Reliability Prelim Exams

Inter-Rater or Inter-Observer Reliability

Used to assess the degree to which different raters/observers give consistent estimates of the same phenomenon.

Test-Retest Reliability

Used to assess the consistency of a measure from one time to another.

Parallel-Forms Reliability

Used to assess the consistency of the results of two tests constructed in the same way from the same content domain.

Internal Consistency Reliability

Used to assess the consistency of results across items within a test.

Average Inter-Item Correlation

The mathematical mean of the correlation coefficients calculated between every possible pair of items on a test.

Average Item-Total Correlation

The average of the correlation coefficients between each individual item's score and the overall test score.

Split-Half Correlations

The correlation between scores calculated from two equal halves of a single test.

Cronbach’s Alpha (α)

Formulated by American psychologist Lee Cronbach in 1951.

Cronbach’s Alpha (α)

A metric representing the mathematical average of all possible split-half reliability coefficients for a given assessment.

Maintenance Program

A document which describes the specific maintenance tasks and their frequency of completion, necessary for the continued safe operation of those aircraft to which it applies.

Maintenance Program

Establishes compliance with instructions for continuing airworthiness issued by type certificate holders, supplemental type certificate holders, and organizations that publish data in accordance with Part 21 (Airworthiness and Environmental Certification), instructions issued by the competent authority, or instructions defined by the owner or operator.

Part 21

Airworthiness and Environmental Certification.

Process-Oriented Approach

A bottom-up approach that begins with the specific and moves to the general.

Task-Oriented Approach

A top-down approach that goes from the general to the specific.

Task-Oriented Approach

Uses predetermined maintenance tasks to avoid in-service failures.

Maintenance Steering Group (MSG) Approach

Began in 1968 (747) with representatives from Boeing’s design and maintenance groups, suppliers, airlines desiring to purchase aircraft, and the FAA using a bottom-up review to determine process usage.

MSG-2

A process-oriented, bottom-up approach that introduced the "condition monitored maintenance" concept.

MSG-3

First published in 1980, introducing a top-down, task-oriented approach focusing on "consequences of failure".

Hard Time (HT)

The removal of an item at a predetermined interval (hours, cycles, calendar time).

Hard Time (HT)

Requires an item to be removed and either completely overhauled, partially overhauled (restored), or discarded before exceeding the specified interval.

On-Condition (OC)

Item will be checked at specific intervals (hours, cycles, or calendar time).

On-Condition (OC)

Requires an item to be periodically inspected or tested to determine if it can continue in service—overhaul, restore, or replace—without performing a teardown inspection.

Condition Monitoring (CM)

Monitors failure rates, removal rates, etc. to facilitate maintenance planning.

Condition Monitoring (CM)

Involves the monitoring of failure rates, removals, etc. of individual components or systems that do not have a definite lifetime or noticeable wear-out period, operating them until failure occurs.

Airframe

The body of an aircraft as distinct from its engine.

Zonal Analysis

The systematic inspection of the geographical locations of the components and interconnections of a system, evaluation of potential system-to-system interactions with and without failures, and assessment of the severity of potential hazards inherent in the system installation.

Maintenance Tasks for Structural Items

Actions—or sets of actions—a maintenance technician is supposed to perform to complete a work order.

Environmental Deterioration

Deterioration caused by climate or environment, which may be time dependent.

Accidental Damage

Damage resulting from human error or impact with an object.

Fatigue Damage

Crack or cracks due to loading or stress.

Inspection

Involves checking something, i.e., examining and assessing something.

General Visual Inspection

Visual examination that will detect obvious conditions or discrepancies.

Detailed Inspection

Use of inspection aids (i.e., mirrors, hand lenses) that may require surface cleaning and detailed access.

Special Detailed Inspection

Use of Nondestructive Inspection (NDI): dye penetrant, high-powered magnification, magnetic particle, or eddy current.

Zonal Maintenance Tasks

Ensures all systems, components, and installations within a specified zone receive adequate screening, security of installation, and general condition assessment.

Maintenance Review Board Report (MRBR)

Contains the initial scheduled maintenance program for US certificated operators.

Maintenance Review Board Report (MRBR)

Provides guidelines for the development of the Maintenance Review Board (MRB) process from which the aircraft maintenance program is derived.

Maintenance Planning Document (MPD)

Contains all maintenance task information from the MRBR report plus additional tasks by the airframe manufacturer.

Maintenance Planning Document (MPD)

Provided by aircraft manufacturers to describe the repetitive tasks that are required to maintain their aircraft.

Monthly Maintenance Plan

Used to identify the projected maintenance workload such as inspections, receipts/transfers, technical directive compliance, etc.

Monthly Maintenance Plan

Provides requirement data for aircraft maintenance, Support Equipment (SE) material, manpower, and training under the cognizance of the Maintenance Officer (MO).

Transit Check

Pre-flight and turn checks consisting of visual inspection, checking for open/loose panels, and fluid leaks.

48-Hour Checks

"Daily" checks inspecting wheels, brakes, fluid levels, and hydraulic fluid.

Hourly Limit Checks

Checks performed at specific operational hours (e.g., 100, 200, 250) for engines and flight control systems.

Operating Cycle Limit Checks

Checks performed based on operating cycles targeting tires, brakes, landing gear, and airframe structures.

Letter Checks

Maintenance intervals divided into A, B, C, and D checks (eliminated in MSG-3 during 777 development).

