Air Systems - Equipment Cooling

Forward equipment cooling supplies cooling air to the equipment in the forward equipment racks and the flight deck avionics.

System Operation is automatic with the EQUIP COOLING selector in AUTO.

On the ground, engine operation, pack operation, air/ground logic, airplane skin temperature and ambient temperature are all factors that determine system configuration.

A supply fan and an exhaust fan either re-circulate the cooling air back through the system, or port it overboard. On the ground, both the supply and one exhaust fan operate. Inflight, only the exhaust fan operates, and the air is recirculated through the system. If the operating fan fails (inflight), the other starts automatically.

As installed

There are three positions on the Equipment Cooling Selector. AUTO is the normal position for both ground and flight operations. STBY and OVRD are used under specific conditions.

• AUTO (Automatic) Operation

  The AUTO position automatically operates the equipment cooling system in various configurations:

  • Ground SCHEMATIC
    The system is automatically configured dependent on engine operation, pack operation, air/ground logic, airplane skin temperature and ambient temperature. Dependent on these conditions, the system will configure itself to provide adequate E/E cooling.

  • Flight SCHEMATIC
    Only the bypass valve is open and the exhaust fan operates. If the exhaust fan fails, the supply fan automatically switches on.

  
  

• STBY (Standby) Operation

  Standby operation is used when an overheat is observed during automatic operation or if a valve light is illuminated after both engines are started.

  Positioning the Equipment Cooling Selector to STBY signals all valves to close except the bypass valve (to OPEN) and both fans to ON while on the ground.

  Inflight, valve positions are the same but only the supply fan is signalled ON. If the supply fan should fail, the exhaust fan will automatically operate.

  
  

• OVRD (Override) Operation

  Override operation is used when overheating is detected after Standby (STBY) is selected or if smoke is detected in the equipment cooling system.

  Selecting OVRD opens the SMOKE/OVRD valve and positions the air conditioning manifold valve to provide conditioned air for cockpit equipment cooling. All other valves close and both supply and exhaust fans are turned OFF.

  Differential pressure causes reverse airflow through the cockpit instrument panels and E/E compartment avionics. The NO COOLING light will illuminate if no reverse airflow, is detected.

  The Ground Crew Call Horn will sound if OVRD is selected on the ground.

Fuel Jettison

Activation of the fuel jettison switch automatically selects the OVERRIDE cooling mode regardless of the equipment cooling selector position. Once the fuel jettison switch is selected off, the equipment cooling system reverts back to the selected equipment cooling mode.

VALVE Light

The VALVE light illuminates and the EICAS advisory message FWD EQPT VAL displays if the system valves fail to reconfigure - are not in their signalled position - during automatic operation.

With the VALVE light illuminated, selecting STBY signals the valves to the inboard configuration. If the overboard valves close and the inboard valves open, the VALVE Light extinguishes and the message disappears.

OVHT Light

The OVHT Light and the EICAS advisory message FWD EQPT OVHT displays if low airflow or high temperature is sensed. Placing the EQUIP COOLING selector in STBY may correct the condition by reconfiguring the system.

When an overheat occurs on the ground the ground crew call horn in the nose wheel well is activated.

When the airflow and temperature conditions are corrected or the selector is moved to STBY or OVRD, the OVHT Light extinguishes and the EICAS message is removed.

Selection of STBY extinguishes and inhibits the OVHT light for five minutes to provide a cooling period. If the overheat condition is not corrected at the end of five minutes the OVHT light illuminates.

Selection of OVRD deactivates the OVHT Light, changes the source of cooling air for the cockpit avionics and exhausts the E/E compartment avionics air overboard through the SMOKE/OVRD Valve.

SMOKE Light

The SMOKE light illuminates and the EICAS advisory message FWD EQPT SMOKE displays if smoke is sensed in the forward equipment cooling ducts. Placing the EQUIP COOLING selector in OVRD allows conditioned air to be drawn in reverse through the system by differential pressure and exhausted overboard through the SMOKE/OVRD valve.

The SMOKE light will extinguish when the smoke clears. Because the smoke sensor is located away from the overboard airflow it may be a period of time before the light extinguishes.

NO COOLING Light

The NO COOLING light and the EICAS caution message FWD EQPT COOLING displays if no cooling airflow is sensed after selecting OVRD and indicate there is no reverse air flow for the E/E compartment avionics. To allow a period of time to establish reverse airflow, the light and message are inhibited for one minute after OVRD is selected.

STBY should be used if the NO COOLING Light illuminates after OVRD is selected. STBY ensures equipment cooling, but smoke removal will be with the normal flow of airplane exhaust air.