MD-11 Flight Crew Operations Manual
k
Air -
Description and Operation
Air.10.4
One of three cooling fans supplies cooling air to the avionics racks. Normally, one
cooling fan operates as primary and a second fan as an automatic backup if the
primary fan fails.
Air flow sensors are installed upstream of the main avionics rack, upstream of the
two forward Display Electronic Units (DEU), and just upstream of an overboard
venturi to detect exhaust flow. These flow sensors detect loss of cooling airflow.
The AVNCS AIR FLO OFF alert is displayed on the EAD when both flow sensors
detect a no flow condition. The AVNCS EXH OFF alert is displayed when not
enough airflow exists in the avionics compartment.
The cooling fans take exhaust air from the cockpit. The exhaust air then flows in
two directions. Some of the cooling air flows to the auxiliary and main avionics
racks. The rest of the cooling air flows to the CRTs and MCDUs in the cockpit.
Piccolo ducts collect compartment air and air that has passed through the avionics
racks. This warm air is routed into an exhaust manifold and dumped overboard
through the venturi using cabin differential pressure across the venturi.
When aircraft differential pressure is less than 1.3 psid (ground operation), the
venturi remains open while an exhaust fan draws air through the manifold and
discharges some of the hot avionics air overboard. The remaining hot avionics air
is exhausted into an area under the cabin floor above the nose wheelwell. During
normal flight operation, the exhaust fan is off.
When the cabin air inflow is too low for cabin pressurization, the venturi shutoff
valve is closed and the exhaust fan is turned on to discharge avionics exhaust air
into the left tunnel and reduce the overboard airflow. This avionics override
function is done automatically by the ESC, or manually by the flight crew with the
AVNCS FAN switch (ESC must be in manual mode) on the AIR control panel.
The AVNCS FAN OVRD alert is displayed when the avionics exhaust fan and
venturi shutoff valve are commanded to override. The avionics exhaust fan
remains in override until the aircraft cabin air inflow and avionics compartment
exhaust airflow are sufficient.
Following takeoff, with 1.3 psid differential pressure and adequate cabin inflow,
the ESC automatically turns off the exhaust fan and opens the venturi shutoff
valve for normal inflight avionics exhaust flow.
Center Accessory Compartment Ventilation System
The Center Accessory Compartment (CAC) ventilation system is similar to the
avionics compartment ventilation system. However, unlike the avionics
compartment, there is no exhaust system. The cooling air blows into one side of
the CAC avionics rack and exits out the top.
October 02, 2006
To Next Page