Unusable fuel determines empty weight and CG in aviation.

What is the amount of fuel used for computing empty weight and corresponding CG?

• A. All fuel in the tanks
• B. Fuel that will be used for flight
• C. Usable fuel
• D. Unusable fuel

Answer: (D)

The amount of fuel used for computing empty weight and the corresponding center of gravity (CG) is based on the unusable fuel. Unusable fuel refers to the quantity of fuel that cannot be used by the aircraft's engines and is typically retained in the tanks due to design features such as the shape of the fuel tanks and the location of the fuel pickups. When calculating empty weight, the focus is on determining the minimum weight of the aircraft without any usable fuel, cargo, or passengers. Since the unusable fuel remains in the aircraft at all times, it is included in the empty weight calculation. Consequently, when computing the CG, the effects of this unusable fuel on the overall weight and balance of the aircraft are taken into account. Using other types of fuel quantity, such as total fuel in the tanks or usable fuel, would not accurately reflect the empty weight category as those amounts can fluctuate with the operational fuel load intended for flight. Evaluating only unusable fuel ensures that the calculation provides a true representation of the aircraft’s weight and balance when it is not intended for flight.

Outline of the story

• Start with a simple question: how do fuel numbers fit into the math of airframe weight and balance?

• Define the key players: empty weight, useful load, usable fuel, and unusable fuel.

• Explain why unusable fuel is the number that matters for empty weight and CG.

• Use a concrete, friendly example to illustrate how the math works in practice.

• Tie the concept back to real-world flight safety and how pilots and operators use the aircraft’s manuals.

• Close with practical takeaways and a light reflection on the bigger picture of weight and balance.

Fuel for Thought: What actually counts when we measure weight and balance?

Let me ask you something simple: when we talk about an airplane’s weight and where that weight sits, which fuel amount should we pretend isn’t there for the empty weight calculation? If you guessed “unusable fuel,” you’re exactly right. It’s a nuance that sounds technical, but it’s all about keeping the math stable and reflecting the aircraft’s real, fixed characteristics.

First, a quick refresher on the basics. An airplane carries several kinds of weight:

• Empty weight: the aircraft’s basic structure plus permanent equipment, and yes, the fuel that can’t be burned.

• Unusable fuel: the small amount of fuel that can’t be retrieved by the engine because of where the fuel pickups sit and how the tanks are shaped. It sits in the tanks no matter what.

• Useful or usable fuel: the fuel you can actually burn in flight.

• Payload: passengers, baggage, cargo.

When we’re calculating empty weight and the corresponding center of gravity (CG), the airplane’s designers want a baseline that doesn’t shift with what you might decide to fuel up for a particular flight. That baseline includes something that never leaves the airplane: unusable fuel. Why? Because it’s effectively a permanent part of the aircraft’s mass. It’s not something you drop in the bag or pour out before takeoff. It’s there, in the tanks, even when the cockpit lights are off.

What makes unusable fuel special

• It’s fixed by design. The geometry of the tanks and the location of the fuel pickups create a smallest amount of fuel that can’t be drawn into the engine. That fixed amount is the “unusable” portion.

• It affects the CG because weight in the tank is a physical lever arm. If that unusable fuel sits forward of the main CG, it nudges the CG forward; if it sits aft, it nudges the CG aft. Either way, it’s a constant that the empty weight calculation must acknowledge.

• It’s not the same as total in-tank fuel or fuel that you’ll take off with. Those numbers can vary flight to flight. The empty weight, by contrast, pretends you’re not taking off with a particular payload or a particular planned fuel load. It wants a constant reference point.

A practical example to bring it home

Imagine an airplane with a small amount of unusable fuel tucked away in a tank near the tail. Let’s say, just for the sake of illustration, that this unusable fuel weighs around 20 pounds. (Fuel weight varies by type; aviation gasoline is roughly 6 pounds per gallon, Jet A a little more; we’re keeping it simple here.)

• Empty weight includes that 20-pound stubborn weight in the tank.

• When you calculate the empty weight CG, you treat that 20 pounds as part of the baseline, just like the airframe and permanent equipment.

• If you later decide to fuel the airplane for a flight, you’ll add usable fuel on top of that baseline, and you’ll compute CG again using the new total weight. But the key point: the baseline itself already accounts for the unusable fuel.

