UNIT II GROUND SERVICING OF VARIOUS SUB SYSTEMS:

Basic Terminology:

Cabin pressure:

The pressure of the air inside the cabin .

Ambient pressure:

The pressure of the air immediately outside the aircraft .

Pressure differential:

The difference between cabin pressure and ambient pressure .

Cabin altitude:

The absolute pressure inside the aircraft expressed in terms of altitude (i.e. feet above sea

level) or the altitude that you would find the same pressure as inside the cabin.

Example: A typical cabin altitude,

Boeing 767's, is maintained at 6,900 feet (2,100 m) when cruising at 39,000 feet (12,000 m).

If the cabin altitude were set to zero then the pressure inside would be the pressure found

at sea level. In practice, it is almost never kept at zero, in order to keep within the design

limits of the fuselage and to manage landing at airfields higher than sea level.

The cabin altitude of an aircraft planning to cruise at 40,000 ft (12,000 m) is programmed

to rise gradually from the altitude of the airport of origin to around a maximum of 8,000 ft

(2,400 m) and to then reduce gently during descent until it matches the ambient air

pressure of the destination.

Keeping the cabin altitude below 8,000 ft (2,400 m) generally avoids significant hypoxia,

altitude sickness, decompression sickness, and barotrauma.

Federal Aviation Administration (FAA) regulations in the U.S. mandate that the cabin

altitude may not exceed this at the maximum operating altitude of the aircraft under

normal operating conditions.

A design goal in newer aircraft is to lower the cabin altitude. For example, the highest

internal cabin altitude of the Boeing 787 Dreamliner is equivalent of 6,000 feet (1,800 m).

Understanding scenario:

An empty bottle, closed during a commercial flight with a cabin altitude of around 8,000 ft,

is crushed by the pressure at ground level after descent.

Need of Cabin Pressurization:

1. Cabin pressurization is used to create a safe and comfortable environment for aircraft

passengers and crew flying at high altitude by pumping conditioned air into the cabin.

2. Pressurization becomes necessary at altitudes above 12,500 feet (3,800 m) to 14,000 feet

(4,300 m) above sea level to protect crew and passengers from the risk of a number of

physiological problems caused by the low outside air pressure above that altitude.

The principal physiological problems are as follows:

(a)Hypoxia. (b) Altitude sickness. (c) Decompression sickness. (d) Barotrauma.

3. Pressurization of the cargo hold is also required to prevent damage to pressure-sensitive

goods that might leak, expand, burst or be crushed on re-pressurization.

Usual source:

This air is usually bled off from the engines at the compressor stage. The air is then cooled,

humidified, mixed with re circulated air if necessary and distributed to the cabin by one or

more environmental control systems. The cabin pressure is regulated by the outflow valve.

Pressurized Airplanes:

Advantages flying at High altitude:

1. When an airplane is flown at a high altitude, it consumes less fuel & more efficient

at a high altitude (as studied in flight dynamics).

2. In addition, bad weather and turbulence can be avoided by flying in the relatively

smooth air above the storms.

Airplanes which do not have pressurization and air conditioning systems are usually

limited to the lower altitudes. Because of the advantages of flying at high altitudes, modern

general aviation type airplanes are being designed to operate in that environment.

Mechanics of pressurization:

Pressurization is achieved by the design of an airtight fuselage engineered to be pressurized

with a source of compressed air and controlled by an environmental control system (ECS).

The most common source of compressed air for pressurization is bleed air extracted from

the compressor stage of a gas turbine engine, from a low or intermediate stage and also

from an additional high stage; the exact stage can vary, depending on engine type.

1. By the time the cold outside air has reached the bleed air valves it is at a very high

pressure and has been heated to around 200 °C. The control and selection of high or

low bleed sources is fully automatic and is governed by the needs of various

pneumatic systems at various stages of flight.

2. The part of the bleed air that is directed to the ECS is then expanded and cooled to a

suitable temperature by passing it through a heat exchanger and air cycle machine

known as the packs system.

3. The hot air from the engine is used to mix with the cold air and make nice warm air

for the passengers. As the air is pumped into the cabin from the packs, the sealed

cabin builds up air pressure as the air is compressed into the cabin.

Compressed air is also obtained from the auxiliary power unit (APU), if fitted, in the event

of an emergency and for cabin air supply on the ground before the main engines are

started.

Most modern commercial aircraft today have fully redundant, duplicated electronic

controllers for maintaining pressurization along with a manual back-up control system.

4. All exhaust air is dumped to atmosphere via an outflow valve, usually at the rear of

the fuselage. This valve controls the cabin pressure and also acts as a safety relief

valve, in addition to other safety relief valves.

5. In the event that the automatic pressure controllers fail, the pilot can manually

control the cabin pressure valve, according to the backup emergency procedure

checklist.

6. The automatic controller normally maintains the proper cabin pressure altitude by

constantly adjusting the outflow valve position so that the cabin altitude is as low as

practical without exceeding the maximum pressure differential limit on the fuselage.

The compressed air means that there is now enough oxygen to breathe.

Because there is air coming in, being compressed to a level where more oxygen is available

to breathe and being vented out, there is a constant supply of fresh air in the pressurized

cabin for the passengers to breath comfortably.

cabin pressure control system:

The cabin pressure control system provides cabin pressure regulation, pressure relief,

vacuum relief, and the means for selecting the desired cabin altitude in the isobaric and

differential range. In addition, dumping of the cabin pressure is a function of the pressure

control system.

A cabin pressure regulator, an outflow valve, and a safety valve are used to accomplish

these functions.

The cabin pressure regulator controls cabin pressure to a selected value in the isobaric

range and limits cabin pressure to a preset differential value in the differential range.

When the airplane reaches the altitude at which the difference between the pressure inside

and outside the cabin is equal to the highest differential pressure for which the fuselage

structure is designed and further increase in airplane altitude will result in a corresponding

increase in cabin altitude.

The cabin air pressure safety valve is a combination pressure relief, vacuum relief, and

dump valve.

The pressure relief valve prevents cabin pressure from exceeding a predetermined

differential pressure above ambient pressure.

The vacuum relief prevents ambient pressure from exceeding cabin pressure by allowing

external air to enter the cabin when ambient pressure exceeds cabin pressure.

The dump valve is actuated by the cockpit control switch. When this switch is positioned to

"ram," a solenoid valve opens, causing the valve to dump cabin air to atmosphere.

The degree of pressurization and, therefore, the operating altitude of the aircraft are

limited by several critical design factors. Primarily the fuselage is designed to withstand a

particular maximum cabin differential pressure.

Several instruments are used in conjunction with the pressurization controller.

The cabin differential pressure gauge indicates the difference between inside and outside

pressure. This gauge should be monitored to assure that the cabin does not exceed the

maximum allowable differential pressure.

A cabin altimeter is also provided as a check on the performance of the system.

In some cases, these two instruments are combined into one. A third instrument indicates

the cabin rate of climb or descent. A cabin rate of climb instrument and a cabin altimeter

are illustrated in Fig. 16-15.

Decompression is defined as the inability of the airplane's pressurization system to

maintain its designed pressure differential. This can be caused by a malfunction in the

pressurization system or structural damage to the airplane. Physiologically,

decompressions fall into two categories:

1. Explosive Decompression. Explosive decompression is defined as a change in cabin

pressure faster than the lungs can decompress.

2. Rapid Decompression. Rapid decompression is defined as a change in cabin pressure

where the lungs can decompress faster than the cabin. Therefore there is no likelihood of

lung damage.

The primary danger of decompression is hypoxia. Unless proper utilization of oxygen

equipment is accomplished quickly, unconsciousness may occur in a very short time.

It is recommended that the crewmembers select the 100% oxygen setting on the oxygen

regulator at high altitude if the airplane is equipped with a demand or pressure demand

oxygen system.

Unplanned decompression:

Unplanned loss of cabin pressure at altitude is rare but has resulted in a number of fatal

accidents.

Failures range from sudden, catastrophic loss of airframe integrity (explosive

decompression) to slow leaks or equipment malfunctions that allow cabin pressure to drop

undetected to levels that can lead to unconsciousness or severe performance degradation of

the aircrew.

Any failure of cabin pressurization above 10,000 feet (3,000 m) requires an emergency

descent to 8,000 feet (2,400 m) or the closest to that while maintaining terrain clearance

(MSA), and the deployment of an oxygen mask for each seat.

ATA21: AIR CONDITIONING SYSTEM:

Procedures Required

(O) – An operational procedure

(M) or (M#) – A maintenance procedure

(O)(M) or (O)(M#) – Both operational and maintenance procedures

Repair Interval Categories

Category A

Items in this category shall be repaired within the time interval specified in the "Remarks

or Exceptions" column of the air operator's approved .

Category B

Items in this category shall be repaired within 3 consecutive calendar days excluding the

day of discovery.

