A maintenance interval assessment method for aircraft structural damage

Abstract

The present invention discloses a maintenance interval evaluation method for aircraft structural damage, including: setting Monte Carlo initial simulation conditions; comparing the initial use time with the randomly generated failure time; if the initial time is less than or equal to the failure time, recording the risk level of the structural crack according to the detection of the aircraft structural damage in previous maintenance inspections and the extension length of the crack; calculating the risk probability of the fleet under different levels of risk according to the number of fleet structural loss risk levels obtained; if the risk probability is within a preset range, it indicates that the currently set maintenance interval meets the requirements; otherwise, adjusting the maintenance interval until the risk probability is within the preset range. By evaluating and formulating reasonable, economical and reliable maintenance intervals, the risk of structural damage is controlled within a reasonable range, thereby effectively reducing the maintenance workload, saving maintenance costs, and improving the aircraft integrity rate.

Description

Technical Field

[0001] The invention relates to the technical field of performance analysis, and in particular to a maintenance interval assessment method for aircraft structural damage. Background Art

[0002] Aircraft maintenance is an effective measure to restore flight performance and reduce flight safety risks. Once an aircraft fails in the air, it will threaten the safety of the aircraft and even cause serious consequences such as the destruction of the aircraft and the death of people. Therefore, even if the aircraft has not failed, it is necessary to carry out preventive maintenance on the aircraft after it has been used for a certain period of time to reduce flight risks. At present, the commonly used maintenance method for aircraft is the "regular maintenance" strategy formulated according to the life of the aircraft. However, in order to ensure the safety of the aircraft, the regular maintenance intervals are usually set more conservatively, wasting maintenance resources. For some aircraft that have defects themselves, regular maintenance cannot detect risks in time. Therefore, in order to balance the relationship between aircraft reliability and economy, domestic and foreign scholars have carried out a lot of research and proposed the "condition-based maintenance" method, which has now become the development direction of aircraft maintenance.

[0003] In order to achieve the transformation of aircraft from "periodic maintenance" to "condition-based maintenance", the most important thing is to accurately assess the flight risk of the aircraft. At present, the main assessment methods for aircraft maintenance intervals include iterative algorithms, random process methods, UGF methods, and simulation algorithms. The risk assessment of aircraft structural damage maintenance intervals mainly adopts hierarchical analysis method and fuzzy evaluation method. Most of these methods are based on expert scoring and lack understanding of the risk mechanism of aircraft structural damage.

[0004] Aircraft structural damage is usually caused by fatigue cracks caused by long-term fatigue loads, and has the characteristics of long-term, hidden, and sudden. When simulating aircraft component failures, there are usually only two states: normal and faulty. However, aircraft structural damage is difficult to detect in the early stages of crack expansion. And after an aircraft component fails, it usually directly leads to related functional disorders and is easy to be discovered in time, but aircraft structural damage will not cause structural failure for a long period of time during the initial expansion. And because fatigue cracks are small and often hidden in parts that are difficult to inspect, the detection rate during each inspection is low. However, once the crack expands to a certain extent, the structure will suddenly break, endangering flight safety.

[0005] At present, for aircraft with structural damage, the risk is usually reduced by shortening the maintenance interval of the aircraft. However, shortening the maintenance interval greatly increases the workload of aircraft maintenance on the one hand, and also affects the aircraft's integrity rate on the other hand.

[0006] Therefore, how to accurately assess the risk of maintenance intervals to aircraft structure damage, and thus develop a reasonable, economical, and reliable maintenance interval plan, has become a problem that needs to be solved urgently. Summary of the invention

[0007] In view of this, an embodiment of the present invention provides a maintenance interval assessment method for aircraft structural damage to solve the problem in the prior art that aircraft are regularly inspected and maintained by shortening aircraft maintenance intervals, resulting in a large workload for maintenance and affecting the aircraft's availability.

