13.2 Jacking with Locking Collars, Chocking & Severe Weather Mooring

13.2 Jacking with Locking Collars, Chocking & Severe Weather Mooring

Approved-Data Control

The figures and hardware examples in this section illustrate principles. For an actual aircraft or component, current approved maintenance data, product instructions, organisation procedures, and applicable law control the material, limit, interval, sequence, tooling, PPE, and acceptance decision.

Lifting an aircraft off the ground for landing gear retraction tests, structural alignment checks, maintenance weighing, or component replacement involves immense concentrated loads and severe airframe tipping hazards. Similarly, securing an aircraft parked on an exposed ramp against high winds, severe gusts, and tropical storms demands rigorous engineering controls. Under EASA Part-66 Module 7 (Maintenance Practices), certifying maintenance engineers must master the mechanical principles, safety hardware, and environmental precautions governing aircraft jacking, wheel chocking, and structural mooring.

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Jack Classifications & Jacking Point Geometry

Two fundamental categories of hydraulic jacks are utilized in aircraft maintenance:

                         AIRCRAFT JACK CLASSIFICATIONS

        [Tripod Airframe Jack]                     [Axle / Bottle Jack]
        - Heavy airframe lifting                  - Single/dual wheel changes
        - Full gear retraction tests              - Brake assembly replacements
        - Multi-point synchronized lift           - Lifts individual landing gear axle
        - Threaded mechanical locking collar      - Quick, portable line maintenance

1. Tripod Airframe Jacks

Tripod airframe jacks are heavy, variable-height hydraulic jacks consisting of three wide-stance tubular steel legs, a central vertical hydraulic cylinder, a hand or pneumatic hydraulic pump, and a precision-ground extendable ram. Tripod jacks lift the entire aircraft clear of the ground, permitting retraction and extension functional tests of the landing gear, structural leveling, or maintenance weighing.

2. Axle Jacks (Bottle & Alligator Jacks)

Axle jacks are low-profile, high-capacity hydraulic bottle jacks designed to engage a dedicated jacking spud located directly on the base of an individual landing gear bogie beam or axle housing. They lift only one wheel or axle assembly by a few centimeters, facilitating rapid tire, wheel, and brake changes during line maintenance without lifting the remainder of the airframe.

3. Jacking Points and Adapters

Every certified aircraft possesses designated, reinforced jacking points engineered to support the concentrated point loads of the lifted airframe without skin wrinkling or spar distortion:

• Standard Locations: Primary jacking points are located beneath the forward fuselage (or nose gear keel) and beneath the main wing front and rear spars inboard or outboard of the engine nacelles. An auxiliary aft jacking point is typically provided on the lower rear fuselage.
• Jacking Pads & Adapters: Aircraft manufacturers install female hemispherical jacking pads or detachable pad adapters onto the airframe. The ram of the hydraulic jack terminates in a matching spherical ball-cup adapter. This ball-and-socket geometry ensures that as the aircraft changes pitch or dihedral angle during lifting, the jack pad self-aligns, eliminating side-loading that could cause the jack ram to bend or the aircraft to slip off the jack head.

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Pre-Jacking Weight, Balance & Environmental Verification

Jacking an aircraft without verifying its internal mass distribution can result in structural failure or violent tipping onto the nose or tail cone.

                     AIRCRAFT JACKING SAFETY ENVELOPE

                     [Center of Gravity (CG)]
                               v
          Fwd Jack          Main Jacks             Aft Jack / Tail Stand
             |                  |                           |
             v                  v                           v
     (O)========================O===========================(O)
             ^                  ^                           ^
      [Nose Jack Pad]    [Wing Jack Pads]           [Mandatory Tail Stand]

1. Center of Gravity (CG) Verification & Tail Support

Before positioning tripod jacks beneath an airframe, the certifying engineer must calculate the aircraft's current Center of Gravity (CG) based on remaining fuel, cargo, and removed components:

• Tail Tipping Risk: On transport aircraft with swept wings and aft-mounted engines (or when engines are removed from wing pylons during overhaul), the empty CG shifts rearward. If the CG is allowed to move aft of the main wing jacking points, raising the aircraft will cause it to pitch violently up and slam its tail onto the hangar floor, inflicting catastrophic structural damage to the rear pressure bulkhead and empennage.
• Mandatory Tail Stand / Trestle: The AMM mandates that whenever an aircraft is jacked on tripod jacks, a certified weighted tail stand or mechanical tail trestle must be installed beneath the aft fuselage tie-down/jacking fitting. The tail stand is preloaded with certified ballast (often 500 to 2,000 kg) to physically hold the empennage down. Alternatively, approved nose ballast must be loaded into the forward cargo bay.