Category C

Items in this category shall be repaired within 10 consecutive calendar days, excluding the

day of discovery.

Category D

Items in this category shall be repaired within 120 consecutive calendar days, excluding the

day of discovery. To be considered for placement in Category D, the item must be of an

optional nature, or excess equipment which an air operator may, at his/her discretion,

deactivate, remove from or install on an aircraft.

ITEM: 21-20-1 RECIRCULATION FANS

System and

Sequence Item

Repair

Interval

Category

Number

Installed

Number

required

for

dispatch

Procedures

required

Remarks or Exceptions

Recirculation

fans

C - 0 (M)(O) May be inoperative provided:

a) Both A/C packs and

pressurization system operate

normally, and

b) Cargo is not carried in the

associated compartment.

ITEM: 21-30-1 AIR CONDITIONING AND PRESSURIZATION CONTROL MODES

Automatic

Pressurization

Control Systems.

C 2 1 May be inoperative provided the

manual pressurization control

system and one autopilot

are operative.

Automatic

Pressurization

Control Systems.

C 2 0 (M)(O) May be inoperative provided:

a) Flight is conducted in an

unpressurized configuration at

or below 10,000 , and

b) Cargo is not carried in

associated compartment.

Automatic and

Manual

Pressurization

Control Systems

C/D - 0 (M)(O) May be inoperative provided:

a) Flight is conducted in an

unpressurized configuration at

or below 10,000 ,

b) Extended overwater

operations are prohibited, and

c) Cargo is not carried in

associated compartment.

(O) addresses unpressurized flight and manual pressurization. (M) addresses operability of

required equipment, outflow valves for smoke clearing, autopilot, if required.

ITEM: 21-30-2 OUTFLOW/SAFETY VALVES

Outflow/Safet

y Valves

C - - (M)(O) May be inoperative provided:

a) Affected valve(s) is

secured OPEN,

b) Flight is conducted in an

unpressurized configuration at or

below 10,000 , and

c) Extended overwater operations

are prohibited.

ITEM: 21-30-3 CABIN ALTITUDE INDICATOR

Cabin

Altitude

Indicator

C - 0 (O) May be inoperative provided:

a) The cabin differential pressure

indicator is operative, and

b) A chart is provided to convert

cabin differential pressure to

cabin altitude.

D - 0 (O) May be inoperative provided flight

is conducted in an unpressurized

configuration at or below 10,000 .

The (O) for unpressurized flight will ensure that the procedures are clearly defined for the

flight crew members.

ITEM: 21-30-4 CABIN ALTITUDE WARNING SYSTEM

Cabin Altitude

Warning System

C 1 0 May be inoperative provided

flight is conducted at or below

10,000 feet .

ITEM: 21-30-5 CABIN RATE OF CLIMB INDICATOR

Cabin Rate of

Climb Indicator

C 1 0 May be inoperative provided

flight is conducted at or below

10,000 feet .

D 1 0 (O) May be inoperative provided

flight is conducted in an

unpressurized configuration at

or below 10,000 .

The cabin rate of climb indicator provides immediate feedback if operating in manual

mode. Without this feedback the workload could be unacceptably high; therefore the

proviso that all other aspects of the pressurization system must operate normally.

Some aircraft may have sufficient redundancy such that the next single failure does not

result in manual mode, an aircraft may have two automatic pressurization systems. For

these aircraft it would not be necessary that all other functions of the pressurization system

be operative, see B767.

ITEM: 21-30-6 DIFFERENTIAL PRESSURE INDICATOR

Differential

Pressure Indicator

C 1 0 (O) May be inoperative provided:

a) The cabin altitude indicator

is operative, and

b) A chart is provided to

convert cabin altitude to cabin

differential pressure.

D 1 0 (O) May be inoperative provided

flight is conducted in an

unpressurized configuration at

or below 10,000 .

ITEM: 21-50-1 AIR CONDITIONING PACKS

Air Conditioning

Packs

C 2 1 (O) Except for extended range

operations, one may be

inoperative provided flight

is conducted at or below

FL XXX.

D 2 0 (M)(O) May be

inoperative provided:

a) Flight is conducted in an

unpressurized configuration

at or below 10,000 ,

b) Both recirculation fans, if

installed, are operative, and

c) Cargo is not carried in

associated compartment.

NOTE:

Unit Load Devices () may be

carried provided cargo is

not carried on or in these

devices. For ballast

purposes, use of bags (made

of glass fibre or kevlar) of

sand or ingots of non-

magnetic metals (such as

lead) is acceptable.

Pack supporting

Class C Fire

Protection/Fire

Detection on Main

Deck Combi/All

Cargo Configurations

C - - May be inoperative

provided main deck cargo is

not carried.

NOTE:

Unit Load Devices () may be

carried provided cargo is

not carried on or in these

devices. For ballast

purposes, use of bags (made

of glass fibre or kevlar) of

sand or ingots of non-

magnetic metals (such as

lead) is acceptable.

ITEM: 21-50-2 EQUIPMENT AND AVIONICS COOLING FANS

Equipment and

Avionics

Cooling Fans

C - - Relief dependent upon

certification requirements

(see discussion).

ITEM: 21-60-1 CREW REST FACILITY - BUNK

1) Temperature C 1 0 (M) May be inoperative provided:

Control a) Heater is deactivated, and

b) Associated crew rest

facility is not occupied.

2) Ventilation C 1 0 (M) May be inoperative provided:

a) Heater is deactivated,

b) Supply/boost fan is

deactivated, and

c) Associated crew rest facility

is not occupied.

3) Temperature

Indicator

D 1 0

This item addresses the air conditioning aspects of the crew rest facility - bunk. Aircraft

certification requirements must be considered when developing relief in this area.

CABIN PRESSURE CONTROL:

The pressurization system (in below figure) described in the following paragraphs is of the

low pressure differential type, and is designed to commence pressurization of the aircraft

cabin at an aircraft altitude of 10,000 ft.

As the aircraft climbs above 10,000 ft, the cabin altitude gradually increases to 18,000 ft

when the aircraft has reached an altitude of 34,000 ft.

Above this altitude, the maximum cabin differential pressure of approximately 4 lbf/in2 is

maintained, and the cabin pressure will fall in proportion with the fall in the pressure of the

outside atmosphere.

1. The control of the air pressure within the aircraft cabin is vested in the cabin pressure

control and discharge valve which senses the pressure of the air within the aircraft

cabin, and the outside atmosphere (ambient pressure) and operates a discharge piston

restricting the outflow of air from the cabin.

2. Pneumatically connected to the pressure control and discharge valve is a slave

discharge valve that increases the amount of air that can be discharged from the

aircraft cabin.

3. Operation of the control part of the valve is regulated by absolute and differential

pressure capsules, which sense changes in the cabin and ambient pressures as they

occur.

4. At a pressure that is dependent upon the position of the differential and absolute

capsules, the discharge part of the valve will open and discharge surplus air from the

aircraft cabin. The valve will close when the cabin pressure has fallen to within the

operating range of the valve and it will continue to open and close as necessary to

maintain the cockpit pressure within its specific operating limits.

5. Operation of the slave discharge valve will always mimic that of the main valve. When

the aircraft has climbed to an altitude of 34,000 ft, the valves will continually discharge

cabin charge air to maintain the constant cabin differential pressure.

SAFETY DEVICES AND WARNINGS

The pressurization system incorporates various safety devices and warnings that ensure the

safe operation of the system during flight.

These can include:

• Safety valve.

• Cabin altimeter.

• Cabin altitude switch.

• Cabin warning indicators.

A safety valve is incorporated into the system to prevent over pressurization of the cabin in

the event of failure of the pressure controller.

If the cabin differential pressure rises above a predetermined level, the valve will open and

discharge excess pressure to atmosphere. In the event of the aircraft rapidly losing height

and insufficient charge air entering the cabin, the safety valve will open inwardly, and

allow atmospheric pressure to flow into the cabin, thus preventing a negative cabin

pressure differential occurring.

SAFETY VALVE

PURPOSE.

The safety valve (sometimes referred to as the outward/inward relief valve) described in

the following text is designed to be entirely automatic in its operation, and performs the

following safety tasks:

• Outward relief - Opens and prevents over pressurization of the cabin should failure by

the cabin pressure control valve occur.

• Inward relief - In the event of insufficient charge air entering the cabin and a rapid loss

aircraft height, the safety valve opens and prevents negative cabin pressure differential.

• Manual outwards relief - Manual operation of the associated ram air valve causes the

safety valve to open automatically and discharge contaminated air from the aircraft cabin.

Construction:

The valve (figure 8) consists of three component parts, these are:

• Discharge valve

• Servo valve

• Relief valve

36. Discharge valve. The discharge valve consists of a valve body that is fitted with a debris

guard. Housed within the body are a piston, a knife-edge seal and a baffle. The piston and

baffle are supported in the base casting by a central guide. A spring holds the knife edge on

its seating.

