An instructor reviews an airplane's CG envelope chart with a student. The chart shows the forward CG limit is 82.0 inches aft of datum at the maximum gross weight of 2,700 lb, and 85.0 inches aft of datum at a weight of 2,000 lb. The airplane is loaded to a gross weight of 2,100 lb with a computed CG of 83.5 inches aft of datum. Using linear interpolation between the two charted weight points, what should the instructor conclude?
- AThe loading is within limits because CG forward limits only apply at maximum gross weight
- BThe loading is within limits because 83.5 inches is aft of the forward limit at maximum gross weight (82.0 inches)
- CThe loading is out of limits because the CG exceeds the aft limit for this weight
- DThe loading is out of limits because the forward limit at 2,100 lb interpolates to approximately 84.6 inches, which is aft of 83.5 inches
Show answer & explanationAnswer & explanation
Correct answer: D. The loading is out of limits because the forward limit at 2,100 lb interpolates to approximately 84.6 inches, which is aft of 83.5 inches
The forward CG limit moves aft as weight decreases. Interpolating between (2,000 lb, 85.0 in) and (2,700 lb, 82.0 in): slope = (82.0−85.0)/(2,700−2,000) = −0.00429 in/lb. At 2,100 lb: 85.0 + (100)(−0.00429) ≈ 84.6 in. Since the actual CG of 83.5 in is forward of the 84.6 in limit, the loading is too far forward and out of limits.
Why the other options are wrong
- A. CG limits vary with weight across the entire envelope, not just at maximum gross weight.
- B. Incorrectly applies the max-gross-weight limit instead of the limit for the actual loaded weight.
- C. The problem involves the forward limit, not the aft limit, so this conclusion misidentifies the boundary being violated.
CG Envelope Limits Vary with Weight
On many airplanes, the forward CG limit moves aft (becomes more restrictive) as gross weight decreases, because a lighter airplane loaded too far forward may exceed elevator authority for landing flare or stall recovery.
- CG limits are read from a graph or table specific to the airplane's actual weight, not just max gross weight.
- Interpolation between charted weight points is required when the exact weight isn't listed.
- A CG forward of the interpolated forward limit is unsafe even if it would be acceptable at max gross weight.
Memory trick: Light and forward is a dangerous pair — the limit creeps aft to keep control fair.