Details

1. Introduction – Why This Module Is Critical
In a modern combat or utility helicopter, the hydraulic system is the nervous system of the aircraft. Every change in main-rotor pitch, every tail-rotor correction, every landing-gear cycle, every brake application, and every deployment of mission equipment ultimately depends on one thing: a stable, uninterrupted supply of hydraulic power.

The Integrated Aircraft Hydraulic Reservoir, Intensifier & Control Module is the unit that guarantees this. It does far more than store oil; it actively ensures that the hydraulic pump is never starved of fluid, even when the aircraft is pulled into aggressive pitch, roll, yaw, or negative-G maneuvers where conventional reservoirs start gulping air and collapsing suction pressure.

Instead of a simple tank-and-filter stack, this module is a self-pressurising reverse-intensifier bootstrap reservoir that continuously maintains around 2.5 bar(g) at the pump suction, irrespective of flight attitude, fluid slosh, or fast transients from flight-control and utility actuators. A drop in suction pressure here doesn’t just show up as a bad number on a gauge – it turns into cavitation, actuator lag, spongy controls and, in the worst case, loss of controllability.

By integrating the reservoir, intensifier, high- and low-pressure relief valves, check valves, filtration, fluid-level sensing, and temperature/pressure monitoring into a single aerospace-grade module, the system eliminates long runs of pipework and multiple failure points. It becomes a single, tightly controlled assurance point for the entire hydraulic system on advanced helicopter and aircraft platforms.

2. System Overview & Functional Role
The Integrated Aircraft Hydraulic Reservoir, Intensifier & Control Module is a compact hydraulic power-conditioning module designed for multi-circuit aircraft hydraulic systems, typically with separate flight-control and utility hydraulic circuits.

Two main reservoir configurations are used:
• 2.75 L module – typically used on primary flight-control systems (main and tail rotor actuators).
• 4.25 L module – typically used on utility systems (landing gear, wheel brakes, hoists, winches, etc.).

Within a single integrated assembly, it combines:
• Self-pressurised bootstrap reservoir
• Differential-area piston intensifier for reservoir pressurisation
• High-pressure and low-pressure relief valves
• Pressure and return filters with automatic shut-off and clog indication
• Pressure transducer and pressure switch
• Temperature switch for fluid thermal monitoring
• Mechanical level gauge and low-level proximity sensor
• Check valves, bleed valve, and service/ground quick-disconnects
• A top manifold block acting as the distribution node for all hydraulic ports
The module is flange-mounted on a 250 × 250 mm base and designed to fit within tight height and weight envelopes, allowing direct integration into the aircraft hydraulic bay.

3. Architecture & Major Sub-Assemblies
3.1 Self-Pressurised Bootstrap Reservoir
• Two reservoir sizes: 2.75 L and 4.25 L maximum fluid volume.
• Useful working volumes: approx. 2.50 L and 4.00 L, with the remaining volume reserved for thermal expansion and emergency capacity.
• Vertically mounted cylindrical reservoir, with cooling fins on the low-pressure (LP) chamber to improve heat dissipation during continuous operation.
• Internal geometry and the fluid-level indication arrangement are designed to avoid air entrapment and give correct level readings during ground checks and in various aircraft attitudes.
• An air filter/breather is provided on the LP side to minimise contamination ingress when exposed to atmosphere.

3.2 Differential-Area Intensifier Assembly
Key dimensional characteristics:
• Low-pressure piston diameter (D): 180 mm
• High-pressure piston diameter (d): 25 mm
• Piston rod diameter (Rd): 15.318 mm
• Area ratio (LP side : HP side): ≈ 82.4 : 1
• Maximum stroke – 2.75 L version: ≈110 mm (gives ~2.75 L max volume, 2.50 L rated, 1.25 L emergency)
• Maximum stroke – 4.25 L version: ≈168 mm (gives ~4.25 L max volume, 4.00 L rated, 1.25 L emergency)

High-pressure fluid from the pump acts on the small-area piston, and this force is transmitted through the rod to the large-area piston acting on the reservoir fluid, creating stable positive suction pressure.

