EQUIPMENT COOLING OFF

P6-4 P18-3

Low Air flow detected (*) Or
Sensor Power Loss

(*) Possible causes for the air mass flow to drop are failure of the cooling fan, a clogged air filter and a even cabin decompression. For the latter the air mass flow through the cooling system drops because of a decreasing differential air pressure between the cooling system inlet (cabin air) and outlet (outside air). Finally, a dirty air flow sensor may also trigger a (spurious) low air flow signal.

On GND MEL 21-27 [Electrical/Electronic Equipment Cooling Blowers], MEL 21-40 [Equipment Cooling Automatic Flow Control Valve...], MEL 21-42 [Equipment Cooling Low Flow Detector Systems], MEL 21-43 [Equipment Cooling Air Filter]
On GND an overtemperature in the E&E-compartment will cause the Ground Crew Call Horn to sound in the nose wheel well.

A failure of the Equipment Cooling Exhaust Fan (normal) may be the result of an unpowered 115v ac Ground Service Bus.

In FLT WX-Radar display will automatically turn off to avoid overheating. WXR DSPLY and EXCESS DATA will annunciate.

According to the NNC, no further action is necessary in flight if the Equipment Cooling OFF amber light(s) remain(s) illuminated. However, according to the operations note in MEL 21-27 :

If the Equipment Cooling OFF amber light illuminates during flight, continued flight beyond 30 minutes can result in loss off :
- (both supply fans INOP) both Captain's DU and the lower center DU
- (both exhaust fans INOP) both F/O's DU and the upper center DU

TR unit maximum rating without forced air cooling drops to 50 amps.
Monitor TR load. Refer to [TR UNIT]

NORMAL MODE
On GND or In FLt with the cabin differential pressure below 2.0 PSI, the fan air for equipment cooling is exhausted via the airplane belly. During pre-flight exterior inspection with ac power to the airplane verify that the air flow is exhausted via the bottom of the aircraft. If no air flow is observed, the Overboard Exhaust Valve may be stuck in closed position from a previous flight.
A few knocks on the airplane's belly just forward of the exhaust hole may free the valve.

When cabin differential pressure increases, the valve closes and warm air from the E&E is diffused under the floor of the forward cargo compartment for additional inflight heating. You notice a substantial increase in air flow noise on the flight deck during descent when the valve reopens at 2.0 PSI cab in differential pressure.

The temperature in the cargo compartment depends on many factors, such as ambient temperature, number of passengers, cargo load and flight time. Boeing guarantees stabilized air temperature shall not descend below 5 degrees C / 40 degrees F in the forward cargo bay and remains above 0 degree C / 32 degrees F in the aft cargo compartment. The area near the cargo doors is excluded from this requirement.

SMOKE MODE
The overboard exhaust valve is an aerodynamically controlled valve that flies from full open to full closed. However, a 28v dc actuator can override the aerodynamic control.

When the actuator is in the SMOKE position, the valve disk is limited to turn from full open to half open (54 degrees) to allow for smoke removal from the E&E and the flight deck. Also, power to the equipment cooling supply fans (both normal and alternate) is removed and the Equipment Cooling Supply OFF amber light is inhibited in the smoke mode.

Left or Right Pack Switch in HIGH AND
(Right) Recirculation Fan Switch OFF AND
Air/Gnd Relay Sensing in Air Mode AND
OR
(As Installed) FWD or AFT cargo fire warning OR
Smoke mode activated (R648 energizes)

P6-D2 :overboard exhaust valve reconfiguration control" is installed for unpressurized dispatch. If the circuit breaker is open, the bypass circuit enables the recirculation fan to operate if the smoke control relay R648 energizes.