AST 473 Cessna Citation II Air Conditioning System Dr. Barnhart

AST 473AST 473Cessna Citation IICessna Citation II

Air Conditioning SystemAir Conditioning System

Dr. BarnhartDr. Barnhart

Air Conditioning GeneralAir Conditioning General

Compressor bleed air provides the air to Compressor bleed air provides the air to operate the systemoperate the system

Mixed with ACM air for coolingMixed with ACM air for cooling Fans circulate cabin airFans circulate cabin air Optional flood cooling system provides a Optional flood cooling system provides a

rapid cool of the cabin.rapid cool of the cabin.

Air Conditioning GeneralAir Conditioning General

Crew has both auto and manual controlCrew has both auto and manual control PRESS SOURCE selector selects the PRESS SOURCE selector selects the

source of the bleed airsource of the bleed air Ambient airflow from tail scoop is an option Ambient airflow from tail scoop is an option

if unpressurizedif unpressurized

Air Conditioning UNs 0482, 0485 Air Conditioning UNs 0482, 0485 and Subsequentand Subsequent

Bleed air passes through the (solenoid Bleed air passes through the (solenoid operated) flow control valves- 6ppmoperated) flow control valves- 6ppm

Bleed from Rt. Engine branches in tail cone; Bleed from Rt. Engine branches in tail cone; one branch- normal flow cnt’l valve, other one branch- normal flow cnt’l valve, other branch to motor operated gnd. Valve (larger branch to motor operated gnd. Valve (larger air draw as described previously for ACM air draw as described previously for ACM ejector)ejector)

Air ConditioningAir Conditioning

PRESS SOURCE selector switch- tilt panel fwd. of PRESS SOURCE selector switch- tilt panel fwd. of throttlethrottle

NORMAL- flow from ea. Eng.- DC fail positionNORMAL- flow from ea. Eng.- DC fail position If rt. eng. > 72% N(2) pressure switch closes the If rt. eng. > 72% N(2) pressure switch closes the

gnd. Valve preventing too much airflow to the gnd. Valve preventing too much airflow to the ACM- BLEED AIR GROUND light extinguishes- ACM- BLEED AIR GROUND light extinguishes- reverses if rt. Throttle below 70%reverses if rt. Throttle below 70%

If primary pressure switch fails and rt. Eng. >74% If primary pressure switch fails and rt. Eng. >74% N(2)- secondary switch closes gnd. Valve and N(2)- secondary switch closes gnd. Valve and illuminates ACM OVER PRESS light- gnd. Valve illuminates ACM OVER PRESS light- gnd. Valve will remain closed until DC is interruptedwill remain closed until DC is interrupted


In EMER left eng. Bleed goes directly into In EMER left eng. Bleed goes directly into cabin- EMER PRESS ON light illuminatescabin- EMER PRESS ON light illuminates

OFF- all valves closes- no pressurization OFF- all valves closes- no pressurization availableavailable


Precooler- bleed air from eng. Passes through a Precooler- bleed air from eng. Passes through a pre-cooler in the ACM ambient air duct- air-to-air pre-cooler in the ACM ambient air duct- air-to-air heat exchanger.heat exchanger.

Cools bleed air prior to the ACM heat exchangersCools bleed air prior to the ACM heat exchangers ACM- after passing via the precooler in the ram air ACM- after passing via the precooler in the ram air

duct then air flows via the ACM primary heat duct then air flows via the ACM primary heat exchanger then via the ACM compressor then exchanger then via the ACM compressor then secondary ht. exchanger and finally across the secondary ht. exchanger and finally across the cooling turbine- this is the cooling process.cooling turbine- this is the cooling process.


Water Separator- removes moisture from air Water Separator- removes moisture from air prior to entering cabin through a check prior to entering cabin through a check valve. This moisture is injected into air valve. This moisture is injected into air flowing over the heat exchangers to flowing over the heat exchangers to increase cooling efficiencyincrease cooling efficiency


Temp. Control- via a temp. select rheostat Temp. Control- via a temp. select rheostat or a manual HOT-MANUAL COLD switchor a manual HOT-MANUAL COLD switch

Temp. is controlled by allowing some of the Temp. is controlled by allowing some of the bleed air to bypass ACM via the mixing bleed air to bypass ACM via the mixing valve. Valve is positioned by elec. Motor- valve. Valve is positioned by elec. Motor- valve opened (warmer) or closed (cooler) by valve opened (warmer) or closed (cooler) by either:either:– Automatic ControlAutomatic Control– Manual temp. controlManual temp. control


Cabin temp. controlled automatically when temp. Cabin temp. controlled automatically when temp. rheostat is in the automatic range. This temp. rheostat is in the automatic range. This temp. selector is a rheostat that sends a variable temp. selector is a rheostat that sends a variable temp. setting to cabin temp. controllersetting to cabin temp. controller– Controller compares desired setting to actual cabin Controller compares desired setting to actual cabin

temp. via the cabin temp. sensor (in top of cabin btn. temp. via the cabin temp. sensor (in top of cabin btn. Main and emer. Exit. Also receives temp. input from Main and emer. Exit. Also receives temp. input from supply duct temp sensor in tailcone; compares inputs supply duct temp sensor in tailcone; compares inputs and tells mixing valve motor to change tempand tells mixing valve motor to change temp


For example- if selector is rotated clockwise For example- if selector is rotated clockwise for warmer- a higher temp- controller for warmer- a higher temp- controller receives this input, compares it to cabin and receives this input, compares it to cabin and supply duct sensors causing the mixing supply duct sensors causing the mixing valve to open allowing more bleed air to valve to open allowing more bleed air to bypass the ACM, boosting the temp.bypass the ACM, boosting the temp.


