Section 5

Hydraulic Systems

Section 5 Hydraulic Systems

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 Left-Hand Hydraulic System Right-Hand Hydraulic System

8 Litres
6 Litres
5 Litres

2 Litres

Left-Hand
Electrical
P P MGB Driven
Right-Hand
MGB Driven P
Main Pump Main Pump
Aux Pump

MAIN SERVO CONTROLS

Standby Upper Cylinder Lower Cylinder
Reservoir

Upper Cylinder Lower Cylinder

Low Level
Low Level Solenoid
Solenoid Valve
Valve Upper Cylinder Lower Cylinder
Hand Pump
P
TAIL SERVO CONTROLS
LH Cylinder RH Cylinder

Normal or
Accumulator
Autopilot
Selector
Valve

Rotor Brake

Utility Services Hoist

Accumulator

N
Wheel Brakes
Flow Restrictor
Check Valve

Unrestricted
Flow in this
Direction

Landing Gear Landing Gear

Normal
EMERGENCY
Retraction
EXTENSION Extension

Figure 1 – Hydraulic Power Generating Systems

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HYDRAULIC POWER GENERATING SYSTEMS

General

The aircraft has two fully Independent hydraulic power generating systems, which ensures aircraft control
can be maintained should one of the systems fail. The systems are as follows –

The Right-Hand system supplies the lower bodies of the dual-cylinder Main Servo controls and the right-
hand body of the dual-cylinder Tail Servo control.

The Left-Hand system supplies the upper bodies of the dual-cylinder Main Servo controls and the left-
hand body of dual-cylinder Tail Servo control.

In addition, the Left-Hand system supplies power for the following -

1. Autopilot Hydraulic System

2. Rotor Brake Hydraulic System
3. Wheel Brake Hydraulic System
4. Landing Gear Hydraulic System
5. Rescue Hoist System.

Included in the Left-Hand system is an emergency reservoir and hand pump to enable the landing gear
to be lowered in the event of hydraulic power failure. The hand pump can also be used to charge an
accumulator to maintain pressure on brakes when parking.

Each system has a reservoir with a total capacity of 12 litres although the actual fluid level in each is less.
The fluid used is MIL H 83282/A.

The nominal operating pressure in the systems is 175 bar (2,500 psi approximately) provided by self-
regulating swash-plate type pumps - one to each system. They are driven by the main gearbox and
mounted on the rear accessory drive. To assist the Left-Hand pump during periods of high demand (such
as landing gear operation) an electrically driven Auxiliary Hydraulic Pump is fitted In the Left-Hand
System. A Block diagram of the systems is shown in Figure 1.

Limitations

Minimum 80 bar
Main System Pressure
Normal 165-196 bar
Both Left and Right
Maximum 210 bar
Minimum 80 bar
Auxiliary System Normal 100-145 bar
Maximum 190 bar

Section 5 Hydraulic Systems

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CONTROLS & INDICATIONS

Refer to Figure 2. Indications of system pressure and operation are displayed at the bottom of the
multiple instrument panel (33 Alpha Panel) situated to left of centre on the main instrument panel.

Autopilot Hydraulic Selector Switches

There is one switch on each pilots collective and it enables the pilot to remove hydraulic pressure from
the autopilot servo pack in the controls cupboard.

Hydraulic System Warning Lights

The Central Warning Panel has a HYD caption to illuminate with any warning light on the hydraulic
warning panel and a SERVO caption. The SERVO caption is discussed in the section dealing with Flying
Controls.

The HYD warning will illuminate in conjunction with an additional warning on the 33 alpha panel as
shown in Figure 2.

The following list interprets the meaning of the numerous hydraulic system warning lights.