2. Hangar Environmental Mandates

Aircraft jacking must take place inside an enclosed hangar on a flat, level, high-strength reinforced concrete floor:

• Hangar Doors Closed: All hangar sliding or rolling doors must remain fully closed throughout the jacking procedure. Ambient cross-winds or sudden thermal gusts acting across high-aspect-ratio wings generate powerful aerodynamic moments and side loads that can push an airframe laterally off its spherical jacking pads.
• Prohibition on Asphalt/Tarmac: Jacking on asphalt, macadam, or soft tarmac is strictly prohibited unless heavy steel load-distribution plates certified in the AMM are placed beneath the tripod jack footpads. Under concentrated loads of tens of thousands of pounds per square inch, tripod jack legs can sink into soft asphalt, causing catastrophic tilting and airframe collapse.

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The Mechanical Locking Collar Safety Protocol

Modern hydraulic jacks utilize fluid pressure to elevate the ram. However, hydraulic systems are vulnerable to hydraulic seal extrusion, hose bursting, manifold cracking, or check valve seat failure. Hydraulic pressure must NEVER be trusted as the sole support mechanism for an elevated aircraft.

                    TRIPOD JACK LOCKING COLLAR OPERATION

              +=========================================+
              |        Spherical Jack Head Adapter      |
              +=========================================+
                                   | |
                                   | | <--- Threaded Ram
                 +-----------------+ +-----------------+
                 |   THREADED MECHANICAL LOCKING COLLAR |
                 |  (Spun down continuously by hand)    |
                 +-----------------+ +-----------------+
                 =========================================
                 |        JACK CYLINDER BARREL           |
                 |      [Hydraulic Oil Reservoir]        |
                 |                                       |

1. Mechanical Locking Collar (Safety Ring)

The extendable ram of an airframe tripod jack features heavy-duty external Acme threads along its entire length. A large cast-steel or bronze mechanical locking collar (threaded safety collar/nut) is threaded onto the ram.

2. Operating Protocol During Lifting

• As the jack ram extends upward under hydraulic pressure, an assigned technician must continuously wind the mechanical locking collar downward by hand, keeping it within 1 to 2 millimeters of the top rim of the hydraulic cylinder barrel.
• If hydraulic pressure suddenly drops due to seal failure, pump line rupture, or operator error, the descending ram travels less than two millimeters before the locking collar crashes solidly against the steel cylinder rim. The entire weight of the aircraft is immediately transferred to mechanical steel-on-steel compression, preventing any perceptible airframe drop.
• Leaving the Collar Loose is Strictly Prohibited: If a technician allows the ram to lift 30 cm into the air while leaving the locking collar at the bottom or top of the ram, a hydraulic failure will cause the aircraft to free-fall 30 cm onto the collar. The resulting kinetic impact shock will shatter the collar threads, buckle the jack ram, and drop the aircraft onto the hangar floor.

3. Lowering Protocol

When lowering the aircraft, the procedure is reversed: the locking collar is unscrewed upward in small, controlled increments (e.g., 20–50 mm), after which the hydraulic release needle valve is opened slightly to lower the ram until it rests near the collar, repeating this sequence synchronously until all wheels contact the floor.

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Aircraft Chocking Practices

Wheel chocks provide primary roll restraint for parked, serviced, or hangared aircraft:

• Placement: Heavy molded rubber or polyurethane chocks must be placed snugly fore and aft of the main landing gear tires. Chocks are generally not placed tightly against nose gear tires during line turns because wind loads against the vertical fin can pivot the aircraft around its main gear, twisting the nose strut against rigid nose chocks.
• The Turnaround Chocking Gap: When an aircraft arrives at the gate from a flight, its tires and multi-disc brake stacks are extremely hot. When placing chocks, maintenance personnel should position them with a small clearance (approximately 25 mm / 1 inch) away from the tire tread before the parking brake is released. As the hot tires cool and deflect under static load, placing chocks drum-tight against hot rubber can cause the tires to expand over the chock edges, wedging them immovably beneath the tire footprint.