In Auto- system has a supply air low temp. In Auto- system has a supply air low temp. Limit of 35 deg. F. to prevent ice in water Limit of 35 deg. F. to prevent ice in water separatorseparator

Input to controller for this is from supply duct Input to controller for this is from supply duct temp. sensor downstream of the water temp. sensor downstream of the water separator in the supply duct.separator in the supply duct.


With temp. rheostat in manual position, With temp. rheostat in manual position, mixing valve is controlled manually by the mixing valve is controlled manually by the MANUAL HOT-MANUAL-COLD switch MANUAL HOT-MANUAL-COLD switch which is spring loaded to OFFwhich is spring loaded to OFF

This causes manual movement of mixing This causes manual movement of mixing valve where it remains until changed- valve valve where it remains until changed- valve has a 10 second travel time- must be has a 10 second travel time- must be cautious to prevent water separator freeze cautious to prevent water separator freeze in this modein this mode


System Protection- If bleed air temp. btn. System Protection- If bleed air temp. btn. Compressor and turbine exceeds 435 deg. F. the Compressor and turbine exceeds 435 deg. F. the overheat relay causes flow control and shutoff overheat relay causes flow control and shutoff valves (or ground valve) to close and the EMER valves (or ground valve) to close and the EMER PRESS valve to openPRESS valve to open– In flight ACM shuts down and noise level in cabin goes In flight ACM shuts down and noise level in cabin goes

up dramaticallyup dramatically– If overheat lasts longer than 12s the emer. Lockout relay If overheat lasts longer than 12s the emer. Lockout relay

energizes and ACM remains inop. If less than 12s it energizes and ACM remains inop. If less than 12s it reverts to previous setting and emer. Valve closesreverts to previous setting and emer. Valve closes


To reset the system to normal after 12s To reset the system to normal after 12s shutdown- rotate the PRESS SOURCE shutdown- rotate the PRESS SOURCE selector to EMER position and then re-selector to EMER position and then re-select a position other than GND or OFFselect a position other than GND or OFF


On ground when ACM overheats the emer. On ground when ACM overheats the emer. press. Valve does not open since it has press. Valve does not open since it has been de-activated by left main squat switch been de-activated by left main squat switch but EMER PRESS light does come on- reset but EMER PRESS light does come on- reset procedure is the sameprocedure is the same


In EMER mode- watch pulling the left In EMER mode- watch pulling the left throttle down too much for temp. control as throttle down too much for temp. control as cabin alt. may rise.cabin alt. may rise.


Air duct from ACM to cabin is protected from Air duct from ACM to cabin is protected from overheat by a duct overheat sensor. If temp. overheat by a duct overheat sensor. If temp. exceeds 157 deg. The AIR DUCT O’HEAT exceeds 157 deg. The AIR DUCT O’HEAT annunciator light illuminatesannunciator light illuminates– Most likely when heat is demanded and most bleed is Most likely when heat is demanded and most bleed is

bypassing the cooling process of ACMbypassing the cooling process of ACM– Pilot should select MANUAL with rheostat and close Pilot should select MANUAL with rheostat and close

mixing valve by holding MANUAL HOT- MANUAL-mixing valve by holding MANUAL HOT- MANUAL-COLD switch to the COLD position- 10 sec.COLD switch to the COLD position- 10 sec.

– Check TEMP circuit breaker in- no power means no Check TEMP circuit breaker in- no power means no temp. control in either mode.temp. control in either mode.


DC failure- defaults as if in NORMAL DC failure- defaults as if in NORMAL position. Emer. Press. Valve fails closedposition. Emer. Press. Valve fails closed

Also temp. control is lost- DC mixing valve Also temp. control is lost- DC mixing valve fails as set at time of power lossfails as set at time of power loss


Air distribution system:Air distribution system:– Overhead conditioned air ducts and outletsOverhead conditioned air ducts and outlets– Footrest and armrest warmer manifolds supplied by Footrest and armrest warmer manifolds supplied by

underfloor conditioned air duct which also supplies the underfloor conditioned air duct which also supplies the windshield defog. Outletswindshield defog. Outlets

– Conditioned air enters cabin via single duct then Conditioned air enters cabin via single duct then branches at a flow divider- cockpit + main and aux. branches at a flow divider- cockpit + main and aux. plenums then to pax. Scn.plenums then to pax. Scn.

