A load on a 200 foot longline completes one full swing every 15.7 seconds. The same load on a 25 foot line completes one every 5.5 seconds and arrives at the bottom of the arc nearly three times faster for the same lateral offset. Line length is the single largest control a lift plan holds over suspended-load motion, and it is chosen before anyone leaves the yard.
A suspended load moves in three distinct ways, and they are not variations on one problem. It swings, which is pendulum motion in the horizontal plane. It spins, which is rotation about the vertical axis. It bounces, which is vertical motion in a mass-spring system formed by the aircraft, the line, and the load. Each has a different driver, a different frequency band, and a different countermeasure. Treating all three as one condition called an unstable load is why crews reach for the same fix three times and get a result once.
The distinction is not academic. Airbus Helicopters published a safety notice in October 2023 on vertical oscillation during external sling load operations and opened it by explicitly carving out the other two motions: the notice "does not address pendulum oscillations or spinning of the load, which is another phenomenon that commonly occurs" during sling work. The manufacturer separates them because the recommended pilot response to one is close to the opposite of the response to another.
Swing: the pendulum, and why line length is the decision
A compact load on a single line is a pendulum, and a pendulum has a period that depends almost entirely on the length of the line. Mass does not appear in the equation. A 900 pound generator and a 9,000 pound transformer on the same 150 foot longline swing at the same rate. What changes with line length is the whole character of the motion, and the numbers are worth having in front of a planner before the line is picked.
| Line length | Full swing period | Frequency | Full offset to bottom of arc | Swing angle for a 10 ft offset | Load speed at bottom of arc |
|---|---|---|---|---|---|
| 25 ft | 5.5 s | 0.181 Hz | 1.4 s | 23.6 deg | 11.6 ft/s (7.9 mph) |
| 50 ft | 7.8 s | 0.128 Hz | 2.0 s | 11.5 deg | 8.1 ft/s (5.5 mph) |
| 75 ft | 9.6 s | 0.104 Hz | 2.4 s | 7.7 deg | 6.6 ft/s (4.5 mph) |
| 100 ft | 11.1 s | 0.090 Hz | 2.8 s | 5.7 deg | 5.7 ft/s (3.9 mph) |
| 125 ft | 12.4 s | 0.081 Hz | 3.1 s | 4.6 deg | 5.1 ft/s (3.5 mph) |
| 150 ft | 13.6 s | 0.074 Hz | 3.4 s | 3.8 deg | 4.6 ft/s (3.2 mph) |
| 200 ft | 15.7 s | 0.064 Hz | 3.9 s | 2.9 deg | 4.0 ft/s (2.7 mph) |
AHLH arithmetic, not measured flight data. Period from T equals 2 pi times the square root of L over g, with g taken as 32.174 ft/s squared. The last two columns model a load displaced 10 feet horizontally from the rest position and released, with speed at the bottom of the arc from conservation of energy. The model treats the load as a point mass on a rigid weightless line and ignores aerodynamic drag, line stretch, and the aircraft's own motion, all of which a real pick has. Line lengths are the standard stock sizes published by Barry Cordage for synthetic helicopter longlines: 25, 50, 75, 100, 125, 150, and 200 feet.
Two things fall out of that table, and they point the same direction. The first is cadence. On a 25 foot line the load reaches the bottom of its arc 1.4 seconds after release, which is inside the reaction and response lag of a pilot flying a correction. On a 200 foot line the same event takes 3.9 seconds, which is enough time to move the aircraft over the load rather than chase it. The second is arrival speed. Displace a load ten feet sideways and let it go, and on a short line it crosses the set point at nearly 8 mph, while on a 200 foot line it crosses at under 3 mph. Long lines do not merely feel calmer. They are quantitatively gentler, because ten feet of offset is a 23.6 degree swing on a short line and a 2.9 degree swing on a long one, and the restoring force scales with that angle.
This is the physical reason a longline is standard equipment on precision placement rather than an accessory, and it sits alongside the reasons already covered in the rigging analysis of slings, angles, and design factors: standoff from downwash, visibility of the set point, and keeping the rotor clear of tall structures. The longline also carries a stiffer design factor precisely because it is a single load path, seven to one for non-human external cargo against five to one for a multi-leg sling arrangement.
The counterweight to all of this is that a longer line takes longer to stop as well as longer to start. Energy put into a 200 foot pendulum stays there a long time, because a pendulum is a low-damping system by construction. The practical technique, which is why vertical reference flying is treated as a discrete skill, is to position the aircraft so the swing is never excited rather than to damp it after the fact. Abrupt lateral cyclic, an over-quick pickup that snatches the load off the ground, and a hover that drifts and then corrects are all inputs at or near the pendulum frequency, and inputs near a natural frequency are what build amplitude.
Spin: rotation about the vertical axis, and the two sources behind it
Spin is a different animal because it has two unrelated sources and the fix for one does nothing for the other.
