🕓 The WOCL – No, It’s Not a Cartoon Character - Why Maintenance Organisations Need to Understand the Window of Circadian Low.

Window of Circadian Low

In aviation, acronyms are everywhere. But one that often slips past maintenance planners and engineers is the WOCL — the Window of Circadian Low. No, it’s not a cartoon character — it’s the time of day when the human body is least alert, and in aviation maintenance, it matters more than most realise. As regulatory reform reshapes the CASA Part 145 environment, human factors are no longer a “tick-the-box” subject. They are fundamental to how maintenance is planned, executed, and verified. And the WOCL is right at the centre of that conversation.

1. What Is the WOCL?

The Window of Circadian Low typically occurs between 0200 and 0600 local time — the hours when human alertness, body temperature, and cognitive performance reach their lowest levels. It’s the period when fatigue is most likely to impair reaction time, decision-making, and error detection — the very skills aircraft maintenance relies upon.

In flight operations, the WOCL has been well understood by flight crew and schedulers for many years. But on the hangar floor, where maintenance continues through the night, it’s a relatively new concept, and is often ignored — or underestimated.

2. The Link Between Human Factors and Maintenance Quality

CASA’s Human Factors (HF) training and the Part 145 framework now formally recognise that humans are not machines. Regulators expect organisations to integrate human performance limitations into scheduling, task allocation, and risk management. Key HF principles such as fatigue awareness, workload management, and team communication are no longer optional “soft” topics. They are hard safety barriers that prevent maintenance errors.

A fatigued engineer working at 3 a.m. is statistically more likely to:

• Miss a torque check.

• Forget to reinstall a fastener.

• Misinterpret a maintenance instruction.

• Sign for work not physically verified.

Each of these can become an airworthiness event if WOCL impacts aren’t mitigated.

3. Maintenance Planning: The WOCL Overlay

Progressive maintenance organisations now apply what’s known as a WOCL overlay to their maintenance planning. This means recognising the physiological troughs in alertness and adjusting accordingly:

• Avoiding critical inspection or certification tasks between 0200–0600 when possible.

• Using buddy checks or independent inspections during WOCL hours.

• Scheduling rest breaks more strategically during overnight shifts.

• Ensuring shift rotations avoid long-term circadian disruption.

• Ensuring facilities are available for adequate rest

• Providing transport home if fatigued

In short, WOCL awareness should be embedded in the same way as tooling control, access control, and documentation compliance — because it directly affects the quality and safety of maintenance outcomes.

4. Regulatory Reform and the Human Element

CASA’s evolving approach to Safety Management Systems (SMS) and Human Factors places clear accountability on maintenance organisations to understand their people. Part 145 is not just about calibrated tools and approved data — it’s about ensuring that the humans using those tools are capable, rested, and supported to make sound decisions. Under a compliant SMS, fatigue risk management is a measurable, reportable component of safety assurance. Organisations that fail to consider WOCL effects in their planning can no longer claim to be “fully compliant” with the intent of Part 145.

5. The Message for Maintenance Leaders

The message is simple:

The human is part of the system. If the human is fatigued, the system is compromised.

As fleets grow and night shifts remain essential, maintenance leaders must treat the WOCL as a critical operational constraint — not an inconvenience. Understanding human physiology is as important as understanding hydraulics, avionics, or structures.

Safety begins with people — and people are bound by biology.

Jotore Aviation Insight

At Jotore Aviation, we help maintenance organisations integrate human factors and fatigue management into their operational frameworks. From shift design to audit readiness, our Human Factors Readiness Reviews identify where WOCL risks exist — and how to mitigate them under CASA Part 145 requirements.

👉 Contact Jotore today on fatigue risk and human performance in your maintenance system.