The Study on Dispatch Reliability Prediction Model of Civil Aircraft

Dispatch reliability is the key parameters to reflect the overall reliability of civil aircraft, and it is also the important technical parameters needs to be considered in the original design of civil aircraft. According to the actual delay rate data of airlines, combined with the relationship between delay rate and dispatch reliability, from the perspective of civil aircraft design, author firstly puts forward that seven design parameters of the aircraft design phase, the design parameters consist of five main parameters and two auxiliary parameters, and then this paper establishes a mathematical relationship between dispatch reliability and aircraft original design parameters, the dispatch reliability of civil aircraft was predicted, prediction model is reasonable and has higher precision accuracy, it laid the theoretical basis and data support for dispatch reliability prediction of our country civil aircraft, meanwhile the prediction model of dispatch reliability can provide reference data for the structure design of civil aircraft.


INTRODUCTION
Dispatch reliability is the only important parameters to characterize the civil aircraft dispatch reliability, and is an important parameter to characterize the whole machine reliability of civil aircraft. Dispatch reliability directly affects the aircraft itself inherent reliability and route operational reliability. Aircraft dispatch reliability is higher, nonscheduled parking time of civil aircraft is less, the day utilization rate is higher, the reliability of flight plan is higher, at the same time the punctuality of flights will be higher. Therefore, in the aircraft original design and development stage, in-depth analysis, accurate and reliable prediction of dispatch reliability is conducted and adequate model validation is made.
Dispatch reliability is a comprehensive parameters evaluating the whole characteristic of civil aircraft, at the same time, the civil aviation transportation is a collection of airline, airport, air traffic control. Three parts must have a collaborated operation. Therefore, the consideration factors of analysis and verification about dispatch reliability is very complex. The consideration factors contain the aircraft's own technical factors, air traffic control operation environment, airport security operation, airlines management. On the one hand, dispatch reliability is affected by its aircraft structure design to a large extent; On the other hand, dispatch reliability needs to combine many non-technical factors such as actual operation environment, maintenance and security conditions.
At present the research on the dispatch reliability of civil aircraft just stay in reliability statistics report of airlines or statistics value about regular operating data in our country [1], in the civil aircraft design stage, we don't have formed to the statistics and evaluation method of dispatch reliability, so we don't have combined the dispatch reliability with aircraft original concept design. Therefore, in the civil aircraft design stage, we shall predict the dispatch reliability level of civil aircraft during the operational phase through the structural design factors, and to determine aircraft design factors affecting flight dispatch based on the expected process and results, to adjust and improve the design method and to increase the aircraft dispatch reliability level, ensure that the economic and commercial value of civil aircraft [2].

Definition of Dispatch Reliability
Dispatch reliability:the flight delays or cancellations caused by non-mechanical failure causes (technical reason) is divided by all the flight numbers, it is a percentage. Sometimes it referred to as the attendance reliability. In the numerical valve, dispatch reliability =1-delays and cancellations rate.
Delays and cancellations rate is during the specified operating hours (or specified cumulative operating landing number), delays and cancellations due to technical reasons is divided by the total the number of operating flights landing. Technical delays and cancellations rate is due to mechanical failure caused by the aircraft itself, it is also known as the ground interrupt rate (ground abnormal rate, aircraft failure technical reasons).

Data Collection
Due to the delay and cancellation rate dispatch reliability numerically exists corresponding relationship. So this paper firstly make a technical investigation on the maintenance engineering department and operational control ministry of domestic airline.
According to the airline's main operational production practices and maintenance reliability management programs, four kinds of short mainstream aircrafts are identified as collection objects, respectively, B737-800, A320, E190, MA60 the reliability data about the four kinds of different short-range models in four different domestic airlines is collected. Airline's annual reliability analysis report contains statistical delay rate data involved ATA subsystem chapters. Delay cancellation rate data are reliable statistics of airlines, it is the actual operating data, the airline's maintenance engineering department is responsible for statistical summary, the aim is monitoring fleet reliability, it is for realtime fleet reliability data trend analysis.

