FAA Private Pilot Airplane (PAR)Aerodynamics and Aircraft SystemsHard
While cruising in a bank, a pilot becomes distracted and fails to notice that the bank angle is slowly increasing while the nose gradually drops, causing airspeed and vertical speed to increase. This is a classic description of which condition?
- ASpiral instability (graveyard spiral)
- BDutch roll
- CAdverse yaw divergence
- DPhugoid oscillation
Show answer & explanationAnswer & explanation
Correct answer: A. Spiral instability (graveyard spiral)
Spiral instability occurs when an airplane, if left undisturbed in a bank, has a tendency for the bank angle to increase further rather than return to wings-level. This produces a descending spiral with increasing airspeed and bank angle if not corrected — often called a graveyard spiral, especially dangerous in IMC or spatial disorientation situations.
Why the other options are wrong
- B. Dutch roll is a coupled yaw-roll oscillation, not a continuously steepening one-directional spiral.
- C. Adverse yaw relates to aileron-induced yaw during roll initiation, not a sustained developing spiral.
- D. A phugoid is a long-period pitch oscillation in airspeed and altitude, not a continuously steepening bank.
Spiral Instability
Spiral instability exists when a banked airplane's bank angle tends to increase further rather than return to wings-level, resulting in a descending, tightening spiral if uncorrected.
- Bank angle self-increases, unlike stable dihedral response
- Airspeed and descent rate increase as spiral tightens
- Often called a 'graveyard spiral,' especially hazardous in IMC
Memory trick: Ignore the bank, and it spirals to the ground.