2 Terms associated with probabilities

  2.1.1 "Occurrence" is a condition involving a potential lowering of the level of safety.

  2.1.2 "Failure" is an occurrence in which a part, or parts, of the craft fail or malfunction, e.g. runaway. A failure includes:

• .1 a single failure;

• .2 independent failures in combination within a system;

• .3 independent failures in combinations involving more than one system, taking into account:

  • .1 any undetected failure that is already present;

  • .2 such further failures^footnote as would be reasonably expected to follow the failure under consideration; and

• .4 common cause failure (failure of more than one component or system due to the same cause).

  2.1.3 "Event" is an occurrence which has its origin outside the craft (e.g. waves).

  2.1.4 "Error" is an occurrence arising as a result of incorrect action by the operating crew or maintenance personnel.

  2.2.1 "Frequent" is one which is likely to occur often during the operational life of a particular craft.

  2.2.2 "Reasonably probable" is one which is unlikely to occur often but which may occur several times during the total operational life of a particular craft.

  2.2.3 "Recurrent" is a term embracing the total range of frequent and reasonably probable.

  2.2.4 "Remote" is one which is unlikely to occur to every craft but may occur to a few craft of a type over the total operational life of a number of craft of the same type.

  2.2.5 "Extremely remote" is one which is unlikely to occur when considering the total operational life of a number of craft of the type, but nevertheless should be considered as being possible.

  2.2.6 "Extremely improbable" is one which is so extremely remote that it should not be considered as possible to occur.

  2.3.1 "Effect" is a situation arising as a result of an occurrence.

  2.3.2 "Minor effect" is an effect which may arise from a failure, an event, or an error, as defined in 2.1.2, 2.1.3, 2.1.4, which can be readily compensated for by the operating crew. It may involve:

• .1 a small increase in the operational duties of the crew or in their difficulty in performing their duties; or

• .2 a moderate degradation in handling characteristics; or

• .3 slight modification of the permissible operating conditions.

  2.3.3 "Major effect" is an effect which produces:

• .1 a significant increase in the operational duties of the crew or in their difficulty in performing their duties which by itself should not be outside the capability of a competent crew provided that another major effect does not occur at the same time; or

• .2 significant degradation in handling characteristics; or

• .3 significant modification of the permissible operating conditions, but will not remove the capability to complete a safe journey without demanding more than normal skill on the part of the operating crew.

  2.3.4 "Hazardous effect" is an effect which produces:

• .1 a dangerous increase in the operational duties of the crew or in their difficulty in performing their duties of such magnitude that they cannot reasonably be expected to cope with them and will probably require outside assistance; or

• .2 dangerous degradation of handling characteristics; or

• .3 dangerous degradation of the strength of the craft; or

• .4 marginal conditions for, or injury to, occupants; or

• .5 an essential need for outside rescue operations.

  2.3.5 "Catastrophic effect" is an effect which results in the loss of the craft and/or in fatalities.

 "Safety level" is a numerical value characterizing the relationship between craft performance represented as horizontal single amplitude acceleration (g) and rate of acceleration (g/s) and the severity of acceleration-load effects on standing and sitting humans.

The safety levels and the corresponding severity of effects on passengers and safety criteria for craft performance should be as defined in table 1.