elements and attachment points for PS-90 engines, which are heavier and more powerful than the 30s, were installed on the right half-span ...
7.

On average, to determine the resource, a flight lasting 3-4 hours and a service life of 20-25 years are taken into account. These values are confirmed first. In the future, to increase the resource, they begin to conduct additional tests, which can last for years. Usually, the resource according to the state of the material (corrosion, fatigue, wear) is less than according to flights, and it is more difficult to extend such a resource. Now, according to the results of tests, the service life of flying Il-76 with D-30 engines has been extended to 10,000 hours ...
8.

A program flight usually lasts 20 minutes and is performed with full load on the structural elements of the wing and fuselage (wing loads are performed with a safety factor of 2). The wing is subjected to pressure and various vibrations through the action of hydraulic cylinders. The load for the calculation is summed from all cylinders. At the final stage of the flight, the wing is subjected to pressure loads that simulate a landing. According to the program, which is now being worked out on the 76th, it is necessary to perform 20,000 such flights ...
9.

If damage occurs, the tests are stopped and the assembly is repaired. After that, the testing process is resumed. Damage to the structure and individual elements is detected in various ways, both visual (if large) and instrumental (there are several special methods) that can find even minimal cracks ...
10.

Usually, the longer the resource is extended, the more restrictions are imposed on the operation of the aircraft. For example, they limit flights due to weather conditions or transfer from passenger flights to cargo transportation ...
11.

We also looked inside the plane. To create a load on the floor, ring weights are laid in various places ...
12.

The place of the navigator and again the loads on the floor ...
13.

Wire harnesses from instrumentation sensors are stretched throughout the cabin ...
14.

View of the wing through the porthole, entangled in a network of beams, brackets and wires ...
15.

The study of the insides of the aircraft was not without an attentive local "controller";)
16.

If cracks are found, their fixation or elimination is now possible with the help of adhesive methods, the so-called "stoppers". Such innovations began with the creation and testing of the wing for the Il-86, which, during development, required different, higher, strength characteristics ...
17.

Today, a very large number of IL-76s are still in operation around the world, including those of foreign operators, which in turn requires additional research on the resource. Therefore, these types of strength tests for this machine will continue further ...
18.

Below are two new pylons for PS-90 engines, handed over by the plant for installation and strength testing ...
19.

The entire wing is hung with various levers and counterweights, combined into one common complex system ...
20.

From this cab, located a few meters above the floor, the operator controls the test programs.
There are two such cabins in the hangar ...
21.

Well, then we got acquainted with another amazing aircraft - a wooden full-size model of the Il-96-300, created mainly for solving problems in the layout of the cabin.
22.

Even part of the wing and the engine are recreated on the mock-up...
23.

Not only the interior, but also the external design features are modeled in sufficient detail ...
24.

Having climbed on board, the first thing we look into is the cockpit, because it is also available in this layout ...
25.

Inside, at first glance, everything looks like in the real 96th. Differences become noticeable only upon closer examination. For the manufacture, in most cases, wood and plywood were used. Although, in some places, real interior trim elements are also installed ...
26.

There are several salons, as in real Il. There is plenty of free space inside. They say that this model was also used in the development of the interior equipment of the presidential IL-96 cabin ...
27.

On the frame below - the 103rd machine (five-seater IL-103), which has already passed the entire scope of tests, and is now also in this department, sheltered next to its older sister ...
28.

And finally, another general view of the laboratory...
29.

The principle of safe damage. Aircraft flight safety is directly related to the durability of structures.

A design is said to be safe to operate if minimal inspection and repair is required while the basic functions are satisfactorily performed. Satisfactory performance means a low probability of structural failure for civil aviation aircraft, or an acceptably low probability of failure for military aircraft. The safety of passengers and crew of civil aviation aircraft is of paramount importance. Methods for calculating structures that are reliable in operation have been developed mainly for civil aviation aircraft.

