"AT RISK TIME": The period of time during which an item must fail in order to cause the failure effect in question. This is usually associated with the final fault in a fault sequence leading to a specific failure condition.

ACCIDENTAL DAMAGE (AD): Physical deterioration of an item caused by contact or impact with an object or influence which is not a part of the aircraft, or by human error during manufacturing, operation of the aircraft, or maintenance practices.

AGE EXPLORATION: A systematic evaluation of an item based on analysis of collected information from inservice experience. It verifies the item's resistance to a deterioration process with respect to increasing age.

AIRWORTHINESS: The condition of an item (aircraft, aircraft system, or part) in which that item operates in a safe manner to accomplish its intended function.

AIRWORTHINESS LIMITATIONS: A section of the Instructions for Continued Airworthiness that contains each mandatory replacement time, structural inspection interval, and related structural inspection task. This section may also be used to define a threshold for the fatigue related inspections and the need to control corrosion to Level 1 or better. The information contained in the Airworthiness Limitations section may be changed to reflect service and/or test experience or new analysis methods.

ANALYSIS: An evaluation based on decomposition into simple elements.

APPROVAL: The act of formal sanction of an implementation by a certification authority.

APPROVED: Accepted by the certification authority as suitable for a particular purpose (ICAO).

ASSESSMENT: An evaluation based upon engineering judgement.

ASSUMPTION: Statements, principles and/or premises offered without proof.

ASSURANCE: The planned and systematic actions necessary to provide adequate confidence that a product or process satisfies given requirements (RTCA DO 178B).

: The organization or person responsible within the State (Country) concerned with the certification of compliance with applicable requirements.

AVAILABILITY: Probability that an item is in a functioning state at a given point in time.

CERTIFICATION: The legal recognition that a product, service, organization, or person complies with the applicable requirements. Such certification comprises the activity of technically checking the product, service, organization or person, and the formal recognition of compliance with the applicable requirements by issue of a certificate, license, approval, or other documents as required by national laws and procedures.

CERTIFICATION AUTHORITY: Organization or person responsible for granting approval on behalf of the nation of manufacture.

COMMON CAUSE: Event or failure which bypasses or invalidates redundancy or independence.

COMMON CAUSE ANALYSIS: Generic term encompassing Zonal Analysis, Particular Risks Analysis and Common Mode Analysis.

COMMON MODE FAILURE: An event which affects a number of elements otherwise considered to be independent.

COMPLEXITY: An attribute of systems or items which makes their operation difficult to comprehend. Increased system complexity is often caused by such items as sophisticated components and multiple interrelationships.

COMPLIANCE: Successful performance of all mandatory activities; agreement between the expected or specified result and the actual result.

COMPONENT: Any self-contained part, combination of parts, subassemblies or units, which perform a distinct function necessary to the operation of the system.

CONDITIONAL PROBABILITY OF FAILURE: The probability that a failure will occur in a specific period provided that the item concerned has survived to the beginning of that period.

CONFORMITY: Agreement of physical realization of the item with the defining document.

CORROSION LEVEL 1: Corrosion damage that does not require structural reinforcement or replacement. Or Corrosion occurring between successive inspections exceeds allowable limit but is local and can be attributed to an event not typical of operator usage of other aircraft in the same fleet (e.g. Mercury spill).

CORROSION PREVENTION AND CONTROL PROGRAM (CPCP): A program of maintenance tasks implemented at a threshold designed to control an aircraft structure to Corrosion Level 1 or better.

CRITICALITY: Indication of the hazard level associated with a function, hardware, software, etc., considering abnormal behavior (of this function, hardware, software, etc.) alone, in combination or in combination with external events.

DAMAGE TOLERANT: A qualification standard for aircraft structure. An item is judged to be damage tolerant if it can sustain damage and the remaining structure can withstand reasonable loads without structural failure or excessive structural deformation until the damage is detected.

DEFECT: State of an item consisting of the non-performance of specified requirements by a characteristics of the item. A defect may, but need not, lead to a failure.

