Why are mass and balance important?

Incorrect mass or CG position is among the most frequent accident causes in general aviation — it causes structural damage, performance loss, stall/spin tendency and control problems. Three areas to distinguish:

1. Structural effects of mass

Maximum Take-Off Mass (MTOM)

The MTOM is the highest mass at which the aircraft may take off.
If exceeded, higher forces act on all structural parts than tested for certification:
- Wing spar: higher bending load,
- Landing gear: higher load on touchdown and on the runway,
- Engine mount: stronger vibrations and dynamic loads,
- Fuselage: more bending in turbulence and manoeuvres.

Maximum Landing Mass (MLM)

On some types lower than MTOM — the landing gear and runway structure is sized for a particular touchdown load.
Exceeding on landing → gear, tyre, and bearing damage.

Maximum Zero Fuel Mass (MZFM)

Maximum mass without fuel in the wing tanks.
Limits wing-root bending load — fuel in the wings relieves the root in flight.

Baggage compartment limits

Each baggage compartment has a specific maximum mass (e.g. C172: Baggage Area 1 = 54 kg, Area 2 = 23 kg).
Exceeding → floor or seat damage.

2. Performance effects of mass

Take-off performance

Higher mass → longer take-off distance (rule of thumb: 10% more mass → about 20% longer TOD).
Liftoff speed scales with the square root of mass — higher speed = more acceleration distance.
Climb gradient decreases drastically — critical at obstacle-rich airfields.

Cruise

Higher mass → higher stall speed → higher minimum speed → less speed margin.
Higher mass → more fuel burn at a given cruise speed (more induced drag).
Higher mass → lower range and endurance.

Landing performance

Higher mass → longer landing distance and higher touchdown speed.
Longer brake-out on the runway.

Manoeuvring

Higher mass → Va may need to be increased (see Va lesson, because the stall-load-factor relationship changes with mass).

3. CG position effects on stability and control

Forward CG

Consequences:

More longitudinal stability — the aircraft pulls back to trim position more strongly.
Higher stall speed — elevator must offset more lift loss.
More elevator pull required for rotation at take-off — take-off distance increases.
Heavier controls — higher control forces.
Reduced flare effectiveness on landing — flatter touchdown.
More difficult deep-stall recovery.

Aft CG

Consequences:

Reduced longitudinal stability — heavy stability oscillations, easier overshoot.
Lower stall speed (stall speed drops with aft CG).
Lighter controls — lower control forces (can be deceptive).
Higher cruise speed at the same fuel burn (less trim drag).
Risk: hard-to-recover spin, because stability is missing.
Beyond the aft CG limit: uncontrollable structurally and aerodynamically — aircraft cannot be recovered from stall.

CG effects on performance

Forward CG → more induced drag at the tailplane (down-force) → higher effective lift demand → more drag → lower TAS and range.
Aft CG → less down-force at the tail → less effective lift required → less drag → slightly higher TAS and range.

But the aft CG is more dangerous, hence the CG envelope is defined tightly.

CG envelope

The AFM defines a manufacturer-approved CG envelope:

Forward CG limit (e.g. C172: 35.0" behind datum at MTOM).
Aft CG limit (e.g. C172: 47.3" behind datum at MTOM).
Within this envelope stability, control and performance are guaranteed.

Practical rule: load the aircraft so CG is forward within the envelope — gives more safety margin against stall and spin.