37. Servo valve. The servo valve is mounted on the valve body and consists of a housing, a

diaphragm, and a schraeder type valve. A tapping on the side of the housing houses

includes a calibrated orifice, through which the control chamber is connected to cabin

pressure. Static pressure if felt on the upper side of the diaphragm and cabin pressure on

the under side.

38. Relief valve. The relief valve is mounted in the valve body and consists of a rubber non-

return valve retained by a disc and circlip.

Operation.

The valve (figure 9) is automatic in operation and depends upon cabin differential pressure

for its operation. The operation sequences are:

• Normal flight conditions

• Outwards relief

• Inwards relief

• Manual outwards relief

Normal flight conditions. The schraeder valve is closed, and cabin pressure is sensed in the

control chamber through the orifice, and on to the underside of the piston through the

mesh guard.

Since the control chamber and underside piston pressures under normal flight conditions

are equal, the knife edge is held on its seating by the action of the spring and air cannot

pass through the valve.

Outward relief. In the event of an uncontrolled increase in cabin pressure following failure

of the cabin pressure control valve, the differential pressure generated moves the

diaphragm against its spring and lifts the schraeder valve from its seating. When the

schraeder valve opens, the control chamber pressure is vented to static at a greater rate

than replenishment occurs through the orifice. Thus the control chamber pressure falls,

and permits the force exerted on the underside of the piston to overcome the spring

pressure and lift the knife edge from its seating. Cabin pressure will now be discharged to

atmosphere and cause the excess pressure in the cabin to be dissipated. A continued fall in

cabin pressure will permit the servo spring to re-assert itself, and move the diaphragm

away from the schraeder valve, allowing the valve to close. Sensed cabin pressure is then

restored in the control chamber, and the piston spring re-asserts itself causing the knife

edge to move onto its seating.

Inward relief. Under normal flight conditions, cabin pressure in the control chamber and

the piston under side is opposed and balanced by atmospheric pressure on the baffle. In

the event of a very rapid descent by the aircraft and insufficient charge air entering the

cabin, there will be a rapid rise in the atmospheric pressure and the possibility of negative

cabin differential pressure. The force exerted on the baffle by the atmospheric pressure

will exceed the force exerted by the piston under side and control chamber pressures and

the baffle will try to lift the knife edge from its seating to allow air to flow into the cabin

from the outside atmosphere. When the baffle lifts the piston, it will compress the air

within the control chamber and cause the relief valve to lift and discharge excess chamber

pressure into the cabin.

Manual outwards relief. Manual selection of the ram air valve to the open position will

cause the cabin pressure to rise. Its selection will also open a Depressurizing valve (figure

10) located on the ram air valve operating shaft. The Depressurizing valve, being

connected by pipeline to the safety valve, will allow control chamber pressure to be vented

to atmosphere. Loss of the control chamber pressure will permit the piston to overcome the

force exerted by the spring, and lift the knife edge from its seating. The excess pressure is

thus discharged to atmosphere. The knife edge will reseat when the ram air valve is closed

and the system returns to normal operation.

Pressure operated cabin altitude switch:

This is located in the pressure cabin. Should the cabin altitude approach levels dangerous

to crew, electrical contacts within the switch operate, and illuminate an „attention getter‟

lamp, a warning caption in the cabin and initiate an audio warning. In the event of cabin

pressurization failure, which causes the cabin altitude switch to operate, a warning caption

(C/PR) will illuminate on the centralized warning panel (figure 3). The pilot may cancel

the flashing lamp and the audio warning by pressing the warning lamp, however, the

centralized warning caption will remain illuminated until the system returns to normal

operation.

CABIN PRESSURE CONTROL AND DISCHARGE VALVE:

Purpose. The purpose of the Cabin Pressure Control and Discharge Valve is to control the

pressure of the air within the aircraft cabin.

As its name suggests, the valve may be considered as two separate valves; a Pressure

Controller which controls the opening and closing of a Discharge Valve which in turn

restricts the outflow of air from the cabin. In some aircraft the valves are physically

separate items.

The Pressure Control and Discharge Valve control the cabin pressure at the following

levels:

• Sea level to 10,000 ft - ambient pressure

• 10,000ft to 34,000 ft - gradual cabin altitude increase from 10,000 ft to 18,000 ft

• 34,000 ft to service ceiling - maintain the cabin differential experienced at 34,000 ft (just

under 4 lbf/in2).

Construction:

The valve is cylindrical in shape and consists of a base mounting casting and mesh guard,

a body, and a cover assembly that houses the following sub-assemblies:

• A pneumatically operated piston is supported on a web-like structure. The piston

incorporates a central spindle that runs within a bush and incorporates a knife edge, which

in turn engages in an annular valve seat.

• The valve control chamber houses a preloaded spring that is located on the piston and

holds the knife edge onto its seating. Located above the spring within the control chamber

are absolute and differential capsules; these are each connected to a main beam, which in

turn rocks about the capsules, depending upon the capsule deflections. The range of

movement of the beam is limited at each end by stop beams.

• The cover assembly houses: static, vent and slave valve connections, a servo valve and a

calibrated orifice connecting cabin pressure to the control chamber.

Operation:

The cabin pressure control valve provides for automatic control of cabin pressure from sea

level up to the service ceiling of the aircraft. Valve operation (figure 5) is influenced by air

pressure acting upon the piston assembly, the preloaded spring and the absolute and

differential pressure capsules.

Valve operation is dependent upon changes in atmospheric pressure and the consequent

expansion/retraction of the capsules.

The valve operation may be divided into three clearly defined stages:

• Sea level – 10,000 ft altitude.

• 10,000 ft – 34,000 ft altitude.

• Over 34,000 ft altitude.

Sea Level – 10,000 ft altitude With the canopy in the closed position, and the air

conditioning system selected ON, air enters the cabin and the pressure is felt through the

mesh onto the underside of the piston tending to lift the knife edge from its seating.

At sea level, the absolute capsule is in the fully contracted condition and the differential

capsule expanded against its stop, causing the main beam to lift the vent valve from its

seating, and permit control chamber pressure to be vented to atmosphere.

The combined force exerted by the preloaded spring and the vented control chamber

pressure on the piston, is overcome by the cabin pressure acting upon the underside of

piston, maintaining the valve in the open position.

Air entering the cabin is thus discharged directly to the equipment compartment cooling

systems. During aircraft take-off and the climb to 10,000 ft, the differential capsule

remains expanded against its stop.

The absolute capsule begins to expand but not enough to reduce the vent valve outlet size to

less than the calibrated orifice size. Hence the vent valve remains in the open position, the

control chamber pressure continues to be at ambient pressure and the discharge valve

remains open.

10,000 ft – 34,000 ft altitude When the aircraft reaches an altitude of 10,000 ft, the absolute

capsule has expanded enough to reduce the vent valve outlet size to less than that of the

calibrated orifice.

This allows the control chamber pressure to rise sufficiently to cause the knife edge to

progressively move onto its seating and control the discharge of air from the cabin.

As the differential pressure increases during the aircraft climb, the differential capsule

contracts and lessens the effect of the expanding absolute capsule.

During further altitude increase up to 34,000 ft, the absolute capsule continues to expand

with corresponding contractions of the differential capsule to gradually increases the cabin

altitude to 18,000 ft

Over 34,000 ft altitude Above 34,000 ft altitude, further expansion of the absolute capsule

is prevented by its stop but contraction of the differential capsule continues and controls

the opening of the vent valve.

The cabin altitude now climbs relative to the aircraft rate of climb and maintains a fixed

differential of just less than 4 lbf/in2.

Heating systems:

Aircraft heating systems range in size and complexity from simple heat exchangers for

small, single-engine aircraft through combustion heaters used with larger aircraft to

compressor bleed-air systems for turbine-engine-powered aircraft. The system is being

used for a complete aircraft environmental control package.

Exhaust heating systems:

The simplest types of heating system, often employed on light aircraft, consists of a heater

muff around the engine exhaust stacks, an air scoop to draw ram air into the heater muff,

ducting to carry the heated air into the cabin, and a valve to control the flow of heated air.

Such a system is the most basic type of heat exchanger.

A heat exchanger is any device by which heat is transferred from one independent system

to another independent system. This is usually accomplished by allowing the heat in one

system to pass into a portion of its containment material and then passing air over that

material, warming the air.

The radiator used by older liquid-cooled aircraft engines is a type of heat exchanger. The

heat of the engine is absorbed the liquid that runs through the engine in a closed system.

The liquid, the primary heat-transfer medium, then passes through the radiator, where the

heat in the liquid is transferred to the metal fins of the radiator.

