Definition
The CG envelope is the range of allowable center-of-gravity locations, plotted against airplane weight, within which the airplane must be loaded for safe and certified flight. It is shown graphically in the Pilot's Operating Handbook as a closed shape with forward and aft CG limits at each weight. Loading must result in a CG that falls inside this shape; loading outside it is prohibited.
Plain English
It's the boundary on a chart that shows where the airplane's balance point is allowed to be for a given weight. If your loaded airplane plots inside the boundary, you're legal and safe to fly. If it plots outside, you have to rearrange the load.
Context Anchor
Seen in weight and balance calculations, usually in the airplane’s flight manual or weight and balance data before a flight.
Derivation
Envelope' here comes from mathematics and engineering, where it means the outer boundary that contains a set of allowable values. It is unrelated to the paper envelope you put a letter in. Pilots will also meet the same word in 'flight envelope' and 'performance envelope,' all using this same boundary meaning.
Why Pilots Care
Staying inside the envelope keeps the airplane stable and responsive; operating outside it can produce uncontrollable pitch or structural overload.
Analogy
Think of it like a marked safe zone on a map. As long as the airplane’s loaded balance point lands inside that zone, the loading is acceptable; outside it, the loading is not acceptable.
Intuition Check
Do not read envelope here as a physical container. In this context, envelope means the approved boundary or safe zone for the airplane’s balance point.
Example Sentence 1
After adding the passengers' bags to the rear baggage compartment, the pilot replotted the loading and confirmed it still fell inside the CG envelope.
Example Sentence 2
An aft CG near the rear limit of the envelope made the airplane more sensitive in pitch during the flare.