AST 473
Cessna Citation II
Air Conditioning System
Dr. Barnhart
Air Conditioning General
 Compressor bleed air provides the air to
operate the system
 Mixed with ACM air for cooling
 Fans circulate cabin air
 Optional flood cooling system provides a
rapid cool of the cabin.
Air Conditioning General
 Crew has both auto and manual control
 PRESS SOURCE selector selects the
source of the bleed air
 Ambient airflow from tail scoop is an option
if unpressurized
Air Conditioning UNs 0482, 0485
and Subsequent
 Bleed air passes through the (solenoid
operated) flow control valves- 6ppm
 Bleed from Rt. Engine branches in tail cone;
one branch- normal flow cnt’l valve, other
branch to motor operated gnd. Valve (larger
air draw as described previously for ACM
ejector)
Air Conditioning
 PRESS SOURCE selector switch- tilt panel fwd. of
throttle
 NORMAL- flow from ea. Eng.- DC fail position
 If rt. eng. > 72% N(2) pressure switch closes the
gnd. Valve preventing too much airflow to the
ACM- BLEED AIR GROUND light extinguishesreverses if rt. Throttle below 70%
 If primary pressure switch fails and rt. Eng. >74%
N(2)- secondary switch closes gnd. Valve and
illuminates ACM OVER PRESS light- gnd. Valve
will remain closed until DC is interrupted
Air Conditioning
 In EMER left eng. Bleed goes directly into
cabin- EMER PRESS ON light illuminates
 OFF- all valves closes- no pressurization
available
Air Conditioning
 Precooler- bleed air from eng. Passes through a
pre-cooler in the ACM ambient air duct- air-to-air
heat exchanger.
 Cools bleed air prior to the ACM heat exchangers
 ACM- after passing via the precooler in the ram air
duct then air flows via the ACM primary heat
exchanger then via the ACM compressor then
secondary ht. exchanger and finally across the
cooling turbine- this is the cooling process.
Air Conditioning
 Water Separator- removes moisture from air
prior to entering cabin through a check
valve. This moisture is injected into air
flowing over the heat exchangers to
increase cooling efficiency
Air Conditioning
 Temp. Control- via a temp. select rheostat or
a manual HOT-MANUAL COLD switch
 Temp. is controlled by allowing some of the
bleed air to bypass ACM via the mixing
valve. Valve is positioned by elec. Motorvalve opened (warmer) or closed (cooler) by
either:
– Automatic Control
– Manual temp. control
Air Conditioning
 Cabin temp. controlled automatically when temp.
rheostat is in the automatic range. This temp.
selector is a rheostat that sends a variable temp.
setting to cabin temp. controller
– Controller compares desired setting to actual cabin
temp. via the cabin temp. sensor (in top of cabin btn.
Main and emer. Exit. Also receives temp. input from
supply duct temp sensor in tailcone; compares inputs
and tells mixing valve motor to change temp
Air Conditioning
 For example- if selector is rotated clockwise
for warmer- a higher temp- controller
receives this input, compares it to cabin and
supply duct sensors causing the mixing
valve to open allowing more bleed air to
bypass the ACM, boosting the temp.
Air Conditioning
 In Auto- system has a supply air low temp.
Limit of 35 deg. F. to prevent ice in water
separator
 Input to controller for this is from supply duct
temp. sensor downstream of the water
separator in the supply duct.
Air Conditioning
 With temp. rheostat in manual position,
mixing valve is controlled manually by the
MANUAL HOT-MANUAL-COLD switch
which is spring loaded to OFF
 This causes manual movement of mixing
valve where it remains until changed- valve
has a 10 second travel time- must be
cautious to prevent water separator freeze
in this mode
Air Conditioning
 System Protection- If bleed air temp. btn.