The modern aircraft has a semi-monocoque type structure, consisting of thin-walled sheets reinforced with beams (trusses) and stringers to prevent buckling. The outer skin or wall forms the aerodynamic contour of the unit - the fuselage, wing, stabilizer. The stiffeners are attached to the inner surface of the skin and perceive concentrated loads. This design has served as the main object of aerodynamic research for many years and significantly distinguishes the vehicles from conventional building structures.

The required service life of a civil aviation aircraft is determined on the basis of comprehensive economic considerations. It is 10-15 years old. The designer first of all tries to ensure a longer operation of the aircraft without the formation of cracks. To do this, he uses a developed calculation method, with which he minimizes the concentration of stresses and tries to keep the stresses as low as possible, based on the requirements for flight characteristics. For parts that are difficult to repair or replace, the designer may try to provide the required crack-free durability equal to the life of the aircraft. For many structures, this is not feasible. In addition, there is a risk of structural damage from service vehicles, runway rocks, and propeller or engine failure. The designer must minimize the loss of strength due to fatigue cracks or damage during the operation of the aircraft. He solves this problem in the following way:

selects materials and determines the dimensions of parts to ensure adequate structural strength in the presence of cracks;

applies elements of reliability (paths of variable loads and plugs that prevent the development of cracks);

selects materials with a low rate of fatigue crack development.

One of the modern means of improving the reliability of structures while increasing the resource, reducing the consumption of materials and improving economic efficiency is the design and determination of the duration of operation according to the principle of safe damage. This takes into account the presence of initial metallurgical and technological defects in structural elements and the formation of cracks in them as operational damage accumulates.

The development and implementation of the principle of safe damage is possible only with the use of methods of fracture mechanics. Determining the stress-strain state of structural elements containing defects such as cracks is the most critical and complex stage of strength analysis. In accordance with generally accepted concepts, the stress-strain state of a body with a crack is completely characterized by the values of the stress intensity factor. Practically all currently known criteria for brittle and quasi-brittle fracture, as well as dependences describing the growth of fatigue cracks, are based on their preliminary determination.

The concept of "safe damage" refers to a structure designed to minimize the possibility of aircraft failure due to the propagation of undetected defects, cracks or other similar damage. In the production of structures in which any damage is allowed, two main problems have to be solved. These problems consist in ensuring controlled safe growth of defects, i.e., safe operation with cracks, and in forced containment of damage, as a result of which either residual durability or residual strength must be ensured. In addition, the calculation of allowable damage does not eliminate the need for careful analysis and calculation of fatigue.

The basic premise on which the concept of safe damage is based is that defects always exist, even in new designs, and that they can go undetected. Thus, the first defect tolerance condition is the condition that any structural element, including all additional load transfer links, must be capable of safe operation in the presence of cracks.

Control of the safe growth of defects. The occurrence of fatigue cracks can be avoided by creating such a structure, at all points of which the stresses would be below a certain level. However, a decrease in the stress level leads to an increase in the weight of the structures. In addition, cracks can occur not only from fatigue, but also for other reasons, for example, due to accidental damage received during operation, or due to material defects. Therefore, in actual design, some small amount of cracks in the structure are allowed at the time of leaving the factory. Larger of these cracks may develop during service.

The most important element of the principle of safe damage becomes the period of time during which a crack can be detected. Due to various accidents, the probability of detecting a crack during inspection is unstable. Sometimes barely visible cracks are found in the most remote areas of the structure, and at the same time, very large ones can be missed. cracks elsewhere. Thus, there is a case when a crack 1800 mm long under the fairing in the pressurized cabin of the aircraft was missed during the inspection of the Boeing-747.

Therefore, for the structural elements that determine the carrying capacity of the airframe, a failure control program must be drawn up. An important element of a failure control program is the development of test methods. For each element, appropriate verification methods should be developed and proposed. For individual parts of the elements, it may be necessary to use non-destructive testing methods of various sensitivities. The timing of the inspection is set based on the analysis of the available information on the growth of the crack, taking into account the specified initial size of the defect and the size of the detected defect, which depends on the sensitivity of the flaw detection method used. The timing of the inspection should be set on the basis that, provided that the required safety factor is ensured, an undetected defect does not reach a critical size until the next inspection. Usually, the time intervals between successive checks are assigned so that two checks pass before any crack reaches the critical size.

