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Pressurisation source selector

The PRESS SOURCE selector determines the amount of bleed air that enters the cabin and determines the source that it comes from, i.e. the left or the right hand engine. The control switch is labeled ‘OFF, LH, RH, BOTH, GRND and EMER.

PRESS SOURCE selector mounted at the bottom of the centre pedestal

The OFF position closes all environmental bleed air valves, i.e. no bleed air enters the air cycle machine or the pressurised cabin. Bleed air is still available though to all of the other pneumatic systems. Ram air is still available to the cabin with the PRESS SOURCE selector in the OFF position and the RAM AIR PULL knob pulled out.

RAM AIR PULL knob for depressurised cabin ram air

The cabin will be operated depressurised with the RAM AIR PULL knob pulled out. Ram air will flow normally throughout the cabin via the distribution ducting.

The GRND position of the pressurisation source selector allows greater airflow into the cabin during ground operations only, and, whilst the engine RPM is below 80%. This is extra flow of bleed air is achieved through the use of the flow restrictor valves which are like venturi tubes that utilise a removable restrictor which reduces the airflow through the venturi once the engine RPM exceeds 80%.

Venturi type flow restrictor valves used during GRND operations

Once the engine RPM exceeds 80% RPM the restrictors are inserted to reduce the amount of bleed air flowing through the air cycle machine manifold and into the cabin. The GRND position of the PRESS SOURCE selector is also unavailable during manual mode operations as the 80% switch which normally controls the function of the restrictors is now controlled by the 60% switch during manual mode operations.

The engines will normally be operated well above 60% RPM during all ground and flight operations in manual mode therefore removing the availability of the GRND function of the PRESS SOURCE selector in manual mode. When the GRND function is selected a BLEED AIR GROUND light will illuminate on the right hand annunciator panel.

BLEED AIR GROUND light illuminated on the right hand annunciator panel

The LH and RH positions of the PRESS SOURCE selector switch allow bleed air from whichever engine the selector switch is positioned to, to enter the cabin via the environmental bleed air valves i.e. LH = left engine. The BOTH position allows bleed air from both engines to enter the cabin.

The EMER position of the PRESS SOURCE selector allows bleed air from the right hand engine to bypass the air cycle machine straight into the cabin for emergency pressurisation. This function is automatically selected should the air cycle machine fail during flight and will be indicated by the EMER PRESS ON light on the right hand annunciator panel and an increase in cabin noise due to the extra airflow into the cabin.

EMER PRESS bleed air from the right engine

EMER PRESS ON light illuminates with automatic selection of the emergency pressurisation during an air cycle machine failure. The EMER PRESS ON light will not come on if manually selected.

If electrical power should be lost during flight, the environmental bleed air valves will automatically default to the BOTH position and the emergency pressurisation valve will automatically close. This will allow normal pressurised operation of the aircraft in flight should an electrical failure occur.

Pressurisation source selector

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