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Aircraft Mooring (Tie-Down) & Severe Weather Protocols

When aircraft must be parked outdoors during severe weather, high surface winds, gales, or tropical cyclones, they must be structurally moored (tied down) to certified ground anchor points.

                     AIRCRAFT MOORING HARDWARE GEOMETRY

         Wing Mooring Ring                          Wing Mooring Ring
               (O)                                        (O)
              /   |                                      /   |
             /    |                                     /    |
            /     v                                    /     v
        [Ground Anchor]                            [Ground Anchor]

1. Mooring Orientation & Control Surface Locks

• Heading into Wind: Whenever operational space permits, the aircraft should be positioned facing directly into the prevailing or forecast wind. This minimizes lateral side loads against the fuselage slab sides and tall vertical fin, reducing overturning moments.
• Gust Locks / Surface Clamps: In high surface winds, free-floating ailerons, elevators, and rudders will violently slam back and forth against their structural travel stops, causing internal control hinge brinelling, bellcrank cracking, and cable stretching. Internal flight deck gust locks must be engaged. On light aircraft or aircraft without internal hydraulic damping, external padded wooden or composite control surface gust clamps (fitted with high-visibility red "REMOVE BEFORE FLIGHT" streamers) must be clamped across every control surface hinge gap.

2. Mooring Rings and Ground Anchors

Mooring lines must be attached ONLY to certified airframe mooring rings located on the lower wing surfaces, nose gear, and aft fuselage. Never attach mooring ropes or chains to landing gear torque links, brake lines, landing gear oleo struts, or pitot masts.

3. Mooring Line Materials & The Rope Shrinkage Phenomenon

Three primary materials are used for mooring lines, each with distinct physical characteristics:

4. Severe Weather Mooring Checklist

• Head aircraft into the prevailing wind; fuel aircraft to maximum permitted ramp weight if heavy storm winds are forecast (the additional fuel mass significantly increases stability and prevents the airframe from becoming airborne on the ramp).
• Install all external landing gear ground downlock safety pins (locking main and nose gear side-struts and drag braces mechanically).
• Secure flight controls with gust locks; install all engine intake/exhaust blanks and pitot covers.
• Attach mooring lines to certified mooring points at an angle of roughly 45 degrees to the ground anchor points.

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Realistic Maintenance Scenario & Common Exam Traps

Realistic Maintenance Scenario

A regional turboprop is moved inside a maintenance hangar for a scheduled landing gear overhaul. To save time, two junior technicians position three tripod jacks under the forward fuselage and wings. They begin pumping the hydraulic jacks simultaneously without checking the cargo manifest or installing a tail stand. While the nose and main gear leave the floor by 10 cm, an avionics team enters the aft cabin to remove communication racks, shifting 180 kg of technician mass into the rear fuselage. Suddenly, the aircraft Center of Gravity crosses aft of the main wing jack heads. The airframe tips rearward, slipping off the spherical wing jack pads with a violent bang. The tail cone crashes into the concrete floor, crushing the rear pressure bulkhead and APU casing. A subsequent investigation confirms that the technicians failed to install the mandatory weighted tail stand specified in the AMM Jacking Section.

Common Exam Traps

• Trap 1: The rope shrinkage paradox. EASA Part-66 exam questions frequently ask how natural fiber (manila/hemp) tie-down ropes should be installed when mooring an aircraft in dry weather. Candidates intuitively choose "as tightly as possible with zero slack." The correct engineering answer is with slight slack (1 to 2 inches) because natural fibers absorb atmospheric moisture during rainfall and contract substantially, exerting destructive downward loads on wing spars.
• Trap 2: Locking collar operation during jacking. Questions often ask whether the mechanical locking collar is tightened only after reaching full jacking height. The answer is NO; the locking collar must be wound down continuously and progressively throughout the entire lifting evolution.
• Trap 3: Jacking inside vs. outside. Candidates often forget that jacking on open ramps is strictly prohibited unless exceptional, tightly controlled conditions (zero wind, specialized outriggers, steel spreader plates) are authorized by the AMM.