13.1 Towing, Taxiing Protocols, Shear Pins & Wing Walkers

13.1 Towing, Taxiing Protocols, Shear Pins & Wing Walkers

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.

Aircraft ground handling and towing operations are among the most frequent maintenance activities conducted on commercial aprons, military flight lines, and maintenance repair organization (MRO) ramps. Because large transport aircraft possess immense mass and high inertia while maneuvering in tight apron environments surrounded by ground support equipment (GSE), strict procedural discipline is vital. Under EASA Part-66 Module 7 (Maintenance Practices), certifying maintenance engineers must possess an exhaustive understanding of towing hardware, mechanical load-limiting devices, hydraulic steering isolation, ground team organization, and ramp hazard zones.

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Towbar Connection, Mechanics & Shear Pin Protection

Moving an aircraft on the ground is primarily accomplished using a specialized aircraft towing tractor (tug) coupled to the aircraft nose landing gear (NLG) via a heavy-duty towbar. Alternative towbarless tractors clamp directly around the nose gear tires, elevating the nose gear slightly off the tarmac.

                    CONVENTIONAL TOWBAR ATTACHMENT SCHEMATIC

  +---------+           Towbar Barrel             Towing Head       NLG Tow Spud
  | Towing  |===O==============================[ Shear Pin ]========(O)  Nose
  | Tractor |   Tug Eyelet                    [ Tension Pin ]      Gear Strut
  +---------+                                 [ Lock Jaws   ]

1. Towbar Construction & Attachment

A conventional towbar consists of a high-strength tubular steel or aluminum alloy barrel supported by a wheeled undercarriage with an adjustable hand-pump hydraulic lift mechanism. The forward end terminates in an eyelet that couples to the towing tractor pintle hitch, while the aft end features a specialized towing head engineered to mate with the specific towing spuds or towing lugs on the aircraft nose gear shock strut.

• Connection Verification: When securing the towing head to the nose gear spud, spring-loaded locking jaws or sliding locking pins must engage completely. Maintenance personnel must visually verify that secondary safety locks (such as ball-lock detent pins or latching cotter clips) are inserted and locked. Never commence towing until mechanical positive locking is physically verified.

2. Calibrated Shear Pins and Tension Pins

During towing and pushback operations, severe mechanical loads can be imparted to the aircraft nose landing gear by aggressive tractor acceleration, abrupt steering inputs, jackknifing the towbar, or traversing runway depressions. To protect the multi-million-euro landing gear assembly from catastrophic structural damage, the towing head incorporates calibrated, sacrificial shear pins and tension pins.

• Design Principle: Shear pins are precision-machined steel fasteners featuring calibrated annular neckings (grooves) turned into their shanks. These neckings are engineered to yield and fracture at precisely determined load thresholds—typically set slightly below the structural yield limit of the nose gear trunnion, shock strut cylinder, steering collar, or torque links.
• Failure Modes Protected:
  • Shear Pins: Fracture when excessive lateral, angular, or torsional twisting loads are applied (such as turning beyond the nose gear steering limit or sharp tug turns).
  • Tension/Compression Pins: Fracture when excessive longitudinal drawbar pull or braking impact forces occur along the towbar centerline.
• Post-Shear Action: If a shear pin fractures during towing, the towing operation must be halted immediately. Maintenance personnel must NEVER replace a sheared pin with a standard airframe bolt (e.g., standard AN/MS hardware), a high-tensile bolt, or uncalibrated rod stock. Doing so removes the structural safety fuse, guaranteeing that subsequent overloads will bend or crack the nose gear trunnion or tear the landing gear from its airframe bulkhead fittings. The nose gear steering mechanism, torque links, and shock strut must undergo a documented NDT and dimensional inspection for deformation before a certified replacement shear pin matching the exact part number from the Aircraft Maintenance Manual (AMM) is installed.

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Nose Gear Steering Bypass & Turning Limits

Modern transport aircraft utilize closed-loop electro-hydraulic or hydro-mechanical nose wheel steering systems. High-pressure hydraulic fluid (typically 3,000 psi / 207 bar) is plumbed directly into powerful hydraulic steering actuators (rack-and-pinion or dual-opposed push-pull cylinders) mounted to the nose gear shock strut.

