Formulating crew scheduling rules, aimed at limiting crew fatigue, is by no means an easy task. Whether it is done by the regulators, or by a crew union in dialogue with the airline management, does not really matter - the basic flaw lies in the idea that a combination of several individual ‘rule limits’, on some roster properties, can effectively separate the ‘safe’ from the ‘unsafe’ patterns of activities on a crew roster.
There’s simply a far too weak correlation between, for example, accumulated duty hours over the last 7 days and the actual (by the crew) experienced fatigue risk. Look at the picture below, from this paper, showing the experienced sleepiness levels by pilots as a function of hours they have worked in the preceding 7 days. The ‘fatigue model’ provided by the regulators (assuming ‘safe’ if all rules are adhered to, and ‘unsafe’ if a single rule is violated) is an oversimplification that is not entirely fit for purpose. The rules are well-intended, but they don’t quite deliver - and airline crew experience this almost daily.
Bio-mathematical fatigue models on the other hand, although by no means perfect, provide a much more nuanced view, scoring ‘fatigue risk’ on a continuous scale between ‘low’ and ‘high’ - and does so using the latest validated, peer-reviewed and published understanding of the mechanisms behind. Not in isolation from each other, but in combination. The interaction between wakefulness, time of day, prior sleep debt, workload, sleep inertia and several other components such as the ‘afternoon dip’ and acclimatisation - are all taking part.
Regulators have attempted to improve their rules mainly through qualitative development working with the sleep scientists, one rule at a time. However, there’s an alternative approach that can be used by operators wishing to improve their rule sets to control fatigue risk in a better and more detailed way, while allowing for good crew efficiency.
Please study this document and feel free to contact your Jeppesen representative for a deep-dive in to improving your hard constraints governing your crew planning. The golden rule is that there is no golden rule. Welcome!
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