Real SPC vs coloured trend bars:
reading the Risk Weather.

See the highlighted block above the contents list. The rest of this article explains why a coloured tile is not a signal, what statistical process control actually computes, the rules that flag a real shift, and how eAviora renders the result as a Risk Weather view.

Most safety dashboards show each indicator as a tile: red, amber or green, often with a small up or down arrow. It looks decisive. The trouble is what the colour actually means. A red/amber/green tile encodes exactly two facts — the number moved, and the latest value is above or below a fixed line someone chose when the dashboard was set up.

That is not a signal. It is a comparison against an arbitrary line. Three problems follow directly:

• It cannot see normal variation.Every indicator wobbles month to month. A go-around rate that bounces between 3 and 7 per thousand landings is not improving in the good months or failing in the bad ones — it is just varying. A coloured tile flips between green and amber on that wobble and manufactures alarm where there is none.
• The threshold is a guess. Someone picked the amber line. Set it too tight and every indicator screams; set it too loose and a genuine drift sits green for months. Either way the colour reflects the line someone drew, not the behaviour of the operation.
• It hides the shape of the change.A single bad month and six consecutive months creeping the same direction can show the same colour. One is noise; the other is the operation genuinely drifting. The tile cannot tell them apart — so neither can the person reading it.

The result is a board that someone has to eyeball and interpret. It tells you a number went up. It does not tell you whether the change is real. That is the gap real statistical process control closes.

Statistical process control (SPC) is a long-established method for telling ordinary variation apart from a real change in the thing being measured. It does not ask anyone to pick a line. It builds the line from the data.

For each Safety Performance Indicator (SPI) — a quantifiable measure of an aspect of safety, such as unstable approaches per thousand sectors — SPC produces a control chart:

• A centre line:the indicator's typical level, computed from its own history. This is what “normal” looks like for this indicator, not a number borrowed from a template.
• Upper and lower control limits:the boundaries of normal variation, also computed from the indicator's own data. As long as the indicator stays inside these limits and shows no out-of-pattern run, the operation is behaving as it always has.

The point of the limits is that they are earned, not assumed. An indicator with a lot of natural month-to-month spread gets wider limits; a stable one gets tighter limits. The chart adapts to how each indicator actually behaves, which is exactly what a fixed colour threshold cannot do.

ICAO Doc 9859, the Safety Management Manual, points operators to this kind of analysis precisely because it removes the argument about where to draw the line. The data draws it. The only question left is the right one: has the indicator stayed inside its own normal envelope, or has it genuinely shifted?

eAviora computes the control limits for each SPI from your own data — not from a benchmark or a guessed threshold — which is what makes the difference between a real special-cause signal and a coloured tile.

A control chart on its own is a picture. What turns it into a signal is a fixed set of tests applied to every new month — the Western Electric Rules. They look for the patterns that mean “this is no longer the same process”, each one a different shape of real change rather than noise:

• A point past the limit. A single month falls outside the upper or lower control limit. The strongest single-month signal: a value this extreme is very unlikely to be ordinary variation.
• A run on one side.Several consecutive months all sit on the same side of the centre line. No single month is alarming, but a sustained run on one side is the operation drifting to a new level — the pattern a coloured tile most often misses.
• A steady trend.A run of months stepping the same direction — up or down — month after month. A consistent climb is a real movement even while every individual month still looks within range.
• Clustering near a limit.Several recent months bunch up close to a control limit. The indicator is creeping toward the edge of normal — an early warning before a point actually crosses it.

The discipline of the rules is the whole point. They are applied the same way every month, to every indicator, so an analyst does not have to squint at a chart and decide by feel whether something looks wrong. A single bad month that is just variation does not trip a run rule; a quiet, sustained drift that no tile would flag does. That is the difference between a number going up and an indicator having genuinely shifted — a real special-cause signal.

eAviora applies these rules to every SPI in line with ICAO Doc 9859 and evaluates them monthly, so the operation always knows which indicators have actually moved — not which ones happen to be the wrong colour this month.

Real SPC produces honest signals, but a list of which indicators tripped which rule is not how a safety leader wants to read risk. eAviora renders the result as Risk Weather— a synoptic view of safety risk built to be read the way a pilot reads a weather chart.

Pilots already read the SIGWX chart: the significant-weather chart that lays out, at a glance, where the real turbulence and hazards sit across a region. It works because it shows where to look, not a colour on every square of the map. Risk Weather borrows that idea for safety:

• It surfaces the genuine shifts.Because the underlying analysis is real SPC, the view foregrounds the indicators where a special-cause signal actually fired — not whichever tile is red because of an arbitrary threshold.
• It is at-a-glance, not a wall of tiles. Like a weather chart, it gives a synoptic read of where the risk weather is rough and where it is calm, so the safety review board spends its time on the indicators that moved.
• It is pilot-readable by design. The whole point of borrowing the SIGWX metaphor is that the people who run the operation can read it without a statistics lesson. The maths is rigorous underneath; the picture on top is familiar.

Underneath the view is the same evaluated SPI library: a curated, ICAO-aligned set of 610 indicators that you activate by maturity tier and that eAviora evaluates monthly with real control charts. A smaller operation starts with a tier it can actually own; a mature one runs the full set. Either way, the Risk Weather shows the same thing — where the operation has genuinely shifted, with the colour earned by the maths rather than chosen by hand.

The relevant surfaces: Safety analytics, SMS module. See the SPI library for airlines or contact us to discuss your operation.