How does fuel burn affect the center of gravity (CG) during flight?

Fuel burn changes the aircraft's mass distribution, causing the CG to shift depending on tank location.

Why is it important to monitor CG movement due to fuel consumption?

Excessive CG shift can lead to stability, controllability, or structural issues, potentially exceeding safe limits.

What happens if the CG moves behind the aft limit due to fuel burn?

The aircraft may become unstable and difficult to control, increasing the risk of loss of control.

How does tank location influence CG shift during fuel burn?

Fuel burned from tanks far from the CG (like in the tail or outer wings) causes greater CG movement than from central tanks.

Does CG position affect aircraft performance during flight?

Yes, a forward CG increases drag and fuel burn, while an aft CG can reduce stability and control authority.

Effects of Fuel Burn on Center of Gravity

Hard4 min readMass & Balance

Moderately Examined

Why this matters

Understanding how fuel burn affects the center of gravity is critical for maintaining safe handling and performance throughout a flight. Unmanaged CG shifts can compromise stability, control, and even the structural integrity of the aircraft.

As fuel is burned during flight, the aircraft's center of gravity (CG) shifts depending on the location of the fuel tanks and the amount of fuel consumed. This movement can affect aircraft stability, controllability, and performance, making it essential for pilots to understand and monitor CG changes throughout the flight.

Quick Check

How does fuel burn from a central fuselage tank typically affect the aircraft's center of gravity (CG) during flight?

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In depth

Explanation

How Fuel Burn Affects Center of Gravity

The position of the aircraft's center of gravity changes as fuel is consumed. The direction and magnitude of this CG shift depend on where the fuel tanks are located relative to the aircraft's datum (reference point):

Wing Tanks (near CG): Burning fuel from wing tanks typically causes minimal CG movement, as these tanks are close to the aircraft's original CG.
Forward Tanks: Fuel consumption from tanks ahead of the CG causes the CG to move aft.
Aft Tanks: Burning fuel from tanks behind the CG moves the CG forward.
Central Tanks: If the tank is on or very close to the CG, fuel burn has little effect on CG position.
Stabiliser or Fin Tanks: These are usually far aft. Burning fuel from these tanks can cause a significant forward CG shift, which may risk exceeding forward CG limits by landing.

Performance and Handling Implications

Forward CG: Increased trim drag, higher stall speed, reduced climb rate, and greater fuel consumption.
Aft CG: Lower trim drag, improved fuel economy, lower stall speed, but reduced longitudinal stability and risk of control issues if limits are exceeded.

Mass and Balance Calculations

Pilots must calculate and plot CG positions for ramp, take-off, and landing conditions, ensuring all points remain within the aircraft's approved envelope. If fuel burn causes the CG to move outside this envelope, the loading arrangement must be adjusted before flight.

Safety and Operational Risks

If the CG moves outside prescribed limits due to fuel consumption, the aircraft may become difficult or unsafe to control, especially near the aft limit where pitch stability is reduced. Special consideration is needed for aircraft with fuel tanks in the stabiliser or fin, as these can cause rapid CG changes.

Key Takeaways

Always consider the sequence of fuel use and tank locations in preflight planning.
Monitor CG movement throughout the flight, especially on long sectors or with unusual fuel tank configurations.

The essentials

Key Points

Fuel burn changes the aircraft's center of gravity, depending on tank location.

Burning fuel from forward tanks moves the CG aft; aft tanks move it forward.

Wing tanks near the CG cause minimal CG movement as fuel is used.

A forward CG increases trim drag, stall speed, and fuel consumption.

An aft CG improves fuel economy but reduces pitch stability.

CG must always remain within aircraft limits for safe operation.

Special care is needed with stabiliser or fin tanks due to significant CG shifts.

Watch out

Exam Traps & Typical Mistakes

Assuming fuel burn always moves the CG aft—direction depends on tank location.

Overlooking the impact of stabiliser or fin tanks on CG movement.

Forgetting that wing tanks near the CG have little effect on CG position.

Believing that CG outside limits only affects performance, not controllability or safety.

Neglecting to check CG at landing, not just at take-off.

Test yourself

Example Exam Questions

Question 2Medium

If an aircraft burns fuel from tanks located in the horizontal stabiliser, what is the primary risk?

Question 3Easy

What is the effect on fuel consumption if the CG shifts forward during flight due to fuel burn?

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