Now, what would happen if you tried to calculate empty weight and CG using usable fuel or total fuel? It would throw things off. Usable fuel, by its nature, can be consumed during flight. If you used that variable amount in the empty weight calculation, you’d be chasing a moving target—your baseline would change every time you fly or refuel a little differently. That wouldn’t give you a stable reference point for the airplane’s weight and balance, and that stability is exactly what pilots rely on for safe flight planning.

Why designers and manuals emphasize unusable fuel

Aircraft manuals—the Airplane Flight Manual or the Pilot’s Operating Handbook—present these numbers with care. They separate fixed, non-consumable components (empty weight) from what you carry for the mission (fuel, passengers, baggage). The unusable fuel line item is the bridge between those two worlds. It’s the inert weight that stays put, week in and week out, regardless of how much you intend to fly.

This distinction matters for safety and performance in a tangible way. The CG must stay within a certified envelope to ensure controllability and stability. If your baseline shifts unpredictably, so does your ability to predict stall characteristics, elevator force, and trim requirements. In short: accurate empty weight and CG calculations mean predictable handling, even before you reach the runway.

How you’d verify this in the real world

• Check the fixed data in the aircraft’s weight and balance documentation. Look for a line item labeled something like “unusable fuel” or a stated weight that represents the amount of fuel that cannot be drained by the engine.

• When you run the numbers, use that fixed unusable fuel amount as part of the empty weight. Then, for any mission, add usable fuel as needed and verify the CG stays within limits.

• If you ever notice a discrepancy between the published empty weight (including unusable fuel) and what you weigh on the ramp, course-correct before you fly. It’s not merely a numbers game; it’s about safe handling.

A few practical takeaways you can bookmark

• The correct fuel used for computing empty weight and CG is the unusable fuel. It’s the constant piece of the puzzle that doesn’t disappear with flight plans.

• Usable fuel and total fuel numbers are flight-dependent; they change with how you load up for the trip. They’re essential for mission planning, but they aren’t what define the fixed baseline used in empty weight.

• The empty weight is a snapshot of the aircraft’s fixed mass, with that fixed unusable fuel included. The CG you derive from that snapshot tells you where the airplane sits in a static sense, ready to be balanced for whatever you plan to carry and burn.

• Always consult the official weight and balance data in the AFM/POH for the aircraft you’re flying. Those documents reflect the range of acceptable loads and the fixed elements you’ll want to understand before you taxi.

A quick analogy to keep it relatable

Think of the airplane like a ship in a harbor with a ballast tank that can’t be pumped out. The ballast represents that unusable fuel—part of the vessel’s fixed mass. It sits there no matter what cargo you load, and its position matters for the ship’s balance just as unusable fuel matters for the aircraft’s CG. You don’t plan your voyage around the ballast, but you need to know it exists to understand where the ship will sit in the water. Likewise, you don’t fly with unusable fuel, but you must count it when you’re figuring out the airframe’s baseline weight and balance.

A few more lines to tie things together

• Remember that weight and balance isn’t a dry spreadsheet exercise. It’s the backbone of safe flight. It links the physical layout of the plane, the way fuel is stored, and the way you’ll handle the aircraft once you’re airborne.

• The concept of unusable fuel is one of those details that seems tiny, but it anchors the whole balancing act. And because it’s fixed, it helps engineers design stiffer, more predictable aircraft, even as fuel loads vary between flights.

• If you ever find yourself curious about why certain numbers sit where they do in the weight and balance charts, take a moment to trace them back to the layout of the tanks and the fuel pickups. The answer almost always comes back to geometry meeting physics in a very practical way.

Closing thought

Weight and balance is a blend of science and common-sense engineering. The decision to use unusable fuel in the empty weight calculation is one of those quiet, steady rules that keeps everything from wobbling in the air. It’s not flashy, but it’s fundamental. And that’s what safe, reliable flight is all about: honoring the steady constants while you plan for the dynamic conditions of the skies.

If you want to keep exploring, look at a few real-world aircraft manuals. Notice how the unusable fuel figure appears and how the published empty weight interacts with it. You’ll start to see the pattern: a fixed baseline, a variable fuel load, and a CG that reflects the airplane’s current moment, all working together to keep you in control from the moment you power up to the moment you roll into the pattern.