HYD 32∝ Caption ON with any hydraulic warning light - Canceled with WARN attention-getter.
SERVO 32∝-Caption - Showing loss of hydraulic pressure at servo-control distributor valve.
LH P Left-Hand System pressure below 80 bar.
AUX HP Auxiliary System pressure below 80 bar for 6 seconds.
RH P Right-Hand System pressure below 80 bar.
LH LEV Left-Hand System fluid level below 4 litres.
AUX P Failure of electrical supply to Aux. Pump - or pump running for >10 consecutive seconds.
RH LEV System fluid level below 2 litres.
LH HMP Output pressure from Left-Hand Main Pump below 80 bar.
APHP Autopilot hydraulic pressure below 70 bar.

Utility Services Accumulator Management Unit

This comprises an accumulator pressure gauge, a Normal or Accessory selector valve and a manual
hand pump. It is situated on the floor, aft of the centre consul.

Section 5 Hydraulic Systems

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 HYD 10
SERVO

S/BY
1
15
10
20

5 25
0 Bar x 10
AUX
2 3 5
15 20 20
15

10 25 25
10
Bar Bar
0 x 10 0 x 10
5 5

7 PRESS

6
LH P AUX HP RH P

LH LEV AUX P RH LEV

LH HMP
4
AP HP AUX P

Figure 2 – Cockpit Controls & Indications

1. Auxiliary Hydraulic system pressure gauge 6. NORM/ACC selector valve

2. Left-Hand Hydraulic system pressure gauge 7. Accumulator pressure gauge
3. Right-Hand Hydraulic system pressure gauge 8. Cover flap for pump and selector valve
4. Aux Hydraulic Pump – ON/OFF push button 9. AP hydraulics selector switch (one per lever)
5. Hand pump (stowed behind Captain’s seat) 10. CWP hydraulic warning lights

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 Right-Hand Hydraulic System
1-Second
2-Litre Filler Cap Delay Timer
Level

Return Filter
With Clogging
Indicator
5 LITRE LEVEL

2 LITRE LEVEL

Right-Hand Main
Hydraulic Pump
12 litres per Minute P
175 bar maximum

PRV Ground
210 bar Coupling

MAIN Filter
SERVO
CONTROLS
Lower Cylinders 0.2 Litre Accumulator

Low Pressure RH P Low Pressure

Switch Warning Light
< 80 bar

Pressure
Transmitter Pressure Gauge
BAR

RH LEV
Low Level
Cut-Off Solenoid Right-Hand
Level Low
Warning Light
TAIL SERVO
CONTROL
Right-Hand
Cylinder - Pressure
- Filtered Return
- Unfiltered Return
- Ground Coupling
- Electrical Connection

Figure 3 – Right-Hand Hydraulic System

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RIGHT-HAND HYDRAULIC SYSTEM

The Right-Hand Hydraulic system is of straight forward design and provides power only to the lower
bodies of the Main Servo controls and the right-hand body of the Tail Servo control. The self-regulating
pump is driven at 4,600 rpm by a quill drive from the rear of the MGB and provides a virtually constant
pressure of 175 bar. It is a swash-plate design with 9 pistons and varies the flow rate according to
system requirements. The nominal flow rate is 12 litres per minute.

Pressure Indication

A pressure transmitter transmits pressure indications to the gauge on the instrument panel.

Low Pressure Warning

A pressure switch operates to illuminate the Low Pressure warning RHP if the pressure in the system
falls below 80 bar.

Low Level Cut-off Solenoid

Should a leak occur and the fluid level drop to 2 litres, the sensor in the reservoir activates the low-level
solenoid after a 1-second delay, cutting off fluid supply to the Tail Servo control. (The long pipe runs to
the tail being the most likely source of a leak). Tail rotor control is still achieved by the left-hand body
being supplied by Left-Hand system. When the solenoid valve is energised the warning RH LEV is
illuminated together with the SERVO caption on 32∝ panel. The SERVO caption indicates loss of right-
hand tail rotor servo.

Should the leak now be contained, pressure will remain as before and Main Servos will still be supplied.
If the leak has not been contained, then when all fluid is exhausted, the pressure will fall and the Low
Pressure warning light RHP will illuminate.