Compressor and turbine exceeds 435 deg. F. the
overheat relay causes flow control and shutoff
valves (or ground valve) to close and the EMER
PRESS valve to open
– In flight ACM shuts down and noise level in cabin goes
up dramatically
– If overheat lasts longer than 12s the emer. Lockout relay
energizes and ACM remains inop. If less than 12s it
reverts to previous setting and emer. Valve closes
Air Conditioning
 To reset the system to normal after 12s
shutdown- rotate the PRESS SOURCE
selector to EMER position and then reselect a position other than GND or OFF
Air Conditioning
 On ground when ACM overheats the emer.
press. Valve does not open since it has
been de-activated by left main squat switch
but EMER PRESS light does come on- reset
procedure is the same
Air Conditioning
 In EMER mode- watch pulling the left
throttle down too much for temp. control as
cabin alt. may rise.
Air Conditioning
 Air duct from ACM to cabin is protected from
overheat by a duct overheat sensor. If temp.
exceeds 157 deg. The AIR DUCT O’HEAT
annunciator light illuminates
– Most likely when heat is demanded and most bleed is
bypassing the cooling process of ACM
– Pilot should select MANUAL with rheostat and close
mixing valve by holding MANUAL HOT- MANUALCOLD switch to the COLD position- 10 sec.
– Check TEMP circuit breaker in- no power means no
temp. control in either mode.
Air Conditioning
 DC failure- defaults as if in NORMAL
position. Emer. Press. Valve fails closed
 Also temp. control is lost- DC mixing valve
fails as set at time of power loss
Air Conditioning
 Air distribution system:
– Overhead conditioned air ducts and outlets
– Footrest and armrest warmer manifolds supplied by
underfloor conditioned air duct which also supplies the
windshield defog. Outlets
– Conditioned air enters cabin via single duct then
branches at a flow divider- cockpit + main and aux.
plenums then to pax. Scn.
– Air circulation enhanced by using overhead fan and
defog. Fan.
Air Conditioning
 Operation- with engines operating select
source- air flows:
– Water separator
– Ducting
– Check Valve (aft bulkhead)
Air Conditioning
 Temp. in supply duct determines position of
recirculating air inlet door. Below 100 Deg.
F. door fully closed- conditioned air flows via
both overhead and underfloor ducts.
 Above 100 deg. F. door fully open- all hot air
from ACM diverted to underfloor duct
 Air from overhead ducts is now recirculated
cabin air only-cooler than incoming air from
ACM
Air Conditioning
 Air flowing through the overhead ducting
system is distributed and controlled by
manipulation of the individual Wemac
outlets
 Increased airflow through these outlets can
be obtained by selecting HI or LOW with the
OVHD fan switch on co-pilot’s instrument
panel
Air Conditioning
 Air flowing to underfloor ducting is divided by the
flow divider assembly; part to flight compartment
part to pass. Footwarmer and armrest warmer
manifolds.
 Position of flow bias valve is determined by AIR
FLOW DISTR selector.- 5 position selector- allows
selection if incr. or decr. Flow to cabin or cockpit;
CKPT- most flow to cockpit- CABIN- most flow to
armrest and footwarmer manifolds
Air Conditioning
 Using defog fan in conjunction with flow divider
increases flow to cockpit
– Max. flow- CKPT with AIR FLOW DISTR selector and
selecting HI with defog fan switch (HI, LOW, OFF)
– Pilot’s footwarmer must be closed for max. defog at
windshields
– When footwarmer outlet is closed, a side window defog
valve is also closed- prevents miost air from getting btn.
Side window panes and condensing as it meets cold
soaked outside air.
Air Conditioning
 Optional Flood Cooling- air outlet grill on upper aft
press. Bulkhead- high volume of cool air from the
ACM
– Bypasses both overhead and underfoot duct systemseliminates heating by hot-soaked ducting system
– Use on ground and in-flight ONLY below 10,000 ft.
– Powered by rt. Gen. directly via a 20 amp breakerprevents use of flood fan on ground without engines
running (battery depletion).
Flood Cooling
 FLOOD COOLING switch- next to
pressurization controller on center panel- 2
position- ON and OFF.
– OFF- Airflow is via normal dist. System
– ON- Normal dist. System blocked- all flow
directed via the flood cooling duct
– Within flood duct is a DC powered axial blowerincreases airflow at low pwr. settings
Flood Cooling
 At low pwr. Settings with Flood Cooling ONACM flow insufficient for blower thus
ambient tailcone air is mixed with ACM air.
 An ambient check valve closes at higher
power settings blocking outside airflow
The End