9.5 Telescopic Boom Attachments: Swing-Away Extensions, Jibs, Erecting & Stowing
Key Takeaways
- Telescopic-boom attachment setup and disassembly is an explicit Crane Knowledge topic on the NCCER Rigger written assessment, and the Basic Rigger focus statement expects comprehension of erecting and stowing jibs and extensions.
- A swing-away extension (fly) is stowed alongside the boom base section and erected by pinning it to the boom head — it is not a telescoping section and does not extend hydraulically.
- Extensions are typically offsettable at fixed detents such as 0, 15 or 17, 30, and 45 degrees; every offset angle has its own load chart column and its own reduced capacity.
- A stowed extension still costs capacity: manufacturers publish a stowed-jib deduction that must be subtracted from main-boom gross capacity, and an erected-but-unused jib costs far more because it adds a forward moment arm.
- Erecting or stowing an extension is a defined assembly/disassembly operation under OSHA 1926.1404, which requires an A/D director, employer procedures, and a qualified rigger whenever workers are in the fall zone.
9.5 Telescopic Boom Attachments: Swing-Away Extensions, Jibs, Erecting & Stowing
The Rigger written assessment lists "Telescopic-Boom Attachment Setup and Disassembly" as a Crane Knowledge topic, and the Basic Rigger focus statement expects "basic comprehension of erecting and stowing jibs/extensions and their relevance to rigging" plus "knowledge of crane operations involving telescopic booms and swinging jibs." NCCER module 21302, Telescopic Boom Attachment Setup and Assembly, is the source curriculum.
Riggers are not usually the ones pinning the jib — but riggers are the ones standing in the fall zone during the operation, and riggers are the ones whose lift plan lives or dies on whether the attachment is stowed or erected.
The Attachment Family
| Attachment | What it is | How it behaves |
|---|---|---|
| Swing-away extension (fly, fly jib) | A lattice or box section stowed along the side of the boom base section and swung out and pinned to the boom head | Adds length and height at a modest capacity. It is not a telescoping section — it does not extend or retract hydraulically with the boom |
| Extendable / two-piece extension | A swing-away that can be pinned at a short or long length, or with an insert added | Each length is a separate load chart column |
| Manual offset jib | Offset angle set on the ground with pins or a bolted link before erecting | Angle is fixed for the entire lift; changing it means bringing the jib back down |
| Power (hydraulic) luffing jib | Offset angle adjusted from the cab through a hydraulic cylinder while erected | Lets the operator change jib angle under controlled conditions, with the chart read at the angle actually set |
| Intermediate boom section / insert | A section pinned into the boom on larger telescopic machines to extend reach | Assembly/disassembly operation with its own procedures |
| Rooster sheave (auxiliary boom nose) | A single sheave pinned to the boom head for a fast, single-part whip line | Small but real chart deduction; lets the crew run a whip line without reeving the main block |
Offset Angles and the Chart
A swing-away extension is erected at a chosen offset angle — the angle between the centerline of the extension and the centerline of the main boom. Typical detents are 0 degrees (in line), 15 or 17 degrees, 30 degrees, and 45 degrees, though the exact values are machine-specific.
The offset matters for two reasons:
- Every offset is its own chart. The capacity at 30 degrees offset is not the capacity at 0 degrees offset. Reading the wrong column is one of the most common causes of an overload on a jib pick.
- Offset changes the radius geometry. The greater the offset, the further the jib head sits horizontally from the center of rotation for the same main-boom angle. Capacity falls accordingly, and the jib head radius must be measured or computed — not assumed to equal the main-boom radius.
Jib capacities are also frequently limited by a maximum main boom angle requirement. Many machines require the main boom above a stated angle before jib operation is allowed at all, because at low main-boom angles the jib produces a moment the boom head simply cannot take.
Erecting and Stowing: The Sequence and the Hazards
STOWED ERECTING WORKING
+===============+ +===============+ +===============+
| Boom base | | Boom base | | Boom base |
| [fly pinned | unpin retaining | [fly swung | pin to boom | |
| alongside] | --> pins, control -->| out on | --> head, set -->| \ fly at |
+===============+ with tag line | its pivot] | offset pins | \ offset|
+===============+ +===============+
The standard sequence is: position the boom per the manufacturer, support the extension before removing any pin, remove the stowing/retaining pins, swing the extension out under control (tag line, never by hand alone), align and pin it to the boom head, install the offset pins or link at the chosen angle, install all keepers and cotters, and re-reeve or transfer the load line as required. Stowing is the same sequence in reverse — and stowing is where crews get hurt, because the extension is being lowered toward them.
The hazards, in the order they injure people:
- Uncontrolled swing. An unpinned extension is a multi-hundred-pound pendulum. It is controlled with a tag line, not with hands.
- Pinch points. Every pin bore is a crush hazard. Fingers never align holes — use a drift pin or an alignment tool.
- Dropped attachment. A partially pinned extension that is released will fall. Nothing is released until the securing pin is fully seated and its keeper is installed.
- Working under a suspended attachment. Nobody stands under the extension during the swing.
OSHA framing: erecting or stowing a boom attachment is assembly/disassembly under 29 CFR 1926.1404. That triggers an A/D director who understands the procedures, use of either manufacturer procedures or employer procedures meeting the standard, and — under 1926.1404(r)(1) — a qualified rigger for assembly/disassembly rigging work, plus a qualified rigger under 1926.1425(c) whenever employees are in the fall zone hooking, unhooking, or guiding.
Why the Rigger Cares: The Capacity Deduction
This is the part that shows up on a rigger exam rather than an operator exam.
| Attachment state | Effect on main-boom capacity |
|---|---|
| Stowed on the boom base section | A specific stowed jib deduction (commonly a few hundred to ~1,500 lb) is subtracted from main-boom gross capacity — the weight is still on the boom |
| Erected at the boom head but lifting on the main boom | A much larger erected jib deduction (commonly 2,000 lb and up), because the jib now hangs out at the boom head on a long moment arm |
| Removed from the crane entirely | No deduction — but removal is itself an assembly/disassembly operation |
| Rooster sheave installed | Manufacturer deduction value, typically 100-350 lb |
The practical planning consequence: if a lift is tight on capacity and the jib is not needed, the correct answer may be to remove the extension entirely rather than accept the erected deduction. That decision belongs in the lift plan, not in the cab at pick time.
What is a swing-away extension (fly jib) on a telescopic boom crane?
A crane is picking on its main boom with the swing-away extension stowed alongside the boom base section. How does the stowed extension affect the lift?
A crew plans to lift with the extension erected at a 30-degree offset. Which statement about reading the load chart is correct?
Under OSHA 29 CFR 1926.1404, how is the operation of erecting or stowing a boom extension classified?