Self Monitoring of Low Level Cut-off

Should the connector plug not be properly connected the RH LEV light will be illuminated regardless of
fluid level. This light is not normally ON with the battery switched ON.

Reservoir

Mounted on it's own manifold the reservoir is situated aft of the MGB on the right-hand side of the
transmission deck. See Figure 6. It contains 5 litres of fluid, which can be checked via a sight-glass and a
low level sensor probe set to activate the cut-off solenoid valve should the fluid level drop to 2 litres.
Contained within the reservoir is a 5μ filter fitted into the fluid return lines from the Main & Tail servo. A
‘tell-tale’ clogging indicator for this filter is fitted adjacent to the filler cap and will pop up should the back
pressure at the filter inlet exceed 1.5 bar.

Note - The return line from the low-level cut-off valve does not pass through the return filter.

A filler cap with a 100μ strainer and a vent plug are also fitted.

Manifold

In addition to the reservoir the manifold also supports the Low Level cut-off solenoid valve, the Pressure
Transmitter, the Pressure Switch, a small (0.2 litre) accumulator charged with nitrogen to 90 bar, self-
sealing connections for a ground power rig, the Pressure Relief Valve (210 bar) and the High Pressure
filter.

The function of the accumulator is to dampen out pressure fluctuations to give a steady reading on the
pressure gauge and to absorb the sudden pressure surge should the Low Level cut-off solenoid valve be
actuated. Internal drillings in the manifold couple these various components to the system.

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 Left-Hand Hydraulic

Filler Cap 4-Litre

Level Sensor

Return Filter
8 LITRE LEVEL With Clogging
Indicator
4 LITRE LEVEL

1
Aux Main
AUX HP P Pump Pump
PRV
P
2 170 bar
LH HMP
AUX P 6 Sec PRV Ground

Filter 3 4 210 bar Coupling

28V 5 Filter
1 Litre 60 bar
Accumulator
Auxiliary 80 -100
bar
Electric Pump 6 7
Control Logic 8
LH P
110 -140
bar
Pressure Pressure
Gauge Gauge
1 9 0.2 Litre 90 bar
Accumulator

BAR BAR

1 1 MAIN SERVO
CONTROLS
LH LEV Upper Cylinders
Low Level
Cut-Off Solenoid

1-Second
Delay Timer

TAIL SERVO
CONTROLS
1 Left Cylinder
Standby
Battery Only Power APHP Reservoir
On The Ground
AUTOPILOT
1PP6 1
1
ROTOR BRAKE

HOIST
Utility Services Accumulator
1 litre 90 bar N LANDING GEAR
Wheel Brakes
1 Extension
Retraction
1
BAR

- Main Pump Pressure

- Filtered Return
- Unfiltered Return
- Aux Pump Pressure
- Ground Coupling Hand Pump
- Electrical Connection

Figure 4 – Left-Hand Hydraulic System

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LEFT HAND HYDRAULIC SYSTEM

The Left-Hand Hydraulic system supplies the Main Servo controls (Upper bodies), the Tall Servo control
(left-hand body), the autopilot hydraulic unit, the rotor brake, wheel brakes and landing gear circuit. It will
also supply the hydraulic hoist if fitted. The Left-Hand system is shown above in Figure 4.

Key to Figure 4.

1. Aux pump failure warning light 10. Pressure transmitter

2. Aux system pressure warning light 11. Left-Hand level low warning light
3. Left hand main pump failure warning light 12. Check valve – opens with main pressure low
4. Low pressure switch for 3 above <80 bar 13. Autopilot low pressure switch <70 bar
5. Low pressure regulation switch 80-100 bar 14. Autopilot low pressure warning light
6. High pressure regulation switch 110-140 bar 15. Autopilot hydraulics solenoid valve
7. Pressure switch <80 bar 16. Utility services accumulator pressure gauge
8. Left-Hand pressure low warning light 17. Flow restrictor
9. Pressure transmitter

It comprises two power supply circuits. The main circuit with a self-regulating, swash-plate type pump
(similar to the Right-Hand pump but with a nominal fluid flow of 27 litres/min) providing power whenever
the rotor is turning. In addition, an auxiliary circuit with an electrical pump that automatically starts
running when the main pump flow is insufficient (e.g. during landing gear operation) or in the event of
main pump failure.

