The initial means of controlling the flow of bleed air is the firewall shutoff valve which is a spring loaded valve and is normally opened by the flow of bleed air out of the second stage compressors. This valve can be manually closed by pressing the F/W SHUTOFF PUSH button in the centre of the annunciator panel.
Firewall shutoff buttons
Pressing the F/W SHUTOFF PUSH buttons electrically closes the firewall bleed air valves and terminates the bleed air flow from that engine to all systems except the engine inlet anti ice. Pressing this button will also closes the fuel firewall shutoff valve, disables the generator and arms the fire bottle.
After pressing the F/W SHUTOFF PUSH button the engine will normally be shut down. However, if the Fuel firewall shutoff circuit breaker is pulled before pressing the respective F/W SHUTOFF PUSH button fuel to the engine will not be cutoff therefore allowing the engine to continue to operate without the generator operating or the bleed air being supplied to the pneumatic systems. The fire bottle will also be armed.
Fuel firewall shutoff CBs mounted on the LH CB panel
This procedure of pulling the circuit breakers can be handy in a couple of scenarios but is not printed in any approved checklists.
First, if the windscreen anti ice safety valves fail with the windscreen heat ON, hot bleed air will flow over the perspex windscreen eventually leading to the windscreen melting.
Windshield Anti ice system
To aid in the control of the flow of bleed air to the windscreen the ‘Right hand’ fuel fire wall shutoff circuit breaker can be pulled followed by pressing the Right hand F/W SHUTOFF PUSH button. This will not terminate the flow of bleed air completely but may reduce the volume of air flow to the point that failure of the windscreen will be delayed.
Windscreen anti ice bleed air from both engines
|(1) Pull this circuit breaker
||(2) Then press this button
Pulling the right hand fuel firewall circuit breaker and pressing the right hand F/W SHUTOFF PUSH button will also remove the emergency pressurisation capability should this be required. However, should the emergency pressurisation fail in the open position causing an uncontrollable rise in cabin temperatures, the same procedure can be applied to terminate the flow of bleed air.
Emerg press air can be removed by terminating the right engine bleed air.