Cable Management – The Pendulum Considerations
Understanding how our inputs affect a pendulum allows us to either control the system—or unintentionally amplify it.
In helicopter hoist operations, we will inevitably create a pendulum anytime we recover a load that is not plumb. At other times, we might create a load pendulum with an abrupt aircraft movement. Most of the time, a pendulum is manageable. Under the wrong conditions, it can become a dangerous situation.
Early in my career, I heard accounts of rescue swimmers being swung nearly level with the aircraft while attached to the hoist. I dismissed it as exaggeration—it didn’t seem physically possible. That changed when I began reading accounts of cables contacting the helicopter undercarriage or wrapping around parts of the aircraft (e.g., landing gear). After digging into the physics of a pendulum—and more importantly, seeing how our control inputs interact with it—I realized not only is it possible, it’s predictable.
These situations often begin in a dynamic, time-pressure environment. A hoist rider is recovered at an angle due to an immediate hazard—a large breaking wave, an aircraft upset, or an unstable platform. The priority is correct: get them clear of danger. But in doing so, crews often do two things simultaneously:
- Rapidly increase the rate of cable being taken in.
- Aggressively increase aircraft altitude.
Individually, each action can be manageable. When combined—and applied without regard to timing—they can significantly amplify the pendulum. In some cases, the result is a rapidly increasing amplitude that becomes difficult to control.
What’s Actually Happening (Simple Physics)
A hoisted load behaves like a classic pendulum:
- The length of the cable determines the timing of the swing.
- The angle of displacement reflects how much energy is in the system.
- Gravity is constantly working to return the load to vertical.
When the load is displaced, energy is stored. As it swings back toward centerline, that energy converts to speed. Left alone, the system will oscillate and gradually dampen.
But here’s the key:
Our inputs can either remove energy from the system—or add to it.
- Shortening the Cable.
When you take in cable on a swinging load:
- The swing rate increases (shorter pendulum = faster oscillation).
- The existing energy is compressed into a smaller radius.
If this is done without regard to timing, the result is often a faster, more aggressive swing with larger angles.
- Climbing at the Wrong Time (Vertical Aircraft Movement).
This is the piece that often gets overlooked.
Think of the helicopter as the pivot point of the pendulum. When you climb, you are moving that pivot.
- If you pull up while the load is moving away from vertical or near the edge of the arc, you are adding energy into the system.
- You are effectively working against gravity at the exact moment it is trying to slow the load.
The result is a larger swing on the opposite side.
Another way to think about it:
You are unintentionally pushing the swing at exactly the wrong moment.
Conversely:
- If vertical input is applied as the load passes directly beneath the aircraft, the likelihood of adding energy is minimized—and in some cases, the system can begin to stabilize.
- Dampening the Pendulum.
If altitude and conditions allow, there is another option that can be missed:
- Paying out cable and/or descending the aircraft.
This effectively does the opposite of what amplifies the pendulum:
- It lengthens the system (slowing the swing rate).
- It reduces tension and energy concentration.
- It allows gravity to do its job and bring the load back toward vertical.
Done smoothly and with awareness of obstacles, this can significantly dampen the pendulum and regain control of the system.
The Combined Effect
When rapid hoisting and aggressive climb occur with a pendulum swing:
- The cable shortens (increasing swing rate).
- The pivot point rises (adding energy).
- The load is already displaced.
That combination can quickly drive the pendulum to large, potentially dangerous angles.
This is how a manageable situation becomes an extreme one.
Try It Yourself (Simple Demonstration)
You can feel this in less than a minute with a cord and a small weight (a computer or phone charger works well).
1. Hold a cord or a line with a weight on the end.
2. Start a pendulum.
3. Observe.
Let it swing naturally and note the rhythm.
4. Shorten the cord.
While it’s swinging, run the cord through your fingers.
What you’ll notice:
- The swing speeds up.
- The arc often becomes more aggressive.
- The motion is less controlled.
5. Start a pendulum again then simulate the helicopter aggressively climbing at the wrong time.
As the weight is moving away from center or near the edge of the arc, raise your hand.
What you’ll notice:
- The next swing is larger.
- The system gains energy.
- It becomes easier to lose control.
If you repeat this a few times, you can drive the swing to surprisingly large angles.
If you combine pulling up with shortening the cord you can see the combined effect.
6. Now do it with timing and coordination.
Let the pendulum stabilize.
This time:
-Shorten the cord only as it passes directly beneath your hand.
-Apply any upward movement at the bottom of the swing.
What you’ll notice:
- The system remains controlled.
- The swing does not amplify.
- The motion may begin to dampen.
7. Now dampen it.
Start the pendulum again.
This time:
-Lengthen the cord slightly (feed it out through your fingers).
-Lower your hand slightly while it swings.
What you’ll notice:
- The swing slows down.
- The arc reduces.
- The system becomes easier to control.
Operational Implication
Cable management during a pendulum is not just about rate—it’s about timing and coordination.
- Taking in cable without timing can amplify the pendulum.
- Climbing at the wrong moment can inject energy into the system.
- Doing both together can escalate the problem rapidly.
At times, the best way to regain control is not to do more—but to do less, or even reverse the input:
- Time your inputs with the natural motion of the pendulum.
- Avoid abrupt, uncoordinated control inputs.
- If conditions allow, pay out cable and/or decrease altitude to reduce—not increase—energy in the system.