Probabilities

Understanding CS-25 Failure Probabilities in Aircraft Systems

In ATPL theory and aircraft systems study, you will repeatedly encounter the CS-25 (formerly JAR-25) framework for system safety. These aviation regulations define how safe large aeroplanes must be, not just qualitatively but with explicit probability targets for different failure condition classes. The aim is to ensure that design, redundancy, and operating procedures together keep the average probability of unsafe outcomes per flight hour at acceptably low levels. For exam purposes and practical understanding, focus on both the quantitative probability bands and the worst credible effects on the aeroplane, flight crew, and occupants.

CS-25/JAR-25 distinguishes four principal failure condition categories, each with a descriptive likelihood term and a numerical target per flight hour. In parallel, each category is defined by the severity of its effects:

• Minor — Probability on the order of 10^-3 to 10^-5 (“probable”). Effects: slight reduction in functional capabilities or safety margins; for the flight crew, a slight increase in workload; for occupants, physical discomfort.
• Major — Probability on the order of 10^-5 to 10^-7 (“remote”). Effects: significant reduction in functional capabilities or safety margins; for the flight crew, physical discomfort or a significant increase in workload; for occupants, physical distress, possibly including injuries.
• Hazardous — Probability on the order of 10^-7 to 10^-9 (“extremely remote”). Effects: large reduction in functional capabilities or safety margins; for the flight crew, physical distress or excessive workload that impairs task performance; for occupants, serious or fatal injury to a small number of passengers or cabin crew.
• Catastrophic — Probability less than 10^-9 (“extremely improbable”). Effects: hull loss; for the flight crew, fatalities or incapacitation; for occupants, multiple fatalities.

These targets are applied as average probabilities per flight hour and are demonstrated through the System Safety Assessment (FHA/PSSA/SSA), using methods such as Fault Tree Analysis, Failure Modes and Effects Analysis, and common-cause analysis. Designers combine redundancy, independence, and alerting to meet the numbers, while accounting for realistic crew response via normal and non-normal procedures. Importantly, credit for pilot action is limited: a failure classified as major or worse must not depend on excessive crew workload or flawless technique to remain safe. For ATPL exams, be precise with the numeric bands, the descriptive likelihood terms (probable, remote, extremely remote, extremely improbable), and the distinct worst-effects wording for the aeroplane, flight crew, and occupants.

What this Probabilities question bank covers

• CS-25/JAR-25 failure condition classifications (Minor, Major, Hazardous, Catastrophic) and their allowable average failure probabilities per flight hour.
• Qualitative likelihood terms aligned to the numbers: probable, remote, extremely remote, extremely improbable.
• Worst credible effects on the aeroplane (functional capability/safety margins up to hull loss), flight crew (workload, distress, incapacitation), and occupants (discomfort through multiple fatalities).
• How these requirements shape aircraft systems design, redundancy, alerts, and crew procedures under EASA/FAA-aligned aviation regulations.
• Key exam distinctions and phrasing frequently tested in ATPL theory questions.