The PFC operates in two modes: normal and secondary. In normal mode, all the functions of the PFC operate. The PFC selects the secondary mode when there is a loss of important air and attitude sensor data from the ADIRU and SAARU. The secondary mode is a degraded mode in which only specified functions of the PFC operate.
Lane Functions
The three lanes can become any one of these three lanes:
*
Command lane
*
Standby lane
*
Monitor lane.
The command lane transmits its command output signals to the flight controls ARINC 629 buses. The command lane also has a lane monitoring function to find and isolate failures in the standby and monitor lanes.
The standby lane transmits test data only. The standby lane becomes the command lane and transmits its command output signals to the buses when a failure causes the command lane to shut down. The standby lane does monitoring functions to find and isolate failures in the command and monitor lanes.
The monitor lane transmits test data only. The monitor lane command output signals are used internally to find failures in the command and standby lanes. The monitor lane becomes the standby lane when the command lane shuts down.
Lane Selection
When the airplane initially gets electrical power, each PFC starts a lane selection procedure. This procedure automatically sets the function of each lane in each PFC. Each PFC must select a different lane for its command lane.
At the end of each flight, the lane selection in each PFC changes to decrease the exposure time for potentially latent failures.
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Input Signal Monitor
Each lane gets input signals from sensors on the captain’s and first officer’s sides of the flight deck. Dual and triple sensors on each side supply input signals. The input signal monitor of each lane sets which input signal to use by mid-value selection and averaging.
The mid-value selection compares the signal values of each side and selects the signal that has the middle value. The input signal monitor then averages the mid-values from the captain’s and the first officer’s sides.
This selection causes each independent process software to receive the same input signal. This selection also removes signals that are out of tolerance.
Control Laws
Each lane has a set of control laws to calculate the proposed command output signals. The command lane sends proposed commands to the other two PFCs.
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Cross-Lane Monitor
The cross-lane monitor does a check of the validity of the proposed signals generated by all three lanes. If a cross-lane monitor finds a lane fault, it temporarily inhibits the transmission of the lane’s data on the flight controls ARINC 629 buses. The monitor lets the faulty lane try to operate as many as four times during a flight. If the lane still cannot operate without failure, then the monitor latches the failed lane so it can no longer operate.
If the command lane fails, the cross-lane monitor shuts it down and causes the standby lane to do the function of the command lane. If a second lane is defective, the PFC shuts down.
PFC Output Selector
Each lane has a PFC output selector to supply a common set of PFC command output signals for use by the ACEs. This function makes sure that all PFC generated command output signals are identical to each other. The PFC output selector operates only in the command lane.
The PFC output selector gets the proposed command output signals from all three PFCs. The PFC output selector compares the three proposed signals and makes a selection by the mid-value process. Only the PFC output selector of the command lane then transmits the selected command output signal to the flight controls ARINC 629 buses and the ACEs.
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PFC Self Test
At the end of each flight, the PFCs do an automatic PFC self test to find component failures and confirm failures found in flight.
Lane Availability and Indications
The three PFCs supply a total of nine lanes on the airplane. When there is no failure, the lane availability is referred to as the 3-3-3 lane configuration. If a PFC has a failed lane, the lane availability is 2. The PFC shuts down if there is a second lane failure or if all three lanes fail. In that case, the lane availability is 1 or 0.
The table below shows the indication on the primary display system and the operating mode for each combination of lane availability.
LANE AVAILABILITY PFCS MODE MESSAGE MESSAGE TYPE
3-3-3 NORMAL NONE NONE
3-3-2 NORMAL NONE NONE
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