The Abu Dhabi Grand Prix highlighted the complexity of F1 fuel systems as both Red Bull and McLaren suffered race-compromising failures - but what makes up the system and why can it fail?
When Sebastian Vettel ground to a halt in qualifying it turned the Abu Dhabi Grand Prix on its head. Vettel, having dominated the last four races, was under pressure from Lewis Hamilton's resurgent McLaren, who had just taken pole position, but he was told to stop on track by the team and when his car was taken to scruteneering after the session it lacked sufficient fuel to pass the FIA test - which requires the car to have one litre of fuel left in the tank at the end of running.
The back-of-the-grid penalty for the Red Bull driver robbed us of a tantalysing battle between the pair - but then in the race a fuel failure for Hamilton cost the McLaren driver a potental victory.
It was not the first time fuel issues have compromised the leading teams this season either - Hamilton was stripped of pole position in the Spanish Grand Prix for doing the same thing as Vettel did last weekend.
Fuel weight is a signifcant factor in the performance of a car, and the more fuel efficient the engine, the less fuel needs to be carried and the more lap time you can gain. Managing that process is crucial but complicated because of the high speeds and forces involved in every corner of every lap.
Fuel tanks like those used in everyday road cars simply would not work because when the fuel begins to be used up, the extra space in the tank would cause it to slosh around, causing increasing problems for the fuel flow into the engine as the fuel levels dropped.
The fuel needs to be more contained, both for that reason and also to prevent fuel leaks which could create fireballs in such tightly packaged vehicles which run at such high temperatures.
Fuel is kept in a protective sack inside a 'fuel cell' (simply another name for a fuel tank) which sits behind the driver. But to get it from there to the engine requires a complex flow run through a series of chambers.
Each chamber drains into the next as fuel is used up - each being one step closer to the engine - and the fuel is prevented from back-flowing by one-way valves. Once the fuel is in the final chamber a number of low-pressure pumps are used to feed small amounts of fuel into the fuel collector, which holds a precise amount (usually a couple of litres) ready to go straight into the engine. From here, the fuel is pumped into the engine at high pressure - and it is also from here that the FIA takes its fuel sample after the race.
If the low-pressure pumps go wrong or there is some problem with the valves between the tank separators, there can be fuel in the main tank but not in the collector itself.
This causes some critical problems.
Firstly, with the FIA fuel test, the rules state that the fuel must be taken from that fuel collector, so it doesn't matter whether there is fuel in the car or not, if the sample fails to reach a litre the rules have not been obeyed.
Secondly, the final high pressure pump which feeds the engine is a coplex part which will fail if it runs dry and that could cause an engine failure - so with the restriction on engines imposed in F1, the teams will monitor this area carefully on their telemetry systems and stop the car before the pump fails if there is a high risk that it will.
Both these issues were demonstrated in Abu Dhabi.
In Red Bull's case, engine manufacturer Renault manages the fuel planning and they use complex programmes to determine just how much fuel they need. They are so precise that they give them the confidence to run right on the limit.
To fill the car they use a fuel trolley which is standard equipment and pumps and measures extremely accurately. Cars will be fully drained of fuel and cleaned before a run to ensure that no contaminants are in the fuel but also to ensure the precise amount in the tank is known.
This closed loop allows teams to take initial predictions of fuel use for the particular circuit then monitor, measure and re-predict their fuel usage throughout the race weekend - as there are many factors such as car set-up, downforce and drag levels, engine gearing, track layout, tyre wear, and so on, that play a part in how much fuel is used.
Red Bull admitted the margin was "a bit fine" in Abu Dhabi but rejected suggestions that they were trying to run underfuelled to gain crucial extra tenths of a second in the tight qualifying session or that they had simply got their sums wrong.
Instead they stated that while engine supplier Renault's fuel figures confirmed enough fuel had gone into the car, they simply could not get it out when the checks required it.
It is likely that Red Bull tried to stop the car and tow it back to the pits in the hope that they would still have enough fuel in the fuel collector to pass the FIA's tests - and they were close, just 200ml away.
Despite the penalty, however, Vettel's recovery on Sunday was strong. With Hamilton, however, the fuel flow issue came in the middle of the race and that is inevitably terminal.
The progress of development in F1 means that fuel systems have come a long way from where they were even a decade ago - but the complexity involved inevitably leads to occasional failures. And as Abu Dhabi showed, that can prove to be very costly indeed...