Mechanical Anti-Skid Systems: The Dunlop Maxaret, Flywheel Inertia, and Apron Realities

Technical Commentary Notice

This is an interpretive technical commentary, not a replacement for official documentation. It does not constitute official maintenance data, approved instruction, or operational field guidance. In accordance with international airworthiness regulations (EASA/CAAM/FAA), all live aircraft maintenance, troubleshooting, and duplicate inspections must be executed strictly following current, approved aircraft maintenance manuals (AMM) and company task cards. The author disclaims all liability for real-world applications of these technical commentaries.


The Dunlop Maxaret is a fully mechanical anti‑skid unit driven off landing gear wheel rotation. It uses flywheel inertia and cam‑displacement poppet valves to sense wheel deceleration rates exceeding 1g. When impending skid is detected, internal valving dumps brake line pressure to a return line, cycling up to 10 times per second to prevent wheel lockup and tyre blowout.

The Reality Gap: Modern Electro‑Hydraulic ABS vs. Pure Mechanical Inertia

Today, we think of anti‑skid systems (ATA 32) in terms of wheel speed transducers, hall‑effect sensors, and anti‑skid control units driving electro‑hydraulic servo valves.

Understanding the purely mechanical Maxaret is where you learn how anti‑skid physics really work. It does not rely on software logic, sensor wiring, or aircraft electrical power. It is a self‑contained mechanical computer mounted inside the wheel axle or driven via brake disc gear teeth. On a rain‑slicked runway at 100 knots, the Maxaret physically overrides pilot hydraulic pressure to prevent flat‑spotted or blown tyres.

Operating Principle: How Flywheel Inertia Overrides Hydraulic Pressure

  1. Normal Rotation (Brakes Off or Normal Braking)

    • A rubber‑tyred drive drum presses against the wheel rim, or a spur gear meshes with the hub.

    • This drum drives an internal flywheel via a clutch.

    • Under stable deceleration, drum and flywheel rotate together. The poppet valve stays seated, allowing pilot hydraulic pressure to reach brake pistons.

  2. Impending Skid (Wheel Lockup)

    • On wet or icy concrete, the wheel stops almost instantly.

    • The drive drum stops, but the flywheel keeps spinning due to angular momentum.

    • Relative displacement drives a ball bearing up a contoured cam.

    • The cam shifts a push‑rod, unseating the poppet valve.

  3. Pressure Release and Cycling

    • Opening the valve closes the pilot supply line and vents fluid back to the return tank.

    • The wheel spins back up, drum matches flywheel RPM, cam resets, valve seats, and brake pressure reapplies.

    • This dump‑and‑reapply cycle repeats 8–10 times per second until speed drops below threshold.

Component Breakdown

Component

Primary Function

Failure Mode / Hangar Impact

Drive Drum / Friction Ring

Transfers wheel rotation into Maxaret

Oil/grease contamination causes slippage → false skid releases or loss of braking

Internal Flywheel

Provides inertial energy source

Bearing wear prevents overrun → wheel lockup

Face Cam & Ball Roller

Converts rotational differential into push‑rod motion

Pitting/brinelling causes erratic valve actuation

Poppet Relief Valve

Blocks supply pressure, opens return

O‑ring failure → continuous bypass, soft brake pedal


Field Inspection & Maintenance Sequences

  1. Drive Interface Inspection Clean drive ring or gear teeth. Remove grease, hydraulic fluid, brake dust. Verify spring tension pressing ring against rim.

  2. Bench / Spin Check Rotate drum by hand. Spin rapidly, snap stop — feel flywheel overrun clutch smoothly.

  3. Hydraulic Static Leakage Test Apply static pressure to inlet. Check return port for leakage. Escaping fluid indicates valve seat or packing failure.

  4. Return Line Clearance Verify flexible hoses are free of twists/kinks. Restricted return line delays dump cycle, risking tyre blowout.

Hangar Floor Bottlenecks: Tacit Experience on ATA 32

  • Grease Contamination: Most “no braking” write‑ups trace to grease slung from wheel bearings contaminating the friction ring. The drum slips, unit assumes skid, dumps pressure, leaving zero braking on that wheel.

  • Return Line Trap: Unlike electronic systems, Maxaret physically pushes fluid back. A kinked flex hose prevents dump, brake stays locked, tyre blows on touchdown.


The Human Element: Discipline on Turnaround

Modern airliners bury anti‑skid logic in avionics racks. Working on mechanical systems like Maxaret forces you to appreciate how design solved high‑speed safety challenges long before microprocessors.

During a wheel change, do not rush the drive ring clean‑down. Take the extra five minutes to wipe the drum, check friction surfaces, and verify return line flexibility. That attention to detail keeps tyres intact and aircraft rolling safely off the runway.