Original Design Parameters
In order to analyze delays and cancellations rate statistically, to establish mathematical relationship between the practical operation data (delays and cancellations rate) and the aircraft itself design parameters. According to the design concept of civil aircraft, the original design parameters were classified, it is divided into five main parameters and two auxiliary parameter, the specific parameters were seen in Table 1 [3].
The five main design parameters mainly cover the basic technical parameters of civil aircraft, including the maximum take-off weight, net thrust, flight time, cruising speed, passenger seats; the two auxiliary design parameters are involved into ratio, contain the ratio of the maximum takeoff weight to flight time, the ratio of the passenger seats to flight time.
Then the design parameters of four types of collection aircraft were defined, the specific value of 7 design parameters about four kinds of plane models were obtained. The 7 design parameters were a fixed value of statistical models, the design parameters of B737-800 model as an example, see Table 2.

Correlation Analysis
The correlation analysis between actual delay rate statistics data of each ATA section and 7 design parameters was conducted, we can conclude that the correlation degree between each ATA section and which design parameters is highest, then each ATA section and related factors were fitted, prediction model formula between delay rate of each ATA section and certain related design factors was obtained. Correlation analysis of each ATA section, see Table 3.
Through correlation analysis, we can draw that the correlation degree between each ATA section and which related factors is maximum, the correlation coefficient r is larger, the greater the correlation degree, delay rate is identified as the dependent variable, design parameters is identified as the independent variable, we can Show that the mathematical relationship between the delays rate and design parameters is closely, the established mathematical expressions were more reliable and accurate. The largest correlation coefficient is 0.976, it can explain that the correlation between the fire prevention system as twentysixth chapter and the passengers' number is largest [4].
The subsystem of maximum correlation is ATA26 fire protection system, ATA35 oxygen system, and ATA80 engine, the correlation coefficients were 0.976, -0.963, -0.949. The correlation coefficients of most subsystem are above 0.5, it shows that the correlation between the actual delay rate value of most subsystem and design parameter is obvious, the correlation degree is high. Only the minimum correlation coefficient of automatic flight system is -0.163 [5].

Prediction Model
Through linear fitting method, the function between delay rate data of each ATA section and related design factors was established [6]. Through the numerical test, fitting degree is higher, it shows that the function expression is reasonable. Delay rate prediction formula of each ATA subsystem is shown in Table 4.
From Table 4 it can be found that fitting degree R2 of most subsystem is higher, correlation analysis can be seen that, the larger relevance of subsystems, the higher the fitting degree, the higher fitting degree of the three sub-systems were ATA26 fire protection systems, ATA35 oxygen system, ATA80 engine, fitting degree was 0.9537, 0.9281, 0.9015 [7,8].
By fitting formula listed in Table 4, the relationship between delay rate and related design factors of each ATA subsystem can be predicted, thereby the delay rate predicted value were obtained. it can pave the way for the next model validation [9,10].

EXAMPLE VERIFICATION
In order to verify the rationality and accuracy of the prediction rate model, based on the 7 design parameters of mainstream models of B737-800 aircraft, we can predict delay rate, we compare the unity between delay rate prediction value with the actual value, Thus we can draw the whole dispatch reliability.
Through the comparison between the actual value and the predicted value of B737-800 delay rate data, from Table 5 it is shown that the delay rate predicted data of various subsystems is basically close to the actual delay rate, because the dispatch reliability=1-ground interrupt rate=1-delay rate, it can draw the relative error between prediction value of the dispatch reliability and the actual value is small, the prediction method about the whole dispatch reliability through the relationship between the delay rate of ATA subsystem and design parameters is feasible, prediction value of dispatch reliability is accurate.

CONCLUSION
According to several main design factors of civil aircraft affecting dispatch reliability, by the actual sample statistics data, it makes a prediction and test on the dispatch reliability using linear regression method, the actual operation data of airline aircraft with main design parameters were effectively combined, the correlation and function relation were found out among them, the design parameters are explanatory variables, delay rate are explained variable. The quantitative relationship between delay rate data and the design parameters was established. Because of delay rate and dispatch reliability have a certain relationship, Dispatch reliability and design parameters have also the same linear relation. The linear regression prediction model of delay rate was constructed. The delay rate was predicted to get the dispatch reliability of certain aircraft, it can put a theory basis for the next step to distribute dispatch reliability.
Finally on the basis of the B738 validation instance, through statistical correlation test it is shown that, the obtained prediction model is correct, the prediction model has certain scientific value and the validity, The research results provide a scientific and reasonable method for the actual dispatch reliability prediction. In the stage of aircraft original design, we can adjust design parameters in order to improve the dispatch reliability, reduce the aircraft delay rate.