ATR 42/72-600 Systems Guide — Chapter M: Hydraulic System

2026-06-23

Chapter M. HYDRAULIC SYSTEM (FCOM DSC 29)¶

1. GENERAL DESCRIPTION & SYSTEM PARAMETERS (Page 143)¶

The Hydraulic System (ATA 29) generates, regulates, and distributes high-pressure fluid power to actuate critical flight control elements, landing gear arrays, and braking sub-systems.

1.1. Core Architectural Characteristics¶

System Isolation: Consists of two completely independent networks running fully isolated from each other: the BLUE System and the GREEN System.
Nominal Operating Pressure: $3000\text{ psi}$ regulated baseline under normal operational profiles.
Fluid Medium: Synthetic fire-resistant phosphate-ester chemical fluid (Skydrol type), color-coded purple for leak tracking.
Accumulator Buffers: Each system is integrated with nitrogen-precharged accumulator flasks to damp pressure surges and provide emergency reserve pressure if generation drops.

2. HYDRAULIC SYSTEM MAIN TOPOLOGY SCHEMATIC (Page 144)¶

The distribution layout maps specific mechanical consumers to either the Blue or Green power networks to preserve redundancy during a single system loss.

      BLUE HYDRAULIC SYSTEM                   GREEN HYDRAULIC SYSTEM
   ┌────────────────────────┐              ┌────────────────────────┐
   │ [ENG 1 DRIVEN PUMP]    │              │    [ENG 2 DRIVEN PUMP] │
   │   (Main 3000 psi Feed) │              │   (Main 3000 psi Feed) │
   └──────────┬─────────────┘              └───────────┬────────────┘
              │                                        │
   ┌──────────┴─────────────┐              ┌───────────┴────────────┐
   │   ELECTRIC AUX PUMP    │              │     MAIN RESERVOIR     │
   │  (Ground/Backup Blue)  │              │ (Fluid Storage Level)  │
   └──────────┬─────────────┘              └───────────┬────────────┘
              │                                        │
              ▼                                        ▼
    [BLUE CONSUMERS AXIS]                    [GREEN CONSUMERS AXIS]
    - Flaps Actuation Motor                  - Landing Gear Actuators
    - Left/Right Spoilers (LH Inner)         - Left/Right Spoilers (RH Outer)
    - Emergency Braking Accumulator          - Normal Main Braking Array
    - Propeller Brake Assembly               - Nose Wheel Steering (NWS)

3. HYD PWR OVERHEAD PANEL LAYOUT SPECIFICATIONS (Page 145)¶

Located on the central overhead control panel structure, managing engine-driven pump relays and backup source activation.

  HYD PWR OVERHEAD PANEL
  ┌────────────────────────────────────────────────────────┐
  │                        HYD PWR                         │
  │                                                        │
  │     ENG 1 PUMP         AUX PUMP          ENG 2 PUMP    │
  │      [ FAULT ]         [  RUN  ]          [ FAULT ]    │
  │      [  OFF  ]                            [  OFF  ]    │
  └────────────────────────────────────────────────────────┘

3.1. Control Mechanics & Light Trigger Rules:¶

ENG 1 / 2 PUMP Pushbuttons:
Released (ON Setting): Main line isolation valves are held open; engine-driven pumps deliver $3000\text{ psi}$ pressure to the grid.
Depressed (OFF State - White text): Closes the intake valve, isolating fluid flow to depressurize the respective engine pump loop.
FAULT Indication (Amber): Illuminates if fluid reservoir levels drop below minimum targets, system temperature breaches safe bounds ($>121^\circ\text{C}$), or delivery pressure decays below operational targets ($<1500\text{ psi}$).
AUX PUMP Pushbutton:
RUN (White): Confirms the ACW-driven auxiliary electric hydraulic pump is active and pressurizing the Blue network.

4. SYSTEM DISPLAY (SD) PAGE PRESSURE READOUTS (Page 146)¶

Selecting the SYS / HYD option key projects real-time system performance variables onto the multi-function display screen.