[0008] An embodiment of the present invention provides a maintenance interval assessment method for aircraft structure damage, comprising:

[0009] Setting the initial Monte Carlo simulation conditions; the initial simulation conditions include the size of the aircraft fleet, the initial service time of each aircraft, the aircraft structure maintenance cycle and the aircraft structure failure probability distribution function;

[0010] Compare the initial usage time with the randomly generated failure time;

[0011] If the initial time is less than or equal to the failure time, the risk level of the structural crack is recorded based on the detection of aircraft structural damage in previous maintenance inspections and the extension length of the crack;

[0012] According to the obtained fleet structure loss risk level times, the risk probability of the fleet under different risk levels is calculated;

[0013] If the risk probability is within the preset range, it indicates that the currently set maintenance interval meets the requirements; otherwise, adjust the maintenance interval until the risk probability is within the preset range;

[0014] If the initial time is greater than the fault time, it means that the aircraft is currently not damaged.

[0015] Optionally, the structural failure probability distribution function of the aircraft is fitted based on the structural crack damage data of the aircraft fleet in historical service:

[0016]

[0017] Where F(t) is the probability distribution function of aircraft structure failure; α and η are Weibull distribution parameters.

[0018] Optionally, the risk level of the structural cracks may include:

[0019] The aircraft structure risk level is determined based on the location and length of the cracks; the aircraft structure risk level is divided into general risk, serious risk and major risk.

[0020] Optionally, the crack extension length is calculated based on the structural crack detection rate, fatigue crack extension equation and fatigue load spectrum; wherein the structural crack detection rate is set separately to simulate the probability of finding aircraft structural cracks during each maintenance inspection; the fatigue crack extension equation is expressed by aN f Curve representation.

[0021] Optionally, after comparing the initial use time with the randomly generated failure time, if the initial time is less than or equal to the failure time, further comprising:

[0022] Calculate the number of crack inspections from the time of structural crack initiation to the time the aircraft is put into service;

[0023] The calculation formula for the number of crack inspections is:

[0024] k = floor(t si / T)-floor(t / T)

[0025] Where, k is the number of crack inspections; t is the structural crack initiation time; t si is the aircraft usage time; T is the aircraft structure maintenance cycle; the floor function is rounded toward negative infinity;

[0026] The condition for judging whether the structural crack is found in k maintenance inspections is:

[0027] If the crack is found in the k'th inspection, the crack extension length a1 of the crack at the k'th inspection is calculated according to the fatigue crack growth equation; where k'≤k;

[0028] If the structural crack is not found in k maintenance inspections, the crack growth rate after service time t is calculated according to the fatigue crack growth equation. si The crack extension length a2 at this time.

[0029] Optionally, the judgment conditions for aircraft structure risk level classification are:

[0030]

[0031] Where a is the crack length.

[0032] Optionally, the fatigue crack growth equation is calculated as:

[0033] Determine the first fatigue crack growth equation of the structure based on fatigue tests of wing structural materials;

[0034] The fatigue load spectrum is obtained by statistical analysis based on the flight training program, and is corrected according to the frequency of large and medium overload tasks of the corresponding aircraft model;

[0035] Obtain the relationship between load peaks and valleys in the fatigue load spectrum and flight time; one flight hour is equivalent to 60 load peaks and valleys.

[0036] Optionally, the aircraft structure maintenance cycle is set to at least 6 types; under different aircraft structure maintenance cycles, the total number of simulations for each aircraft is set to at least 100,000 times.

[0037] Optionally, the risk probability is obtained based on the number of aircraft structure damages obtained through simulation, the average service life of all aircraft in the fleet, and the total number of simulations.

[0038] The beneficial effects of the embodiments of the present invention are as follows: by evaluating and formulating reasonable, economical and reliable maintenance intervals, the risk of structural damage is controlled within a reasonable range, thereby effectively reducing the maintenance workload, saving maintenance costs, and improving the aircraft availability rate. This is of great significance for realizing the transformation of aircraft structures from "regular maintenance" to "condition-based maintenance" and improving the maintainability, reliability and economy of weapons and equipment. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The features and advantages of the present invention will be more clearly understood by referring to the accompanying drawings, which are schematic and should not be construed as limiting the present invention in any way. In the accompanying drawings:

[0040] Figure 1 A flow chart of a maintenance interval assessment method for aircraft structure damage in embodiment 1 of the present invention is shown.

