UNIT II
AIRCRAFT SYSTEMS
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Aircraft Systems and Instruments
Hydraulic System
Hydraulics :
Greek word for water
Study of behavior of water at rest and in motion
Current context the meaning has been expanded to include
the physical behavior of all liquids, including hydraulic
fluid.
Early aircraft had hydraulic brake systems
Invent of aircraft sophistication newer systems with
hydraulic power were developed.
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Advantages:
Light Weight
Ease of Installation
Simplification of Inspection
Minimum Maintenance requirements
100% efficient with negligible loss due to fluid friction
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Applications:
Landing Gear
Wing Flaps
Speed and Wheel Brakes
Flight Control Surfaces
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Hydraulic Fluid:
Primarily used to transmit and distribute forces to various units to be
actuated
Liquids are Incompressible
Pascals Law : Confined hydraulic fluid exerts equal pressure at every point
and in every direction in the fluid.
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Selection of Hydraulic Fluids:
Working Conditions
Service required
Temperatures expected inside and outside the system
Pressure the liquid must withstand
Possibilities of corrosion and other conditions
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Criteria For Selection:
Viscosity :
Internal resistance to flow
Viscosity decrease with increase in temperature
Satisfactory liquid must have enough body to give a good seal
at pumps, valves and pistons, but not so thick that it offers
resistance to flow, leading to powerloss and higher operating
temperatures which will lead to excessive wear of parts.
It is measured with viscosimeter (or) viscometer (Saybolt
Universal Viscosimeter)
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Criteria For Selection:
Chemical Stability :
Ability to resist oxidation and deterioration for long periods
All liquids tend to undergo unfavorable chemical changes under severe
operating conditions
Excessive temperatures have a great effect on the life of a liquid
Localized hot spots occur on bearings, gear teeth or at the point where
liquid under pressure is forced through a small orifice.
Small metals such as zinc, lead, brass and copper have a undesirable
chemical reaction on certain liquids resulting in formation of sludge,
gums and carbon (or) other deposits which clog openings, causes valves
and pistons to stick or leak and give poor lubrication to moving parts.
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Criteria For Selection:
Flash Point:
Temperature at which the liquid gives off vapour in sufficient quantity to ignite
momentarily or flash when a flame is supplied.
High flash point is desirable for hydraulic fluids because,
It indicates good resistance to combustion
Low degree of evaporation at normal temperature.
Fire Point:
Temperature at which a substance gives off vapour in sufficient quantity to ignite
and continue to burn when exposed to a spark or flame.
High fire point is required of desirable hydraulic fluids.
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Types of Hydraulic Fluids
To assure proper system operations
To avoid damage to non-metallic components of hydraulic system
Use the type specified in aircraft manufacturers maintenance manual (or)
instruction plate affixed to the reservoir or unit being serviced.
Types:
Vegetable Base Hydraulic Fluid
Mineral Base Hydraulic Fluid
Phosphate ester base Hydraulic Fluid
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Vegetable Base Hydraulic Fluid
Composed of caster oil and alcohol
Pungent alcoholic odor and generally dyed blue
Used in older aircrafts (Brake Systems)
Natural rubber seals are used with this type of fluid
If contaminated with petroleum base (or) phosphate ester base fluids, the
seals will swell, break down and block the system
Fluid is Flammable
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Mineral Base Hydraulic Fluid
Processed from Petroleum
Odor similar to penetrating oil and is dyed red
(Neoprene)Synthetic rubber seals are used with this type of fluid
Do not mix with other types
Fluid is Flammable
In small aircraft, Wheel brakes, Flaps and Landing gear are operated using
these fluids. Used on World War II aircrafts.
Less Corrosive and Less Damaging
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Phosphate Ester Base Fluids
In 1946, United Airlines DC-6 flying from San Francisco to Denver
experienced Hydraulic fire that rapidly destroyed the wing and the aircraft
crashed shot of the runway.
Today's larger aircraft use phosphate ester base non flammable fluids such
as Skydrol.
They are approximately seven times as expensive as mineral base fluids.
Specially designed to operate at very high loads and temperature.