The first source is torsional wind-up in the rigging itself. A rope or wire longline under load carries stored twist, and a load hung from a single point has nothing resisting rotation. The military manuals treat this as a hardware problem and solve it with a swivel. The Marine Corps and Navy releasable swivel hook pendant, an 8 foot assembly weighing about 53 pounds with a 12,000 pound safe working load, is specified explicitly so that "the swivel prevents the load lifting sling from wrapping up if the load twists." That is the honest description of what a swivel does. It protects the line from being wound into itself. It does not stop the load rotating.
The field criterion for a single-point pickup with a reach pendant, published in the multiservice sling load manual, is one of the few hard numbers in the literature on rotation. If there is no twisting the lift may continue, but "if the load spins more than 360 degrees the strength of the reach pendant weakens and the lift should be stopped immediately," and if the load turns a full revolution and then does not begin to unwind in the opposite direction, the load is unacceptable for that pendant.
The second source is aerodynamic, and it scales with airspeed rather than with rigging. A bluff, flat-sided load in an airstream sheds vortices alternately off its sides, and the alternating pressure produces a yawing couple. Once the load is turned even slightly out of the airstream that couple can reinforce rather than correct, and the object will rotate continuously. NASA Ames work on a UH-60 carrying a 6 by 6 by 8 foot CONEX container found the container starts spinning in the hover and that the spin rate increases with airspeed, which is why the load was limited to 60 KIAS in service. The same study found that fitting a 3 by 5 foot vertical fin to the container extended the usable airspeed to 110 knots.
That result is the whole lesson in one line. A swivel would not have moved the 60 knot limit at all. A surface giving the load a preferred heading in the airstream moved it by 50 knots. The commercial equivalent is the drogue, a modified round or cruciform parachute canopy tethered to the trailing end of a load, described by suppliers as a device "to induce drag and maintain directional stability to an underslung load" and sized against the weight, surface area, and planned airspeed of the specific object. A drogue and a fin are the same idea executed in fabric or in steel, and both share the same limitation: they need airflow. In a hover over the set point, at the moment placement precision matters most, a drogue is limp and a fin is inert.
The remaining countermeasure is the ground crew. Tag lines let a crew orient a load through the final feet of a placement, and they work in the hover where aerodynamic devices do not. The constraint on them is written into the OSHA helicopter crane standard, which requires that "tag lines shall be of a length that will not permit their being drawn up into rotors." The same standard governs the rest of the receiving crew's exposure, including the requirement at paragraph (j) that static charge on the suspended load be dissipated with a grounding device before anyone touches it. Tag line handling is a rehearsed job rather than an improvised one, and it belongs in the same briefing as the PPE the receiving crew wears under 1926.551.
OSHA 1926.551 Ground crew rule
Bounce: the mass-spring system, and the one the pilot is part of
Vertical oscillation is not pendulum motion at all. Airbus describes it plainly: a slung load "behaves somewhat like a mass hanging on a spring", and the sensitivity and frequency of the resulting bounce depend on the elasticity of the longline, the damping in the line and its connectors, the mass of the load, and the aerodynamics of the object. Turbulence, a control input, rotor airflow on the load, or the moment the load leaves the ground can all start it. Usually it damps out. Sometimes it does not, and the manufacturer's account of what happens then is blunt: "in most cases, releasing the external load was the only way to get rid of the bouncing."
What turns a damped bounce into a divergent one is usually the pilot, and the notice separates two distinct mechanisms by frequency. Pilot induced oscillation is active participation in the control loop and occurs up to 2 Hz, where the pilot is attempting to correct an oscillation and the lag between input and aircraft response puts each correction out of phase with the motion. Pilot assisted oscillation is passive and occurs from 2 Hz to 8 Hz, where the pilot's own body, shaken by the aircraft, feeds involuntary motion back through a hand resting on the collective. The pilot's muscular system, in the manufacturer's description, "acts like its own spring mass that amplifies the up and down motion of the seat."
Both bands sit an order of magnitude above the pendulum frequencies in the table above, which run from 0.064 Hz to 0.181 Hz. That gap is the reason the two problems cannot be flown the same way. A swing is countered by moving the aircraft. A bounce is countered by taking the hand off the collective. Airbus lists the response in order: disconnect any autopilot upper modes, relax the grip on the controls, avoid jerky inputs, release the collective lever entirely if operationally possible, reduce airspeed in cruise, and as an ultimate measure jettison the load. It also notes that adding friction to the collective, on aircraft with an adjustment for it, damps the feedback path before an oscillation can build.
One design implication follows for the rigging chain. Compliance in the load path sets where the bounce frequency lands, so a nylon web sling, a polyester roundsling, a synthetic longline, and a wire rope longline are not interchangeable from an oscillation standpoint even when their working load limits match. That is a question worth asking of a lift plan rather than assuming, and it belongs with the other questions raised when a lift is planned from site survey to load sign-off.