The principle of safe damageability of aircraft structures has necessitated a wider application of methods for non-destructive monitoring of the technical condition of all functional systems. Possibilities of various methods of non-destructive testing for the detection of fatigue cracks. Non-destructive testing methods are constantly being improved.

Fatigue, corrosion and crack resistance. In the practice of aircraft operation, numerous cases of destruction of parts of elements and assemblies due to material fatigue are known. Such failure is the result of variable or repeated loads. Moreover, for fatigue failure, a significantly lower maximum load is required than for static failure. In flight and on the ground, many parts and structural elements of an aircraft are subjected to variable loads and, although stress ratings are often low, stress concentrations that do not normally reduce static strength can lead to fatigue. destruction. This is confirmed by the practice of operating not only aircraft, but also ground vehicles. Indeed, one can almost always observe fatigue failures and very rarely - failures from static loads.

A feature of fatigue fracture is the absence of deformations in the fracture zone. Similar phenomena are observed even in materials such as mild steels, which are highly ductile under static failure. This is a dangerous feature of fatigue failure, since there are no signs that precede failure. Incipient fatigue symptoms are usually very small and difficult to detect until they reach macroscopic size. Then they spread rapidly and in a short period of time complete destruction occurs. Thus, timely detection of fatigue cracks is a difficult task. Most often, fatigue cracks originate in the zone of shape change or surface defects of parts.

Such defects, as well as a small change in the working section of the parts, do not affect the static strength, since plastic deformation reduces the effect of stress concentration. At the same time, during fatigue failure of parts, plastic deformations are usually small, as a result of which there is no decrease in stresses in the concentration zone and the concentration is taken into account stress is essential, so it is important when designing parts operating under variable loads to make them easier and safer in terms of fatigue failure.

Thus, factors influencing fatigue resistance include: stress concentrators, part sizes, the relative importance of static and cyclic loads, as well as corrosion, especially frictional corrosion, which is the result of small repeated movements of two contact surfaces.

Fatigue failures are usually caused by many thousands or millions of loading cycles. However, they can also occur after hundreds or even tens of cycles.

All elements, parts and assemblies of the aircraft are subject to dynamic loads when moving on the ground and in flight. Variable loads of a different nature, acting on structural elements, parts of units and devices, cause the corresponding variable stresses, which ultimately lead to fatigue failures. The rate of processes of mechanical destruction of loaded parts and assemblies, respectively, and the time to failure depend on the structure and properties of materials, on stresses caused by acting loads, temperature and other factors. However, the nature of fracture due to material fatigue has a peculiar form, different from brittle fracture.

Fatigue failure of a part usually begins near a metallurgical or technological defect, a zone of stress concentration, and also in the presence of technological defects in products.

As is known, static destruction is determined mainly by the probability of a large load occurring in flight, for example, from an air blast, as a result of which a load will act on the aircraft that exceeds the static strength limit of the structure, i.e. the possibility of static failure is essentially a matter of the likelihood of a large load occurring.

Fatigue failure under these assumptions is the result of applying a sufficient number of load cycles or a sufficient number of aircraft flights over a certain distance.

The main difference between fatigue and static loading is as follows:

the main factor in fatigue strength for a given load distribution, even with data scatter, is the number of load changes or service life; for static strength and destruction - acting load;

the nature of the probabilistic approach to fatigue loading differs significantly from the nature of the probabilistic approach to static loading - for specific operating conditions, the probability of the impact of a single large load on the aircraft, for example, from an air blast exceeding the static destructive one, does not depend on the operating time. This can happen at the beginning and at the end of the service life. The probability of fatigue failure changes during operation, increasing significantly towards the end of the service life. At the same time, designers and scientists believe that the assigned resource or life limit and the corresponding level of probability should be such that the frequency of occurrence of failure is small enough, which, if possible, would be generally accepted. This probability value is 10 9, which is taken as a basis by leading foreign and domestic aviation firms.