Touchy stuff (Make sure reservoir cap is fastened)
Old fluid smells sour and colour darkens
Drain old fluid, flush with Varsol or Stoddard solvent and replace with
fresh fluid.
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Phosphate Ester Base Fluids
Intolerant of contamination , fluids has to be inspected with a microscope.
When system component fails, drain the fluid, flush the system and refill
with fresh fluid.
Contaminated fluid eat the seals and grind the surfaces.
Used in most transport category aircraft and are very fire resistant
Not Fire Proof, under certain conditions fluids will burn.
They are coloured Purple.
Butyl synthetic rubber (or) Teflon Flurocarbon resin seals, gaskets and
hoses
Fire resistant fluids will soften or dissolve many types of paints, lacquers
and enamels.
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Performing Work with a Liquid
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Simple Hydraulic System
Requirements:
Reservoir
Pump
Valve Mechanism
Pipes (or) Hoses
Actuating Cylinder
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Hydraulic System Components
Hydraulic Reservoirs
Inline Reservoir
Pressurized
Unpressurized
Integral Reservoir
Hydraulic Filters
Micronic Type (3m)
Porous Metal Type (5m)
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Hydraulic System Components Contd..
Heat Exchanger
Equipped with Temperature operated Bypass valve
Hydraulic Pumps
Heart of the hydraulic system
Various methods are employed to power a pump ( Electricity,
Bleed air from a turbine engine or even by a hand)
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Hydraulic System Components Contd..
Classification of Hydraulic Pumps:
Hand Driven Pump
Single-acting Hand Pump
Double-acting Piston Displacement Pump
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Hydraulic System Components Contd..
Power Driven Pump
Constant Delivery Pump
Variable Delivery Pump
Pumping Mechanism
Gears
Gerotors
Vanes
Pistons
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Pumping Mechanism
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Hydraulic System Components Contd..
Ram Air Turbine (RAT)
Air Turbine
Speed Governing Device
Variable Volume Pump
Pressure Control Devices
Pressure Switches
Pressure Regulators
Relief Valves (Thermal Relief Valve)
Pressure Reducing Valves
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Hydraulic System Components Contd..
Accumulators
Diaphragm
Bladder
Piston
Selector Valves
Rotary Valve
Poppet
Spool (or) Piston
Open-Center System
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Automatic Operating Control Valves
Orifice (or) Restrictor Valve
Check Valve
Orifice Check Valve
Metering Check Valve
Hydraulic Fuse
Sequence Valve
Shuttle Valve
Priority Valve
Flow Equalizers
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Hydraulic System Components Contd..
Hydraulic Actuators
Hydraulic Plumbing Components
Prevent leakage and Preserve pressure in the system
Seals (Packings and Gaskets)
Packings : Installed in parts containing moving parts (Actuating
Cylinders, Valves and Pumps)
Gaskets: Seal between two stationary parts
O ring seal commonly used for sealing pistons and rods
Identification of O-ring usage using Colour Codes
Red Fuel System : Blue Hydraulic System ; Yellow Oil System
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Hydraulic System
Classified into two major sections
Power Section
Actuating Section
Power Section :
Provides fluid flow
Regulates and limits pressure
Carries fluid to various selector values in the system
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Hydraulic System
Actuating Section
Contains various operating units such as wing
flaps, landing gear, brakes, boost systems and
steering mechanisms.
Power Section may be further classified as
Open System
Closed System
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Open System
It has fluid flow but no appreciable pressure in the system
whenever the actuating mechanisms are idle.
Selector valves are always connected in series with each other,
whereby the pressure line goes through each selector valve.
Pressure developed by the pump is controlled by one of the
three valves:
Open-center valve
Power-control valve
Pump-control valve
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Open System
It develops no pressure except when a mechanism is being
operated; the pressure is then metered by selector valve and
limited by a relief valve.
Advantages:
It does not require expensive or complicated pressure
regulators.
The power pump can be a simple gear pump, although a
fixed displacement piston pump may be used
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Open System
Disadvantages:
Operation of only one subsystem at a time is possible
without interference from other systems.
Transport-category aircraft require more complex
systems, which may have several units operating at the
same time.
Uses:
Light, general aviation aircraft.
