Utility Hydraulic Flashcards
General
System has several ways for hydraulic power to be supplied to subsystems, mainly through the utility hydraulic pump being driven by the AGB when rotors are turning. During ground operations with rotors stopped/engines not running, an APU motor/pump can be used to drive the AGB, which will power the utility hydraulic pump and therefore the utility hydraulic subsystems
Those systems are:
- Cargo hook
- APU start system
- Main engine start system
- Wheel brakes
- Swivel lock
- Power steering
- Ramp
- Hoist (when installed)
EPUSHA
Installed on right side of ramp below manual hand pump, is used to pressurize the 200 cu inch accumulator or fill utility hydraulic reservoir
It is a fixed displacement hydraulic pump rated at 0.71 gym, attached to an explosion proof DC electric motor with a reduction gearbox, has filter assemblies, an electric relay, and control panel
To pressurize 200 cu inch accumulator:
- Turn aircraft battery ON
- Set manual control valve to NORMAL
- Move arming switch on control panel to ARMED
- Hold spring loaded ON/OFF switch to ON until reaching desired pressure (max 3000psi)
To refill utility hydraulic reservoir:
- Turn aircraft battery ON
- Set manual control valve to FILL
- Fill filler tank with fluid
- Move arming switch to ARMED
- Hold spring loaded ON/OFF switch to ON until reservoir is filled
Power is provided by No. 1 28vdc emergency bus through CB on No. 1 PDP labeled AUX UTILITY PUMP
200 Cu. In. Accumulator
Stainless steel sealed tubular assembly complete with internal floating separator piston
It is pre charged with nitrogen or dry air to a preload pressure of 1400 psi
Functions is to store pressurized fluid for APU starting and can supply pressure for the limited operation of utility subsystems
Also provides backup pressure when large demands are made on system
Utility Reservoir
Stainless steel tube sealed with quantity indicating dipstick
To provide positive supply to utility pump, reservoir is pressurized whenever utility system is pressurized. This is accomplished by taking pressurized fluid from 25 cu in accumulator and passes this pressurized fluid tor rear face of internal separator piston within reservoir.
Position of separator piston will always be dictated by quantity of fluid within reservoir. Attached to piston is dipstick, which is marked FULL or REFILL.
Utility Hydraulic Pump
Mounted to aft end of AGB and supplies pressure fluid to all utility hydraulic systems
Constant pressure type that is self adjusting to produce 3350psi
25 cu. in. Accumulator
Smol version of 200 cu in accumulator, provides a constant pressure for “Boot Strap” pressurization of utility system reservoir. Pre load air or nitrogen is 1400psi
Utility System Isolation Valve
Solenoid valve used to retain pressure within 200 cu in accumulator when rotors and APU shut down
Automatically closes when APU or main engines are started to remove all utility pressure from subsystems to allow max pump output for start sequence
APU must not be started during ground taxiing because all braking and steering will cease to function
Utility Hydraulic System Switch
On overhead panel marked UTIL-ON/OFF
With switch ON, DC emergency bus supplies power to open normally closed solenoid valve. This directs 200 cu in accumulator pressure to utility subsystems, and permits limited use of of hydraulic equipment on ground without external hydraulic power or APU running
With switch OFF, solenoid valve closes
Cooler Unit
Found on RH ramp area, unit is positioned between return manifold and return filter in utility system return line
Cooler has a duct, hydraulic motor fan, and cooler
Pressurized fluid flows from utility system valve through motor to operate fan and then flows into return line. Hot hydraulic fluid flows through cooler and is cooled by air drawn in by the fan
Fan Control Valve
Valve operates with thermostatic switch to allow pressurized fluid to enter IN port of cooler fan motor
Switch closes when hydraulic fluid temperature reaches 68* C
Fluid Cooler Thermostatic Switch
Unit is threaded into low pressure manifold next to hydraulic fluid cooler
When temp is high, the switch contacts close to energize the fan control valve and operates the fan
When fluid temp decreases, switch contacts open to de energize fan control valve and stops the fan
Contacts close 65.5 to 71.1*C