The heating system consists of the muffler and heat shroud, ducting to the air box and

windshield defroster outlets, and ducting to the heat outlets in the cabin. The amount of

heat delivered to the cabin is controlled from the cockpit.

Combustion heater systems

Electric heating systems

Heating with bleed air

Heating pressurized aircraft

Cabin-cooling systems:

Aircraft-cooling systems, also called air-conditioning systems, are used to reduce the

temperature inside an aircraft for crew and passenger comfort. The two basic methods of

reducing the temperature of aircraft are the Freon vapour-cycle machine and the air-cycle

machine.

The vapour-cycle machine is a closed system using the evaporation and condensation of

Freon to remove heat from the cabin interior. The air-cycle machine uses the compression

and expansion of air to lower the temperature of the cabin air.

Vapour-cycle cooling systems:

The vapour-cycle air-conditioning system is used in reciprocating-engine-powered aircraft

and in smaller turboprop aircraft that do not make use of air-cycle machines to reduce the

cabin interior temperature.

These cut-out and interrupt systems are used to disengage the refrigerant compressor

during demand for high engine power output, such as during takeoff operations or when

one engine of a multi engine has failed. This interruption in compressor operation allows all

available power to be used to maintain controllable flight.

In addition, as heat is added to a solid, it becomes liquid and then a gas. Conversely, as heat

is extracted from a gas, it becomes a liquid and then a solid.

A vapour-cycle cooling system takes advantage of these laws of nature using two heat

exchangers to control the temperature of the cockpit and cabin.

The cooling process starts at the compressor, where the refrigerant is in a gaseous form.

The function of the compressor is to push the refrigerant, under pressure, through the

entire system. As the gas enters the condenser, heat is drawn from the refrigerant and

passed to the atmosphere. The cooling of the refrigerant causes it to condense into a liquid.

Because of the compressor, the liquid is under pressure.

The pressurized liquid is then metered into tiny droplets by an expansion valve. The

droplets then enter the evaporator, where they draw heat from the air and then change into

a gas. As a result of heat being drawn from the air, the temperature of the air is decreased.

It is this cooled air that is introduced to the cabin for cooling.

When it becomes necessary to change a unit in vapour-cycle refrigeration system, the

refrigerant must be released. This is called purging the system.

The recommended procedure for purging the refrigeration system is first to connect the

service manifold to the low side and high side service valves. The manifold port should be

connected to a vacuum pump, connected to a closed container. As mentioned previously, the

technician must assure that the valve fittings are correct for the type of valves involved.

The service instructions provide this information. The manual valves on the service

manifold must be closed before connecting the manifold to the system. After the service

manifold is connected, either one or both of the service valves are “cracked”, with the

vacuum pump on, to allow a slow escape of the refrigerant into a closed container. The

escaping gas should be monitored for escaping compressor oil, which indicates that the

system is being purged too quickly. If oil is discharging with the gas, the discharge is too

rapid and the manual valves should be closed slightly. When both pressure gauges on the

service manifold indicate zero pressure, the system is purged and can be disassembled.

Air-cycle cooling:

Modern large turbine-powered aircraft make use of air-cycle machines to adjust the

temperature of the air directed into the passenger and crew compartments of these large

aircraft. Although this portion of the discussion of air-cycle machines is directed to the

ability to provide cabin-cooling air, it should be noted that the cabin can also be heated and

pressurized by the use of an air-cycle machine.

These large aircraft utilize air-cycle cooling because of its simplicity, freedom from

troubles, and economy. In these systems, the refrigerant is air. Air-cycle cooling systems

utilize the same principles of thermodynamics and the same laws of gases involved in

vapour-cycle systems. One principal difference is that the air is not reduced to a liquid, as is

the refrigerant in a vapour-cycle system.

The principle of cooling by means of a gas is rather simple. When a gas is compressed, it

becomes heated and when the pressure is reduced, the gas becomes cooled.

In an air-cycle system, the air is continuously compressed and then cooled by means of heat

exchangers through which ram air is passed; then the pressure is reduced by passing the

air through an expansion turbine. The air leaving the expansion turbine is at low pressure

and low temperature. The cooled air is directed through ducting with control valves to

regulate the amount of cooling air needed to produce the desired cabin temperature.

The turbine-compressor unit by which air is cooled is called an air-cycle machine. Got

compressed air from the compressor of one of the turbine engines flows through the

primary heat exchanger. The heat exchanger is exposed to ram air, which removes the heat

from the air. The cooled but still compressed air is then ducted to the compressor inlet of

the ACM. The compressor further compresses the air and causes it to rise in temperature.

This air is directed to the secondary heat exchanger, which, being exposed to ram air,

removes heat from the compressed air. The compressed air is then directed to the expansion

turbine. The expansion turbine absorbs energy from the air and utilizes this energy to drive

the compressor. As the air exits the expansion turbine, it enters a large chamber, which

allows the air to expand and causes a further reduction in the air temperature. Thus the air

leaving the turbine is cooled by the loss of heat energy and by the expansion that takes

place. The freat reduction in temperature causes the moisture in the air to condense, and

this moisture is removed by means of a water separator. The dried, cold air is then routed

to ducting to be utilized as required to provide the desired temperature in the cabin. Note

that a bypass duct with the cabin-temperature-control valve will bypass air around the

cooling system when cooling is not required.

Pressurization-control operation:

The control of the cabin pressurization is through various valves. These valves are operated

by differences in air pressure in order to achieve the desired level of pressurization or to

prevent overstressing the aircraft structure. To understand the basic principles of the

pressurization-control operation, look at a safety valve and then move on to an outflow

valve and the controller.

Cabin environmental system for a jet airliner:

A typical air-conditioning system for a jet airliner is represented by the system employed

for one model of the Boeing 727 airplane. This system provides conditioned, pressurized air

to the crew cabin, passenger cabin, lower nose compartment, forward cargo compartment,

air-conditioning distribution bay, and aft cargo compartment. The air supply is furnished

by engine bleed air when in flight and from engine bleed air, a ground pneumatic supply

cart, or a ground conditioned air supply cart during ground operation. Part of the warm

air supply from the engines is passed through the air-conditioning packs to be cooled. The

cold air is then mixed with the remainder of the warm air as required to obtain the

temperature of air called for by the temperature control system. The outflow valves are

regulated to exhaust only that additional quantity of air required to maintain the desired

pressure in the cabin.

Engines 1 and 3 furnish eighth or thirteenth-stage compressor bleed air, depending upon

engine power setting and air-conditioning demand. Eighth stage bleed air from engine 2 is

also available as an alternative source. Bleed air from engines 1 and 3 passes through a heat

exchanger to reduce the air temperature to approximately 370⁰F. A precooler controller

and modulating valve combine to maintain the correct temperature. If difficulties occur in

the controller, a thermo switch prevents excessively hot sir form entering the air-

conditioning system by closing the bleed-air shutoff valve. Airflow to each air-conditioning

system is regulated by a flow-regulating servo and modulating valve.

The air-conditioning system for the airplane described here consists, by function, of 4

subsystems. These are for cooling, temperature control and distribution and pressurization

control.

Cooling packs:

The cooling of air for this model of the Boeing 727 airplane is provided by means of 2

cooling packs. These packs also remove excess moisture from the air. With the exception of

the water separator, which is located in the distribution bay, all cooling pack equipment is

contained inside the centre fuselage fairing.

The cooling devices used in the cooling packs consist of a primary heat exchanger, a

secondary heat exchanger, and an air-cycle machine (ACM). The heat exchangers are of the

air-to-air type, with heat being transferred from the air going through the ram air system.

The ACM consists of a turbine and a compressor. Air expanding through the turbine drops

in temperature as the energy is extracted for the major cooling in the pack. It can be seen

that engine bleed air passes through the primary heat exchanger for initial cooling,

through he secondary heat exchanger, and then through the expansion turbine of the air-

cycle machine. At this point the air is at its lowest temperature, since the heat energy has

been extracted by means of the heat exchangers and the expansion turbine.

As the air cools, its moisture content condenses. The moisture is atomized so finely,

however, that it will stay in suspension unless a special moisture removing device is

employed. This is the function of the water separator. Moisture entering the water

separator is prevented from freezing by an anti-icing system. An anti-icing thermostat in

the water separator actuates a 35⁰F control valve in a duct between the primary heat-

exchanger exit and the water-separator inlet. The valve opens to add warm air if the

turbine discharges temperature approaches the freezing temperature of water.

The primary heat exchanger id the first unit of the cooling packs through which engine

bleed air passes to be cooled. The unit is rectangular and is located between 2 sections of

the ram air duct. Two plenum chambers in the heat exchanger are connected by a bank of

tubes to allow maximum surface exposure of each tube to ram air passing across the

outside of the tubes.