DELAMINATION / DISBOND: Structural separation or cracking that occurs at or in the bond plane of a structural element, within a structural assembly, caused by in service accidental damage, environmental effects and/or cyclic loading.

DEMONSTRATION: A method of proof of performance by observation.

DERIVED REQUIREMENTS: Additional requirements resulting from design or implementation decisions during the development process. Derived requirements are not directly traceable to higher level requirements; though derived requirements can influence higher level requirements.

DESIGN: The result of the design process.

DESIGN PROCESS: The process of creating a system or an item from a set of requirements.

DEVELOPMENT ASSURANCE: All those planned and systematic actions used to substantiate, to an adequate level of confidence, that development errors have been identified and corrected such that the system satisfies the applicable certification basis.

DEVELOPMENT ERROR: A mistake in requirements determination or design.


To be direct, the functional failure or resulting secondary damage must achieve its effect by itself, not in combination with other functional failures (no redundancy exists and it is a primary dispatch item). Adverse Effect on Safety: Safety shall be considered as adversely affected if the consequences of the failure condition would prevent the continued safe flight and landing of the aircraft and/or might cause serious or fatal injury to human occupants. Operating: This is defined as the time interval during which passengers and crew are on board for the purpose of flight.

DISCARD: The removal from service of an item at a specified life limit.

ECONOMIC EFFECTS: Failure effects which do not prevent aircraft operation, but are economically undesirable due to added labor and material cost for aircraft or shop repair.

ELECTRICAL WIRING INTERCONNECTION SYSTEM (EWIS): An electrical connection between two or more points including the associated terminal devices (e.g., connectors, terminal blocks, splices) and the necessary means for its installation and identification.

ENVIRONMENTAL DETERIORATION (ED): Physical deterioration of an item's strength or resistance to failure as a result of chemical interaction with its climate or environment.

ERROR: An occurrence arising as a result of an incorrect action or decision by personnel operating or maintaining a system. (JAA AMJ 25.1309) (2) A mistake in specification, design,or implementation.

EVENT: An occurrence which has its origin distinct from the aircraft, such as atmospheric conditions (e.g., wind gusts, temperature variations, icing, lighting strikes), runaway conditions, cabin and baggage fires. The term is not intended to cover sabotage. (JAA AMJ 25.1309) Note: This definition, as it is stated here, describes an “External Event”. There are other uses of “event” that covers other aspects (e.g., FTA events).

EXCHANGED FUNCTION: Interdependencies between functions.

EXPOSURE TIME: The period of time between when an item was last known to be operating properly and when it will be known to be operating properly again.

FAILURE: A loss of function or a malfunction of a system or a part thereof. Note: This differs from the ARP 4754 definition and conforms to the AC/AMJ 25.1309 definition.

FAILURE: The inability of an item to perform within previously specified limits.

FAILURE CAUSE: Why the functional failure occurs.

FAILURE CONDITION: A condition with an effect on the aircraft and its occupants, both direct and consequential, caused or contributed to by one or more failures, considering relevant adverse operation or environmental conditions. A Failure Condition is classified in accordance to the severity of its effects as defined in FAA AC 25.1309-1A or JAA AMJ 25.1309.

FAILURE EFFECT (FE): A description of the operation of a system or an item as the result of a failure; i.e., the consequence(s) a failure mode has on the operation, function or status of a system or an item.

FAILURE MODE (FM): The way in which the failure of an item occurs. FAILURE RATE: The gradient of the failure distribution function divided by the reliability distribution function at time t. λt = F’(t)/(1-F(t)) If the failure distribution function is exponential, the failure rate is constant and the failure rate can be approximately calculated by dividing the number of failures within a hardware item population, by the total unit operating hours. Note: Failure rate could also be expressed in terms of failures per flight hour or per cycle.

FAILURE RATE: The gradient of the failure distribution function divided by the reliability distribution function at time t. λt = F’(t)/(1-F(t)). If the failure distribution function is exponential, the failure rate is constant and the failure rate can be approximately calculated by dividing the number of failures within a hardware item population, by the total unit operating hours. Note: Failure rate could also be expressed in terms of failures per flight hour or per cycle.