The auxiliary pump maximum flow rate of 10 litres per minute can also supply power to the system with
rotors stopped to enable pre-start control checks or rigging to be carried out. The automatic control of this
pump is described later.

The main pump draws it's fluid from the 2 litre level of the reservoir, whilst the auxiliary pump draws from
the bottom, thus if the main pump leaks a reserve of fluid remains.

As long as the main pump supplies the required flow the system pressure varies only slightly and the
auxiliary pump remains off. If the main pump output is inadequate to meet the demand the system
pressure will fall and the auxiliary pump will be switched on. A green indicating light shows when pump
is running.

AUX P

The auxiliary pump has two pressure-regulation switches to control its operation.

1. High Pressure Switch (aircraft in flight or with external electrical ground power rig)

Pump operates when pressure falls below 110 bars and stops running when pressure reaches 140
bar

2. Low Pressure Switch (ground condition with battery only power supply)

Pump operates when pressure falls below 80 bar and stops running when pressure reaches 100 bar.

Note - Occasionally the hydraulic pressure regulation switch sticks at the 140 bar position and the
auxiliary pump can not be switched on during start up. Should this occur, a light tap on the switch will
clear the problem.

Pressure Indications

Pressure transmitters in the Main and Auxiliary systems transmit the systems pressures to their
respective gauges. Note that the Auxiliary pump transmitter is upstream of the check valve and should
only read 140 bars. Higher readings indicate failure of the check valve

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Low Pressure Warnings

If both the Main pump and the Auxiliary pump fail (if a leak has emptied the reservoir) a pressure switch
activates the LHP light when pressure falls below 80 bar. Confirmation of the warning can be obtained
from the readings on the pressure gauges.

Low Level Solenoid Valve

Should the fluid level in the reservoir drop below 4 litres, the low level solenoid valve will be energised
(after 1 second delay) cutting off the supply to all equipment downstream of the Main Servo controls
(upper bodies) and activating the LH LEV and APHP warning lights. The SERVO caption on 32∝ panel
will also illuminate and indicates loss of left-hand body of the tail rotor servo.

The solenoid valve will also be operated in the battery only configuration on the ground when the
auxiliary pump is switched on. This is to prevent excessive running of auxiliary pump thus conserving
battery power. In this case the LH LEV light is not illuminated.

Note - Should the LH LEV light Illuminate when battery is switched on, it indicates that the sensor plug is
not correctly connected.

Reservoir

Similar to Right-Hand reservoir, mounted on it's own manifold and situated aft of the MGB on the left-
hand side or the transmission deck. It contains 8 litres of fluid and a low level sensor set to activate the
cut-off solenoid valve should the fluid level fall below 4 litres. See Figure 6. The fluid level can be
checked by 2 sight glasses. The upper one being used when the utility services accumulator is empty of
fluid (or the landing gear is in the ‘kneeled' position) and the lower one when accumulator is full or
landing gear in normal down position. A 5μ filter is fitted in the reservoir in the return lines from the
services with a pop-up ‘tell-tale’ clogging indicator situated adjacent to the filler cap. Note that some
return lines do not pass through the return filter but feed back via a stack-pipe at the 6-litre level in the
reservoir.

Manifold

The manifold in this system is similar to the Right-Hand system manifold but has additional drillings and
connections for the extra services supplied including the auxiliary pump supply and return. An additional
pressure switch is mounted to operate the LH HMP light. The accumulator, pressure switch, transmitter,
cut-off solenoid, filter and PRV fulfill the same functions as those in the Right-Hand system.

