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The engineer was confused by the term ‘ppm’ – meaning ‘parts per million’ – while conducting a biocidal shock treatment of the Titan Airways A321’s fuel tanks, as the jet neared the end of a month-long maintenance check.

While the anti-contamination treatment required a biocide concentration of 100ppm, no explanation or instruction for calculating the overall quantity featured in the aircraft maintenance manual task, and the engineer resorted to an internet calculator.

But he miscalculated the dose and, instead of the correct quantity of just under 0.8kg, he poured 30kg of biocide into each of the right- and left-hand wing tanks – equivalent to more than 37 times the maximum permitted dosage, given that the tanks both held 6,200kg of fuel.

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