Spin vs. Oscillation vs. Pendulum
Spins, Oscillations, and Pendulums – Know the Difference
Not all load movement beneath the aircraft is the same—and treating it the same can make things worse.
In a previous posts, we covered the prevention and management of spins and pendulums. This follow-on is about recognition: understanding the type of motion you’re seeing so you can apply the right correction.
Crews will typically encounter three primary types of load movement:
Spin
A rotation about the vertical axis of the cable.
This is driven primarily by rotor wash interacting with the surface area of the load—most commonly in the turbulent flight zone (TFZ).
Oscillation (Circular Motion)
A wider, circular movement that may include rotation.
This is usually caused by a combination of rotor wash and aircraft position, often influenced by terrain or obstacles that funnel or distort airflow.
Pendulum (Linear Swing)
A side-to-side or fore-aft swing along a relatively straight path.
This is most often caused by hoisting a load that is not plumb, and/or by abrupt changes in aircraft motion (abrupt or aggressive position corrections).
These distinctions matter because the corrections are different.
A pendulum is primarily an aircraft-driven problem:
• Caused by position (not plumb on pickup) and/or abrupt control inputs.
• Managed by pausing the hoist or paying out cable to help dampen the swing.
Spins and oscillations are primarily airflow-driven problems:
• Caused by rotor wash, especially in or near the TFZ.
• Managed by changing the airflow around the load.
• Minimizing time in the TFZ reduces the likelihood and severity.
A common mistake is treating a spin like a pendulum—pausing the hoist while remaining in the TFZ—or treating a pendulum like a spin by rapidly hoisting to “get through” the TFZ with minimal impact. Both can make the situation worse.
In both cases, a smooth transition to forward or lateral airspeed will have a stabilizing effect.
The bottom line:
Pendulums generally come from aircraft movement or taking a load that isn’t plumb. Spins and oscillations generally come from the rotor wash.
Correctly identifying the type of motion is the first step in applying the right solution—and preventing a manageable situation from escalating.