Main Pump Failure

A pressure switch fitted in the pump output line, upstream of a check valve, is set to activate the LH HMP
warning light if the pump output pressure falls below 80 bar. This could occur due to a leak in the system
or a mechanical failure of the pump. In either case the Auxiliary pump will run to maintain pressure in the
system at 140 bar.

Auxiliary Circuit Pressure Drop

A pressure drop below 80 bars is sensed by the left-hand pressure regulation switch and if this continues
for more than 6 seconds the switch activates the AUX HP warning light. The time delay prevents the
light from illuminating during periods of transient pressure drops before the pump cuts in.

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 2 3 4 9 10

Reservoir
7
8
5

Manifold

Figure 5 – Right-Hand Hydraulic Tank and Manifold

1. Filler cap 6. Ground couplings

2. 100μ filter 7. Accumulator
3. Low level sensor 8. Pressure transmitter
4. Vent 9. Filter clogging warning line
5. Sight glass 10. Return line filter of 5μ

2 3 8 9
1 14
Reservoir

7 13
12

11
4 15
5 6

10
Auxiliary Pump

Manifold

Figure 6 – Left-Hand Hydraulic Tank and Manifold

1. Vent 9. Low level sensor

2. Filter clogging warning line 10. High pressure regulation switch 110-140 bar
3. Return line filter of 5μ 11. Low pressure regulation switch 80-100 bar
4. Upper sight glass (if accumulator is empty) 12. Pressure transmitter
5. Lower sight glass (if accumulator is full) 13. Accumulator
6. Pressure transmitter 14. Cooling air duct
7. Accumulator 15. Auxiliary pump motor
8. 100μ filter

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 +
AUX P +
AUX P

10 Sec
Timer

Aux Pump
P >1200 amps

P P
2
A B
Pressure Switches
Controlling Relay ‘C’
High Pressure
110 - 140 bar
AUX P • In Flight
C • On Ground with GPU
• On Ground with ALTN.
ON
Hyd Low Pressure
Aux Pump 80 - 100 bar
2 Ground • On the ground
P Flight • with battery
Logics power Low Level
P l Cut-Off Solenoid
6

Hyd
LH Low level
Un-powered
During
Battery Only
Conditions LH LEV +
2
P
P Hyd 1-Second
< 4 Litres
5 Aux Pump Delay

Figure 7 - Hydraulic System Electrical Circuit (‘in flight’)

Auxiliary Hydraulic Pump Power Supplies and Control Logics

Refer to Figure 7. The pump is powered by a 28V supply from the Primary DC Bus (PP2) via a 1200 amp
circuit breaker. The ON/OFF control push button, situated on the Hydraulic warning panel, contains a
green light which illuminates whenever the pump is running and a white mechanical marker to indicate
the ON position of the switch.

When the switch is selected ON the supply from bus 2PP6 energises relay (A) and makes it's contact
thus allowing the PP2 supply to be available at the contacts of relay (B) – the regulation relay.

The energising of relay (B) is controlled by the pressure regulation switches to maintain the output
pressure either between 110 to 140 bar or 80 to 100 bar depending upon the position of the contact of
logic relay (C).

In flight or with external power supply the contact is as shown in Figure 7 and the High Pressure switch
controls the regulation relay, energising it up to 140 bars then de-energising and re-energising when
pressure falls to 110 bars. When relay (B) is energised one contact allows the PP2 supply to power the
pump and the other contact provides an earth for the green indicating light.

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Should the current through the pump motor exceed 1200 amps the circuit breaker on PP2 will trip, cutting
off the earth for relay ‘A’, thus de-energising it, and at the same time making the earth to illuminate the
amber AUX P light.