Pressure Gauges: Render digital pressure values and color-coded graphical dial needles tracking actual line pressures. Normal parameters register between $2900\text{ psi}$ and $3100\text{ psi}$ (Green spectrum).
Reservoir Level Scales: Dynamic vertical bars showing current fluid volumes. Dropping below low-level sensors triggers an amber transition and generates the HYD LO LVL Master Caution alert.
Accumulator Status Blocks: Monitor the pressure status of the brake and emergency accumulators to verify backup performance margins prior to flight terminal entrance.

5. HYDRAULIC ON-RAMP COLD START CONFIGURATIONS (Page 147)¶

This section defines system behaviors when the aircraft is powered on the ground prior to starting the engine cores.

Battery Only Status: With engine generators stationary, the main pumps are inactive ($0\text{ psi}$). The Green and Blue grids are completely unpressurized.
Aux Pump Ground Routing: Pressing the pedestal or overhead AUX PUMP button commands the electric pump to draw current directly from the ground power network or batteries. This pressurizes the Blue system manifold to $3000\text{ psi}$, enabling ground crews to operate the forward cargo door or set the parking brake without initializing the main turboprop spools.

6. NORMAL IN-FLIGHT BALANCED MANIFOLD OPERATION (Page 148)¶

Following dual engine synchronization and takeoff dispatch, the hydraulic system operates in a balanced configuration.

Engine Pump Supremacy: Engine 1 drives the Blue main pump; Engine 2 drives the Green main pump. Both channels generate stable $3000\text{ psi}$ lines.
Aux Pump Auto-Rest State: The electric AUX PUMP is automatically unpowered and rests in a standby state during normal flight tracking to conserve system component lifecycle wear.
Isolation Boundary Enforcement: The Blue and Green systems operate as independent loops. Fluid mixing is physically impossible under standard tracking, preventing a single pipeline burst from draining both networks simultaneously.

7. SINGLE SYSTEM PUMP FAILURE RECOVERY LAWS (Page 149)¶

If an engine-driven pump experiences a mechanical failure or line pressure drops below safety boundaries, the FWS initiates corrective feedback loops.

Alert Verification: Delivery drop below $1500\text{ psi}$ triggers the Master Caution single chime and flashes the amber HYD ENG 1(2) PUMP LO PR alert line on the EWD.
Automated Component Reconfiguration: The system loop does not cross-feed raw fluid between Blue and Green channels to preserve system isolation. Instead:
If the Green Pump fails, systems requiring high-volume assistance (like landing gear retraction) function at slower velocities via accumulator reserve drawdown.
If the Blue Pump fails, the Multi-Function Computer (MFC) automatically sends a start current command to the standby electric AUX PUMP to fully restore Blue manifold pressure, keeping the flaps and spoilers fully operational.

8. BOTH MAIN HYDRAULIC PUMPS LOSS EMERGENCY MANAGEMENT (Page 150)¶

A compounding double failure event involving the loss of both Engine 1 and Engine 2 driven pumps forces the hydraulic network into a critical survival configuration.

8.1. Degradation Trajectory & Essential Flight Controls¶

Manifold State: Both Green and Blue primary engine-driven lines bleed down to $0\text{ psi}$.
Green Utility Loss: Nose wheel steering, landing gear retraction loops, and normal main brakes are completely lost. Landing gear must now be deployed via manual gravity free-fall extension release handles.
Blue Utility Survival Line: The electric AUX PUMP remains the absolute final defense line for the airframe. As long as electrical ACW generation or advanced battery inversion networks remain live, the Aux Pump will continuously pressurize the Blue system manifold. This preserves flap actuation and roll spoiler authority, and charges the emergency braking accumulator to allow a safe halt on the runway layout after touchdown.

Chapter M. HYDRAULIC SYSTEM (CONTINUED)¶

8.2. BOTH HYDRAULIC SYSTEMS LOSS EMERGENCY MANAGEMENT (Page 151)¶

A simultaneous failure of both the Blue and Green hydraulic networks leads to massive system degradation. Crews must immediately shift focus toward passive safety fallback operations and backup energy reservoirs.