As explained previously, the ACM cools compressed air by expansion. When the air is

originally compressed by the engine compressor, its temperature rises in approximate

proportion to the rise in air pressure. The heated compressed air is passed through the

primary heat exchanger, where some of the heat energy is removed. It is then directed to

the compressor section of the ACM.

In the compressor section the air id further compressed and heated. This additional heat is

reduced in the secondary heat exchanger, which is located between the compressor and

turbine of the air-cycle machine.

As the air expands across the turbine, heat energy is expended in driving the turbine and

through the expansion process. Thus the air leaving the turbine is at its lowest temperature.

OXYGEN SYSTEMS:

Oxygen systems are required on airplanes that fly for extended periods at altitudes

substantially above 10,000ft. Although the normal human body can survive without a

special supply of oxygen at altitude of over 15,000ft, the mental and physical capacities of a

human being are reduced when the usual supply of oxygen is not available in the air.

A lack of oxygen causes a person to experience a condition called hypoxia. This condition

results in light headedness ,”headaches ,dizziness ,nausea, unconsciousness, or death,

depending upon its duration and degree. When permanent physical damage results from

lack of oxygen, the condition is defined as anoxia.

Two principal factors affect the amount of oxygen that a person will absorb. These are

1) the amount of oxygen in the air the person is breathing and 2) the pressure of the air and

oxygen mixture.

Normal air contains approx 21 % oxygen, and this provides adequate oxygen for the hu-

man body at lower altitudes. At 34,000ft altitude, a person must be breathing 100%oxygen

to absorb the same amount of oxygen as when breathing air at sea level.

To adjust for variations in cabin altitude , oxygen systems are often equipped with baro-

metric regulators, which increase the flow of oxygen as cabin altitude increases. In a non

pressurized aircraft , the cabin altitude is the same as the aircraft altitude , and the oxy-

gen flow is adjusted for aircraft altitude.

TYPES OF OXYGEN SYSTEMS:

Oxygen systems, classified according to source of oxygen supply , may be described as

stored –gas , chemical or solid –state , and liquid oxygen systems (LOX). Systems for pri-

vate and commercial aircraft are of the stored –gas or chemical type.

Oxygen systems may be portable or fixed. The fixed system is permanently installed in an

airplane where a need for oxygen may exist at any time during flight at high altitudes.

Commercial airplanes are always equipped with fixed systems, augmented by a few porta-

ble units for crew members, who must be mobile, and for emergency situations.

The simplest type of portable oxygen system includes a Department of Transporta-

tion(DOT) approved oxygen cylinder of either 11ft3 capacity or 22ft

3 capacity , a regulator

assembly, a pressure gauge , an ON - OFF valve, hose couplings, flow indicator ,and one or

two oronasal masks. This system is charges to 1800psi and is suitable are available with au-

tomatic flow –control regulators, which adjust oxygen flow in accordance with altitude.

Oxygen system are also classified to the type of regulator that controls the flow of oxygen

.The mask employed must be compatible with the type of regulator .The majority of oxygen

systems for both private and commercial aircraft are of the continuous flow type.

The regulator on the oxygen supply provides a continuous flow of oxygen to the mask .The

mask valving provides for mixing of ambient air with the oxygen during the breathing

process. As mentioned previously, some continuous – flow systems adjust flow rate in ac-

cordance with altitude.

Demand and diluter -0demand regulators used with demand masks supply oxygen to the

user during inhalation. When the individual using the equipment inhales, if causes a reduc-

tion of pressure in a chamber in the regulator. This reduction in pressure activates the oxy-

gen valve and supplies oxygen to the mask. A flow indicator shows when oxygen flow is tak-

ing place.

Pressure – demand regulators contain an aneroid mechanism, which automatically increas-

es the flow of oxygen into the mask under positive pressure. This enables the user to absorb

more oxygen under the conditions at very high altitudes. This type of equipment is normal-

ly used at altitudes above 40,000ft . The additional pressure is needed to enable the user to

absorb oxygen at a greater rate demand mask must be worn with a pressure –demand reg-

ulator.

OXYGEN BOTTLES :

Oxygen cylinders, also called oxygen bottles , are the containers used to hold the aircraft

gaseous oxygen supply. The cylinders may be designed to carry oxygen at a high or low

pressure.

High –pressure cylinders are designed to contain oxygen at a pressure of approx 1800psi.

These cylinders can be identified by their green color and by the words “Aviators‟ breath-

ing oxygen ”on the side of the cylinder.

Three types of construction are used: a high strength, heat treated, steel alloy cylinder: a

wire –wrapped metal cylinder: and a Kevlar –wrapped aluminum cylinder.

High pressure cylinders are manufactures in several sizes and shapes. The cylinders are

designed for a maximum operating pressure of about 2000psi but are normally serviced to

a pressure of between 1800 &1850psi. To obtain the proper pressure for the ambient air

temperature, aircraft manufactures .When using one of these charts, make sure that it is

the correct one for the system you are servicing.

There are several types of cylinder valves in use. The hand-wheel type has a wheel on the

top of the valve and operates like a water faucet.

Another type of valve is of the self opening design. When the valve is attached to the oxygen

systems, a check valve is moved off of its seal, allowing the cylinder to charge the system.

A third type of valve uses a cabin operated push –pull control to operate a control lever on

the top of the valve. This eliminates the necessity of always having the oxygen system

charges but allows the pilot to activate the system whenever needed.

Oxygen cylinders are often fitted with safety disks, which rupture if the pressure in the

cylinder becomes too great. The released oxygen is vented overboard through a discharge

line.

REGULATORS:

Regulators for the pressure and flow of oxygen are incorporated in stored-gas systems

because the oxygen is stored in high-pressure cylinders under pressure of 1800psig or more.

The high pressure must be reduced to a valve suitable for application directly to a mask or

to a breathing regulator. This lower pressure is usually in the range of 40 to 75 psig, de-

pending upon the system.

This pressure regulator is similar in design to many other gas- or- air- pressure regulators

in that it utilizes a diaphragm balanced against a spring to control the flow of gas. This

regulator consists of a housing, diaphragm, regulator spring, link actuator assembly, relief

valve, and an inlet valve. With no inlet pressure on the regulator, spring tension on the dia-

phragm through the link actuator assembly forces the inlet valve to the open position.

When oxygen is flowing, regulated pressure in the lower diaphragm chamber acts against

the diaphragm, causing it to move upward & compress the regulator spring. The link actu-

ator assembly then mechanically causes the regulator valve to move toward the closed posi-

tion, thus reducing the flow of oxygen. When the pressure in the lower chamber of the dia-

phragm is equal to the regulator spring force, the diaphragm ceases to move and positions

the inlet valve to maintain the proper oxygen flow.

Regulator for demand systems include both pressure regulators and demand or diluter –

demand regulators.

The diluter –demand regulator “dilutes” the pure oxygen with air in accordance with the

cabin altitude.

When the user inhales, a slight negative pressure is created in the chamber to the right of

the demand diaphragm. This pressure reduction causes the diaphragm to move to the right

and opens the demand valve .This causes a negative pressure to be applied to the chamber

under the reducing valve diaphragm, moving the diaphragm to the left. When the dia-

phragm moves to the left, the pressure –reducing valve is lifted off its seat, allowing oxygen

to enter the regulator and flow toward the mask.

The mixing of air with oxygen is caused by the aneroid in the mixing chamber. The aneroid

is a sealed metal bellows. At sea level the aneroid is compressed by atm pressure so that the

oxygen – metering port is closed and the air-metering port is open. As atm pressure is de-

creased, the aneroid expands, opening the oxygen –metering port and reducing the air-

metering port. At an altitude of approx 34000ft, the air – metering port is completely closed

and the user is receiving only oxygen.

The diluter –control closing mechanism can be used to override the diluter –control opera-

tion of the system by mechanically closing off the air-metering port and fully opening the

oxygen –metering port. Additionally, if the mechanism should malfunction, the user can

open the emergency metering control, bypass the diluter mechanism, and be supplied with

pure oxygen.

OXYGEN MASKS:

Oxygen masks vary considerably in size, shape, and design, however, each is designed for

either a demand system or a continuous –flow system.

An oxygen mask for a demand system must fit the face closely, enclosing both the mouth

and nose, & must form an airtight seal with the face. Inhalation by the user will then cause

a low pressure in the demand regulator, which results in opening of an oxygen valve & a

flow of oxygen to the mask. When the user exhales, the flow of oxygen is cut off.

An oxygen mask for a constant –flow system is designed so that some ambient air is mixed

with the oxygen. The complete mask usually includes an oronasal face piece, a reservoir

bag, valves, a supply hose, and a coupling fitting. Some models include a flow indicator in

the supply hose.

Oxygen masks on airliners are slowed in overhead compartments or in a compartment at

the top of the seat back. If the cabin should depressive, the compartments open automati-

cally and present oxygen masks to the passengers. If the automatic system fails to work, a

backup electrical system can be activated by a member of the crew to open the oxygen

compartments.