In order to conserve the battery power, but still enable the auxiliary pump to supply pressure for a pre-
start servo control check, logic relay (C) is brought into operation. When 'Ground' logics are made, (via
undercarriage proximity switches), and 2PP5 is un-powered (battery only conditions) relay (C) is
energised by the supply from 2PP6 and it's 2 contacts made.

The top contact changes over the regulation control to the Low Pressure switch and the system will be
pressurised up to 100 bars only. The Auxiliary pump will cut in again when pressure falls to 80 bar. The
lower contact enables the supply from 2PP6 (via Circuit Breaker HYD LH Low Level) to energise the Low
Level cut-off valve and limit the pressure to upper bodies of the Main Servos only. Thus the pump is run
for only short periods and the drain on the battery power reduced.

Note - With the loss of 2PP5 for any reason, when the aircraft lands, logic relay (C) will be made and the
flying controls revert to ‘Manual’ with the loss of the autopilot hydraulic supply.

The Auxiliary Hydraulic pump normally runs intermittently as and when the hydraulic system pressure
variations demand. Should a fault occur, such as a leak or regulation failure, which causes the pump to
run for 10 consecutive seconds then the timer logic will be activated giving Illumination of the amber
AUX P light and extinguishing of the green operating light.

The pump should be switched ‘OFF’.

Note - On some aircraft the contactors are designed to cut off the supply to the pump automatically
should this fault occur. However, the pump should still be switched OFF.

Auxiliary Pump Manifold

Situated aft of the Left-Hand reservoir, alongside the MGB oil cooler, the manifold supports the two
pressure regulation switches, a pressure transmitter, an accumulator and the pump. See Figure 6. The
DC motor is cooled by air ducts. A pressure relief valve set at 170 bar, high-pressure filter and check
valves are incorporated in the manifold. The 1-litre accumulator is charged with nitrogen to 60 bar and
provides initial supply of pressure when Left-Hand system pressure falls until the pump cuts in at 110
bar.

Emergency Landing Gear Lowering and Utility Services

Refer to Figure 8. The Hand Pump situated under a flap aft of the centre console, draws hydraulic fluid
from a 2-litre standby reservoir located in the flying control cabinet (Broom cupboard). The reservoir is
maintained full by the unfiltered return flow from the autopilot, rotor brake and undercarriage services as
it passes back to the Left-Hand reservoir. The hand pump is a double action type operated from side to
side.

Included In the system are a NORM/ACC selector valve, fitted at right-hand side of the pump under the
flap and the utility services accumulator of 1-litre capacity charged with nitrogen to 60 bar. Check valves
ensure flow is in one direction only. The Hydraulic pressure in the accumulator is indicated by a pressure
gauge fitted in the cockpit floor to the left of the flap. (Minimum pressure before start up should normally
be 100 bar). The accumulator is situated under the cabin floor aft of the broom cupboard.

With the selector valve in it's ‘NORMAL’ position (aft), the hand pump may be used to lower the
undercarriage in the event of a Left-Hand hydraulic system failure. When the selector valve is moved
forward to its ‘ACC’ position, the utility services accumulator may be pressurised by the hand pump. In
the forward position the flap cannot he closed thus indicating that the valve should be selected to ‘NORM’
prior to start-up. The accumulator is charged automatically by Left-Hand system when the pumps are
running.

When the accumulator is fully charged, the rotor and wheel brakes pressures will he maintained for
approximately 24 hours (parking).

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 Left-Hand Hydraulic
Reservoir

Left-Hand
Electrical
P P MGB Driven Lever
Main Pump
Aux Pump

Standby
Reservoir
NORM / ACC
Selector Lever

Accumulator
Accumulator
Hand Pump
P Pressure
Gauge Pressure
Gauge

BAR
Hand Pump

Normal or
Accumulator
Selector
Valve

Rotor Brake

Utility Hoist
Services

N
Wheel Brakes
Flow Restrictor

Landing Gear Landing Gear

Normal
EMERGENCY Retraction
EXTENSION Extension

Figure 8 – Emergency and Utility System

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