How big a sample?
Taken when and from where?
What do you do with the fuel after 7 days?
What would you do if an aircraft crashed that had been refuelled 6 days ago?
1. 3 litres
2. Daily, before first delivery of the day, from the forecourt unit ( the thing with the nozzle and trigger on). It's dispensed to an approved glass jar for inspection then kept in a blue tin marked with the day of the week.
Weekly, ALL the several drains at the bottom of the tank are sampled, inspected for contamination, then re-cycled. The only sample we don't recycle is from the bottom of one of the filter units, which we keep for fire training.
3. Daily, last week's sample returned through the filter system
4. Impound the sample from that day awaiting collection by the AAIB. Sadly, we had a fatal crash at an event I was helping with, a few years ago. We immediately sealed the fuel system and impounded all the samples until the AAIB arrived. We were able to say from the fuel dispensing records exactly when the fuel had been put in the aircraft and which was the relevant sample. In the event, the accident wasn't fuel related.
All our fuel activity is recorded, including SG checks of bulk deliveries. We don't deem it necessary to check SG of fuel in the tank, but would happily do so if a customer or Club member asked for it.
We have an above-ground tank and are mindful of the issues surrounding contamination from water. However, the regime of sampling we have shows that in fact we don't suffer from condensation. The fuel is actually taken by a floating device from the top of the fuel. The tank has a 200-litre 'un usable' space at the bottom, from which we draw the weekly test samples. This is where we would expect to see the first indications of contamination. We don't just rely on visual inspection for water; we use a water-finding paste which is very sensitive.
Being right on the coast, our diurnal temperate range is small. However, the downside is everything rusts like crazy in the salt air.
Another point about above-ground versus underground tanks - insurance. Apparently it's cheaper to insure an above-ground unit as any likely accident to it is going to be a single, insurable event, whereas an underground unit might leak into the surrounding soil un-noticed and the owner might be faced with an big un-insured clean-up bill.
TOO