Shape governs stability, and weight does not
The most counterintuitive finding in the sling load literature is that heavy loads are frequently more stable than light ones. Military sling load certification records a maximum tested stable airspeed for every certified load, defined as the fastest the aircraft flew in straight and level flight during the evaluation before the load became unstable or the aircraft ran out of power. Read down that record and the correlation with weight is close to absent.
| Load | Rigged weight | Shape class | Published stable airspeed |
|---|---|---|---|
| Collapsible fabric tank, 10,000 gallon, empty | 1,040 lbs | Light, large area, limp | May become unstable above 35 kt |
| HB2000 Heli-Basket, empty | 640 lbs | Open frame basket | 120 kt |
| HB2000 Heli-Basket, loaded | 5,100 lbs | Open frame basket | 100 kt |
| Loaded armament carrier, M1151, under a CH-47 | 11,500 lbs | Dense wheeled vehicle | 90 kt |
| CONEX container, 6 x 6 x 8 ft, under a UH-60, unmodified | Study configuration | Bluff flat-sided box | 60 KIAS |
| Same CONEX with a 3 x 5 ft vertical fin fitted | Study configuration | Bluff box with a tail surface | 110 kt |
Compiled by AHLH from published sources, not measured here. The fabric tank caution, the Heli-Basket speeds, and the M1151 entry are from the multiservice helicopter sling load manuals TM 4-48.09 (23 July 2012) and TM 4-48.10 (5 July 2013). CONEX figures are from Cicolani and Ehlers, NASA Ames, NTRS 20030064141. The manuals state that for both certified and suitable loads the listed airspeed "is a recommendation and not a restriction, unless so stated", and that it reflects a single flight evaluation rather than a certified limit. These figures describe military airframes and military rigging, and are used here as published aerodynamic evidence, not as civil operating limits.
A 1,040 pound empty fabric tank is cautioned above 35 knots. An 11,500 pound armored vehicle flies at 90. The manual states the underlying rule directly: low-density equipment with low weight and large flat surface area, naming shelters, empty trailers, pallet loads, boat-shaped items, and empty fuel or water drums, is "likely to become extremely unstable when flown during sling load operations, even at low airspeeds." Density, not mass, is the predictor, because an object with a large surface area and little inertia is easily pushed around by the air it presents to.
The manual's own remedy for that condition is the one contractors least expect: add weight. Where a trailer is unstable when light, a minimum rigged weight is published, and crews are instructed to add cargo or dummy weight as near the centre of the item as possible until that minimum is reached. Ballast costs hook capacity, which has to come out of the payload budget after density altitude has already taken its share off the top, and it is one reason a light bulky object can consume more aircraft than its weight suggests. The same reasoning runs in reverse through the capacity tables from light twins up to the Mi-26, where a rated hook figure describes what the aircraft can hold, not what a given shape can be flown with.
What a lift plan should say about motion
Load motion is decided long before the aircraft arrives, and it is legible in a plan to anyone who knows what to look for. Six items cover it.
- Line length, stated with a reason. Not simply the word longline. The length chosen, and whether it was chosen for standoff, for visibility of the set point, for rotor clearance from the structure, or for the slower pendulum period the table above quantifies.
- An anti-rotation measure named for each source. A swivel addresses line wind-up. A fin or drogue addresses aerodynamic yaw in forward flight. Tag lines address orientation in the hover. A plan that names one of the three and calls rotation handled has covered a third of the problem.
- A planned transit airspeed for the specific load shape. Bluff, flat-sided, or low-density objects need a stated speed and a stated response if the load begins to turn, which per the manufacturer's guidance is to slow down, since aerodynamic excitation in forward flight usually reduces when airspeed reduces.
- A ballast decision for light bulky items. Either a stated minimum rigged weight and how it is made up, or an explicit finding that the item does not need one.
- The compliance in the load path. Which line and which slings, since their elasticity sets where the vertical bounce frequency falls, and the pilot's ability to stay out of that loop depends on where it falls.
- The jettison plan. Every source consulted here, military and manufacturer alike, ends the divergent-oscillation sequence at the same place: release the load. That means the fall zone under the transit route is a planned area with nobody in it, which is a requirement a congested-area operation carries anyway.
The jettison item deserves emphasis because it reframes the other five. A load that has to be released is a load lost, a schedule broken, and a fall zone tested for real. Every countermeasure above, from line length through fin sizing to keeping a loose hand on the collective, exists to keep the sequence from reaching that step. The regulatory frame around that release, and around the routes and fall zones it presumes, sits under the external-load certificate itself.
14 CFR 133 External load certificate 14 CFR 133.33 Operating rules
None of this is exotic engineering. It is a set of questions with published answers, asked in the right order, before a load leaves the ground. The physics of a swinging line has not changed since the first external load, and the discipline that keeps a suspended object flying where it is pointed is the same discipline that governs external load operations generally and that shapes how aerial construction work is bid and sequenced.
