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All aircraft systems, using anemometric data, have been built-in fault accommodation logics. The fault accommodation logics are not the same for various systems but, all rely on voting principle whereby when one source diverges from the average value, it is automatically rejected and the system continues to operate normally with the remaining two sources. This principle applies to flight controls and flight guidance systems.
NORMAL SITUATION
Each PRIM receives speed information from all ADIRUs and compares the 3 values. Pressure altitude information is not used by the PRIM. Each FE (Flight Envelope computer) receives speed and pressure information from all ADIRUs and compares the 3 values.
ONE ADR OUTPUT IS ERRONEOUS AND THE TWO REMAINING ARE CORRECT
The PRIMs and the FEs eliminate it.
On basic A330 and A340-200/300, there is no cockpit effect (no caution, normal operation is continued), except that one display is wrong and CAT3 DUAL is displayed as INOP SYS on STATUS page.
On the A340-500/600 and enhanced 330 and 340-300, if one ADR deviates, and if this ADR is used to display the speed information on either the CAPT or F/O PFD, a NAV IAS DISCREPANCY
caution is triggered. Furthermore, as with the A340-200/300, CAT3 DUAL is displayed as INOP SYS on STATUS page.
TWO ADR OUTPUTS ARE ERRONEOUS, BUT DIFFERENT, AND THE REMAINING ADR IS CORRECT, OR IF ALL THREE ARE ERRONEOUS, BUT DIFFERENT :
The autopilot and the auto thrust are disconnected by the FE (whichever autopilot is engaged). If the
disagree lasts for more than 10 s, the PRIM triggers the NAV ADR DISAGREE ECAM caution. Flight controls revert to Alternate 2 law (without high and low speed protection). On both PFDs : The "SPD LIM" flag is shown ; no VLS and no VSW is displayed. This situation is latched, until a PRIM reset is performed on ground, without any hydraulic pressure.
However, if the anomaly was only transient, the autopilot and the autothrust can be re-engaged when the disagree has disappeared.
ONE ADR IS CORRECT, BUT THE OTHER TWO ADRS PROVIDE THE SAME ERRONEOUS OUTPUT, OR IF ALL THREE ADRS PROVIDE CONSISTENT AND ERRONEOUS DATA :
The systems will reject the "good" ADR and will continue to operate normally using the two "bad" ADRs. This condition can be met when, for example, two or all three pitot tubes are obstructed at the same time, to the same degree, and in the same way. (Flight through a cloud of volcanic ash, takeoff with two pitots obstructed by foreign matter (mud, insects)).
The following chart provides a non-exhaustive list of the consequences of various cases of partially or totally obstructed pitot tubes and static ports on airspeed and altitude indications. It should
ABNORMAL OPERATIONS
NAVIGATION
be noted that the cases described below cover extreme situations (e.g. totally obstructed or unobstructed drain holes), and that there could be multiple intermediate configurations with similar, but not identical, consequences.
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A330/A340 机组训练手册 FCTM Flight Crew Training Manual 2(44)
