Air Systems - Equipment Cooling 
- Limitations - Air Conditioning
General
There are two separate equipment cooling systems. The forward system cools avionic equipment in the cockpit and the forward Electrical/Electronic (E/E) compartment. The aft system cools equipment in the aft E/E compartment.
Forward Equipment Cooling System
Forward equipment cooling supplies cooling air to the equipment in the forward equipment racks and the flight deck avionics.
The system has a supply fan that draws cooling air from the cabin and forces it through the equipment racks and avionics. The air is then exhausted into the Air Conditioning Mix Manifold by the Left Recirculation Fan. If the differential pressure is less than 2.5 psi, the Overboard Exhaust Valve automatically opens and most of the air is exhausted overboard.
If the operating fan fails, the alternate fan must be selected.
When the left Recirculation Fan is inoperative, the Overboard Exhaust Valve is automatically opened so differential pressure can produce airflow for equipment cooling.
Equipment Cooling Overheat Light
The equipment cooling OVHT Light illuminates and the EICAS advisory message EQPT OVHT displays if:
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The selected supply fan fails. An alternate supply fan is activated by pushing the ALTN EQUIP COOLING Switch. The OVHT Light extinguishes and the EICAS message blanks when the alternate supply fan operates. The OVHT light re-illuminates and the EQPT OVHT message re-displays if the alternate supply fan fails.
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Insufficient Airflow (Ground Only). On the ground, the OVHT light illuminates, the EQPT OVHT message displays and the Ground Crew Call Horn sounds if insufficient airflow is detected.
The Overboard Exhaust Valve does not open when the Left Recirculation Fan stops.
- Both Aft Lavatory/Galley Ventilation Fans inoperative.TASK 21-58-00-735-101 SUBTASK 865-155 (System Test - Equipment Cooling System)
When the situation is corrected the light is extinguished and the message cancelled.
- Avionics and electrical equipment and displays not powered by standby buses, are subject to imminent failure.
- Avionics and electrical equipment on Standby Buses are reliable for 90 minutes. Continued flight beyond 90 minutes can result in loss of essential avionics and electrical equipment.
Auxiliary Fan
ETOPs
An auxiliary fan automatically operates to provide cooling for essential avionics and electrical equipment if the alternate supply fan has been selected but both supply fans are inoperative. However, non-essential avionics and electrical equipment are subject to imminent failure. The Auxiliary Fan can be powered by the Hydraulic Driven Generator.
On the ground, the OVHT light illuminates, the EQPT OVHT message displays and the ground crew call horn sounds if insufficient cooling airflow is detected.
Alternate Equipment Cooling Switch
ALTN
- Selects alternate supply fan.
- Opens Overboard Exhaust Valve (if Left Recirculation Fan is not operating and the Overboard Exhaust Valve failed to open automatically).
- ETOPs ONLY - Activates Auxiliary Fan, if both supply fans fail.
Equipment Smoke
The Equipment Cooling SMOKE Light illuminates and the EICAS advisory message FWD EQPT SMOKE displays when smoke is detected in the Forward Equipment Cooling System.
The following items, which may be observed in the flight deck, occur automatically to help evacuate the smoke:- the recirculation fan may turn off
- one or both air conditioning packs switch to High Flow
- the Overboard Exhaust Valve is latched open.
When smoke is no longer detected, the Recirculation Fans and Air Conditioning Packs automatically return to normal operation.
Smoke Clearance - Forward System
When SMOKE is detected in the forward equipment Exhaust Duct, the following actions automatically occur:
- Overboard Exhaust Valve is latched OPEN.
- (In-flight, Slats UP) LEFT Recirculation Fan is turned OFF
- LEFT air conditioning Pack is switched to HIGH flow.
When SMOKE is detected in the forward equipment Supply Duct, the following actions automatically occur:
- Overboard Exhaust Valve is latched OPEN.
- (In-flight, Slats UP) LEFT Recirculation Fan is turned OFF.
- RIGHT Recirculation Fan is turned OFF
- BOTH supply fans are turned OFF
- OVHT light inhibited for 5 minutes (pushing the ALTN switch will start the alternate supply fan)
- BOTH Air Conditioning Packs are switched to HIGH flow
Aft Equipment Cooling System
- APU control unit, APU Battery and APU Battery Charger
- ADF receivers
- The left and right (if installed) HF communication transceivers
- (-300) Brake Temperature Monitor Unit
Air from the Aft E/E equipment racks is ducted into the Lavatory/Galley Vent System. The vent system exhaust fans discharge the exhaust air in the vicinity of the Outflow Valve.
If fan 1 does not run, fan 2 will automatically provide back-up ventilation.