                    NOSE GEAR STEERING BYPASS CIRCUIT

  Normal Operation:                      Towing Operation (Pin Installed):
  [Hydraulic Supply: 3000 psi]           [Hydraulic Supply: Isolated]
            |                                      |
            v                                      X  Bypass Valve Actuated
    [Metering Valve]                               |
       |         |                         +-------+-------+
       v         v                         |               |
   [Cylinder A] [Cylinder B]           [Cylinder A] <====> [Cylinder B]
   (Rigid Hydraulic Lock)              (Fluid Circulates Freely to Return)

1. Steering Bypass Pin (Lockout Pin)

When an aircraft is towed, the towing tractor turns the nose wheels externally. If the aircraft hydraulic steering system remains pressurized or hydraulically locked, the forced displacement of the steering cylinders will encounter immovable fluid barriers. This creates massive hydrostatic pressures exceeding 5,000–8,000 psi within the cylinders, leading to:

• Blown cylinder end seals and burst hydraulic lines.
• Ruptured actuator barrels and cracked steering collars.
• Severe damage to the mechanical feedback cables or rotary variable differential transformers (RVDTs).

To prevent this, a dedicated steering bypass pin (often termed a ground lockout pin or towing pin) must be inserted into the nose gear steering control valve module or bypass deactivation valve prior to connecting the towbar:

• Mechanism: Inserting the pin mechanically drives an internal spool valve that disconnects the steering metering valve from aircraft hydraulic supply pressure and opens a cross-flow bypass channel between the working chambers of the left and right steering actuators, routing displaced hydraulic fluid freely back to the low-pressure hydraulic return reservoir.
• Warning Streamer: The bypass pin is equipped with an unmistakable high-visibility red streamer labeled "REMOVE BEFORE FLIGHT". It is the certifying engineer's and flight crew's strict legal responsibility to verify that the steering bypass pin is removed after towbar disconnection and securely stowed in the flight deck or nose wheel well bracket.

2. Maximum Turn Angle Index Limits

Every aircraft has strict structural limits regarding how far the nose landing gear can be turned to either side of the center longitudinal line:

• Painted Index Markings: Red or yellow index stripes and pointer arrows are painted onto the stationary nose gear strut outer cylinder and the rotating steering collar or nose wheel doors. These indicate the Maximum Permissible Towing Turn Angle (typically between 50° and 78° on commercial airliners).
• Structural Stops: If the tug driver turns the nose wheels past these markings, the steering mechanism will crash into the mechanical internal stops, shearing the steering collar, bending the torque links, or buckling the strut trunnion.
• Torque Link Disconnection: When extreme turns or hangar maneuvers require 360-degree nose wheel castering, certain aircraft maintenance manuals mandate physically disconnecting the upper and lower nose gear torque links (scissors). When the torque links are disconnected, ground personnel must ensure that electrical harness and hydraulic brake hose service loops have sufficient clearance to prevent them from being severed during full rotation.

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Ground Towing Team Composition & Safety Rules

Aircraft ground towing requires precise crew coordination. A formal towing team consists of qualified personnel assigned to specific, non-overlapping safety roles:

                           TOWING TEAM POSITIONS

                               [Flight Deck Rider]
                                        |
                                        v
   [Left Wing Walker] <=======> [Towing Tractor] <=======> [Right Wing Walker]
    (Illuminated Wand)           (Tug Driver)               (Illuminated Wand)
                                        |
                                        v
                                  [Tail Walker]
                               (Illuminated Wand)