[0041] Figure 2 A line graph showing the aircraft structure damage risk probability at different maintenance intervals in Example 2 of the present invention is shown. DETAILED DESCRIPTION

[0042] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present invention.

[0043] Example 1

[0044] The embodiment of the present invention provides a maintenance interval assessment method for aircraft structure damage, such as Figure 1 As shown, including:

[0045] Step S10, setting the Monte Carlo initial simulation conditions; the initial simulation conditions include the size of the aircraft fleet, the initial use time of each aircraft, the aircraft structure maintenance cycle and the aircraft structure failure probability distribution function.

[0046] In this embodiment, the initial simulation conditions are set as follows: the size of the aircraft fleet is n; the initial use time of the i-th aircraft in the fleet is t si , aircraft structure maintenance cycle T, aircraft structure failure probability distribution function F(t).

[0047] Step S20, comparing the initial use time with the randomly generated failure time.

[0048] In this embodiment, a structural damage risk simulation is performed on the i-th aircraft in the fleet. The structural failure probability distribution function F(t) obeys the uniform distribution of (0, 1), and the structural failure probability of the i-th aircraft is obtained by random sampling, and the structural crack initiation time (i.e., failure time) t in this sampling simulation is calculated by formula (1).

[0049] Step S31, if the initial time is less than or equal to the failure time, then the risk level of the structural crack is recorded according to the detection of aircraft structural damage in previous maintenance inspections and the extension length of the crack.

[0050] In this embodiment, for the i-th aircraft, if t≤t si , it means that the aircraft structure has been damaged, but due to the structural crack detection rate F jc If the crack is too low, the structural damage may not be detected. Therefore, it is necessary to determine whether the structural damage has been detected in previous maintenance inspections and the extension length of the crack.

[0051] Step S32: If the initial time is greater than the fault time, it indicates that the aircraft is not currently damaged.

[0052] In this embodiment, t≥t si , indicating that there is no damage to the aircraft structure.

[0053] Step S41, calculating the risk probability of the fleet at different risk levels according to the obtained fleet structure loss risk level times.

[0054] In this embodiment, multiple sampling simulations are performed on the fleet according to the set total number of simulations, and the number of risk levels to which the cracks belong is calculated according to the structural damage risk level. The number of risk levels corresponding to the structural damage of n aircraft in the fleet is counted.

[0055] If the risk probability is within the preset range in step S51, it indicates that the currently set maintenance interval meets the requirements; otherwise, step S52 is executed to adjust the maintenance interval until the risk probability is within the preset range.

[0056] In this embodiment, for the risk index of aircraft flight safety, it is generally believed that when the flight risk probability is less than 1×10 -7The risk is acceptable when the maintenance interval in the simulation model is adjusted to make the simulation results change to within the acceptable risk range, thereby determining the actual maintenance interval.

[0057] The embodiments of the present invention control the risk of structural damage within a reasonable range by evaluating and formulating reasonable, economical and reliable maintenance intervals, thereby effectively reducing the maintenance workload, saving maintenance costs, and improving the aircraft availability rate. This is of great significance for realizing the transformation of aircraft structures from "periodic maintenance" to "condition-based maintenance" and improving the maintainability, reliability and economy of weapon equipment.

[0058] 1. Analysis of factors affecting aircraft structure damage risk

[0059] 1.1 In reliability theory, the probability of product failure usually conforms to the Weibull distribution. Using the historical service data of crack damage on the fleet structure, the failure probability distribution of the fleet structure is fitted.

[0060]

[0061] Where F(t) is the probability distribution function of aircraft structure failure; α and η are Weibull distribution parameters.

