CG envelope (center of gravity envelope) defines the forward and aft limits within which an aircraft's center of gravity must fall during all phases of flight.
How It Works#
Every aircraft has a center of gravity (CG), the single point where its total weight effectively acts. The manufacturer calculates safe forward and aft limits for this point. Together, those limits form the CG envelope.
The envelope is plotted on a graph. One axis shows CG position, usually in inches from a reference point called the datum. The other axis shows gross weight. A loaded aircraft's CG must fall inside the enclosed area on that graph.
CG position changes as fuel burns, passengers move, or cargo shifts. Pilots calculate CG at takeoff, but they must also consider how it moves during the flight. A CG that starts inside the envelope can drift outside it as conditions change.
Example in Aviation#
A Cessna 172 pilot prepares for a cross-country flight with two passengers and full fuel. She plots the aircraft's weight and CG on the loading graph in the Pilot's Operating Handbook (POH). The point falls inside the envelope, so the loading is legal and safe.
Mid-flight, one passenger moves to the rear seat. The pilot mentally recalculates. The CG shifts aft but remains within limits. She continues the flight with confidence, knowing the aircraft will respond normally to control inputs.
Why It Matters#
Flying outside the CG envelope is dangerous. A forward CG makes the nose heavy. The aircraft needs more back-pressure to maintain level flight, and stall speed increases. A rearward CG is more hazardous. The aircraft becomes unstable, control authority decreases, and recovery from a stall may be impossible.
Every preflight weight-and-balance calculation exists to confirm the aircraft is inside its envelope. Regulatory compliance matters too: 14 CFR §91.9 prohibits operating an aircraft outside its weight and balance limits. Understanding the envelope is not just exam knowledge. It is a direct safety skill.
Key Takeaways#
- The CG envelope defines the safe forward and aft limits for an aircraft's center of gravity.
- CG position is plotted against gross weight on a loading graph from the POH.
- CG shifts during flight as fuel burns and passengers or cargo move.
- A forward CG increases control forces; a rearward CG reduces stability and control.
- Operating outside the envelope violates 14 CFR §91.9 and creates serious safety risks.