Aviation experts believe that corrosion, like fatigue damage, determines the service life of an aircraft structure to the same extent. Often the sources of corrosion are damage to the structure when loading the aircraft on the ground and scratches on the skin.

It is known that corrosion damage to the structure depends entirely on the operating conditions of the aircraft and the quality of maintenance.

In the instructions, first of all, attention is drawn to the corrosion of the main structural structural elements. It has been established that corrosion is more caused by internal than external factors. So, the cause of corrosion is liquids spilled in the buffet area (especially fruit juices) and toilets.

Areas of the fuselage structure most prone to corrosion and fatigue cracks (shaded).

The least dangerous in terms of fatigue is general (uniform) corrosion. But in real operating conditions, uniform corrosion in its pure form is rare and is usually supplemented by pitting. Effect of such corrosion on fatigue resistance.

It can be seen that, depending on the area and depth of corrosion damage, the fatigue life of the D16T alloy is significantly reduced. In this case, the area of corrosion damage reduces fatigue resistance to a lesser extent than the diameter and depth of corrosion pits.

During operation, the processes of accumulation of fatigue and corrosion damage alternate with partial overlap. It is generally believed that corrosion damage develops in parking lots, while fatigue damage develops in flight. Corrosion damages are stress concentrators.

Provisions and approaches used in the justification of resources within 103 liters. h for 20-25 years of operation, necessitate the use of the progressive principle of "safe damage" in ensuring flight safety at the present stage, along with the principle of "safe resource".

This last principle allows fatigue damage to structural members during the time interval between two successive inspections, provided that this interval is not too long, the damage does not reach its limit state and does not lead to the destruction of the structure as a whole.

Consequently, the criterion of aircraft strength, which states the inadmissibility of cracking, is incorrect for the structure as a whole, since under conditions of long-term operation of aircraft it is practically impossible to avoid fatigue cracks in its individual elements. It is necessary to detect cracks in time and prevent their further development beyond the maximum allowable dimensions.

Thus, the strength resource of an aircraft should be determined on the basis of a strength criterion that takes into account the intensity of the initiation and development of cracks for the structure as a whole and in elements that do not lead to a catastrophic outcome.

There is a concept according to which it is considered that within 30 minutes. 101 l. h security must be ensured, and then up to 60 * 103 l. h - operation is ensured due to the property of the survivability of structures.

Recall that the survivability of an aircraft or its functional systems is understood as a property that ensures the normal performance of the specified functions in flight (or flights) with individual malfunctions or damage to their elements or assemblies. It is provided by the presence of a reserve, specific design solutions that favor a fairly slow development of damage and sufficient strength in the presence of a malfunction, easy accessibility for damage detection and objective control, if possible.

Experience shows that in the process of long-term operation wear of components, fatigue and corrosion damage are the most common failures.

Fatigue cracks lead to a decrease in the strength of the structure and determine its strength reliability. Therefore, when designing, it is necessary to provide for the following conditions: the development and propagation of a crack in structural elements must be so slow that the residual static strength during the development of cracks to the size of its visual detection is sufficient for trouble-free operation of the aircraft without restrictions.

Let us consider some test results of aircraft fuselage skin samples with pressurized cabin. Thus, a diagram of the development of a fatigue crack in the fuselage panels of the DC-10 aircraft is shown. The residual strength of the DC-10 aircraft fuselage was studied on panels measuring 4267 x 2642 mm with a curvature radius of 300 mm. The tests were carried out under conditions of combined loading, simulating inertial loads and boost pressure in the passenger cabin. For this, a panel was taken from the upper part of the skin with an existing initial crack equal to 12 mm. As can be seen, at the first stage of testing at a nominal pressure of 0.65 Pa up to 15,000 cycles, crack growth was practically not observed. After making an incision in the power element and some increase in internal pressure, the crack growth rate began to increase, without reaching, however, a dangerous value. At 46,000 cycles, the destruction of the central frame occurred, followed by the destruction of both frames, which led to a sharp increase in the rate of crack development and the destruction of other power elements. Complete destruction of the panel occurred at a crack length of 1157 mm and at a pressure exceeding 1.53 times the nominal pressure in the cabin.