[0062] 1.2 Structural damage to aircraft is usually graded based on the degree of damage and its impact on aircraft flight safety. Based on the location of structural cracks, crack size, etc., the risk level is divided into general risk, serious risk and major risk. At present, there is no clear standard specification for the classification of structural crack risk levels, which is usually determined based on the assessment of equipment support departments and aircraft manufacturers. This article determines the aircraft structure risk level based on the location and length of the cracks.

[0063] 1.3 After an aircraft component fails, it usually directly leads to related functional disorders of the aircraft and is easy to be discovered in time. However, the aircraft structure damage will not lead to structural failure for a long period of time during the initial expansion. In addition, since fatigue cracks are small and often hidden in parts that are difficult to inspect, the detection rate during each inspection is low. Therefore, this paper introduces the detection rate Fjc to simulate the probability of discovering aircraft structure cracks during each maintenance inspection.

[0064] 1.4 Due to the hidden characteristics of aircraft structural damage, the detection rate during each maintenance inspection is low. When the fuselage structural crack is discovered, the crack length has often reached a relatively serious level. This means that during the period of time after the fuselage structural crack has not been detected, although it has undergone multiple maintenance inspections, it is still gradually expanding. Therefore, this paper introduces the fatigue crack growth equation to simulate the length of the crack that continues to grow during the period of time after the crack has been detected. Fatigue crack growth is usually expressed as aN fThe curve shows that according to the fatigue load spectrum of the aircraft structure, the relationship between fatigue cracks and flight time can be obtained. f Indicates the material life.

[0065] Monte Carlo simulation calculation process:

[0066] (1) Set the initial simulation conditions: the size of the aircraft fleet is n; the initial use time of the i-th aircraft in the fleet is t si , aircraft structure maintenance cycle T, aircraft structure failure probability distribution function F(t).

[0067] (2) Perform structural damage risk simulation on the i-th aircraft in the fleet. The structural failure probability distribution function F(t) obeys the uniform distribution of (0, 1). The structural failure probability of the i-th aircraft is obtained by random sampling, and the structural crack initiation time t in this sampling simulation is calculated by formula (1). The structural crack initiation time t is a random number generated by Weibull distribution before each simulation starts.

[0068] (3) For the i-th aircraft, if t≤t si , it means that the aircraft structure has been damaged, but due to the structural crack detection rate F jc If t ≥ t si , it means that there is no damage to the aircraft structure.

[0069] (4) When t≤t si , calculate the time from the structural crack initiation time t to the aircraft service time t si The number of crack inspections up to this point is k.

[0070] k = floor(t si / T)-floor(t / T) (2)

[0071] Where the floor function rounds toward negative infinity.

[0072] Determine whether the structural crack is found in the kth maintenance inspection: If the crack is found in the k'th inspection (k'≤k), the crack extension length a of the crack at the k'th inspection is calculated according to the fatigue crack growth equation; if the structural crack is not found in the kth maintenance inspection, the crack extension length a of the crack at the service time t is calculated according to the fatigue crack growth equation. si The crack extension length a at .

[0073] (5) Repeat steps (2) to (4) to perform multiple sampling simulations, and calculate the number of risk levels N to which the crack belongs based on the structural damage risk level. jc1 、Njc2 、N jc3 、N wjc1 、N wjc2 、N wjc3 . N jc1 、N jc2 、N jc3 Respectively represent the number of times the cracks are detected corresponding to the risk level of general, serious, and major; N wjc1 、N wjc2 、N wjc3 They represent the number of times that undetected cracks correspond to the risk levels of general, severe and major.

[0074] (6) Simulate steps (2) to (4) for other aircraft in the fleet. Count the number of risk levels corresponding to structural damage of n aircraft in the fleet.

[0075] Example 2

[0076] There are 24 aircraft of a certain type at an airport. Due to the tight development time of this type of aircraft, the fatigue load test of the fuselage structure was not carried out in the early stage of development, resulting in frequent structural damage in the fleet. In order to accurately assess the structural damage risk of the fleet and formulate reasonable maintenance intervals, this paper takes the fleet as an example and combines the historical structural damage data of the fleet to carry out risk analysis, and verifies the aircraft structural damage maintenance interval risk assessment method proposed in this paper.