FATIGUE DAMAGE (FD): The initiation of a crack or cracks due to cyclic loading and subsequent propagation.

FATIGUE RELATED SAMPLING INSPECTION: Inspections on specific aircraft selected from those which have the highest operating age/usage in order to identify the first evidence of deterioration in their condition caused by fatigue damage.

FAULT: An identifiable condition in which one element of a redundant system has failed (no longer available) without impact on the required function output of the system (MSI). At the system level, a fault is not considered a functional failure.

FAULT: An undesired anomaly in an item or system.

FAULT-TOLERANT SYSTEM: A system that is designed with redundant elements that can fail without impact on safety or operating capability. Redundant elements of the system may fail (fault), but the system itself has not failed. Individually, and in some combinations, these faults may not be annunciated to the operating crew, but by design the aircraft may be operated indefinitely with the fault(s) while still satisfying all certification and airworthiness requirements.

FUNCTION: The normal characteristic actions of an item.

FUNCTIONAL CHECK: A quantitative check to determine if one or more functions of an item performs within specified limits.

FUNCTIONAL FAILURE: Failure of an item to perform its intended function within specified limits.

FUNCTIONAL HAZARD ASSESSMENT (FHA): A systematic, comprehensive examination of functions to identify and classify Failure Conditions of those functions according to their severity.

GUIDELINES: Recommended procedures for complying with regulations.

HARDWARE: An object that has physical being. Generally refers to LRUs, circuit cards, power supplies, etc.

HAZARD: A potentially unsafe condition resulting from failures, malfunctions, external events,errors, or a combination thereof.

HIDDEN FUNCTION 1: A function which is normally active and whose cessation will not be evident to the operating crew during performance of normal duties. 2. A function which is normally inactive and whose readiness to perform, prior to it being needed, will not be evident to the operating crew during performance of normal duties.

IMPLEMENTATION: The act of creating a physical reality from a specification.

INDEPENDENCE: (1) A design concept which ensures that the failure of one item does not cause a failure of another item. (Derived from JAA AMJ 25.1309.) (2) Separation of responsibilities that assures the accomplishment of objective evaluation.

INHERENT LEVEL OF RELIABILITY AND SAFETY: That level which is built into the unit and, therefore, inherent in its design. This is the highest level of reliability and safety that can be expected from a unit, system, or aircraft if it receives effective maintenance to achieve higher levels of reliability generally requires modification or redesign.

INSPECTION: An examination of an item against a specific standard.

INSPECTION - DETAILED (DET): An intensive examination of a specific item, installation or assembly to detect damage, failure or irregularity. Available lighting is normally supplemented with a direct source of good lighting at an intensity deemed appropriate. Inspection aids such as mirrors, magnifying lenses, etc. may be necessary. Surface cleaning and elaborate access procedures may be required.

INSPECTION - GENERAL VISUAL (GVI): A visual examination of an interior or exterior area, installation or assembly to detect obvious damage, failure or irregularity. This level of inspection is made from within touching distance unless otherwise specified. A mirror may be necessary to enhance visual access to all exposed surfaces in the inspection area. This level of inspection is made under normally available lighting conditions such as daylight, hangar lighting, flashlight or drop-light and may require removal or opening of access panels or doors. Stands, ladders or platforms may be required to gain proximity to the area being checked.

INSPECTION - SPECIAL DETAILED (SDI): An intensive examination of a specific item, installation, or assembly to detect damage, failure or irregularity. The examination is likely to make extensive use of specialized Inspection Techniques and/or equipment. Intricate cleaning and substantial access or disassembly procedure may be required.

INSPECTION - ZONAL: A collective term comprising selected general visual inspections and visual checks that is applied to each zone, defined by access and area, to check system and powerplant installations and structure for security and general condition.

INTEGRATION: (1) The act of causing elements of an item to function together. (2) The act of gathering a number of separate functions within a single implementation.

INTERVAL (INITIAL - REPEAT): Initial Interval - Interval between the start of service-life and the first task accomplishment. Repeat Interval - The interval (after the initial interval) between successive accomplishments of a specific maintenance task.

