Definition
A steep, descending turn in which the airplane is banked, the nose is well below the horizon, airspeed is increasing rapidly, and altitude is being lost at a high rate. The wings are still producing lift (the airplane is not stalled), which distinguishes a spiral dive from a spin. If not corrected, airspeed and load factor can quickly exceed structural limits.
Plain English
The airplane is banked over and diving toward the ground in a tightening turn, picking up speed fast and losing altitude fast. The wings are still flying, but the situation will get dangerous in a hurry if the pilot doesn't recover.
Context Anchor
Encountered in stall, spin, and unusual-attitude training, especially when learning to tell a true spin from a steep, fast descending turn.
Derivation
Spiral' comes from the Latin 'spira', meaning a coil or twist. The flight path coils downward around an imaginary vertical line, which is exactly what a spiral dive looks like from above.
Why Pilots Care
If not recovered promptly by reducing power, leveling the wings, and easing out of the dive, airspeed can exceed structural limits and cause damage or loss of control.
Grounding Statement
Picture the airplane circling downward with the nose low, getting faster each second unless the pilot stops the turn and raises the nose carefully.
Intuition Check
Do not assume a spiral dive is the same as a spin. In a spin, the wing is stalled; in a spiral dive, the airplane is still flying but descending in a steep turn and accelerating.
Example Sentence 1
After entering cloud without an instrument rating, the pilot let a shallow turn steepen into a spiral dive and had to reduce power and level the wings before pulling out.
Example Sentence 2
The instructor demonstrated recovery from a spiral dive by first reducing power and then rolling the wings level before pulling out.