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The probability that a system will be able to perform its designated function when required for use.

Equipment Elements

The Site & Building Infrastructure, Building Plant, ATS Operational Equipment, Human Interface, External Services and Facilities elements of a system.


A discrepancy between a computed, observed or measured value or condition and the true, specified or theoretically correct value or condition. An error is that part of a system state that is liable to lead to a failure. A failure occurs because there is an error in the system.


Termination of the ability of a system or component to perform a required function.

Functional Failure Mode

A way a function can fail. Functional Failure Modes are used in the derivation of safety requirements. Functional failure modes appropriate to most systems include Loss, Corruption, Delay and Misdirection.


The ability of an item under given conditions to be retained in, or restored to, a state in which it can perform the required function more effectively.

People Elements

The ATS Personnel, both Engineering and ATC together with relevant training and development functions in their role as elements of a system.

Procedure Elements

The Airspace sectorisation, Engineering maintenance and ATC procedural elements of a system.

Random Error

An error that is introduced at random into the system. For example, errors caused by component failure or violation of maintenance instructions.

Random Failure

A failure that appears at random. This may be due to random error, or to a combination of systematic error and random exposure of the error. This includes pseudo-random errors, which are entirely deterministic but for which the combination of circumstances.


The probability that during a certain period of time a system will perform the functions described in the requirements under the stated conditions.


The probability of occurrence, together with the severity of the consequences, of a hazardous event.


Freedom from unacceptable risk.

Safety Assessment

The undertaking of an investigation, in order to arrive at a judgment, based on evidence, of the functional safety achieved by a system.

Safety Case

A documented account of the evidence, arguments and assumptions to show that system hazards have been identified and controlled, both in engineering and operational areas, and that qualitative and quantitative safety requirements have been met.

Safety Plan

Documentation of a safety programme that is planned, integrated and developed in conjunction with other design, development, production and quality control activities.

Safety Related Software

Software that implements functions, the failure of which may comprise a hazard.

Safety Related System

A system which:
  * Implements functions necessary to achieve or maintain a safe state; and
  * Is intended to achieve, on its own or with other safety related systems or external risk reduction facilities, the level of safety integrity necessary for the implementation of the required functions


Programs, procedures, data rules and associated documentation pertaining to the operation of a computer system.


A combination of physical components, procedures and human resources organised to achieve a function. Systems encompass a number of elements including: *Site and building infrastructure;
* Building plant;
*ATS operational equipment;
* Human interface;
* Airspace sectorisation and ATS procedures;
* ATS personnel;
* Training and development functions;
* External services and facilities.

Systematic Error

An error that is always present in a given version of a system. For example, software design errors or maintenance procedure errors.

Systematic Failure

A failure that is not a random failure.


Confirmation by examination and provision of objective evidence that the requirements have been fulfilled. Validation is generally used to refer to a larger process than verification (see below). In particular, whereas verification tests against specifications, validation is concerned with whether the operation of the system provides the results needed by the users. Validation therefore includes the consideration of whether the specification of a system sufficiently and accurately represents the needs of the intended user.


Confirmation by examination and provision of objective evidence that the requirements have been fulfilled. Verification is generally used to refer to the process of determining whether the product of each phase of the development process is consistent with the requirements specified in a previous phase.

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Last updated: 07-08-2005.