Contacts open 61.1 to 65.5*C
Utility Hydraulic Cooler Test Switch
Located on RH side of aft fuselage
Has 2 positions, TEST and spring loaded OFF
With switch at test the cooler fan solenoid valve energizes and provides utility hydraulic pressure to cooler hydraulic motor driving the cooler fan. During TEST, cooler fan solenoid thermal switch is bypassed allowing checkout of system
Utility Hydraulic Overtemperature Switch
Switch closes when fluid temp reaches 134.4 to 141.1*C, which illuminates UTIL HOT on overhead panel and HYD OVERTEMP caution light on annunciator panel
When temp falls below 116 to 125.5*C, the switch opens
Utility Hydraulic Pressure Indicating System
Consists of pressure transmitter, indicator gauge, and wiring
System powered by 26v AC supplied by No. 2 instrument bus
Normal operating range (green arc) is 2700 to 3500 psi
Main Engine Start System
Hydraulic starters are motored by 4000psi from utility hydraulic pump
During engine start, utility pump output increases from 3000psi to 4000psi to provide pressure to drive starters and all other subsystems are isolated
After engine start, and start switch placed OFF, pump output pressure returns to 3000psi and isolation valve opens
Cargo Hook General
Consists of:
- Cargo hook actuating cylinder
- Transverse beam
- Solenoid operated control valve
- Control relay
- CARGO HOOK control panel on overhead panel
Electrical power required is through No. 2 DC BU
Hook actuating cylinder can be operated by hydraulic pressure, emergency release air charge in actuating cylinder, or by manual emergency release mechanism on hook actuating cylinder
Cargo Hook Transverse Beam and Hook Assembly
Hook is suspended by center of transverse beam, which is mounted on bearings that allow beam and hook assembly to rock fore and aft
Cargo hook assembly is attached to the beam by a pivot bolt and is free to swing from side to side
Hook actuating cylinder is mounted to left side of hook assembly, and the utility hydraulic system provides pressurized fluid to hydraulically operate hook actuating cylinder
Hook has spring loaded keeper to prevent load from slipping off hook arm
Cargo Hook Release Valve
Hook release valve is solenoid operated 3 way valve in drive shaft tunnel area
When valve is de energized, utility hydraulic pressure is cut off to system and ports No. 1 and 2 are allowed to return
When valve is energized, pressurized fluid flows through the No. 3 port to cargo hook actuating cylinder
Cargo Hook Actuating Cylinder
Comprises of:
- Housing
- Floating piston assembly
- Solenoid valve
Actuating cylinder contains 4 chambers:
- Chamber A…is annular chamber between piston assembly and bulkhead of housing, normally at or near atmospheric pressure
- Chamber B…located at lower end of actuating cylinder, charged with dry air or nitrogen to 2100psi, air pressure acts solely against the center area of floating piston which forces piston downward, closing the arms
- Chamber C…annular chamber between upper end of housing and floating piston, chamber is vented to atmosphere except when emergency hook release switch is operated
- Chamber D…between piston assembly and floating piston, utility hydraulic goes to this chamber for normal release
Cargo Hook Normal Operation
Cargo hook arm is normally forced closed by air pressure acting on floating piston center area, contained within chamber B
The hook opens as follows:
- CARGO HOOK MASTER switch on overhead panel ARM, then upper hook release button is pressed on cyclic head. Depressing this button energizes control relay and hook release valve
- With control relay energized, cargo hook release valve allows 3000psi utility system pressure to flow to cargo hook actuating cylinder. In cylinder, this pressure is ducted to chamber D between the floating piston flange and piston assembly. This hydraulic pressure overcomes 2100psi of air pressure in chamber B, causing piston flange to move upward, retracting floating piston and opening hook
- Placing CARGO HOOK MASTER switch to OFF opens circuit to control relay, deenergizing control relay and hook release valve. Deenergized hook release valve allows the pressurized fluid in actuating cylinder to return to utility hydraulic system reservoir. With no hydraulic pressure in chamber D, air pressure in chamber B returns the piston to downward position, forcing hook closed.
