Reading a chart

A PHASE chart tells one story and one only: how the situation evolved while the aircraft kept flying. It can be read without a legend, once you know its two axes and its three bands.

Time on the horizontal axis

From left to right runs flight time. One second is the same width everywhere. The axis shows sequence, simultaneity, duration and overlap.

A few light markers sit on the axis: a procedural gate, such as 1,000 ft, or a change of flight phase. They belong to the axis. They carry no information about the objects.

Function on the vertical axis

From bottom to top, the vertical axis is neither an organisation chart nor a hierarchy. It carries a function: how far the information has got in its processing.

Position says what, not who. It measures nothing: the distance between two strata is not a duration, and height indicates neither severity nor importance. Within a stratum, an object’s height means nothing: two stacked objects simply coexist, in no particular order.

Ten strata, three bands

The chart has ten strata, always the same, always in the same order. None is added, removed or reordered.

Band	Stratum	Evidence required
Crew	Internal modulators	reconstructed conditions
Crew	Non-technical competencies	indirect trace
Crew	Technical competencies	direct trace
Crew	Error	—
Reference corridor	Aircraft state	direct and documentary trace
Threats	Aircraft	direct and documentary trace
Threats	Weather and environment	direct and documentary trace
Threats	ATC and operations	direct and documentary trace
Threats	Airport and infrastructure	direct and documentary trace
Threats	Organisation and documentation	direct and documentary trace

In the middle, the aircraft state. This is the chart’s reference line: speed, path, track, energy, configuration, modes. Everything is read against it. Its corridor is drawn more strongly than the two bands so the eye finds it without searching.

Below, the threats. Weather, runway, traffic, organisation. Whatever is present in the situation and has to be managed, whether or not the crew sees it. A threat is external to the crew.

Above, the crew at work. The Error stratum holds the finding that an action was not the expected one. Above it, hands-on control and correction in the technical competencies; conferring, deciding and sharing the workload in the non-technical competencies. At the top, internal modulators: the conditions under which all of this takes place.

The right-hand column matters. The more internal the level, the stronger the evidence required. What is enough to establish a control input is not enough to establish where someone’s attention was. The required level is written once, at the left of the stratum. The level each object actually reaches is in its record.

Three movements

A chart reads like a loop that moves forward in time.

Going up. A piece of information appears at the bottom of the chart: a threat, an aircraft state. It travels up, or it doesn’t. If it is not perceived, you can see it: nothing lights up above.

Processing. If the information travels up, it is processed, at different heights. It may stop at the technical competencies: a correction, and that’s it. Or it may go higher: the crew has to understand, talk and decide before acting. The higher it goes, the more it costs.

Coming back down. The response returns to the central corridor and changes the aircraft state, or doesn’t. A deviation that closes is a loop that closed. A deviation that persists is a loop left open.

Then it starts again. The new state produces new information, which travels up or not, while the aircraft keeps flying. The loop demo shows these three movements on a constructed case.

What an empty stratum means

All information travels up; the chart shows only what matters for the analysis. A sparse stratum does not mean the crew processed nothing. It means the analysis retained nothing there.

Normal flight deck activity is not drawn. A quiet chart is not a poor one.

Reading aloud

Each sign has a short phrase. Saying them aloud is the most reliable way to read a chart, and to have trainees read one. Each sign is detailed in The signs; the cursor in The moment T.

Sign	Read aloud
Diamond	“at the moment when”
Bar	“while”
Delta	“this is not what was expected”
White	“thanks to this”
Grey	“because of this”
Hatched	“neither thanks to this nor because of it, but worth knowing”
Dashed outline	“they didn’t know yet”
Label in italics	“can I state this, or only suggest it?”
Cursor	“what is going on at this moment?”

A typical reading, bottom up, under the cursor: “While the shortened approach path is in progress, the aircraft is above the profile. They didn’t know yet. At the moment when the deviation is called, the outline turns solid. While the decision is being made, the crew is holding on to the original plan, because of this…”

What the chart does not say

Some limits apply from the very first chart:

• the length of a bar expresses neither intensity nor severity;
• vertical alignment and proximity do not establish causation;
• no arrow links a cause to its effect on an analysis chart;
• the source, the status and the uncertainty of time bounds are in the record.

The chart makes the dynamics visible and open to discussion. It does not prove them.