ITEM: Any level of hardware assembly (i.e., system, sub-system, module, accessory, component, unit, part, etc.)., One or more hardware and/or software elements treated as a unit.

L/HIRF: Lightning/High Intensity Radiated Field L/HIRF Characteristics Those properties of L/HIRF protection components that are necessary to perform their intended L/HIRF protection function(s). L/HIRF Protection Components Any self-contained part, combination of parts, subassemblies, units, or structures that perform a distinctive function necessary to provide L/HIRF protection. L/HIRF Protection Systems Systems comprised of components that avoid, eliminate, or reduce the consequences of an L/HIRF event.

LATENT FAILURE: A failure which is not detected and/or annunciated when it occurs.

LETTER CHECKS: Letter checks are named collections of tasks (e.g., A-Check, C-Check, etc.) assigned the same interval.

LUBRICATION AND SERVICING: Any act of lubricating or servicing for the purpose of maintaining inherent design capabilities.

MAINTENANCE SIGNIFICANT ITEM (MSI): Items identified by the manufacturer whose failure a. could affect safety (on ground or in flight), and/or b. is undetectable during operations, and/or c. could have significant operational impact, and/or d. could have significant economic impact

MALFUNCTION: The occurrence of a condition whereby the operation is outside specified limits.

MULTIPLE ELEMENT FATIGUE DAMAGE :The simultaneous cracking of multiple load path discrete elements working at similar stress levels.

MULTIPLE SITE FATIGUE DAMAGE: The presence of a number of adjacent, small cracks that might coalesce to form a single long crack.

NON - METALLICS: Any structural material made from fibrous or laminated components bonded together by a medium. Materials such as graphite epoxy, boron epoxy, fiber glass, Kevlar epoxy, acrylics and the like are nonmetallics. Non-metallics include adhesives used to join other metallic or non-metallic structural materials.

NOVELTY: Applicable to systems using new technology and to systems using a conventional technology not previously used in connection with the particular function in question.


Qualified flight compartment and cabin attendant personnel who are on duty. Normal Duties a. Procedures and checks performed during aircraft operation in accordance with the Aircraft Flight Manual. b. Recognition of abnormalities or failures by the operating crew through the use of normal physical senses (e.g., odor, noise, vibration, temperature, visual observation of damage or failure, changes in physical input force requirements, etc.).

OPERATIONAL CHECK: An operational check is a task to determine that an item is fulfilling its intended purpose. Does not require quantitative tolerances. This is a failure finding task.

OPERATIONAL EFFECTS: Failure effects which interfere with the completion of the aircraft mission. These failures cause delays, cancellations, ground or flight interruptions, high drag coefficients, altitude restrictions, etc.

OTHER STRUCTURE: Structure which is judged not to be a Structural Significant Item. "Other Structure" is defined both externally and internally within zonal boundaries.

P To F INTERVAL: Interval between the point at which a potential failure becomes detectable and the point at which it degrades into a functional failure.

POTENTIAL FAILURE: A defined identifiable condition that indicates that a degradation process is taking place that will lead to a functional failure.

PRELIMINARY SYSTEM SAFETY ASSESSMENT (PSSA): A systematic evaluation of a proposed system architecture and implementation based on the Functional Hazard Assessment and failure condition classification to determine safety requirements for all items.

PRODUCT: An item generated in response to a defined set of requirements.

PROTECTIVE DEVICE: Any device or system that has a function to avoid, eliminate or reduce the consequences of an event or the failure of some other function.

REDUNDANCY: Multiple independent means incorporated to accomplish a given function.

REDUNDANT FUNCTIONAL ELEMENTS:  Two or more independent physical elements of a system/item providing the same function.

RELIABILITY: The probability that an item will perform a required function under specified conditions, without failure, for a specified period of time.

REQUIREMENT: An identifiable element of a specification that can be validated and against which an implementation can be verified.

RESIDUAL STRENGTH: The strength of a damaged structure.

RESTORATION: That work necessary to return the item to a specific standard. Restoration may vary from cleaning or replacement of single parts up to a complete overhaul.

RISK: The frequency (probability) of occurrence and the associated level of hazard.

