Safety in design risk zoning

All of the interactions that occur every day have some level of risk involved in them. This risk can be thought of as a series of bands that surround any object that could interact with another object.

The zones can be though of as expanding 3D bubbles that surround objects. These zones can be identified in different ways, however the most useful way is to grade them as:

  • negligible
  • low
  • medium
  • high
  • extreme

The zones are not static throughout the entire period that an object exists, but are in fact dynamic relative to both the objects and influences from the environment they relate to. For example zones can expand or contract when different parameters such as speed or time change, and include qualitative factors such as colour and fatigue (in humans, components, etc). The easiest way to explain the principle surrounding risk zoning is to use a series of examples to demonstrate the zoning of risk and how it can change dynamically

Example

The initial example relates to aircraft and how they can have different risk profiles depending on the properties they take and the environment they relate to.

1. Firstly think about an aircraft that is sitting on the ground that is switched off and is not being used. When you think about the risk profiles associated with negligible, low, medium, high, and extreme you can see that the general area around the entirety of the aircraft:

  • would be negligible in risk up until the surface of the aircraft
  • the low, medium, high, and extreme zones would all lie immediately on the surface of the aircraft

2. Then think about the aircraft as it is taxiing along the taxiways, as the aircraft does so the dimensions of the negligible, low, medium, high, and extreme zones will all gradually increase with an increase in groundspeed. But they will also be different for based on elements such as:

  • different types and sizes of aircraft
  • different pilot (including factors such as experience, fatigue, awareness, etc)
  • fauna (such as birds, etc)
  • weather (is it raining, sunny, icy, etc)
  • etc.

3. The aircraft then takes off with the risk zones increasing in dimension until a plane levels out and cruises along. At this point the aircraft will have relatively stable risk zones. However these zones will vary based around:

  • different types and sizes of aircraft
  • different pilot (including factors such as experience, fatigue, awareness, etc)
  • fauna (such as birds, etc) depending on the altitude
  • weather (is it raining, sunny, icy, turbulence etc)
  • etc.

The aircraft risk zones could look something like those shown in Figure 01 below

2 SID risk zoning_aircraft risk zones_flying

Figure 01: Aircraft risk zones when an aircraft is flying

The thing to remember is that while these zones can be considered relatively stable in a simple sense, the reality is that they are dynamic and will change throughout the flight depending on the object and the influences from the environment. For example, a pilot may be suitably refreshed immediately after reaching the cruising altitude, but a short time later may begin to experience fatigue. It is anticipated that the pilot who remains fresh would have a smaller set of risk zones than a fatigued pilot.

4. But the risk zones can also change when considering a change in the properties associated with the aircraft such as speed. As an aircraft increases its speed, the aircraft risk zones would begin to expand because the aircraft can cover more distance in the same than it could at a slower speed. This is shown in Figure 02 below.

2 SID risk zoning_aircraft risk zones_effect of increasing speed

Figure 02: Effect of an increase in speed on risk zoning