The environmental control system (ECS) regulates temperature, humidity, ventilation, and pressurization in the aircraft cabin. It also provides cooling for avionics and transfers heat from systems like hydraulics. The ECS uses air conditioning and pressurization systems. Air conditioning controls temperature, humidity, and ventilation, while pressurization maintains safe oxygen levels at high altitudes. Pressurization systems also include safety valves and controls for rate of cabin pressure change. The ECS protects against icing and provides anti-g protection for pilots during maneuvers.

1. Environmental Control System

2. • Environmental Control System deals with the
following requirements to be met for the aircraft
cabin.
• Ventilation
• Temperature control
• Humidity control
• Pressurization
Also,
• ECS system provides cooling for the Avionics Bay
as well as heat load transfer from hydraulic, fuel and
oil systems.
• ECS system provides de-misting, anti-icing, anti-g
and rain dispersal services.

3. Air Conditioning & Pressurization
Air conditioning system refers to the first 3 of the
above requirements ,namely, control of ventilation,
temperature and humidity.
• For aircraft flying at low altitudes up to 10,000 ft., air
conditioning alone is adequate whereas
pressurization is required for high altitude aircraft to
ensure required pressure of oxygen for breathing.
• ARP 1270 outlines aircraft cabin pressurization
control criteria.

4. Air conditioning system
• Air Conditioning refers to adequate supply of air to
the cabin with controlled temperature and humidity.
As the aircraft is operated at different levels in
different temperature zones, the air conditioning
system must be capable of taking extremely cold air
and warming it, or extremely hot humid air and
cooling it and dehumidifying it.
• For comfort the average requirements are
 About 1lb of air per minute per person for ventilation
 Temperature between 18° C to 24° C
 Relative humidity of 30% to 60%

5. • Some light aircraft have only ventilation along with
ram air heating systems using exhaust gas from
engines. The hot air supplied as required to the
cabin through a control box.
• Some other light aircraft uses dedicated combustion
heater to heat ram air.
• These systems are suitable upto a maximum of
16000 ft.

6. • For aircraft flying at higher altitudes above 16000 ft.,
some kind of air conditioning system is used.
• As the ambient temperature is a variable, the air
conditioning systems use heated ambient air for its
cycle.
• Heating of the ambient air is either by dedicated
means, or hot bleed air is taken from a gas turbine
compressor.
• This hot air is then split and a portion cooled before it
is mixed together to achieve the desired temperature
and supplied to the cabin.

7. Heating system
Fig. Ram air muffler type heat exchanger

8. Cooling system
• Cooling of air is done by two ways
•Air cycle machines
•Vapor cycle machines
Air cycle machines
• In this, heat is removed by a heat exchanger from
compressed hot air which is then expanded in a
turbine resulting in cold air. An Air Cycle machine is
also called Cold Air Unit (CAU)
Different types of CAU are:
– Bootstrap
– Brake turbine
– Turbofan

9. Air conditioning system schematic
Fig. Bootstrap system with blower

10. Fig. Bootstrap system with bleed air

11. • In the bootstrap CAU, the hot charge air is generated
by a blower or bleed air from gas turbine engine is
taken. The air is passed through flow control valve,
bypass valve, primary heat exchanger, cold air unit
and water separator. The cold air is then mixed with
hot air as required and supplied to cabin.
• The charge air from blower or from compressor is at a
high pressure and temperature.
• Bypass valve controls the amount of air that enters the
heat exchanger
• One more bypass valve controls the amount of air that
enters the cold air unit. Both are controlled by
temperature sensors.
• The primary heat exchanger cooled by ram air cools
the charge air at constant pressure.

12. • The bootstrap cold air unit has 3 components,
compressor, heat exchanger and turbine.
Compressor and turbine are linked together.
• The system is referred to as a bootstrap as it is able
to self start. As soon as air flows across the turbine
it starts to rotate and then its compressor increases
the airflow which accelerates the turbine. Due to
increased load from the turbine they self regulate.
• The output from the compressor is passed through
secondary heat exchanger before supplied to
turbine. The air that is expanded in the turbine is
cold air.

13. Vapor Cycle system for cooling
• Vapor Cycle system is a closed loop system where heat
load is absorbed by evaporation of a liquid refrigerant
such as Freon.
• The refrigerant passes through a compressor and then
cooled in a condenser. It flows back to the evaporator
via an expansion valve.
• Vapor cycle machines, though more efficient, are
heavier than Air cycle machines.

14. Cabin Air conditioning
• Water extractor removes excess moisture.
• Water particles are removed through a diffuser that
slows the airflow and guides it over a coalescer and
the water is extracted.
• Dried air leaving water separator is then mixed with
hot air and supplied to the cabin.

15. Pressurization system
• Cabin pressurization is done to safeguard humans
from effects of hypoxia at high altitudes.
• Hypoxia is the sickness caused due to reduced
partial pressure of oxygen.
• Partial pressure of oxygen above 8000ft. altitude is
not comfortable, and hence ambient pressure in
cabin is to be maintained at 10.92 psi (8000ft. cabin
altitude) or higher.
• Maximum safe altitude for pilots to operate without
supplementary oxygen is 15000ft. Above 20,000ft.
loss of consciousness sets in.

16. • Maintaining cabin differential pressure causes stress
on aircraft structure and an upper limit of 9.5 psi is
fixed by airworthiness regulations. So to protect
aircraft from structural damage due to excessive
pressure differential, two outward relief valves called
safety valves and two inward relief valves are fitted.
• Cabin is pressurized by using ventilation air from the
air conditioning system. The inflow is considered
fixed. The cabin pressure is determined by
controlling the outflow of this air.