– Air circulation enhanced by using overhead fan and Air circulation enhanced by using overhead fan and defog. Fan.defog. Fan.


Operation- with engines operating select Operation- with engines operating select source- air flows:source- air flows:– Water separatorWater separator– DuctingDucting– Check Valve (aft bulkhead)Check Valve (aft bulkhead)


Temp. in supply duct determines position of Temp. in supply duct determines position of recirculating air inlet door. Below 100 Deg. recirculating air inlet door. Below 100 Deg. F. door fully closed- conditioned air flows via F. door fully closed- conditioned air flows via both overhead and underfloor ducts.both overhead and underfloor ducts.

Above 100 deg. F. door fully open- all hot air Above 100 deg. F. door fully open- all hot air from ACM diverted to underfloor ductfrom ACM diverted to underfloor duct

Air from overhead ducts is now recirculated Air from overhead ducts is now recirculated cabin air only-cooler than incoming air from cabin air only-cooler than incoming air from ACMACM


Air flowing through the overhead ducting Air flowing through the overhead ducting system is distributed and controlled by system is distributed and controlled by manipulation of the individual Wemac manipulation of the individual Wemac outletsoutlets

Increased airflow through these outlets can Increased airflow through these outlets can be obtained by selecting HI or LOW with the be obtained by selecting HI or LOW with the OVHD fan switch on co-pilot’s instrument OVHD fan switch on co-pilot’s instrument panelpanel


Air flowing to underfloor ducting is divided by the Air flowing to underfloor ducting is divided by the flow divider assembly; part to flight compartment flow divider assembly; part to flight compartment part to pass. Footwarmer and armrest warmer part to pass. Footwarmer and armrest warmer manifolds.manifolds.

Position of flow bias valve is determined by AIR Position of flow bias valve is determined by AIR FLOW DISTR selector.- 5 position selector- allows FLOW DISTR selector.- 5 position selector- allows selection if incr. or decr. Flow to cabin or cockpit; selection if incr. or decr. Flow to cabin or cockpit; CKPT- most flow to cockpit- CABIN- most flow to CKPT- most flow to cockpit- CABIN- most flow to armrest and footwarmer manifoldsarmrest and footwarmer manifolds


Using defog fan in conjunction with flow divider Using defog fan in conjunction with flow divider increases flow to cockpitincreases flow to cockpit– Max. flow- CKPT with AIR FLOW DISTR selector and Max. flow- CKPT with AIR FLOW DISTR selector and

selecting HI with defog fan switch (HI, LOW, OFF)selecting HI with defog fan switch (HI, LOW, OFF)– Pilot’s footwarmer must be closed for max. defog at Pilot’s footwarmer must be closed for max. defog at

windshieldswindshields– When footwarmer outlet is closed, a side window defog When footwarmer outlet is closed, a side window defog

valve is also closed- prevents miost air from getting btn. valve is also closed- prevents miost air from getting btn. Side window panes and condensing as it meets cold Side window panes and condensing as it meets cold soaked outside air. soaked outside air.


Optional Flood Cooling- air outlet grill on upper aft Optional Flood Cooling- air outlet grill on upper aft press. Bulkhead- high volume of cool air from the press. Bulkhead- high volume of cool air from the ACMACM– Bypasses both overhead and underfoot duct systems- Bypasses both overhead and underfoot duct systems-

eliminates heating by hot-soaked ducting systemeliminates heating by hot-soaked ducting system– Use on ground and in-flight ONLY below 10,000 ft.Use on ground and in-flight ONLY below 10,000 ft.– Powered by rt. Gen. directly via a 20 amp breaker- Powered by rt. Gen. directly via a 20 amp breaker-

prevents use of flood fan on ground without engines prevents use of flood fan on ground without engines running (battery depletion).running (battery depletion).

Flood CoolingFlood Cooling

FLOOD COOLING switch- next to FLOOD COOLING switch- next to pressurization controller on center panel- 2 pressurization controller on center panel- 2 position- ON and OFF.position- ON and OFF.– OFF- Airflow is via normal dist. SystemOFF- Airflow is via normal dist. System– ON- Normal dist. System blocked- all flow ON- Normal dist. System blocked- all flow

directed via the flood cooling ductdirected via the flood cooling duct– Within flood duct is a DC powered axial blower- Within flood duct is a DC powered axial blower-

increases airflow at low pwr. settingsincreases airflow at low pwr. settings

Flood CoolingFlood Cooling

At low pwr. Settings with Flood Cooling ON- At low pwr. Settings with Flood Cooling ON- ACM flow insufficient for blower thus ACM flow insufficient for blower thus ambient tailcone air is mixed with ACM air.ambient tailcone air is mixed with ACM air.

An ambient check valve closes at higher An ambient check valve closes at higher power settings blocking outside airflowpower settings blocking outside airflow

The EndThe End

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AST 473 Cessna Citation II Air Conditioning System Dr. Barnhart