1. Towing Team Positions & Duties

• Towing Supervisor / Tug Driver: Controls the speed and direction of the movement. Must maintain continuous visual and audio contact with the flight deck rider and wing walkers. Maximum towing speed must never exceed a brisk walking pace (approximately 5 km/h / 3 mph) in congested ramp or hangar environments, and must not exceed 15–25 km/h (10–15 mph) on open, unobstructed airport taxiways.
• Flight Deck Brake Rider: Must be a licensed maintenance engineer, authorized technician, or flight crew member certified on the specific aircraft type. Duties include:
  • Checking that normal and alternate brake hydraulic pressure gauges are within the green operating band (recharging accumulators via electric auxiliary pumps if required).
  • Ensuring the parking brake is fully released prior to tractor motion.
  • Operating navigation and anti-collision lights.
  • Monitoring ATC ground control frequencies via the VHF communications radio.
  • Maintaining continuous interphone communication with the tug driver.
• Wing Walkers: Positioned outboard of each wingtip slightly forward of the leading edge. Mandatory whenever an aircraft is moved within 7.5 meters (25 feet) of any obstacle, building, or other aircraft, and during all hangar movements. In poor visibility or night operations, each wing walker must carry illuminated wands (day-glow red/orange or lighted LED wands). Their sole duty is to watch wingtip obstacle clearance and immediately signal the driver to stop if clearance is threatened.
• Tail Walker: Positioned aft of the horizontal stabilizer and tail cone when backing into hangars or narrow cul-de-sacs to ensure empennage and APU exhaust clearance.

2. The Cardinal Rule of the Brake Rider

Under standard operating conditions, the towing tractor's brakes are designed to decelerate and stop both the tractor and the aircraft. The flight deck brake rider must NEVER apply the aircraft brakes while the tractor is in motion, unless an absolute catastrophe or uncommanded towbar disconnection occurs:

• Consequences of Abrupt Aircraft Braking: If the brake rider steps on the aircraft brake pedals while being towed forward or pushed aft, the massive braking torque instantly halts the aircraft main landing gear. However, the high momentum of the moving towing tractor continues pulling or pushing the towbar. This imparts a catastrophic compressive or tensile shock load into the towbar, instantly snapping the shear pins, crushing the towbar barrel, or violently twisting and collapsing the nose gear shock strut.
• Emergency Exception: Aircraft brakes are applied ONLY when the tug driver explicitly commands an emergency stop over the headset, or if the towbar physically breaks or decouples from the aircraft, causing the airframe to roll free toward an obstacle or personnel.

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Ramp Communication & Marshalling Protocols

Reliable communication is the primary barrier against ramp collisions and ground worker injuries:

• Interphone Headset System: Direct, wired interphone communication between the flight deck brake rider and the tug driver/ground supervisor via a long umbilical cord plugged into the aircraft external service interphone jack (located in the nose wheel well or lower forward fuselage). Standardized, unambiguous aviation phraseology must be used:
  • Tug: "Flight deck, confirm nose gear steering bypass pin installed and brakes released."
  • Flight Deck: "Steering bypass pin installed, brake accumulator in green, parking brake released, cleared to push."
  • Tug: "Commencing pushback."
• ICAO Annex 2 Marshalling Hand Signals: When voice interphone fails or when marshalling taxiing aircraft into gate positions, ground personnel utilize standard ICAO Annex 2 / EASA CS-ADR-DSN hand signals with day-glow bats or illuminated wands. The universal emergency stop signal consists of crossing both wands/arms rapidly above the head in an 'X' pattern.

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Ramp Engine Danger Areas & Ground Hazards

Certifying engineers working on active aprons must recognize the lethal ground hazard envelopes generated by operating powerplants, auxiliary units, and avionics:

                      JET ENGINE RAMP HAZARD ZONES

            [Suction Zone]           Turbofan Engine           [Jet Blast Zone]
             (Ingestion)                                         (Overturning / Thermal)
           <--- 3 to 10m --->                                  <---- 30 to 100m ---->
             |             |     +=================+     |                          |
             |   HAZARD    |     |   POWERPLANT    |     |     HIGH VELOCITY        |
             +=============+     +=================+     +==========================+
                                                         50-100+ knots / 150-300°C

1. Turbofan Engine Suction Danger Zones

Modern high-bypass turbofan engines move thousands of kilograms of air per second. The resulting depression in front of and around the engine inlet cowl creates a powerful vortex capable of ingesting ground personnel, baggage containers, toolboxes, and loose ramp debris:

• Idle Power: The suction hazard area extends in a semi-circular arc approximately 3 to 4 meters (10 to 13 feet) forward and to the sides of the inlet cowl.
• Takeoff / Breakaway Thrust: When high thrust is applied to initiate taxiing, the suction danger zone expands drastically to 8 to 10+ meters (26 to 33 feet) forward and lateral to the inlet. Anyone stepping inside this zone will be ingested into the spinning titanium fan blades within milliseconds.