Emergency Landing Gear Extension¶

Mechanical Free-Fall Alignment: With no system pressure available to drive the actuation cylinders, the normal landing gear lever is bypassed. Pilots must pull the manual LDG GEAR EMER EXTENSION handle located on the cockpit floor.
Gravity Drop Mechanics: Pulling the handle unlatches the gear door locking hooks via mechanical teleflex cables. The structural mass of the main gear legs forces them to drop down via gravity, swinging into position until mechanical over-center drag links lock the gear assemblies securely into the down-and-locked configuration.

Accumulator Braking Limitations¶

Finite Capacity Bounds: Once the Green system pressure drops to zero, normal power braking is lost. The system switches automatically to the emergency brake circuit supplied by the Blue system accumulator.
Application Constraints: The pressurized nitrogen pre-charge inside the accumulator flask holds enough stored fluid to provide a finite limit of 6 complete manual brake applications. Pilots must use smooth, continuous pressure onto the toe pedals during the landing rollout; pumping the brakes will bleed off the accumulator pressure prematurely, leading to a total loss of stopping authority.

9. MAIN BRAKING SYSTEM & ANTI-SKID INTERFACE LAWS (Page 152)¶

The main landing gear wheels feature multi-disc carbon brake assemblies powered normally by the Green hydraulic system and monitored electronically to optimize stopping metrics on contaminated runways.

  NORMAL BRAKING CONTACTOR CONTROL LINE
  ┌────────────────────────────────────────────────────────┐
  │               PILOT RUDDER PEDAL TOE CUPS              │
  └───────────────────────────┬────────────────────────────┘
                              │
                              ▼ [Direct Mechanical Brake Metering]
  ┌────────────────────────────────────────────────────────┐
  │                 GREEN HYDRAULIC MANIFOLD               │
  │               (Delivers 3000 psi Line Feed)            │
  └───────────────────────────┬────────────────────────────┘
                              │
                              ▼ [Anti-Skid Control Modulation Valve]
  ┌────────────────────────────────────────────────────────┐
  │            WHEEL BRAKE DISCS & MULTI-PLATES            │
  └────────────────────────────────────────────────────────┘

Anti-Skid Protection Logic¶

Sensing Core Network: Tachometer transducers inside each main wheel axle measure real-time rotational speed parameters, feeding data to the Brake Control Unit (BCU).
Pressure Modulation Loop: If the BCU detects a wheel deceleration trend that matches a sudden slide or lockup configuration relative to the aircraft's ground speed, it commands an electric dump valve to open. This temporarily bleeds hydraulic line pressure away from the slipping wheel to let it spin back up to matching track parameters, preserving directional tracking control.
Low-Speed Deactivation Threshold: The anti-skid modulation loop cuts out automatically when ground speed drops below 10 kt. This allows pilots to lock the wheels completely for tight ramp maneuvers or static parking holds.

10. NOSE WHEEL STEERING & MECHANICAL DISCONNECT LAWS (Page 153)¶

Directional steering on the ground utilizes a hydraulically driven rack-and-pinion assembly mounted directly to the nose landing gear shock strut cylinder.

Control Authority Limits¶

Captain Steering Handwheel: Located exclusively on the left lateral console structure. Provides high-authority steering commands, allowing the nose wheel to swivel up to $45^\circ$ either side of the absolute aircraft centerline for tight airport ramp maneuvering.
Rudder Pedal Interconnection: Deflecting the primary flight deck rudder pedals commands low-authority nose wheel adjustments, limited to a maximum travel boundary of $7^\circ$ deflection either side. This allows for precise, stable tracking corrections during high-speed takeoff and landing tracking runs.

Ground Towing Disconnect Safeties¶

Bay Bypass Lever: A mechanical bypass toggle link is integrated into the external nose gear torque arm assembly.
Castering Freedom: Inserting the safety lockout pin locks the hydraulic steering loop valves open. This bypasses cockpit control commands and allows the nose wheel assembly to caster freely through a full $360^\circ$ arc during ground tug pushbacks, preventing internal piston structural damage or towbar snapping event profiles.