3.3 Valve & Filtration Manifold
Mounted on the reservoir top, the manifold integrates:
• High-pressure relief valve (two-stage, cartridge type)
  ▹ Opens at approximately 1.25–1.33 × nominal system pressure to protect the system from over-pressure.
  ▹ Sized to pass full pump flow (~25 L/min).
• Low-pressure overboard relief valve
  ▹ Protects the reservoir and LP chamber from over-pressurisation due to return-line blockage or thermal expansion.
  ▹ Vents to atmosphere at approximately 4–5 × normal return pressure, also for full pump flow.
• Pressure filter (without bypass)
  ▹ Located in the system pressure line.
  ▹ Automatic shut-off prevents draining the reservoir when the filter element is removed.
  ▹ Integrated clog indicator gives early warning of restriction.
• Return filter (with bypass)
  ▹ Located in the return line to the reservoir.
  ▹ Automatic shut-off and clog indicator.
  ▹ Bypass function ensures continued flow even when the element is clogged, while still signalling the maintenance requirement.
• Check valves
  ▹ In the pump pressure line (unfiltered branch) to maintain pressure on the intensifier and hence reservoir pressurisation after pump shutdown.
  ▹ In the pump case-drain line (with filter) to control back-flow and protect the pump internals.

3.4 Sensors & Instrumentation
• Pressure transducer for continuous monitoring of system pressure.
• Pressure switch for discrete pressure alarms and redundancy.
• Temperature switch to protect against excessive hydraulic fluid temperature.
• Mechanical fluid-level indicator visible during inspections.
• Low-level proximity sensor typically set to:
  ▹ Switch ON a warning when volume drops below ≈ 1.3 L
  ▹ Switch OFF when volume recovers above ≈ 1.5 L
• All wiring from sensors and switches is consolidated into a single multi-pin electrical connector, simplifying harness design and reducing installation errors.

3.5 Hydraulic Ports & Interfaces
The manifold typically provides:
• PS – Pump suction
• PP – Pump pressure line
• PC – Pump case drain
• SP / SR – Service pressure and service return to aircraft actuators
• GP / GR – Ground pressure and ground return for maintenance rigs
• DP – Dump / overboard outlet
Service and ground ports use quick-disconnect couplings with dust caps, allowing rapid connection of ground test stands, flushing rigs, or external hydraulic power units without disturbing fixed aircraft piping.

4. Working Principle – Reverse Intensifier Bootstrap
4.1 Normal Operation
• The engine-driven pump draws fluid from the reservoir via PS, boosts it to nominal system pressure (~206 bar), and supplies it to the aircraft hydraulic system via PP.
• A branch from the high-pressure line feeds the high-pressure side of the intensifier piston.
• The force on the small piston area AHPis transmitted via the rod to the larger area ALP, which acts on the reservoir fluid.

Using Pascal’s principle:

Reservoir Pressure = System Pressure × AHP
ALP

With an area ratio of about 82.4 : 1, this yields:
≈ 2.5 bar = 206 bar ×
1
82.4

This configuration ensures:
• The pump consistently sees a positive inlet pressure well above cavitation threshold.
• Fluid pressure on the suction side remains stable across a wide range of actuator demands.
• No suction-side pressure collapse during engine start, idle, or fast transients.
• Check valves can hold residual pressure on the intensifier side after shutdown, preserving suction head for controlled restarts.

4.2 Aggressive / Negative-G Flight
In conventional non-pressurised reservoirs, sharp maneuvers or negative-G conditions can cause the fluid to move away from the suction pick-up, leading to air ingestion and cavitation. In this system, the entire reservoir volume is held under positive pressure, so even with fluid motion, the pump inlet still sees ~2.5 bar(g), dramatically reducing the risk of cavitation or vapour lock.

4.3 Bleeding & Air Removal
• A press-to-bleed valve on the LP side allows technicians to vent trapped air and draw fluid samples during maintenance.
• Additional bleed nipples can be arranged at local high points to ensure complete de-aeration of the connected pipework.

4.4 Over-Pressure Protection 
• If discharge pressure rises beyond the acceptable range, the two-stage high-pressure relief valve opens and diverts flow from pressure to return, rotecting the pump, actuators and piping.
• If return-line blockage or thermal expansion increases LP chamber pressure, the low-pressure overboard relief valve vents to atmosphere, preventing structural overload of the reservoir shell.