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Basic Open-Center System
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Closed System
It directs fluid flow to the main system manifold and
builds up pressure in that portion of the system that leads
to all selector valves.
Two basic types:
Constant-volume pump and a pressure regulator
Variable-volume pump
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Closed System
Constant-volume pump and a pressure regulator:
To control pressure at working range .
To unload the pump when there is no flow requirement.
Pressure builds up in the system manifold.
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Closed System
Variable-volume pump:
Directs the flow to the system manifold
Output of the valve is controlled by an integral control
valve.
The valve reduces the pump flow to zero when no units are
operating in the system and pressure is built up in the
storage chambers, called accumulators.
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Closed System
Any number of subsystems may be incorporated in a closed
system
Selector valves are arranged in parallel
Based on system pressure the fluid is directed either to the system
or to the reservoir by the pressure regulator.
System pressure is maintained between kick-out and kick-in
settings of the regulator when the actuating mechanisms are not in
operation.
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Closed System
Accumulator: It stores fluid under pressure, Stabilizes system
pressure, ensures smooth operation of the regulator.
Relief valve safeguards the system if the regulator fails.
Multiple power pumps are used in multiengine aircrafts, where
they can be driven by separate engines.
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Closed System with Dual Pumps
Closed System with Dual Pumps
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Closed- Center System
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Aircraft Brake Systems
Mode of Actuation:
Mechanical (Older Small Planes)
Hydraulic (Sub system or Independent)
Pneumatic (In case of Hydraulic failure)
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Independent Brake System
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Power Boost Brake System
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Debooster Valve
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Aircraft Pneumatic System
Used Primarily as emergency source of pressure.
Principle of Operation is same with one exception AIR in a
pneumatic system is compressible therefore pressure can reduce
gradually.
In hydraulic system : Pressure drops from accumulator pressure
to zero pressure when pump is not operating.
Entire system, including air storage bottles can store air pressure,
in hydraulic system only accumulator stores fluid under pressure.
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Aircraft Pneumatic System
Air must be kept clean by means of filters and also kept free
from moisture and oil droplets (or) vapours.
Liquid separators and Chemical driers are used.
Moisture in the system may freeze in low temperatures at high
altitudes, resulting in serious system malfunction.
No return lines, air is dumped overboard saves tubing, fittings
and valves.
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Aircraft Pneumatic System
System utilized in Fairchild F-27 aircraft
Used to operate : Landing Gear Retraction and Extension,
Nose wheel centering, Propeller Brakes, Main wheel brakes
and Passenger entrance door retraction.
Source of Power :
Primary System
Emergency System
Fokker F - 27
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Pneumatic System Components:
Power Section located in each engine nacelle:
Gear Box driven Compressor
Bleed Valve
Unloading Valve
Moisture Seperator
Chemical Drier
Back Pressure Valve (Right Side Only)
Filter
Shuttle Valve
Disk Type Relief Valve
Ground Charging Connection for Ground maintenance (or) Initial Filling.
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Pneumatic System
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Aircraft Landing-Gear System
Configuration
Conventional Geared Aircraft
Tricycle Landing Gear
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Classification
Non - Absorbing Landing Gear
Rigid Landing Gear
Shock-Cord Landing Gear
Spring-Type Gear
Shock-Absorbing Landing Gear
Spring - Oleo
Air- Oleo
Fixed Gear
Retractable Gear
Hulls and Floats
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Landing-Gear Components
Trunnion
Struts
Torque Links
Truck or Bogie
Drag Link or Drag Strut
Side Brace Link or Side Strut
Overcenter Link or Downlock
Swivel Gland
Shimmy Dampers
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Landing-Gear Components contd..
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Hydraulic Landing Gear Retraction System
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Landing Gear System
(Retractive Mechanism)
Electrically Operated Landing Gear System
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Reference:
Aircraft Maintenance and Repair, Kroes, Watkins and Delp, Sixth
Edition, Tata McGraw Hill Education Pvt Ltd.
Page Nos: 341 378 ; 395 401; 404 414; 417 420; 452 456
Airframe and Powerplant Mechanics, Airframe HandBook, U.S
Department of Transportation (FAA) 15A
Page Nos: 349 - 351
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