Pressurized aircraft are normally equipped with diluter-demand oxygen systems for use by

the flight –deck crew.

GASEOUS OXYGEN SYSTEMS FOR UNPRESSURIZED AIRCRAFT ;

Unpressurized aircraft that are capable of flying at altitudes requiring the use of oxygen

by crew and passengers may be equipped with portable or fixed gaseous systems. The sys-

tem includes a high pressure regulator, pressure gauge, manifold, and various types of out-

lets to which tubing connected to masks may be attached. The regulators may be of the de-

mand type or the constant –flow type.

A permanently installed , stored –gas oxygen system for a light twin airplane .This system

consists of a high –pressure oxygen cylinder with a regulator , an altitude –compensating

regulator , a filler valve, an overboard discharge indicator, a control cable and knob, a cyl-

inder pressure gauge, outlets, oxygen masks, and a required plumbing. The supply regula-

tor attached to the oxygen cylinder reduces the high cylinder pressure to a lower, constant

pressure. The altitude-compensating regulator reduces oxygen expenditure at lower alti-

tudes, thus increasing oxygen supply duration. The pressure gauge shows actual cylinder

pressure.

The control knob on the instrument panel is used to open a valve and allow controlled

oxygen pressure to flow the mask outlets. When a mask fitting is plugged into an outlet a

continuous flow of oxygen is available at the mask. The masks include flow indicators for

visual verification of oxygen flow. The masks, hoses, and flow indicators are stored in plas-

tic bags, where they are readily available to crew and passengers.

The oxygen outlets are installed in the overhead console and above the passenger seats.

Each outlet contains a spring-loaded valve that prevents oxygen flow until the mask hose is

engaged with the outlet.

The oxygen filler valve is usually located under an access panel on the outside of the fuse-

lage and near the oxygen cylinder. The filler valve consist of the valve incorporating a filter

and valve cap. A check valve is installed in the high –pressure line at the regulator to pre-

vent the escape of oxygen from the cylinder at the filler line port.

The overboard discharge indicator is located on the bottom or side of the aircraft near the

oxygen bottle. A low pressure green disk is provided to prevent dust and contamination

from entering the line.

GASEOUS OXYGEN SYSTEMS FOR PRESSURIZED AIRCRAFT:

Oxygen systems for pressurized aircraft are primarily installed for emergency use in case

of cabin-pressurization failure or cabin decompression. The oxygen supply is sufficient to

take care of all passengers and crew until the airplane is at a low altitude, where oxygen is

no longer necessary.

The crew oxygen system consists of a high pressure supply cylinder, a shut off valve with a

cylinder pressure gauge, a pressure regulator ,automatic pressure breathing –demand regu-

lators, oronasal masks, and a quick –disconnect test fitting.

The capacity of the crew oxygen –supply cylinder is 48ft3 of oxygen at standard atm and

pressure.

The pressure gauge, installed in the body of the shut off valve, indicates cylinder pressure

with the valve in either the open or closed position. The pressure- reducing regulator, at-

tached by means of a union fitting to the cylinder shut off valve, reduces the high cylinder

pressure of 1850psig to a constant supply pressure of approx 65psig .

The diluter –demand pressure breathing regulator installed at each flight –crew station

automatically controls the mixture ratio of air to oxygen the ratio varying with cabin pres-

sure.

A light for panel pressure is installed in the regulator. The supply –line pressure is indicated

by a pressure gauge on each regulator.

An oxygen mask is provided for each crew member and for each passenger.

A breathing tube extends from the mask housing. The tube is equipped with a quick-

disconnect fitting, which connects into the crew oxygen-supply connector or the portable

cylinder.

A portion of the passenger oxygen system includes high-pressure oxygen cylinder, passen-

ger outlets, and associated parts. The cylinder has a capacity of 64ft, weighs approx 28lb,&

is charged to a pressure of 1850psig under normal atm conditions.the cylinder is secured

with strap clamps in an upright position in the aft right –hand corner of the flight com-

partment.

A direct-reading pressure gauge is installed in the shut off valve body between the valve

and the cylinder. The pressure gauge indicates pressure with the valves in either the OPEN

or CLOSED position. An adapter containing a frangible blowout disk will rupture and al-

low oxygen to escape overboard in the event that cylinder pressure should exceed approx

2650psig

The passenger oxygen masks are of the oronasal type and are made of a plastic –type

rubber, which forms around the mouth and nose area.

The passenger masks are stowed in oxygen stowage boxes above each seat row in the

overhead stowage –rack utility panel, in each lavatory washstand, and at each attendant‟s

station.

The attendant‟s forward-station masks will fall when the door is opened.

SERVICE AND MAINTENANCE OF OXYGEN SYSTEM : Refer Xerox copy of kroes,

Watkins, delp.Page no:547

Ground support equipment (GSE) is the support equipment found at an airport, usually on

the ramp, the servicing area by the terminal. This equipment is used to service the aircraft

between flights.

The functions this equipment plays generally involve ground power operations, aircraft

mobility, and loading operations (for both cargo and passengers).

Speed, efficiency, and accuracy are important in ground handling services in order to

minimize the turnaround time (the time during which the aircraft remains parked at the

gate).

Some airlines may enter into a Maintenance and Ground Support Agreement (MAGSA) with

each other, which is used by airlines to assess costs for maintenance and support to aircraft.

For example, activities undertaken during a typical aircraft gate period include: cargo

loading and unloading, passenger loading and unloading, potable water storage, lavatory

waste tank drainage, aircraft refueling, engine and fuselage examination and maintenance,

and food and beverage catering.

Ground units

Non-powered:

Chocks

Baggage carts

Dollies for containers and pallets

Powered:

Refuelers

Tugs and tractors

Ground power units

Container loader

Conveyor belt loaders

Potable water trucks

Air Start Unit (ASU)

Transporters

Lavatory service vehicles

Catering vehicle

Conveyor belt loaders

Passenger boarding stairs

De/anti-icing vehicles

Chocks are used to prevent an aircraft from moving while parked at the gate or in a

hangar. Chocks are placed in the front ('fore') and back ('aft') of the wheels of landing

gear.

They are made out of hard wood or hard rubber.

Corporate safety guidelines in the USA almost always specify that chocks must be used in a

pair on the same wheel and they must be placed in physical contact with the wheel.

Baggage carts:

Baggage carts are used for the transportation of luggage, mail, cargo and other materials

between the aircraft and the terminal or sorting facility.

Carts are fitted with a brake system which blocks the wheels from moving when the

connecting rod is not attached to a tug.

Dollies for containers and pallets:

The dollies or trolleys are specialized equipment to carry containers (Unit load device for

aircraft) and pallets which are both designed to save weight and thus do not have wheels

for their easy moving. Advanced dollies, such as those used in airport apron, have the

following specialized facilities:

Rollers - Dollies have in-built rollers or balls on the deck for the acceptance of containers or

pallets for their easier moving.

Revolving platform - Dollies have revolving platform to rotate containers so as to facilitate

transfer onto a conveyor belt which further move the containers into an aircraft. Some

platforms are power driven.

Brakes - Dollies have mechanical brakes which automatically locks the wheels when the

towbar is in a parked (pointing vertically upward) orientation, and release when the towing

bar is in a towing (horizontal) orientation.

dolly for containers

dolly for pallets

Powered equipment

Refuelers

Aircraft refuelers can be either a self-contained fuel truck, or a hydrant truck or cart.

Fuel trucks are self-contained, typically containing up to 10,000 US gallons of fuel and

have their own pumps, filters, hoses, and other equipment.

A hydrant cart or truck hooks into a central pipeline network and provides fuel to the

aircraft. There is a significant advantage with hydrant systems when compared to fuel

trucks, as fuel trucks must be periodically replenished.

Tugs and tractors

The tugs and tractors at an airport have several purposes and represent the essential part

of ground support services.

They are used to move any equipment that can not move itself. This includes bag carts,

mobile air conditioning units, air starters, lavatory carts, and other equipment.

Ground power units

A ground power unit (GPU) is a vehicle capable of supplying power to aircraft parked on

the ground. Ground power units may also be built into the jetway, making it even easier to

supply electrical power to aircraft. Many aircraft require 28 V of direct current and 115 V

400 Hz of alternating current. The electric energy is carried from a generator to a

connection on the aircraft via 3 phase 4-wire insulated cable capable of handling 261 amps

(90 kVA). These connectors are standard for all aircraft, as defined in ISO 6858.

Buses at airports are used to move people from the terminal to either an aircraft or another

terminal. The specific term for airport buses that drive on the apron only is apron bus.

Container loader

The loader for wide bodied aircraft (cargo platform) is used for loading and unloading of

cargo placed in containers or on pallet.