5. Technical Specifications

    

        
Parameter
        
2.75 L Module
        
4.25 L Module
    

    

        
Base dimensions
        
250 × 250 mm
        
250 × 250 mm
    
    

        
Overall height
        
340 mm
        
470 mm
    
    

        
Dry weight
        
≈ 9 kg
        
≈ 10 kg
    
    

        
Maximum fluid volume
        
2.75 L
        
4.25 L
    
    

        
Rated useful volume
        
2.50 L
        
4.00 L
    
    

        
Minimum / emergency volume
        
1.25 L
        
1.25 L
    
    

        
Working temperature range
        
−20 °C to +120 °C
        
−20 °C to +120 °C
    
    

        
Nominal system pressure
        
206 bar
        
206 bar
    
    

        
Useful operating pressure range
        
180–220 bar
        
180–220 bar
    
    

        
Nominal suction chamber pressure
        
2.5 bar(g)
        
2.5 bar(g)
    
    

        
Proof pressure – system lines
        
310 bar
        
310 bar
    
    

        
Proof pressure – return lines
        
155 bar
        
155 bar
    
    

        
Proof pressure – reservoir LP chamber
        
20 bar
        
20 bar
    
    

        
Burst – system lines (design)
        
525 bar
        
525 bar
    
    

        
Burst – return lines (design)
        
265 bar
        
265 bar
    
    

        
Burst – reservoir LP chamber (design)
        
35 bar
        
35 bar
    
    

        
Rated flow through relief valves
        
25 L/min
        
25 L/min
    
    

        
Working fluid
        
MIL-H-5606G aircraft hydraulic fluid
        
MIL-H-5606G aircraft hydraulic fluid
    


6. Typical Applications
• Primary flight-control hydraulic systems on advanced twin-engine helicopters and similar aircraft.
• Utility hydraulic systems operating:
  ▹ Landing gear deployment/retraction
  ▹ Wheel-brake and parking-brake systems
  ▹ Rescue and cargo hoists
  ▹ Sonar/harpoon and other mission equipment winches
• Any aerospace platform requiring a compact, self-pressurised hydraulic reservoir with integrated intensifier and control functions.

7. Operational Advantages
• Direct impact on flight safety by preventing pump cavitation and actuator performance loss.
• Attitude-independent operation, including aggressive manoeuvres and negative-G flight.
• Highly integrated architecture reduces external pipework, leak paths and potential failure points.
• Compact and lightweight compared with distributed reservoirs, accumulators and external pressurisation systems.
• Maintenance-friendly design: quick-disconnect ports, single electrical connector, automatic filter shut-off, and clog indication.
• Qualified for harsh environments: vibration, shock, temperature extremes, sand/dust, icing, humidity and EMI/EMC conditions.

8. Testing & Qualification Approach
A comprehensive test philosophy typically covers:
• Routine acceptance tests on every production unit:
  ▹ Pressure and leak testing
  ▹ Functional checks on valves, switches and sensors
• Extended qualification tests on representative units:
  ▹ Fatigue/endurance cycling under full-range pressure and flow 
  ▹ Proof and burst pressure tests on HP, return and reservoir sections
  ▹ Environmental tests: altitude, acceleration, vibration, shock, salt fog, fungus, sand/dust, icing/freezing rain, humidity, high/low temperature and thermal shock
  ▹ EMI/EMC compliance with aircraft electrical/avionic systems

9. Summary
The Integrated Aircraft Hydraulic Reservoir, Intensifier & Control Module is not just a reservoir; it is a flight-critical hydraulic power-conditioning and protection system. By combining reverse-intensifier pressurisation, robust filtration, comprehensive protection valving and integrated sensing into one compact, aerospace-grade assembly, it ensures that the aircraft’s hydraulic system remains stable, responsive and safe across the entire flight envelope.

In simple terms, this is the module that makes sure rotor blades obey the pilot, landing gear and brakes respond when commanded, and mission equipment operates reliably – even in the harshest and most dynamic operating conditions.