Here is something you don’t hear every day: “Hey, we need to add some gas to get the last passenger on.” It’s a rare statement indeed, and in the typical general aviation world, you won’t hear it at all. The cause is something called Zero Fuel Weight (ZFW). On the Canadair Regional Jet the ZFW is 44,000 pounds, and the max landing weight is 47,000 pounds, and both can play a role in determining the maximum takeoff weight. What’s the difference? An example might make this easier, but remember this: Anything added above the ZFW limit must be fuel.
Assume that on a short flight (35 minutes or so), the fuel burn will be 1,500 pounds. That means the maximum takeoff weight would be 48,500 pounds, since your 1,500-pound burn would get you down to the structural max landing weight of 47,000 just when you touch down. But what if the planned fuel load for the flight is 4,000 pounds (which would be typical for such a flight)? Well, then you would be limited to 44,000+4000=48,000 pounds, or in payload terms, three fewer passengers (or an equivalent weight in bags). But, because everything added above 44,000 pounds must be fuel, you can add fuel, and thus add passengers. In this case, we can add up to 500 pounds of Jet-A, and our ZFW limiting weight and our structural landing weight plus fuel burn would be the same (47,000+1500=48,500 and 44,000+4500=48,500). And if the fueler inadvertently brought the fuel up to 4,700 pounds, we would then use the lower of the ZFW and the fuel burn plus landing weight. In other words, we’d still be limited to 48,500 pounds, and assuming that the three extra passengers combined with the extra fuel didn’t put us over that limit, we could still take them.
This is a rare occurrence, and as you can surmise, on the CRJ it only happens on short flights with light fuel loads in VFR conditions (a typical flight carries at least 6000 pounds, and 8000 is not unusual). An easy way to understand ZFW is to think of an airplane with a center tank between the wings, under the fuselage. The walls of the tanks are only designed to support a certain amount of weight, and at a certain point, the only weight that can be added is fuel. Think of it as an empty box you try to stand on. If it’s empty, it might collapse. But if you start to fill it with dirt, and give it some rigidity, it will gain strength, and you can stand on it with no worries. But if your buddy wants to stand on it with you, you may need to put some more dirt in it. Add a third person, and with luck, it holds. But if it collapses, then you have exceeded the maximum structural weight. The same holds for an airplane.
Adding fuel to add passengers…it should always be so easy.