Exposure

 Exposure

By Catalina9

Exposure in the Safety Management System is an integrated part of a risk assessment and risk analysis. A risk assessment involves several steps and forms the backbone of an overall risk oversight plan. Included in a risk assessment is one or several risk analyses to determine the defining characteristics of each hazard and to assign risk level scores based on the analysis. Key components of a risk analysis are likelihood, severity and exposure. Likelihood is a definition of times between intervals of an active hazard, severity is a defined outcome of the occurrence, and exposure is the variable, defined as common cause variation or special cause variation and a assigned a function, or weight score, between 0 to 1. If the exposure is zero, the hazard does not exist or has been eliminated. When the exposure is one, the impact of a hazard is inevitable.

When common cause variations are treated as special cause variations, the risk analysis has taken the wrong turn at the fork in the road. Common cause variations are integrated in a process, they are necessary for the process and the process would fail if one or more common cause variations were eliminated. An example of common cause variation is ice in clouds and thunderstorms. For ice to form on an aircraft in flight, the air must be cold and contain moisture. Icing conditions frequently occur when moist air is forced upward. As the air rises, it expands and cools. If the air cools to the saturation point, where the temperature equals the dew point, the moisture will condense into clouds or precipitation. For ice to form there must be clouds or precipitation and icing can be most intense near the cloud tops, where the amount of liquid water is often greatest. This part of the cloud has the greatest amount of lifting, cooling, and condensation. Encountering inflight icing in clouds is therefore a common cause variation, while non inflight icing in clouds is a special cause variation.

When applying exposure in a risk analysis the task is to analyse in 3D and measured

in time (hours-minutes-seconds), space (geographical location) and compass (direction). A 3D analysis is to analyse a moving object within the tube itself, rather than from behind, below, above, beside or in front of a moving object. A 3D analysis is the expected view as observed by the pilot at a specific moment in time, location, and direction.

Exposure paints a picture of the past to plan for the future.

The first step when analyzing exposure is to determine if the variation is a common cause variation or a special cause variation. When traveling to or from work, people conduct a mental exposure analysis by leaving at a certain time to avoid the heaviest traffic. In aviation common cause variation analyses are also conducted for arrivals at major airports or during special events. Comprehension of systems is therefore vital to correctly identify the true variation and develop the proper corrective action plan. If encountering inflight icing in clouds was assigned as a special cause variation with a root cause analysis, the analysis would be derailed from the beginning. The analysis could easily take a turn to explain that icing in clouds were not in the forecast. While this might be true, does not make it a special cause variation, since icing in clouds is to be expected anytime an aircraft is flying above freezing level. 

If the freezing level was lower than forecasted still makes it a common cause variation, since this is what the freezing levels do every day. The forecasted freezing level is nothing else but a risk assessed model of what altitude the level might be in the future. Pilots and dispatches often blindfolded accept icing and freezing level computer models, which then could be mistaken for a special cause variation. Level of exposure changes with time, location, and direction of an aircraft. An aircraft on the ground has a zero-exposure level to inflight icing. The exposure level begins when the reach the rotation speed. Inflight icing could be from ice accumulated on the ground and the exposure level for inflight icing is therefore 1,or 100% likelihood, or probability, that the ice will affect aircraft performance. 

One reason for ground de-icing and anti-icing is to reduce the exposure level of inflight icing to an acceptable level and defined as holdover time. Research of anti-ice fluids has determined that the fluid remains effective for a short period of time and when an aircraft is airborne prior to the time expires, the exposure probability, or likelihood, to inflight icing is inconceivable, or times between intervals are imaginary, theoretical, virtual, or fictional.

After it has been determined that a variation is a common cause, the next step is to analyse how the hazard could be exposed, or how the hazard could affect operations. If the hazard is icing in cloud, the analysis shows a likelihood of 1 that flight into known icing will expose the aircraft to inflight icing. The analysis is both a part of the pre-flight planning and inflight operational observations. The severity of icing is determined by several conditions, but for the purpose of icing when entering clouds at a flight level above freezing level, the likelihood of exposure is methodical, planned and dependable, without defining the operational system or processes involved. When analysing flight crew, aircraft and expected level of icing severity, available operational systems play a role. Encountering icing may vary from a level of informational, which is a severity level that is not compatible with another fact or claim of the hazard, to catastrophic which is a severity level where functions, movements, or operations cease to exist, or it could be any level between these two extreme severity levels. Exposure level in SMS is a pre-flight, or pre-task operational tool with actions defined in applicable safety cases.

Special cause variation Beatty NV 1981-03-18

The third step and an analysis of level of exposure to a special cause variation is a totally different approach, since a

special cause variation is unexpected, it is an abnormal condition and a variation that is irrelevant for the process to function as expected. A special cause variation could be a

malfunctioning ITT or Inter Turbine Temperature during takeoff. Special cause variations are excluded from pre-flight planning since they are items covered by other levels of protections. When using the ITT example above, aircraft engines are regularly inspected and found acceptable, or it is removed from the aircraft if unacceptable. When the pilot takes off, the engine is expected to perform as it should without malfunctioning. However, a principle in aviation is to expect the best but to be prepared for the worst. Preparing for the worst at every takeoff is not exposure to an engine failure or other system failures but is a part of an ongoing recurrent training program. Below is an example of how a malfunctioning ITT is identified in a control chart and when this is identified a root cause analysis must be performed. Normally the ITT is running 680°, but one day it was 681°.

This variation did not trigger an incident or ITT exceedance, but it is a variation that is not common within the system itself and must be investigated with a root cause analysis. Exposure levels triggers two actions: The first action is to prepare for common cause variations and the second action is to conduct a root cause analysis of a special cause variation.

Catalina9