PHAK Ch. 5 · study note

Stability

Stability is the airplane’s inherent tendency to return toward a flight condition after a disturbance (gust, brief control input). It is related to—but not the same as—controllability and maneuverability. Design trades stability against how “snappy” the airplane feels.

Axes

Axis (through CG)	Motion	Stability name	Primary control
Lateral	Pitch	Longitudinal stability	Elevator / stabilator
Longitudinal	Roll	Lateral stability	Ailerons
Vertical	Yaw	Directional stability	Rudder

Static vs dynamic

Ball in bowl positive static, flat neutral, hill negative static stability — **Figure 2.** Classic ball analogy for static stability. Dynamic stability asks whether the return is damped over time.

	Positive	Neutral	Negative
Static (initial tendency)	Returns toward original attitude/path	Stays in new position	Diverges further
Dynamic (over time)	Oscillations damp out	Oscillations continue undamped	Oscillations grow

Designers want positive static and positive dynamic stability for training airplanes. Too much stability can make an airplane sluggish; too little is dangerous.

Longitudinal stability (pitch)

About the lateral axis. Strongly affected by CG location relative to the wing’s center of lift and by the horizontal stabilizer.

Forward CG more stable versus aft CG less stable side views — **Figure 3.** Forward CG: more nose-down tendency, more stable, but can need more elevator for flare / raise stall speed slightly. Aft CG: less stable, can reduce stick force and hurt stall recovery—stay inside AFM CG limits.

Lateral stability (roll)

About the longitudinal axis. Contributors include:

Dihedral — upward wing angle; sideslip raises the low wing.
Keel effect / high wing — side area and wing position act like a weathercock in roll/sideslip coupling.
Sweepback — more advanced designs; less critical for basic trainers.

Directional stability (yaw)

About the vertical axis. Usually the easiest stability to provide: more side area aft of the CG (fuselage + vertical stabilizer) weathervanes the nose into the relative wind. A larger vertical fin → stronger directional stability.

Stability vs controllability (pilot view)

Stable airplanes tend to hold attitude; good for IFR/cruise, easier to trim.
Less stable / aft CG can feel lighter on the stick—easy to overcontrol; respect loading.
Weight & balance is not paperwork only—it is a stability and control limit. See weight and balance and the W&B Lab.

Sources

FAA PHAK Ch. 5 — Stability; longitudinal / lateral / directional; axes; CG effects

Figures are original educational SVGs.

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