SAFE LIFE STRUCTURE: Structure which is not practical to design or qualify as damage tolerant. Its reliability is protected by discard limits which remove items from service before fatigue cracking is expected.

SAFETY (ADVERSE EFFECT): Safety shall be considered as adversely affected if the consequences of the failure condition would prevent the continued safe flight and landing of the aircraft and/or might cause serious or fatal injury to human occupants.

SAFETY/EMERGENCY SYSTEMS OR EQUIPMENT: A device or system that: 1) enhances the evacuation of the aircraft in an emergency or, 2) if it does not function when required, results in a Failure Condition that might have an adverse effect on safety.

SCHEDULED MAINTENANCE CHECK: Any of the maintenance opportunities which are prepackaged and are accomplished on a regular basis.

SCHEDULED STRUCTURAL HEALTH MONITORING (SSHM): The act to use/run/read-out a SHM device at an interval set at a fixed schedule

SEGREGATION: The maintenance of independence by means of a physical barrier between two hardware components.

SEPARATION: The maintenance of independence by means of physical distance between two hardware components.

SIMILARITY: Applicable to systems similar in characteristics and usage to systems used on previously certified aircraft. In principle, there are no parts of the subject system more at risk (due to environment or installation) and that operational stresses are no more severe than on the previously certified system.

SOFTWARE: Computer programs, procedures, rules, and any associated documentation pertaining to the operation of a computer system.

SPECIFICATION: A collection of requirements which, when taken together, constitute the criteria which define the functions and attributes of a system, or an item.

STRUCTURAL ASSEMBLY: One or more structural elements which together provide a basic structural function.

STRUCTURAL DETAIL: The lowest functional level in an aircraft structure. A discrete region or area of a structural element, or a boundary intersection of two or more elements.

STRUCTURAL ELEMENT: Two or more structural details which together form an identified manufacturer's assembly part.
Structural Function The mode of action of aircraft structure. It includes acceptance and transfer of specified loads in items (details /elements /assemblies) and provides consistently adequate aircraft response and flight characteristics.

STRUCTURAL HEALTH MONITORING (SHM): The concept of checking or watching a specific structural item, detail, installation or assembly using on board mechanical, optical or electronic devices specifically designed for the application used. SHM does not name any specific method or technology

STRUCTURAL SIGNIFICANT ITEM - (SSI): Any detail, element or assembly, which contributes significantly to carrying flight, ground, pressure or control loads and whose failure could affect the structural integrity necessary for the safety of the aircraft.

SYSTEM: A combination of inter-related items arranged to perform a specific function(s).

SYSTEM SAFETY ASSESSMENT (SSA): A systematic, comprehensive evaluation of the implemented system to show that the relevant requirements are met.

SYSTEM SAFETY ASSESSMENT PROCESS: The complete process applied during the design of the system to establish safety objectives and to demonstrate compliance with FAR/JAA 25.1309 and other safety related requirements.

TASK APPLICABILITY: A set of conditions that leads to the identification of a task type when a specific set of characteristics of the failure cause being analyzed would be discovered and/or corrected as a result of the task being accomplished.

TASK EFFECTIVENESS: A specific set of conditions that leads to the selection of a task already identified to be applicable. Avoids, eliminates, or reduces the negative consequences of the failure to an extent that justifies doing the task at the selected interval.

TASKS - MAINTENANCE: An action or set of actions required to achieve a desired outcome which restores an item to or maintains an item in serviceable condition, including inspection and determination of condition.

THRESHOLD : See "Interval - Initial".

THRESHOLD PERIOD: A period during which no occurrences of the failure can reasonably be expected to occur after the item enters into service.

VALIDATION: The determination that the requirements for a product are sufficiently correct and complete.

VERIFICATION: The evaluation of an implementation to determine that applicable requirements are met.

VISUAL CHECK: A visual check is an observation to determine that an item is fulfilling its intended purpose. Does not require quantitative tolerances. This is a failure finding task.

WEAR DAMAGE: Physical deterioration of the surface of an item due to relative motion between two parts in contact.

ALD Reliability and Safety Tools Overview