Similar tests carried out on other panels with a set of load-bearing elements showed the possibility of creating structures of increased survivability and applying the principle of “safe” structure damage with ensuring control of its technical condition during maintenance.

However, fatigue fractures of the fuselage structural elements are the most dangerous. Thus, cracks in the fuselage skin of the Kometa aircraft, which arose near the window cutouts, were the cause of two accidents of aircraft of this type.

The main reason for the appearance of cracks is repeated loads of the fuselage skin with a pressurized cabin of the Kometa aircraft and design flaws. As you know, the aircraft skin experiences repeated tensile-compression loads. They caused the development of cracks in places of stress concentration. No cracks of this type were observed after completion of the skin modifications.

The survivability design allows for certain damage sizes that must meet more general regulatory requirements. So, for example, Douglas believes that the residual strength of a passenger aircraft structure should be ensured with a crack in the wing up to 400 mm long with a stringer destroyed in the middle, and in the fuselage with a longitudinal crack up to 1000 mm long with a titanium stopper destroyed in the middle or with a transverse a crack up to 400 mm long with a spar destroyed in the middle.

Lockheed defines the following permissible damage for the fuselage: a 300 mm long crack is allowed in the skin with a frame or stringer destroyed in the middle; longitudinal crack in the skin - up to 500 mm; a crack extending from the corner of any notch, up to 300 mm with the destruction of one frame or stringer.

The ICAO requirements specify that the minimum level of residual strength of damaged structures should correspond to the value of the maximum operational load equal to 66.6% of the design for the most important design load cases.

GOST 27.002 83 defines durability as the property of an object to remain operational up to a certain state with the installed MRO AT system. The limit state may be due to: fatal violation of flight safety requirements due to a violation of the strength of the structure; fatal departure of the parameters of units and devices beyond the limits of tolerances; irreparable decrease in efficiency; the need to perform a major overhaul in accordance with the current regulatory and technical documentation.

Like reliability, durability is built into the design of an aircraft, is ensured in production, and is maintained during operation. For AT, the durability is determined from the condition of flight safety and the expediency of its further use based on the comparative efficiency and the possibility of replacing it with more advanced models. When designing AT products, possible loads during operation, operating modes are taken into account; choose the appropriate material for parts, processing methods. For elements operating under friction conditions, materials are selected that are most wear-resistant under the expected operating conditions, etc.

All this allows designers not only to create workable structures, but also to carry out appropriate calculations and ensure the required standards of durability of the designed equipment.

Durability as a design property depends on numerous factors that can be divided into strength, operational and organizational.

Strength factors include design, production, technological, load and temperature factors. Among them: stress concentrators in structural elements and residual stresses arising from imperfect technology and due to plastic deformations during assembly and repair; properties of materials and their change during operation, including initial static strength; fatigue limit; stress intensity factor for fractures such as separation and shear.

Experts believe that, using modern achievements in science, engineering and technology, it is possible to ensure the durability of parts of the structure of long-haul aircraft up to 40,103 hp. h. Without the appearance of cracks, the aircraft can fly 30 x x 103 l. h. If we assume that the economically viable resource (or duration of operation) is 60,103 liters. h, then it is guaranteed that approximately half of this period can be provided, and the remaining half of the aircraft will be operated with permissible damage to parts and assemblies and their replacement during repairs.

Recently, I was able to visit a closed shop for static strength testing of aircraft. Such tests are necessary so that the aircraft does not suddenly fall apart in the air from the loads. Having visited this workshop, I realized that not all aircraft are equally strong.

Static testing is an experimental method for studying the stress-strain state and static strength of an aircraft structure. Static testing is carried out to assess the actual strength of an aircraft by testing the structure to failure.