[0077] According to the records of fuselage structural cracks found in the maintenance inspection of the fleet in recent years, it is found that the structural cracks of the fleet are mainly concentrated in the wing structure, so the wing structure is selected as the risk assessment object of this paper. According to the historical data of wing structure damage of the fleet in recent years, the parameters such as crack length and flight hours of the aircraft when the crack was found were selected, and the failure probability distribution parameters of the fleet structure were obtained by fitting, and α = 1.6, η = 600 were obtained.

[0078] Based on the location of structural cracks in the fleet, the size of the cracks, and the degree of impact on flight safety, the risk levels are divided into general risk, serious risk, and major risk.

[0079]

[0080] The crack distribution of the wing of this fleet is relatively hidden. When the crack is first discovered, the crack extension length is long. Therefore, it is often difficult to find it during maintenance inspection, and the crack detection rate is low. According to the crack distribution location and the size of the crack when it is first discovered, the crack detection rate F is set. jc =0.1.

[0081] In this embodiment, the crack detection rate is a fixed value. In an optional embodiment, the crack detection rate can be set separately as needed.

[0082] The fatigue crack growth equation of the structure is determined based on the fatigue test of the wing structure material. The fatigue load spectrum is obtained by statistical analysis based on the flight training outline, and the load spectrum is corrected according to the frequency of large and medium overload missions of this aircraft type to obtain the relationship between the load peak and valley values ​​in the load spectrum and the flight time. One flight hour is equivalent to approximately 60 load peak and valley values. The fatigue crack growth aq equation is then obtained based on the wing structure crack detection data of the fleet:

[0083] a=2.818×10 -6 q 2.565 -2.818×10 -6

[0084] Where q = N f / 60 represents the flight time, and the specific parameters are determined by the characteristics of the aircraft materials.

[0085] The initial use time of the 24 aircraft in the fleet was substituted into the simulation model, and different maintenance intervals T were set. The fleet was simulated using the above parameters. Under different maintenance intervals T, each aircraft was simulated 100,000 times, and the fleet was simulated 2.4 million times. The simulation results are shown in Table 1.

[0086] Table 1 Cumulative number of risk simulations under different maintenance intervals

[0087]

[0088] It can be seen from Table 1 that the maintenance interval T has no effect on the total number of structural damages in the fleet. However, as the maintenance interval T decreases, the number of severe and major risks decreases significantly. This shows that the reduction of maintenance intervals can significantly reduce the severe and major level risks of aircraft structural damage.

[0089] The average service time of the 24 aircraft in the fleet is 486.833 hours. The risk probability of structural damage in the fleet per flight hour can be calculated from the risk probability of structural damage in the fleet. The calculated risk probability of structural damage in the aircraft under different maintenance intervals T is as follows: Figure 2 shown.

[0090] The formula for calculating the probability of aircraft structure damage risk is as follows:

[0091]

[0092] In the formula, F d is the risk probability per flight hour of the fleet aircraft; N d is the number of aircraft structure damages obtained by simulation; t m is the average usage time of all aircraft in the fleet; N is the total number of simulations.

[0093] For the risk index of aircraft flight safety, it is generally believed that when the flight risk probability is less than 1×10- 7 , the risk is acceptable. Figure 2 It can be seen that for the wing structure damage risk of this fleet, when the aircraft maintenance interval T≤4, the probability of major risk is ≤2.14×10 -8 ; When the maintenance interval T≤3, the probability of serious risk≤2.74×10 -8 Therefore, if the aircraft major risk probability is to be guaranteed to meet the requirements, the maintenance interval should be less than 4 hours; if the aircraft serious and major risk probabilities are to be guaranteed to meet the requirements, the maintenance interval should be less than 3 hours.

