The Aircraft reliability programme is a combination of statistical monitoring and recording of the events associated with the airworthiness of an aircraft. The results obtained by monitoring reliability in operation may serve as a basis for supplementing or modifying the aircraft maintenance programme; such changes would indicate the malfunction of components or systems manifesting lacks and the need for early control, or replacement during utilization.
In the aviation industry maintenance is considered, the key parameter for successful airlines. Noteworthy efforts needed to achieve maximum aircraft utilization with a safety level as high as possible and with minimal operative and ecological costs. To achieve increased profitability, airline needs to monitor the reliability of an aircraft, its components and systems.
The primary purpose of any aircraft reliability programme is the collection of data and information and their statistical analysis to get valuable feedback with the data on irregularities developed in the operation. The programme of reliability monitoring serves as a supplement to the standard maintenance programme of an aircraft in order to maintain its continuous airworthiness.
Parameters for Monitoring Aircraft Reliability
For monitoring the aircraft reliability technical data obtained in operation as well as regular and extra inspections of an aircraft, its systems and components are recorded. The data collected in that way are the basis for each reliability monitoring and the conclusions obtained and based on them depend on their accuracy and so the total result. The obligation of collecting the data lies with the air carrier according to the requirements of aviation regulations for keeping continuous airworthiness of aircrafts EASA PART M* or CAMO or Continuous Airworthiness Department.
The parameters monitored for aircraft reliability in operation are the following:
Figure 1: The “Iceberg” of parameters for monitoring aircraft reliability. Source: Airbus Customer Services: Reliability Control Program Training.
After collecting the data for a given month, data is statistically analysed, and aircraft availability and reliability is obtained. The parameters are to be monitored as a large whole, so that the reliability programme would be as effective as possible. This may be well illustrated by an “iceberg”, Figure 1.
In Figure 1 it is evident that the reports and complaints (of pilot/technical staff) themselves do not indicate which problem has the largest effect on aircraft operations and that operative interruptions (technical occurrence, interrupts in flight, failures) show only the tip of the “iceberg” and not the cause of the problem.
Interpretation of the Results of Reliability Monitoring
When all the data for a given month are collected, calculations follow showing the reliability level of the aircraft, its components and systems. The systems and the components of an aircraft are classified according to ATA-100 specification, which is a standard documentation for all commercial aircraft. According to ATA-100 specification, an aircraft is divided into systems and subsystems. The systems are constituted of a power plant, structure, electric systems and similar. Subsystems include particular parts, which together make one system. They are all specified by a multi-digit ATA number, where the subsystems of one system have the same two digits as the ATA number of the system they form.
Our Definitions & Formulae:
AIRCRAFT DAYS: It is the number of days aircraft is available for the period. Aircraft Days = No. of Aircraft in Fleet X Total days in the Month – Total days of Aircraft in ground
AIRCRAFT IN SERVICE: It is the average aircraft available per day for the period. Aircraft in Service = Sum of Aircraft Days / Total Days in the Month
FLIGHT HOURS: The flight hours are airborne flight hours (Take OFF - Touch DOWN)
AVERAGE AIRCRAFT UTILIZATION: It is the average flying hours per day per aircraft considering aircraft in-service only. Average Aircraft Utilization = Total Flight Hours / Aircraft Days
FLEET FLIGHT HOURS PER LANDING: It is the average flight hours per landing (Flight Hours / Flight Landings)
PILOT REPORT (PIREP): It is a known or suspected malfunction or defect as reported by flight crew in the Aircraft Technical Log book.
PIREP RATE: It is the average PIREP per 1000 revenue flight hour. Pireps Rate = (Total No. of Pireps / Total Flight Hours) x 1000
CANCELLATION: A cancellation is counted when a scheduled flight does not occur. Cancellation of a multi-segment flight is counted as a single cancellation.
TECHNICAL DISPATCH RELIABILITY: It is the probability of scheduled flights departing on time due to technical reasons. Dispatch reliability = 100 – (No. of Delays x 100 / Total Flights)
FLIGHT COMPLETION RATE: Flight Completion Rate = 100 – (No. of Cancellation x 100 / Total Flights)
AVERAGE FLIGHT DURATION: Duration in minute of average flight during reporting period. Calculated by dividing fleet revenue flight hrs. by fleet cycles and multiplying by 60 minutes.
ON TIME DEPARTURE: The percentage of schedule flights which depart without mechanical delays of greater than 14 minutes or cancellations
AVERAGE FLIGHT CYCLES: The percentage of flights capable of performing schedule service. When several consecutive flights are delayed or cancelled due to single condition by one event is counted. The average no. of flight cycles performed per A/C per period of time (cycles/month)
OCCURANCE: The report of some actions such as inspect, repair, remove or replace, service, calibrate or adjust being accomplished on a component or system.
R/R: Removed and Replaced
ATBO (AVERAGE TIME BETWEEN OCCURANCE): The average flight time a component has experienced before maintenance was required. ATBO is calculated by dividing the sum of the time in service on each component by the number of reported occurrences.
MTBUR: Mean Time Between Unscheduled Removals (in hours), calculated as: =Flight hours*Quantity per Aircraft / Number of Unscheduled Removals
MTBF: Mean Time Between Failures (in hours), calculated as: =Flight hours*Quantity per Aircraft / Number of Confirmed Removals
System Defect Rates and Alert Levels are calculated in accordance with UK CAA CAP 562 Leaflet 1-7 (formerly CAP 418) "Condition Monitored Maintenance" 3.8.3 (e) (i), Calculation 2 (see also Appendix B Example 1 of the same leaflet. The Alert levels are taken as the mean of the three-monthly moving average Pirep rates plus three (3) standard deviations.
Analysis of Reliability Monitoring by Manufacturer
The air carrier will monitor the reliability of aircraft in operation and send the collected data to the aircraft manufacturer. Four times a year, on the basis of the collected data received from air carriers, the manufacturer issues a report on the reliability for a certain aircraft type available to all air carriers having that type of aircraft in their fleet. In the relevant report, collected and analysed data indicating reliability of aircraft components and system are presented. This is of great importance to the air carrier, since this can draw attention to an increased trend of failures of particular components, the untimely checking of which and, as needed, possible early service or replacement could cause non-airworthiness of aircraft due to technical malfunction. Each of the cases influences the safety of operations and represents high costs – hence, it is in the interest of the air carrier to prevent them.
The problems occurred during aircraft operation plays a big role and is of high importance in aircraft maintenance. Information that the air carrier can and must deliver to the aircraft manufacturer gives the manufacturer a possibility to collect and process data for a large number of aircraft seen as a fleet. On the basis of these data the manufacturer may identify the systems and/or components due to which problems in aircraft operation most often arise. If a defect in manufacture is established, modifications and installation of new components are introduced by the aircraft manufacturer, which should provide an increase of reliability in maintenance and operation. Also, an air carrier is obliged to deliver reports on reliability monitoring to aviation authorities as a proof that aircraft structure, power plant, its components and other systems are capable of achieving and maintaining reliability levels high enough, which is the basis for safety of carrying out aircraft operations.
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