We're now moving into the heart of how we classify failures on an aeroplane — Page 37, Lesson 53

We're now moving into the heart of how we classify failures on an aeroplane. This is the severity classification system from CS 25.1309, and it's absolutely central to how we design, certify, and operate aircraft. Let me walk you through the five categories, because each one is a legal definition with very specific consequences. We start with the lower end of the scale. A Minor Failure Condition is one that would cause a slight reduction in safety margins or functional capabilities, a slight increase in crew workload — think of something like a routine flight plan change — or some physical discomfort to passengers or cabin crew. The key word here is "slight." It's an annoyance, not a threat. Next we step up to Major. A Major Failure Condition would reduce the capability of the aeroplane, or the ability of the crew to cope with adverse operating conditions, to the extent that there would be a significant reduction in safety margins or functional capabilities. Notice the escalation: from "slight" to "significant." It also means a significant increase in crew workload, or conditions that impair crew efficiency, or discomfort to the flight crew, or physical distress to passengers or cabin crew — and that distress could possibly include injuries. Then we have Hazardous. This is where things get serious. A Hazardous Failure Condition reduces the capability of the aeroplane or the crew to cope with adverse operating conditions to the extent that there would be one of three things. First, a large reduction in safety margins or functional capabilities — again, note the escalation from "significant" to "large." Second, physical distress or excessive workload such that the flight crew cannot be relied upon to perform their tasks accurately or completely. That's a critical point: the crew becomes unreliable. Third, serious or fatal injury to a relatively small number of occupants other than the flight crew. And finally, at the top of the scale, Catastrophic. A Catastrophic Failure Condition would result in multiple fatalities, usually with the loss of the aeroplane. There's an important note here: in previous versions of the rule, "Catastrophic" was defined as a failure condition which would prevent continued safe flight and landing. That older definition is worth remembering because you'll see it in older documentation, but the current definition is multiple fatalities, usually with loss of the aeroplane. Now, here's the crucial part about how we actually use these categories. When we're doing qualitative analyses to show compliance with CS 25.1309(b), we use what are called Qualitative Probability Terms. These are commonly accepted descriptions that aid engineering judgement. The first one is Probable — and the excerpt cuts off right as it's defining it, but the pattern is set: each severity category has a corresponding probability term that tells us how likely a failure of that severity is allowed to be. So the structure you need to hold in your mind is this: we have five severity levels — Minor, Major, Hazardous, Catastrophic, and the one that's just starting to be defined, Probable. Each one escalates in terms of safety margin reduction, crew workload, and occupant injury. And each one will be paired with a probability limit that tells us how often such a failure is permitted to occur. That pairing — severity with probability — is the entire logic of CS 25.1309.