Let's look at some of the facts from the AIB report.
1) Analysis of GPS and radar data indicates that the flight took a total of 2 hrs and 13 minutes and 55.5 USG would have been burned …
The total fuel capacity was 128 US gallons, with 5 US gallons unusable.
So assuming he had about 60 USG on departure, the gauges for both sides should have read about half full. According to the accident report "After refuelling the pilot noted that the fuel gauges indicated 'full' on the left and 'three quarters full' on the right."
2) Having completed the calculation, the pilot asked the refueller to put 45 litres (12 US gallons) of fuel in each wing. So, his calculation error was 3 US gals per wing. 6 USG total would be about 15-20 minutes flight time- assuming 20-22 USG per hour consumption.
3) The pilot based his fuel plan on a 'rule of thumb' provided to him during his type-conversion training. In common with many such 'rules' nothing was written formally, but the 'rule' was based on cruising at about 140kt IAS with 2,200 RPM and 30 to 32 inches of MAP giving fuel consumption of about 20 USG per hour. However, the pilot planned to fly at 154 kt TAS which the AFM indicates requires 23.3 USG per hour. Analysis of GPS and radar data indicates that the flight took a total of 2 hrs and 13 minutes and 55.5 USG would have been burned compared to the 2 hrs flying time and 40 USG fuel consumption calculated by the pilot.
The pilot calculated his reserve fuel by adding 10 USG to the fuel burn and by adding a further safety factor by converting the final figure into Imperial gallons. He thus planned to carry a total of 60 USG compared to the 73 USG that would have been required if the flight had been planned in accordance with the AFM and the FOB.
I cannot see that the conversion error in the fuel calculation is the primary cause of this accident. At most. It constituted about 15-20 minutes flying time, which would be far too small a margin for a planned 2-hour flight.
The pilot's calculations were based on the 'rule of thumb', which was about 10% on the wrong side of the operating conditions. However he planned for 60 USG, which would have been 50% more than his predicted fuel burn. Although it would not have met with the 73 USG which would have been required for the AFM and FOB, it would have been enough for the flight, without diversion.
Based on the aircraft technical log, the fuel on board prior to refuelling ought to have been around 30 USG. Thus he assumed residual fuel existed which would have been a further 50% margin above his planning.
He was also aware that G-OMAR was occasionally used for charter flights and that in order to avoid potential weight problems the aircraft operator had an unwritten policy that the aircraft should not be returned with very high residual fuel loads. So he is under pressure not to take unnecessary fuel.
The fuel gauges indicated 'full' on the left and 'three quarters full' on the right in other words, about 7/8 of 128 USG - or well over 100 USG - at least 4 hours for a 2 hour flight. Based on the tech log, 30 USG would be around 90 minutes flying time. So he thinks he has loaded 2 hours of fuel, he has 1.5 hours residual fuel from the tech log, and his fuel gauges are showing at least 4 hours. Look pretty consistent and not unreasonable for the flight. The flight to Sheffield was uneventful with an airborne time of 1 hour 2 minutes and a block time (start of taxi at Shoreham to end of taxi at destination) of 1 hour and 5 minutes. So there should be about 3 hours fuel left for a 1 hour flight.
Now, the fact that he has loaded 6 USG less than he thought (about 15-20 minutes flying) is hardly going to make a big difference to the 3 hours that looks to be in the tanks.
I can't see that this is negligence. It looks to me like this guy has been caught out by faulty fuel gauges - remember he can't visually check the tanks - and a reluctance to fill the aircraft because of the operator's policy.
The only fault I can see is that his original fuel planning of 60 US Gal, instead of the 73 USG as recommended was re-miss, because he used rule-of thumb values (who doesn't?), and he doesn't seem to have planned for sufficient diversion fuel (but he wasn't in a diversion situation).
I think he has been unlucky, and perhaps a little lax in diversion planning, but I don't think he should have been prosecuted.