2 Terms associated with probabilities

 Different undesirable events may have different orders of acceptable probability. In connection with this, it is convenient to agree on standardized expressions to be used to convey the relatively acceptable probabilities of various occurrences, i.e. to perform a qualitative ranking process.

  2.1 Occurrences

  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:

  • .3.1 any undetected failure that is already present;

  • .3.2 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 Probability of occurrences

  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 shall be considered as being possible.

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

  2.3 Effects

  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 shall 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.

  2.4 Safety level

Safety level is a numerical value characterizing the relationship between craft performance represented as horizontal single-amplitude acceleration (g) 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 shall be as defined in table 1.