The 02:40 Torque Setting
At 02:40 on the fourth consecutive 10-hour night shift, a line maintenance technician is replacing a hydraulic return-line union on a narrow-body aircraft. The aircraft is scheduled to depart at 05:30 and the gate is congested. He slept about 4.5 hours before the first night and has had two 15-minute breaks in the last eight hours. The manual specifies a torque of 45 N·m; the wrench scale has major marks every 10 N·m and minor marks every 2 N·m. He sets the wrench, tightens the union, then looks at the scale again and cannot remember whether he set 45 or 54. He is alone at the gate; the supervisor is in another bay. He re-torques "to be sure" and signs the task card.
1. Identify the human-factor contributors in this situation and explain how at least three of them interact to raise the probability of error.
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Contributors include:
- Fatigue and circadian low – fourth consecutive night shift, about 4.5 h sleep before the first night, only two short breaks; performance at 02:40 is near the circadian trough.
- Time pressure – 05:30 departure and a congested gate push toward fast completion.
- Lone working / reduced supervision – no second person to cross-check the wrench setting.
- Poor task design for perception – a analogue scale with 10 N·m major marks and 2 N·m minor marks invites misreading at low light.
- Memory demand – he must hold the set value in working memory while tightening, then recall it.
These do not act separately. Fatigue degrades working memory and vigilance, so the set value is lost; time pressure discourages stopping to re-read the manual; lone working removes the independent check that would have caught the uncertainty. The combined effect is much greater than any single factor, which is why fatigue risk management treats night work, staffing and deadlines together rather than as separate issues.
2. What immediate actions should the supervisor take when told about the uncertainty, and why is "re-torque to be sure" not a reliable fix?
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The supervisor should:
- Stop the task and treat the joint as unverified until it is positively confirmed.
- Break the joint and remake it to the manual procedure with a correctly set, calibrated wrench, rather than simply adding torque to an already-tightened union.
- Use two people – one to set and read the wrench, one to verify – or a wrench with a clear digital or breakaway indication.
- Check for damage from possible over-torque (deformed fitting, stretched threads, crushed seal) and replace parts as needed.
- Re-plan the departure and record the interruption honestly.
Re-torquing is unreliable because a fastener already at or above the target requires breakaway torque to move; the wrench may click without the joint turning, giving false reassurance. Adding torque to an over-torqued joint increases the risk of damage. It also does not answer the original question – whether the first application was 45 or 54 N·m – so it converts a known uncertainty into an unverified assumption.
3. If the union is later found over-torqued, classify the likely error type and state one organisational barrier that would have prevented it.
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The most likely classification is a slip (action error) – the intended action was correct but the wrench was set or read incorrectly – possibly combined with a lapse if he simply forgot the setting. It is not best described as a deliberate violation, because there is no evidence he chose to depart from the procedure.
An organisational barrier that would have helped:
- A two-person independent check for safety-critical torque tasks, or a digital torque wrench that displays and logs the set value, removing the need to read a fine analogue scale under fatigue.
- Equally valid: a fatigue risk management schedule that avoids placing safety-critical, fine-discrimination tasks at 02:40 on the fourth night, or a task-card hold point requiring supervisor sign-off before the joint is closed.
The key reasoning is that the barrier should address the latent condition (task design, staffing, scheduling), not merely remind the individual to be more careful.