Failure of the ATG system was the other immediate cause of the incident. The servo-gauge had stuck (causing the level gauge to ‘flatline’) – and not for the first time. In fact it had stuck 14 times between 31 August 2005, when the tank was returned to service after maintenance, and 11 December 2005. Sometimes supervisors rectified the symptoms of sticking by raising the gauge to its highest position then letting it settle again, a practice known as ‘stowing’. On other occasions Motherwell was called in to rectify the matter, although the definitive cause of the sticking was never properly identified. Sometimes the sticking was logged as a fault by supervisors and other times it was not.
27 Thefailuretohaveaneffectivefaultloggingprocessandthelackofamaintenance regime that could reliably respond to those faults were two of the most important ‘root cause’ managerial and organisational failures underlying the incident. Further, Motherwell staff never saw that the unreliable gauge should be investigated. They did not analyse why they had been called out so frequently nor questioned the reliability of the system.