[0094] Although the embodiments of the present invention have been described in conjunction with the accompanying drawings, those skilled in the art may make various modifications and variations without departing from the spirit and scope of the present invention, and such modifications and variations are all within the scope defined by the appended claims.

Claims

1. A maintenance interval assessment method for aircraft structural damage, characterized in that: include: Set the initial Monte Carlo simulation conditions; The initial simulation conditions include the size of the aircraft fleet, the initial service time of each aircraft, the aircraft structure maintenance cycle and the aircraft structure failure probability distribution function; comparing the initial usage time with a randomly generated failure time; If the initial time is less than or equal to the failure time, the risk level of the structural crack is recorded according to the detection of aircraft structural damage in previous maintenance inspections and the extension length of the crack; According to the obtained fleet structure loss risk level times, the risk probability of the fleet under different risk levels is calculated; If the risk probability is within the preset range, it indicates that the currently set maintenance interval meets the requirements; otherwise, adjust the maintenance interval until the risk probability is within the preset range; If the initial time is greater than the fault time, it indicates that the aircraft is not currently damaged; The crack extension length is calculated based on the structural crack detection rate, fatigue crack extension equation and fatigue load spectrum; wherein the structural crack detection rate is set separately to simulate the probability of finding aircraft structural cracks during each maintenance inspection; the fatigue crack extension equation is expressed by aN f The curve shows that a is the crack length, N f Indicates material life After comparing the initial use time with the randomly generated failure time, if the initial time is less than or equal to the failure time, further comprising: Calculate the number of crack inspections from the time of structural crack initiation to the time the aircraft is put into service; The calculation formula for the crack inspection times is: k=floor(t si / T)-floor(t / T) Wherein, k is the number of crack inspections; t is the structural crack initiation time; t si is the aircraft usage time; T is the aircraft structure maintenance cycle; the floor function is rounded toward negative infinity; The condition for judging whether the structural crack is found in k maintenance inspections is: If the crack is found in the k'th inspection, the crack extension length a1 of the crack at the k'th inspection is calculated according to the fatigue crack growth equation; wherein k'≤k; If the structural crack is not found in k maintenance inspections, the crack growth rate after service time t is calculated according to the fatigue crack growth equation. si The crack extension length a2 at this time.

2. The maintenance interval assessment method for aircraft structure damage according to claim 1, characterized in that: According to the structural crack damage data of the aircraft fleet in historical service, the aircraft structure failure probability distribution function is fitted: Where F(t) is the probability distribution function of aircraft structure failure; α and η are Weibull distribution parameters.

3. The maintenance interval assessment method for aircraft structural damage according to claim 1, characterized in that: The risk levels to which the structural cracks belong include: The aircraft structure risk level is determined based on the location and length of the cracks; the aircraft structure risk level is divided into general risk, serious risk and major risk.

4. The maintenance interval assessment method for aircraft structure damage according to claim 3, characterized in that: The judgment conditions for the aircraft structure risk level classification are: Where a is the crack length.

5. The maintenance interval assessment method for aircraft structure damage according to claim 1, characterized in that: The calculation process of the fatigue crack growth equation is: Determine the first fatigue crack growth equation of the structure based on fatigue tests of wing structural materials; The fatigue load spectrum is obtained by statistically sorting and analyzing the flight training program, and the fatigue load spectrum is corrected according to the frequency of large and medium overload tasks of the corresponding aircraft model; The relationship between the load peak-valley values ​​and the flight time in the fatigue load spectrum is obtained; one flight hour is equivalent to 60 load peak-valley values.

6. The maintenance interval assessment method for aircraft structural damage according to claim 1, characterized in that: The aircraft structure maintenance cycle is set to at least 6 types; under different aircraft structure maintenance cycles, the total number of simulations for each of the aircraft is set to at least 100,000 times.

7. The maintenance interval assessment method for aircraft structure damage according to claim 6, characterized in that: The risk probability is obtained according to the number of aircraft structure damages obtained by simulation, the average service time of all aircraft in the fleet and the total number of simulations.

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