The loader has two platforms which independently raise or come down. The containers or

palettes on the loader are moved with the help of built-in rollers or wheels, and are carried

in aircraft across the platforms. There are different container and pallet loaders.

• 3.5 T

• 7 T (standard version, wide-body, universal, high)

• 14 T

• 30 T

Transporters

The transporters are cargo platforms constructed so that beside loading and unloading can

transport cargo. Depending on the type and load capacity the containers could be

transported, and the same is valid for greater transporters and palettes.

Air Start Unit (ASU)

An Air Start Unit or air starter is a vehicle with a built-in gas turbine engine which, during

the start of an aircraft engine, gives the necessary quantity of air to start the engine. When

a compressor cannot deliver the necessary quantity of air for its own work, the air is

provided by an air starter. An air starter blows air in by one or two hoses attached to the

aircraft.

Potable water trucks

Potable water trucks are special vehicles that fill up drinking water tanks in aircraft. The

water is filtered and protected from the elements while being stored on the vehicle. A pump

in the vehicle assists in moving the water from the truck to the aircraft.

Lavatory service vehicles

Lavatory service vehicles empty and refill lavatories onboard aircraft. Waste is stored in

tanks on the aircraft until these vehicles can empty them and get rid of the waste. After the

tank is emptied, it is refilled with a mixture of water and a disinfecting concentrate,

commonly called 'blue juice'.

Catering vehicle

Catering includes the unloading of unused food and drink from the aircraft, and the

loading of fresh food and drinks for passengers and crew. The meals are typically delivered

in standardized carts. Meals are prepared mostly on the ground in order to minimize the

amount of preparation (apart from chilling or reheating) required in the air.

The catering vehicle consists of a rear body, lifting system, platform and an electro-

hydraulic control mechanism. The vehicle can be lifted up, down and the platform can be

moved to place in front of the aircraft.

Conveyor belt loaders

Conveyor belt loaders are vehicles with movable belts for unloading and loading of baggage

and cargo of aircraft. A Conveyor belt loader is positioned to the door sill of an aircraft

hold (baggage compartment) for the operation. Conveyor belt loaders are used for narrow

body aircraft (e.g. 737) and bulk hold of wide body aircraft (e.g. 767 and 747). Baggage

stored without containers is known as bulk loading.

Passenger boarding stairs

Passenger boarding stairs, sometimes referred to as 'air-stairs', 'boarding ramps', 'stair

car' or 'aircraft steps', provide a mobile means to traverse between aircraft doors and the

ground. Because larger aircraft have door sills 5 to 20 feet high, stairs facilitate safe

boarding and deboarding. While smaller units are generally moved by being towed or

pushed, larger units are self-powered. Most models have adjustable height to accommodate

various aircraft.

De/anti-icing vehicles

The procedure of de/anti-icing, protection from fluids freezing up on aircraft, is done from

special vehicles. These vehicles have booms, like a cherry picker, to allow easy access to the

entire aircraft. A hose sprays a special mixture that melts current ice on the aircraft and

also prevents some ice from building up while waiting on the ground.

Aircraft Engine Lubrication

Major functions of lubricants:

1. Lubricants are used to reduce friction and wear, whether it's in aviation.

2. Other major functions of a lubricant include cleaning, cooling and sealing, in addition to

helping fight corrosion and rust in the engine.

The more frequently and consistently an airplane is flown, the easier it is to properly

maintain and lubricate.

Benefits of using a lubricant that cleans the engine:

All aviation oils clean. When we say aviation oil cleans, we think of removing sludge,

varnishes, and grunge accumulations in the oil pan, on plugs, or in the screen.

In aviation engines, the oil must carry off a greater percentage of the engine's heat.

Air-cooled aircraft engines rely on their oil for cooling far more than water-cooled

automotive engines.

Oil is a heat-transfer medium which flows through the crankcase and oil coolers, and

dissipates the heat from moving parts, thus constantly cooling engine bearings and piston

rings.

The engine‟s oil is the life blood of the engine and it is very important for the engine to

perform its function and to extend the length between overhauls.

Requirements and Characteristics of Reciprocating Engine Lubricants While there are

several important properties that satisfactory reciprocating engine oil must possess, its

viscosity is most important in engine operation

Unfortunately, the viscosity of oil is affected by temperature.

The oil selected for aircraft engine lubrication must be light enough to circulate freely at

cold temperatures, yet heavy enough to provide the proper oil film at engine operating

temperatures.

Since lubricants vary in properties and since no one oil is satisfactory for all engines and all

operating conditions, it is extremely important that only the approved grade or Society of

Automotive Engineers (SAE) rating be used.

Several factors must be considered in determining the proper grade of oil to use in a

particular engine, the most important of which are

1. The operating load,

2. Rotational speeds, and

3. Operating temperatures.

The grade of the lubricating oil to be used is determined by the operating conditions to be

met in the various types of engines.

The oil used in aircraft reciprocating engines has a relatively high viscosity required by:

1. Large engine operating clearances due to the relatively large size of the moving parts, the

different materials used, and the different rates of expansion of the various materials

2. High operating temperatures; and

3. High bearing pressures.

To simplify the selection of oils, they are often classified under an SAE system that divides

all oils into seven groups (SAE 10 to 70) according to viscosity at either 130 °F or 210 °F.

SAE ratings are purely arbitrary and bear no direct relationship to the Saybolt or other

ratings.

The SAE letters on an oil container are not an endorsement or recommendation of the oil

by the SAE. Although each grade of oil is rated by an SAE number, depending on its

specific use, it may be rated with a commercial aviation grade number or an Army and

Navy specification number. The correlation between these grade numbering systems is

shown in Figure 6-3.

Commercial Aviation No. Commercial SAE No. Army and Navy

Specification No.

65, 80, 100, 120, 140 30, 40, 50, 60, 70 1065, 1080, 1100, 1120

table 6-3. Grade designations for aviation oils

Lubrication System Requirements:

The lubrication system of the engine must be designed and constructed so that it functions

properly within all flight attitudes and atmospheric conditions that the aircraft is expected

to operate.

In wet sump engines, this requirement must be met when only half of the maximum

lubricant supply is in the engine.

Dry Sump Oil Systems Many reciprocating and turbine aircraft engines have pressure dry

sump lubrication systems. The oil supply in this type of system is carried in a tank. A

pressure pump circulates the oil through the engine. Scavenger pumps then return it to the

tank as quickly as it accumulates in the engine sumps. The need for a separate supply tank

is apparent when considering the complications that would result if large quantities of oil

were carried in the engine crankcase.

The principal units in a typical reciprocating engine dry sump oil system include an

Oil supply tank,

An Engine-driven pressure oil pump,

A scavenge pump, an oil cooler with an oil cooler control valve,

Oil tank vent,

Necessary tubing and

Pressure and temperature indicators. [Figure 6-4]

Oil Tanks:

Oil tanks are generally associated with a dry sump lubrication system, while a wet sump

system uses the crankcase of the engine to store the oil.

Oil tanks are usually constructed of aluminum alloy and must withstand any vibration,

inertia, and fluid loads expected in operation.

Each oil tank used with a reciprocating engine must have expansion space of not less than

the greater of 10 percent of the tank capacity or 0.5 gallons. Each filler cap of an oil tank

that is used with an engine must provide an oil-tight seal. The oil tank usually is placed

close to the engine and high enough above the oil pump inlet to ensure gravity feed.

Oil tank capacity varies with the different types of aircraft, but it is usually sufficient to

ensure an adequate supply of oil for the total fuel supply. The tank filler neck is positioned

to provide sufficient room for oil expansion and for foam to collect.

The filler cap or cover is marked with the word OIL. A drain in the filler cap well disposes

of any overflow caused by the filling operation. Oil tank vent lines are provided to ensure

proper tank ventilation in all attitudes of flight. These lines are usually connected to the

engine crankcase to prevent the loss of oil through the vents. This indirectly vents the tanks

to the atmosphere through the crankcase breather.

Early large radial engines had many gallons of oil in their tank. To help with engine warm

up, some oil tanks had a built- in hopper or temperature accelerating well. [Figure 6-5]

This well extended from the oil return fitting on top of the oil tank to the outlet fitting in

the sump in the bottom of the tank. In some systems, the hopper tank is open to the main

oil supply at the lower end.

Generally, the return line in the top of the tank is positioned to discharge the returned oil

against the wall of the tank in a swirling motion. This method considerably reduces

foaming that occurs when oil mixes with air.

Baffles in the bottom of the oil tank break up this swirling action to prevent air from being

drawn into the inlet line of the oil pressure pump. Foaming oil increases in volume and

reduces its ability to provide proper lubrication

An oil tank sump, attached to the undersurface of the tank, acts as a trap for moisture and

sediment. [Figure 6-4] The water and sludge can be drained by manually opening the drain

valve in the bottom of the sump.

