we used to tell the fuel guy to put in XXX "big gallons"
I often get asked, for example "a good 47 tonne please eng".
We both know what the Captain means and the a/c will have something like 47.4 on the gauges.
If you're (the 'general' form of
you) going to put more fuel on the airplane, at least have the courage to call it what it is. If you want 47.4 tonnes on the gauges, then ask for it. Using words like "healthy", "big", "good", to describe more than the requested number seems somewhat cowardly. It's nothing like turning your back on your fellow troops and watching them get slaughtered while you retreat - but it is still a form of cowardice. Maybe indecisiveness too?
Fiddlefactor notwithstanding, how many actually check the fuel S.G.
I do. The maximum weights are the ones that are normally published for conversion (i.e. 7.00 lbs/US GAL or 1.85 lbs/litre). I find that the actual S.G. almost every single time I call for fuel is right around 6.81 lbs/US GAL.
The airplane I fly has a capacitance-type fuel measurement system. It measures the volume by capacitance and then corrects the reading for temperature. It'd be correct if they compensated for S.G. but it doesn't. That being said, the difference is quite small as the fuel capacity is not exceedingly large.
However on a small turboprop, a 100kg can be a major difference.
The airplane I'm referring too, and flying now, is considered a large aeroplane but just barely. I would consider 100 kg to be significant.
A little more background:
The airplane has a way to measure the volume of fuel in the tanks to confirm what is on the gauges in the cockpit. When I check the tanks on the walkaround, the tanks always have had between 100-200 lbs (usually around 125 lbs) more fuel than what's on the gauges. So when we're taking off, we have ~250 lbs more fuel than what's on paper... assuming the Captain hasn't added granny fuel. Most add around 200 lbs meaning that we're taking off 450 lbs overweight. When the manufacturer flight tested the airplane, they did so at precise weights. When performance is specified for a particular weight, the airplane was tested at that weight... not at that weight plus a few hundred pounds for the allowed discrepancy on the fuel gauges, which happens to be 200 lbs per side. That being said, when you're 450 lbs overweight for a field or obstacle limited airfield, in an airplane that weighs ~10,000 lbs and 15,000 lbs, that will significantly affect your performance. Two engine isn't much of a problem but when you're single-engine, you might encounter some serious problems.