In general aviation, fuel is ordered in gallons. Fuel capacity is stated in gallons, and the fuel burn of a Cessna 172 is eight gallons an hour. The only time fuel is measured in pounds is for the sake of weight and balance, and most of those problems (if we’re honest with ourselves) are only done during checkrides.
But when you start burning Jet A, the rules change. Jet fuel is measured in pounds because the volume of the fuel can change based on temperature, whether it’s Jet A, JP-4, etc. This is especially critical at high altitudes where the temperature will be minus-40 to minus-50 degrees Fahrenheit. Further, fuel is burned by mass, not by volume.
But where this really comes into play is in weight and balance. In larger airplanes, the weight of the fuel becomes a much greater part of the total equation, and therefore it becomes a major consideration. The 737-800/900 burns roughly 6,000 pounds of fuel an hour. The CRJ 100/200 burns roughly 3,500 pounds the first hour and 2,500 pounds an hour subsequently. That’s a lot of weight, and it needs to be properly accounted.
Another consideration that will be new to you as move up the ranks is the concept of zero fuel weight (ZFW). This is a number computed by the manufacturer, and it simply means that all weight above that number must be in the form of fuel. The basic operating weight, passenger load, and cargo added together must come in below the ZFW. At times, it may even be the limiting weight for takeoff. On the CRJ, this would happen on very short flights flown at low altitudes, but it rarely created a problem in terms of payload.
Pounds has become the universal standard in fuel units. Part of this is due to the famous ‘Gimli Glider’ accident, Air Canada 143, a Boeing 767 that ran out of fuel in 1983 during a transcon. Part of the problem was confusion in ordering fuel in kilograms, which could be measured in liters, and then converting it into pounds for the sake of operation, along with a chain of other events. One of the results of the accident was the movement to an industry standard of pounds in order to mitigate the risks of another airplane running out of fuel.
In the United States, using pounds also helps when it comes to doing quick-and-dirty calculations regarding max landing weight for a diversion or return to the departure airport, etc. There is no need to convert from one unit to another, which is especially helpful during a busy, stressful event.
So, get used to using fuel totalizers and computing fuel in pounds. It’s a sign of the step up to the big leagues, and it really does make your job much easier.