2. Jet Blast and Thermal Exhaust Danger Zones

The exhaust gas exiting the core and fan nozzles reaches temperatures exceeding 300°C to 600°C at velocity speeds between 50 and 150+ knots:

• Idle Thrust: The high-velocity, high-temperature hazard footprint extends 30 meters (100 feet) aft of the engine exhaust pipe.
• Takeoff / Breakaway Thrust: The hazardous exhaust footprint extends 60 to 100+ meters (200 to 330+ feet) directly behind the aircraft. Jet blast can blow over ground service vehicles, launch loose baggage carts across the apron, shatter hangar windows, and cause fatal burns and blunt trauma.

3. Propeller Danger Arcs & Anti-Collision Beacons

• Propeller Arc: The rotational plane of a turboprop or piston propeller is virtually invisible when spinning. Maintenance personnel must NEVER enter the propeller rotational arc under any circumstances while engines are operating. Even with an engine shut down, propellers must be treated as live: a faulty magneto ground P-lead or hot ignition system can cause the engine to kick and fire if the propeller is turned by hand.
• Anti-Collision Beacons: High-intensity flashing red beacons mounted on the upper and lower fuselage crown and belly must be illuminated immediately prior to engine start, APU start, or pushback, serving as a universal visual warning to ramp workers that machinery is active or imminent movement is underway.
• Weather Radar RF Radiation Hazard: High-power weather radars operating in the X-band (8–12 GHz) emit intense microwave pulse energy. When transmitting on the ground, radar beams can induce electrical currents in fueling equipment that ignite volatile aviation fuel vapors, trigger electro-explosive devices (squibs) in fire bottles, and inflict thermal tissue damage (especially cataracts) on personnel within the transmitter-specific hazard area defined by approved aircraft and site procedures. Weather radar systems must remain switched to OFF or STANDBY during all ramp and hangar operations.

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Realistic Maintenance Scenario & Common Exam Traps

Realistic Maintenance Scenario

A night-shift line maintenance crew is tasked with repositioning a twin-engine commercial airliner from the active passenger terminal to an engine run-up bay. The ground handler attaches a towbar to the nose landing gear spud and connects to the tug. In a rush, the technician forgets to insert the nose gear steering bypass pin into the steering deactivation valve. The flight deck brake rider pressurizes the yellow hydraulic system to verify brake pressure, leaving 3,000 psi in the steering manifold. As the tug initiates a sharp 40-degree turn out of the gate, the unbypassed hydraulic steering actuators attempt to resist the external turning moment. The resulting trapped hydrostatic pressure surges past 6,000 psi, violently bursting the actuator cylinder barrel and blowing hydraulic fluid across the tarmac. The operation is aborted, the towbar shear pin fractures under the resultant side-load shock, and the aircraft is grounded for nose gear actuator replacement.

Common Exam Traps

• Trap 1: Confusing shear pin replacement rules. Exam questions frequently ask whether a sheared towbar pin can be temporarily replaced with a standard aerospace-grade bolt (such as an AN or MS bolt) of matching diameter to finish a tow. The answer is strictly NO; only the exact, calibrated manufacturer part-numbered shear pin can be installed.
• Trap 2: Brake rider braking actions. Questions often ask what the brake rider should do if the tug begins to slow down approaching an intersection. Candidates often mistakenly select "apply aircraft brakes gently to assist the tug." The rule is absolute: the brake rider must NEVER touch the aircraft brakes while in motion unless an emergency towbar failure occurs.
• Trap 3: Function of the steering lockout pin. Candidates often think the steering bypass pin disconnects electrical power to the steering computer. In reality, it mechanically or hydraulically opens a fluid bypass valve between the cylinder working chambers, allowing hydraulic fluid to circulate freely back to the return line.