- With hook forced closed (either pneumatically or manually), the over center position of latching cam linkage locks hook closed, this ensures loss of air pressure won’t cause hook to open
Emergency Hook Release System (Pneumatic)
CARGO HOOK EMERG RELEASE switch on overhead panel or T handle mounted on RH side of center console opens emergency release valve on actuating cylinder, which will:
- Dump portion of pressurized air contained in chamber B to atmosphere
- Route portion of pressurized air to chamber C, which forces floating piston flange upward, opening hook
When EMERG RELEASE switch is released, emergency solenoid valve is deenergized closed. Chamber C is vented to atmosphere and remaining pressure in chamber B extends piston and closes hook. Actuating cylinder must be reserviced with pressurized air/nitrogen after each emergency release
Emergency Hook System (Manual)
Consists of:
- Handle
- Release cable
- Release lever and cam
When handle is pulled, weight on hook causes cams to rotate, pulling hook spindle out of pivot sockets in the formers. The hook falls free from the formers, and pivots at the point where is attached to the actuating cylinders, causing hook to open and load released
Cargo Hook Annunciator
HOOK OPEN light comes on when one of two micro switches become activated
One micro switch operated by hook rotating cam, the other by movement of the hook actuating cylinder
Wheel Brake System
Power assisted servo system where brake pressure is proportionate to foot pressure applied at foot pedals. Hydraulic pressure assists in operating brakes
Each pedal operates its own master cylinder, with transfer valves to allow either pilot to apply brake pressure through common braking lines.
Parking brake is operated by pressing brake pedals, pulling out parking brake handle to hold pressure in wheel cylinders, and releasing pedals. Parking brake is released by tapping either right toe pedal
Pressure Reducing Valve
Incorporated into wheel brake system to reduce utility hydraulic pressure from 3350psi to 1390 psi.
Located in forward transmission area
Has 2 valves:
- One to reduce high pressure input
- Another to relieve excess pressure fluid to system return
Brake Master Cylinders
4 cylinders mounted to pedals, 2 cylinders per pilot, both sets interconnected via transfer valves
Brakes are initially applied, hydraulic fluid is pushed out brake port. Further pushing of the brakes downward causes master cylinder to telescope, which aligns ports on slide and sleeve assembly.
Pressurized utility hydraulic fluid then enters the aligned open ports and flow to the top of the piston. This pressurized fluid acting on top of the piston reduces manual effort required for increased braking action
Parking Brake Valve and Handle
Parking brake is spring loaded thermal compensating 4 way valve
In unparked position ports are open from the brake master cylinder and normal braking is permitted
To set parking brake, depress toe pedals to create braking pressure, then pull parking brake handle out to close ports in master cylinder, trapping pressure in master cylinder
Press either right toe brake to reseat parking brake handle and release trapped pressure
Thermal compensators (one for each wheel brake) prevents excess pressure caused by thermal expansion of fluid within brake lines to prevent rupturing of brake lines
Brake Pressure Switch
Installed on each parking brake valve outlet line
Pressure in left or right brake lines will close respective switch and illuminate PARKING BRAKE ON annunciator
Switches close at 620 psi and reopen at 550-600 psi
Landing Gear System
Front 2 are fixed cantilevers with twin wheels
Back two are single, full swivel (360*) which can be centered hydraulically and locked in trail position
Aft right landing gear can be steered from cockpit by a STEERING CONTROL knob on center console
Power Steering Switch
3 position switch with LOCK, UNLOCK, and STEER
LOCK…28vdc from No. 2 DC bus applied to swivel lock solenoid valves. Utility hydraulic pressure at 3000psi is then applied to centering cams on both rear wheel to center and lock them into trailing position
UNLOCK…swivel lock solenoid is de energized closed, and swivel unlock solenoid is energized open. This unlocks swivel locks and allows them to rotate 360* freely
STEER…28vdc is applied to power steering control box. Safety interlock between swivel lock and power steering prevents operation of power steering while swivel locks engaged
AFCS heading hold is disengaged when switch is set to UNLOCK
Power Steering System
Power steering is accomplished by rotating the control knob, which causes a servo valve in the power steering actuator to regulate pressure in order to extend or retract the actuator
Feedback potentiometer mounted on the power steering actuator stops actuator when desired turn radius is reached
STEERING FAIL Annunciator
Illuminates when right rear wheel has exceeded its limits
58* for left turn, 82* for a right turn are the limits
Beyond these limits the power steering control is de activated and the wheel become free swiveling
When steerable wheel exceeds steering range it enters a wheel out of phase zone and the wheel out of phase switch removes electrical power from the hydraulic control valve through a fail safe relay in the control box. This switch is what illuminates the STEERING FAIL annunciator
To energize the power steering system, the landing gear must be recentered and LOCK selected on power steering switch. Wheels may be recentered by:
- Taxi forward
- Physically force right rear wheel to center position
- Lift to hover and select LOCK
