MD-11 Flight Crew Operations Manual
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Fire Protection -
Description and Operation
Fire.10.3
Since the bleed air temperature from engine 2 can vary over a wide range
depending on engine operating conditions, and since the fire detection alarm
temperature is above the nominal bleed air temperature expected, a ruptured duct
will be detected only if it is hazardous.
Fire Detection Control Unit (FDCU)
One FDCU for each main engine and the APU is located in the avionics
compartment directly below the cockpit. The four FDCUs are identical, however,
the external circuitry for the engines is different than for the APU.
The FDCU contains circuits that will:
• Detect signals from the sensing elements.
• Differentiate between a fire signal and a fault.
• Receive inputs from the LOOPS A/B and LOOPS A/B TEST switches.
• Energize the MASTER WARNING lights, LOOP A/B lights, alerts, and
aural fire warning or APU horn.
The FDCU incorporates two loop circuits (loop A and loop B) which provide
continuous monitoring of the detectors. These loop circuits provide warning
signals to the flight crew in the event of any fire/overheat alarms, or loop faults.
In addition to the loop circuits, the FDCU has a microprocessor and ARINC 429
transceiver circuit.
The microprocessor monitors detectors wiring, and FDCU integrity. It is capable
of isolating a fault to the Line Replaceable Unit (LRU) level.
The FDCU has Built-in-test Equipment (BITE) and interfaces with the centralized
fault display system (CFDS) via the ARINC 429 bus.
The FDCU continuously receives and analyzes both fire detection loops signals
and is capable of detecting one or more failures in the fire detection loops.
In the event of a failure, the FDCU will:
• Store the failure in a memory.
• Isolate the failure to an LRU.
• Provide alerts regarding the system status including failures.
• Enable single loop fire detection capability on the remaining good loop
when a single loop fails.
When a fire condition is sensed, the FDCU will generate two fire outputs. Fire
output 1 is powered by the loop A power source, and fire output 2 is powered by
the loop B power source.
The two fire outputs eliminate the possibility of a single point failure preventing
an alarm during a fire condition.
October 02, 2006
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