17. • During aircraft’s climb, air has to be allowed to
escape from the cabin at a greater rate than inflow
from the air conditioning system to allow the cabin
altitude to climb. When the aircraft and the cabin
altitudes arrive at their predetermined level, the
outflow must equal inflow.
• The major component in the cabin pressurization
system is the outflow or discharge valve.
Rate of Change
• Rate of change is the value in ft/minute by which the
cabin altitude is allowed to ascend or descend.
• The maximum allowed rate of change for human
comfort due to human ear physiology is 500 fpm for
ascent, and 300 fpm for descent.

18. Pressurization system schedule
Fig . Rate of Change of Cabin Pressure

19. Pressurization system methods
Cabin pressure control systems
• There are 3 types
– Pneumatic
– Electro Pneumatic
– Electronic
• The cabin pressure controller has controls for rate of
change and cabin altitude, according to selected
cabin altitude and rate of change.
• The controller also ensures that the valve is fully
open on landing to ensure that the aircraft does not
land pressurized.

20. Electronic pressure control system
• In this, the discharge valve is a totally electronically
operated system with electrical motors. This system
has the advantage of reduced pilot work load and more
comfort.
Decompression
• Loss of cabin pressure is called decompression.
- Explosive : in 0 to 4 seconds due to structure
failure
- Rapid : in 5 to 7 seconds, (supplementary oxygen
to be given)
- Normal

21. Anti ‘g’ system
• Another use of ECS, air in a fighter aircraft is to
provide anti-g protection to the pilot.
• At high ‘g’ levels encountered in a highly
maneuverable fighter aircraft, heart becomes unable
to supply adequate supply of oxygenated blood to
the brain leading to a black out.
• Anti ‘g’ trousers consisting of inflatable air bladders
with ECS air and control valves are used to restrict
the flow of blood away from brain.
• With anti ‘g’ trousers, pilot can perform maneuvers
up to 8 g.

22. Aircraft Icing
• Icing is caused either by freezing on to aircraft
surfaces of some form of precipitation, this usually
occurs on the ground; or by supercooled liquid
water droplets found in clouds or rain solidifying on
impact with aircraft structure, which is at a
sufficiently low temperature during flight.
• This ‘accretion’ occurs on areas of the airframe
where the airflow is near to stagnation, i.e., close to
a rest such as wing or tail plane leading edges and
engine intakes.
• For the ground icing hazard de-icing fluids are
universally used to ensure the aircraft surface is free
of ice at take-off.

23. • The method of countering the airborne icing hazard
varies depending on the nature of the aircraft and its
operational requirements as those aircrafts requiring
ice protection and those not.
• A distinction is made between an anti-icing system
where ice accretion is prevented, and de-icing
system where a limited amount of accretion is
allowed before some action is taken to shed it. These
systems are used in conjunction with an ice
detector.

24. Anti-ice Systems
• Hot bleed air where continuously or when icing
conditions are present, hot air is projected on the
inside of a surface subject to ice accretion such as
wing leading edge or engine bullet.
• Electrical heating where elements are embedded in
the structure susceptible to icing to achieve a
continuous surface temperature above freezing
level.
• Liquid ice protection where a freezing point
depressant liquid is deposited on a surface or
extruded through a porous surface to prevent
freezing

25. De-Icing System
• Pneumatic boots where a reinforced synthetic rubber
layer is overlaid on the susceptible surface and
periodically inflated in conditions of ice accretion thereby
breaking and shedding the ice.
• Electrical heating which can be used in de-icing made by
switching on and off periodically during exposure to
icing conditions.
• Electro – expulsive system which utilises opposing
magnetic fields or eddy currents induced by conductors
embedded in a flexible surface to create relative
movement and hence the breakage and shedding of the
accreted ice.
• Electro-magnetic impulse de-icing which utilises coils
inside the leading edge inducing eddy currents in metal
skin with the result that the surface is deformed, breaking
the ice.

26. Rain Dispersal
• A pilot must have clear vision through the windscreen
under all weather conditions, particularly on approach to
landing. The use of windscreen wipers can be effective
up to higher subsonic speeds, particularly on large
screen. Wipers used in conjunction with washing fluid to
clean the screen of insect debris, dust, dirt and salt spray
etc. However wipers are not suitable for use with plastic
windscreen since they tend to scratch the surface and
disadvantage of increasing drag.
• Hot air jets for rain dispersal can be used up to much
higher speeds than wipers and are suitable for use on
glass and plastic. The air is discharged at high velocity
over the outside surface of the screen from a row of
nozzles at the base. The hot air is supplied from ECS
system at temperature of at least 1000C. Such high
temperatures are to evaporate the water.

27. Anti-Misting and De-Misting
• Misting will occur when the surface temperature of
the transparency falls below the dew point
temperature of the surrounding air.
• Misting typically occurs when an aircraft which has
been cruising at an altitude where air is cold and
relatively dry. When aircraft descends into a warmer
and more humid atmosphere, misting will occur on
the surfaces which have not had enough time to
warm up to a temperature above the dew point of the
air.
• An anti-misting system can be provided to keep the
surface temperature of the transparency above the
dew point and thus preventing misting.

28. • A system of nozzles blowing air at about 1000C ocer
the canopy from its base can be used, or
alternatively an electrically heated gold or metal
oxide film can be deposited on the transparency
surface or placed between laminations.
• A transparency de-mist system can be provided to
clear the transparency of mist should misting occur
suddenly or if the anti-mist system fails.
• The de-misting system consists of nozzles blowing
ECS air at high flow rate across the transparency.

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