A Check

Performed approximately every 400–600 flight hours, or every 200–300 flights, depending on aircraft type.

B Check

Performed approximately every 6–8 months, taking about 160–180 man-hours, and usually completed within 1–3 days at an airport hangar.

C Check

Performed approximately every 20–24 months, or a specific number of actual flight hours (FH), or as defined by the manufacturer.

D Check

Sometimes known as a "heavy maintenance visit" (HMV), by far the most comprehensive and demanding check for an airplane.

Components to Reliability

Probability

Intended function

Satisfactory

Specific period of time

Specified conditions

IEC 60050

Who DEFINITION OF RELIABaILITY:

“ability of an item to perform a required function under given conditions for a given time interval”

Smith, D.J. (2005)

Who DEFINITION OF RELIABILITY:

“… the probability that an item will perform a required function without failure under stated conditions for a stated period of time…”

RELIABILITY

The probability that an item/system will perform a required function without failure under stated conditions for a stated period of time.

RELIABILITY

expressed as the number of failures over a period

RELIABILITY

It is the probability that a product or service will operate properly for a specified period of time (design life) under the design operating conditions without failure

First generation maintenance (pre-WW2)

• Industry not highly mechanized so downtime not particularly important.• Most equipment simple and over-designed

• Only systematic maintenance required - simple cleaning, servicing and lubrication.

Second generation maintenance (WW2 to mid-70’s)

• More mechanisation and concern over downtime.

• Idea that failures could and should be prevented.

• Awareness of “burn-in” failure mode

• Increase in maintenance costs as a proportion of total operating costs.

• Philosophy: Preventative overhauls

3rd generation maintenance (mid-70’s +)

• Aircraft data identifies 6 different failure mode patterns.

• Growth of mechanisation and automation increases focus on plant availability and reliability.

• Effect of failures on HS&E becomes important as regulations tighten.

• Cost of maintenance increases: Often 1st or 2nd highest of all operating costs.

Predictive Maintenance

To predict failures

Asset Management

 Optimise maintenance decisions

survival probability

The probability of an item that does not fail in a time interval (0,t)

failure rate

Mean number of failures in a given time

Mean time between failures (MTBF)

the average time between successive failures

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Mean time to failure (MTTF)

the average time that elapses until a failure occurs.

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Objectives of Reliability Engineering

• To apply engineering knowledge and specialist techniques to prevent or reduce the likelihood or frequency of failure.

• To identify and correct the causes of failures that do occur, despite of the efforts to prevent them.

• To determine ways of coping with failures that do occur, if their causes have not been corrected.

• To apply methods for estimating the likely reliability of new designs, and for analysing reliability data.

Fault Tree Analysis (FTA)

Failure Mode and Effects Analysis (FMEA)

Failure Mode, Effects, and Criticality Analysis (FMECA)

Qualitative reliability Theories

• Statistical reliability

• Physics-based reliability

Quantitative reliability Theories

• Time-based maintenance

• Condition-based maintenance

Maintenance Types

• makes routine inspections, documents inspections and repairs

• performs scheduled maintenance

• runs tests after making repairs or performing maintenance

Role of a Mechanic

develop new technological solutions

• includes defining problems

• conducting and narrowing research

• analyzing criteria

• finding and analyzing solutions and making decisions.

Role of an Engineer (Design)

Maintenance

The objective of maintenance is to maintain the assets of a company so that they meet reliability needs at an optimal cost.

Maintenance

Webster's Dictionary definition: to maintain, keep in existing condition, preserve, protect, or keep from failure or decline.

Optimal Reliability

The ultimate goal of maintenance designed to directly meet the business needs of the company.

Breakdown maintenance

A reactive maintenance strategy where people wait until equipment fails (including partial failure where a machine is not anymore functional) and repair it.

Preventive maintenance

Daily proactive maintenance activities (cleaning, inspection, oiling, and re-tightening) designed to retain the healthy condition of equipment and prevent failure by addressing deterioration early through periodic inspections or diagnoses.

Periodic maintenance (Time based maintenance - TBM)

A preventive maintenance method consisting of periodically inspecting, servicing, and cleaning equipment and replacing parts to prevent sudden failure and process problems.

Predictive maintenance

A condition-based maintenance method where the service life of an important part is predicted based on inspection or diagnosis to use the part to its maximum limit.

Predictive maintenance

A method that manages trend values by measuring and analysing data about deterioration and employs a surveillance system designed to monitor conditions through an on-line system.

Corrective maintenance

Improving equipment and its components so that preventive maintenance can be carried out reliably, which involves redesigning equipment with design weaknesses to improve reliability or maintainability.

Corrective maintenance

Planned or unplanned tasks performed when there is a detection during monitoring, when potential faults are uncovered during inspection, or during a breakdown.

Maintenance prevention

A design-phase practice indicating the design of new equipment, where weaknesses of current machines are sufficiently studied (on site information leading to failure prevention, easier maintenance, prevention of defects, safety, and ease of manufacturing) and are incorporated before commissioning new equipment.

Zero Hours Maintenance (Overhaul)

The scheduled set of preventive and corrective tasks aimed at reviewing equipment at scheduled intervals before appearing any failure, or when equipment reliability has decreased considerably making capacity forecasting risky.

Zero Hours Maintenance (Overhaul)

A review based on leaving equipment at zero hours of operation (as if it were new) by repairing or replacing all items subject to wear to ensure, with high probability, a good working time fixed in advance.