Investigators have described as fortunate a refuelling error involving a Singapore Airlines Boeing 777-200ER because the aircraft, unknown to the crew, departed with a large surplus of fuel rather than a deficiency.
The aircraft was being fuelled at Singapore ahead of a service to Johannesburg. Its crew had calculated the jet needed to carry 86t of fuel.
For reasons that could not be determined, the aircraft's internal fuel-quantity indicator had registered the aircraft as a 777-200, which features a smaller centre fuel tank than the -200ER. This caused the aircraft's instruments to under-measure the amount of fuel on board.
Although the aircraft's cockpit and refuelling-panel indicators showed total fuel of 86t, the refuelling had taken longer than expected, and the ground personnel found that the dispenser had apparently delivered 121.5t to the aircraft.
The ground team was uncertain about the discrepancy, initially believing that the fuel-flow counter might not have been reset before the fuel was dispensed.
In order to check the quantity of fuel on the aircraft, the team's lead technician performed a 'magnastick' check – a manual reading using floating gauges. This was only carried out on the centre tank, because the two wing tanks were assumed to be full as a result of the way the refuelling system was programmed.
From the lead technician's readings, and the wing-tank assumption, the certifying technician calculated that the fuel quantity on board was 86t.
But the Singapore Transport Safety Investigation Bureau inquiry says that, despite the apparent match in the calculated figure, the magnastick readings were "grossly inaccurate". The technicians, it says, had "limited" experience in performing the check and that the magnastick readings were "likely not correct".
Combined with the 5.5t of fuel which had remained on the 777 from the previous flight, the uplift of 121.5t meant the aircraft departed with 127t of fuel on board – some 41t above the intended figure of 86t.
"It was fortuitous that the aircraft had been fuelled with much more fuel than it needed," says the inquiry. "Had the magnastick reading errors been in the other [direction], the aircraft could have ended up in a fuel starvation situation in flight."
The aircraft was fitted with a programme switch module which had been correctly configured for a 777-200ER.
But investigators believe a fault in the module – the nature of which could not be established – resulted in its incorrectly interpreting the aircraft as a 777-200, which meant that the fuel-quantity processor did not take into account eight fuel sensors in the enlarged centre tank. This meant the aircraft was under-reading the actual quantity of fuel on board.
About 1h into the flight the aircraft's crew received an alert stating that the amount of fuel on board – based on burn-off calculations – was less than the figure on the fuel-quantity indicator, and that the discrepancy was increasing. The crew opted to turn around and return to Singapore where, after landing, the fuelling error was discovered.
The inquiry points out that, while too little fuel risks starvation, excessive fuel can result in incorrect calculations for take-off speeds, higher fuel-burn, or reduced controllability.
Investigators state that the crew did not experience handling difficulties. The aircraft involved (9V-SVC) was undamaged during the 16 April 2014 flight and none of its occupants was injured.