Most aircraft oil systems are equipped with the dipstick-type quantity gauge, often called a

bayonet gauge. Some larger aircraft systems also have an oil quantity indicating system

that shows the quantity of oil during flight. One type system consists essentially of an arm

and float mechanism that rides the level of the oil and actuates an electric transmitter on

top of the tank. The transmitter is connected to a cockpit gauge that indicates the quantity

of oil.

Oil Pump:

Oil entering the engine is pressurized, filtered, and regulated by units within the engine.

They are discussed along with the external oil system to provide a concept of the complete

oil system.

As oil enters the engine, it is pressurized by a gear-type pump. [Figure 6-6] This pump is a

positive displacement pump that consists of two meshed gears that revolve inside the

housing. The clearance between the teeth and housing is small. The pump inlet is located on

the left and the discharge port is connected to the engine‟s system pressure line. One gear is

attached to a splined drive shaft that extends from the pump housing to an accessory drive

shaft on the engine. Seals are used to prevent leakage around the drive shaft. As the lower

gear is rotated counterclockwise, the driven idler gear turns clockwise.

As oil enters the gear chamber, it is picked up by the gear teeth, trapped between them and

the sides of the gear chamber, is carried around the outside of the gears, and discharged

from the pressure port into the oil screen passage. The pressurized oil flows to the oil filter,

where any solid particles suspended in the oil are separated from it, preventing possible

damage to moving parts of the engine.

Oil under pressure then opens the oil filter check valve mounted in the top of the filter. This

valve is used mostly with dry sump radial engines and is closed by a light spring loading of

1 to 3 pounds per square inch (psi) when the engine is not operating to prevent gravity-fed

oil from entering the engine and settling in the lower cylinders or sump area of the engine.

If oil were allowed to gradually seep by the rings of the piston and fill the combustion

chamber, it could cause a liquid lock. This could happen if the valves on the cylinder were

both closed and the engine was cranked for start. Damage could occur to the engine.

The oil filter bypass valve, located between the pressure side of the oil pump and the oil

filter, permits unfiltered oil to bypass the filter and enter the engine if the oil filter is

clogged or during cold weather if congealed oil is blocking the filter during engine start.

The spring loading on the bypass valve allows the valve to open before the oil pressure

collapses the filter; in the case of cold, congealed oil, it provides a low-resistance path

around the filter. Dirty oil in an engine is better than no lubrication.

Oil Filters:

The oil filter used on an aircraft engine is usually one of four types:

1. screen

2. Cuno

3. canister

4. spin-on

A screen-type filter with its double-walled construction provides a large filtering area in a

compact unit. [Figure 6-6] As oil passes through the fine-mesh screen, dirt, sediment, and

other foreign matter are removed and settle to the bottom of the housing. At regular

intervals, the cover is removed and the screen and housing cleaned with a solvent. Oil

screen filters are used mostly as suction filters on the inlet of the oil pump.

The Cuno oil filter has a cartridge made of disks and spacers. A cleaner blade fits between

each pair of disks. The cleaner blades are stationary, but the disks rotate when the shaft is

turned. Oil from the pump enters the cartridge well that surrounds the cartridge and

passes through the spaces between the closely spaced disks of the cartridge, then through

the hollow center, and on to the engine. Any foreign particles in the oil are deposited on the

outer surface of the cartridge. This filter is not often used on modern aircraft.

A canister housing filter has a replaceable filter element that is replaced with rest of the

components other than seals and gaskets being reused. [Figure 6-7] The filter element is

designed with a corrugated, strong steel center tube supporting each convoluted pleat of

the filter media, resulting in a higher collapse pressure rating. The filter provides excellent

filtration, because the oil flows through many layers of locked-in-fibers.

Full flow spin-on filters are the most widely used oil filters for reciprocating engines. In a

full flow system, the filter is positioned between the oil pump and the engine bearings,

which filters the oil of any contaminants before they pass through the engine bearing

surfaces.

A cutaway of the micronic filter element shows the resin-impregnated cellulosic full-pleat

media that is used to trap harmful particles, keeping them from entering the engine.

[Figure 6-9]

Oil Pressure Regulating Valve

An oil pressure regulating valve limits oil pressure to a predetermined value, depending on

the installation. [Figure 6-6] This valve is sometimes referred to as a relief valve but its real

function is to regulate the oil pressure at a present pressure level. The oil pressure must be

sufficiently high to ensure adequate lubrication of the engine and its accessories at high

speeds and powers.

Oil Pressure Gauge Usually, the oil pressure gauge indicates the pressure that oil enters the

engine from the pump. This gauge warns of possible engine failure caused by an exhausted

oil supply, failure of the oil pump, burned-out bearings, ruptured oil lines, or other causes

that may be indicated by a loss of oil pressure.

Oil Temperature Indicator In dry-sump lubricating systems, the oil temperature bulb may

be anywhere in the oil inlet line between the supply tank and the engine.

Oil systems for wet-sump engines have the temperature bulb located where it senses oil

temperature after the oil passes through the oil cooler.

In either system, the bulb is located so that it measures the temperature of the oil before it

enters the engine‟s hot sections. An oil temperature gauge in the cockpit is connected to the

oil temperature bulb by electrical leads .

Any malfunction of the oil cooling system appears as an abnormal reading.

Oil Cooler

The cooler, either cylindrical or elliptical shaped, consists of a core enclosed in a double-

walled shell. The core is built of copper or aluminum tubes with the tube ends formed to a

hexagonal shape and joined together in the honeycomb effect. [Figure 6-11] The ends of the

copper tubes of the core are soldered, whereas aluminum tubes are brazed or mechanically

joined. The tubes touch only at the ends so that a space exists between them along most of

their lengths. This allows oil to flow through the spaces between the tubes while the cooling

air passes through the tubes.

Dry Sump Lubrication System Operation

The following lubrication system is typical of those on small, single-engine aircraft. The oil

system and components are those used to lubricate a 225 horsepower (hp) six-cylinder,

horizontally opposed, air-cooled engine. In a typical dry sump pressure-lubrication system,

a mechanical pump supplies oil under pressure to the bearings throughout the engine.

[Figure 6-4] The oil flows into the inlet or suction side of the oil pump through a suction

screen and a line connected to the external tank at a point higher than the bottom of the oil

sump. This prevents sediment that falls into the sump from being drawn into the pump.

The tank outlet is higher than the pump inlet, so gravity can assist the flow into the pump.

The engine-driven, positive-displacement, gear-type pump forces the oil into the full flow

filter. [Figure 6-6] The oil either passes through the filter under normal conditions or, if the

filter were to become clogged, the filter bypass valve would open as mentioned earlier. In

the bypass position, the oil would not be filtered. As seen in Figure 6-6, the regulating

(relief) valve senses when system pressure is reached and opens enough to bypass oil to the

inlet side of the oil pump. Then, the oil flows into a manifold that distributes the oil through

drilled passages to the crankshaft bearings and other bearings throughout the engine. Oil

flows from the main bearings through holes drilled in the crankshaft to the lower

connecting rod bearings. [Figure 6-15]

Oil reaches a hollow camshaft (in an inline or opposed engine), or a cam plate or cam drum

(in a radial engine), through a connection with the end bearing or the main oil manifold; it

then flows out to the various camshaft, cam drum, or cam plate bearings and the cams.

The engine cylinder surfaces receive oil sprayed from the crankshaft and also from the

crankpin bearings. Since oil seeps slowly through the small crankpin clearances before it is

sprayed on the cylinder walls, considerable time is required for enough oil to reach the

cylinder walls, especially on a cold day when the oil flow is more sluggish. This is one of the

chief reasons for using modern multiviscosity oils that flow well at low temperatures.

When the circulating oil has performed its function of lubricating and cooling the moving

parts of the engine, it drains into the sumps in the lowest parts of the engine. Oil collected

in these sumps is picked up by gear or gerotor-type scavenger pumps as quickly as it

accumulates. These pumps have a greater capacity than the pressure pump. This is needed

because the volume of the oil has generally increased due to foaming (mixing with air). On

dry sump engines, this oil leaves the engine, passes through the oil cooler, and returns to the

supply tank.

A thermostat attached to the oil cooler controls oil temperature by allowing part of the oil

to flow through the cooler and part to flow directly into the oil supply tank. This

arrangement allows hot engine oil with a temperature still below 65 °C (150 °F) to mix with

the cold uncirculated oil in the tank. This raises the complete engine oil supply to operating

temperature in a shorter period of time.

Troubleshooting Oil Systems The outline of malfunctions and their remedies listed in

Figure 6-30 can expedite troubleshooting of the lubrication system. The purpose of this

section is to present typical troubles. It is not intended to imply that any of the troubles are

exactly as they may be in a particular airplane.