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 Post SB 21-1121 For A/C 001-049,051-060,101-102,
 A. Flow Control Valve
 (1)
 General Information Each air conditioning pack has a flow control valve which gives variable flow and shut off control. These valves are electro-pneumatically operated. They will open when the PACK 1 (2) pushbutton switches are pushed in (the OFF legends go off) if the conditions which follow exist:

 -the engines are not in a starting sequence,
 -an ENGINE FIRE pushbutton switch is in the normal position (the light in the pushbutton switch is off),
 -
the DITCHING pushbutton switch 13HL is released (the ON legend is off),

 -
the bleed air pressure is more than the minimum pressure needed to open the flow control valves.

 (2)
 Normal Operation (packs running on engine bleed supply) When the PACK 1 (2) pushbutton switches 7HB (6HB) are pushed in (the OFF legends go off) the flow control valves open. The crew selects the PACK FLOW selector 5HB to the LO, NORM or HI position.

 -This position selects the flow control reference in the zone controller 8HK to 80%, 100% or 120%.
 -
The zone controller calculates the necessary flow and transmits this value to the pack controllers 7HH (27HH).

 -
The pack controllers then set the flow control valves 11HB (8HB) to

 the demanded position. When the PACK FLOW selector is set to the LO position, and the temperature demand cannot be reached, the zone controller will increase the normal airflow of the flow control valve from 80% to 100%.
 (3) APU in Operation (packs running on APU bleed air supply) When the PACK 1 (2) pushbutton switches 7HB (6HB) are pushed in (the OFF legends go off) the flow control valves open. The zone controller 8HK receives the APU bleed valve open signal. The flow control reference in the zone controller is set to 120% and the flow control valves are commanded to the fully open position.
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 (4)
 Single Pack Operation (pack running on engine or APU bleed air supply) When a pack pushbutton switch is pushed in (the OFF legend goes off) the respective flow control valve opens. Irrespective of which position the PACK FLOW selector is set to, the zone controller and the pack controller will modulate the pack to the 120% (HI) mode. Thus the flow control valve is commanded to the fully open position.

 (5)
 Operation with Failures

 (a)
 Single Pack Failure When the PACK FLOW selector is set to the NORM or LO position and there is a failure in one pack, the zone controller and the pack controllers modulate the other pack to the 120% (HI) mode. Thus the flow control valve is commanded to the fully open position.

 (b)
 Engine Fire If there is an engine fire, and you push the applicable FIRE pushbutton switch, the respective flow control valve closes. This stops the hot bleed air flow in order to prevent damage to the components of the air conditioning system and to prevent entry of smoke into the cabin.

 (c)
 Ditching In case of landing on water the DITCHING pushbutton switch must be pushed. This closes the flow control valves which prevents the entry of water into the cabin through the air conditioning system.

 (d)
 Air-Cycle Machine Compressor Overheat If the compressor of an air-cycle machine overheats, the applicable flow control valve starts to close. When the compressor temperature reaches 260 deg.C (500.00 deg.F) the flow control valve will fully close.

 (e)
 Pack Shut-down in the Engine-Start Sequence In the engine-start sequence (the EIU gives an engine rpm-signal of N1<50%), normally both air-packs close. To prevent the complete pack shut-down in flight because of a wrong EIU-signal, the crossbleed valve indication relay 7HV monitors the position of the Cross Bleed Valve (CBV) 6HV:

 -If the CBV is in the closed position, only one pack instead of both closes.
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 B. Mixer Flap Actuator
 (1)
 Normal Operation When the PACK 1 and PACK 2 pushbutton switches or only the PACK 1 pushbutton switch is(are) 'on' the mixer flap actuator 20HB is in the open position. Thus air is supplied from the pack 1 mixer chamber to the cockpit. When the PACK 1 pushbutton switch is released (the OFF legend is on) the mixer flap actuator closes and the air is supplied from the pack 2 mixer chamber to the cockpit.
　
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