“... I’d remembered 8 lbs/gal but apparently that’s for water. In those planes, and I believe in passenger jets, the gauges display lbs., ... in a small plane is fuel quantity displayed in pounds or in gallons?”
Thanks for your support in instrumentation.
I served on B-52 aircrews in the 1970s and early 1980s - assisted the copilot in mission planning, estimating the required fuel load, and calculating weight & balance. So my memory got reinforced.
Water was used in the B-52G during some takeoffs - sprayed into engine intakes to cool the air and increase density, which in turn boosted thrust. Delivered a perceptible kick when turned on. We used something like 1100 gallons of it in 90 seconds.
The B-52 had so many fuel tanks and so many gauges that crews were warned not to trust the gauges when the total fell below a certain figure. I forget the exact value, but it was pretty big. Maximum fuel allowed was over 300,000 pounds.
Fuel volume changing with temperature was a serious concern, just as you mentioned: engines sucked fuel in pounds per hour, not gallons. After ground refueling, the crew chief used measuring sticks to dip each tank. He (later she) would record the level in inches, record the ambient temperature, then refer to conversion charts to determine the pounds onboard.
Light aircraft not used for conveying passengers nor cargo were required to have a “means to determine fuel remaining” but the mechanical specifics were left up to the manufacturer. On the Aeronca 7AC, the tank was right in front of the pilot seat: a simple cork float with a metal rod sticking out of the top, marked with fuel-level gradations, projected up through the filler cap and the pilot could see the marks. Later, more “advanced” aircraft like the Cessna 150 had electrical gauges just like one’s average automobile: marked “E” and “F”, with tick marks in between. No numbers.
Haven’t flown any since the mid 1990s; not sure if any of this is current.
I worked on F-4s and KC/EC-135s. One of the F-4 pilots came in pretty low on fuel one day, low enough to turn on the "Low Fuel" warning light, and thinking that when the gauge read zero he actually had about 500 lbs. remaining.
So at maintenance debriefing he asked another instrument guy (not I) how much he really had when the gauge read zero, and the answer is ZERO. He was stunned to realize that he almost let the aircraft run out of fuel for no good reason.
The calibration procedure was to pull the plane into the Fuel Shop and they would completely drain all tanks. Then we instrument guys used our equipment to calibrate the gauge(s) to...zero!