The need for static tests is determined by the fact that the methods for designing and calculating strength of aircraft use, as a rule, some idealized design schemes that differ from the real design. During tests, the values and distribution of design loads acting on the aircraft in various loading cases are reproduced - during maneuvers, during landing, etc.

1. The place made a very strong first impression. I still didn’t really know what would be waiting for me there, and the first thing I saw was a huge IL-76TD (long-range transport). Il-76TD (long-range transport) - civil modification of the Il-76MD. Military equipment was dismantled. The carrying capacity of the machine was 50 tons. The maximum take-off weight was 190 tons. The flight range with a maximum load was 3600 km. First flight 05/05/1982.

2. The workshop is very high. Here you can test both small and large airliners.

3. IL-103 hid behind a large IL. IL-103 - a five-seat single-engine piston passenger aircraft-air taxi. The aircraft was certified to AP-23 IAC in 1996. This is a relatively recent development. I would like to believe that it has successfully passed the tests, and the planes of this model will not fall apart in the air.

4. Next to the cockpit, there are all kinds of recording equipment and a large number of cables coming from the aircraft itself.

5. Supports in the workshop. Massive metal structures served as the basis for testing.

6. View from the fuselage of the IL-76 to the remains of the fuselage of the IL-96-300.

7. These things have airplane wings on them, I think they were tested for reliability here. The wing experiences some of the heaviest loads during the flight, since the aircraft actually hangs on them.

8. Parts of the wings are still on the test frames, albeit covered in a healthy layer of dust.

9. In the booth at the top of the frame, they apparently received data on the test results. This was the heart of the test complex.

10. Racks with some kind of equipment? The purpose of this line of lockers remains a mystery.

11. Wing.

12. There was a huge number of representatives of the cat family in the workshop. They behaved very freely there, apparently with the departure of people they became the main ones in the shop.

13. A small panorama.

15. Wooden plane for processing the ergonomics of the layout and refinement of the drawings.

17. Looked through the glass inside the booths. The feeling that people are just on their lunch break, but a layer of dust in the entire workshop suggests that no one has been here for a couple of years.

18. After inspecting the workshop, he returned to Il and climbed inside.

19. Cabin.

20. Through the hatch climbed onto the fuselage of the aircraft. If you look closely, you can see a bunch of small cat footprints.

21. The axis of symmetry of the aircraft.

22. Fasteners for cargo.

23. Tail.

24. Jacking cart.

25. Once again IL-103.

28. Panoramas.

We were met and given a tour by the deputy head of the laboratory of the complex of strength tests "AK named after S.V. Ilyushin", candidate of technical sciences - Vladimir Ivanovich Tkachenko ...

Vladimir Ivanovich spoke about the types of strength tests, and specifically about the studies that are carried out in this laboratory.

We also looked inside the plane. To create a load on the floor, ring weights are laid in various places ...

The place of the navigator and again the loads on the floor ...

Wire harnesses from instrumentation sensors are stretched throughout the cabin ...

View of the wing through the porthole, entangled in a network of beams, brackets and wires ...

The study of the insides of the aircraft was not without an attentive local "controller";)

Below are two new pylons for PS-90 engines, handed over by the plant for installation and strength testing ...

The entire wing is hung with various levers and counterweights, combined into one common complex system ...

From this cab, located a few meters above the floor, the operator controls the test programs.
There are two such cabins in the hangar ...

Well, then we got acquainted with another amazing aircraft - a wooden full-size model of the Il-96-300, created mainly to solve problems in the layout of the cabin.

Even part of the wing and the engine are recreated on the mock-up...

Not only the interior, but also the external design features are modeled in sufficient detail ...

Having climbed on board, the first thing we look into is the cockpit, because it is also available in this layout ...

There are several salons, as in real Il. There is plenty of free space inside. They say that this model was also used in the development of the interior equipment of the presidential IL-96 cabin ...

On the frame below - the 103rd machine (five-seater IL-103), which has already passed the entire scope of tests, and is now also in this department, sheltered next to its older sister ...

And finally, another general view of the laboratory...