Greybeard:
Thanks for the TCAS block diagram on post 2650, PJ2. It confirms that airspeed is not an input to TCAS, hence not a reason for the NAV TCAS Fail on the ACARS list.
[...]
Altitude on the PFD was still valid during the Delta/NW event, so the altitude to the TCAS would still have been valid. The ADR will not send different altitudes to these two destinations, except below the QNH transition level. The TCAS Fail message from AF447 remains unexplained, and not in concert with the other reported failures that can all be explained by erroneous airspeed.
One case of unreliable airspeed is always different from another. Look at the two cases investigated by the NTSB:
NTSB INVESTIGATING TWO RECENT INCIDENTS INVOLVING POSSIBLE A-330 SPEED AND ALTITUDE INDICATION ANOMALIES
The National Transportation Safety Board is investigating two recent incidents in which airspeed and altitude indications in the cockpits of Airbus A-330 aircraft may have malfunctioned.
The first incident occurred May 21, 2009, when TAM Airlines flight 8091 (Brazilian registration PT-MVB) flying from Miami, Florida to Sao Paulo, Brazil, experienced a loss of primary speed and altitude information while in cruise flight. Initial reports indicate that the flight crew noted an abrupt drop in indicated outside air temperature, followed by the loss of the Air Data Reference System and disconnections of the autopilot and autothrust, along with the loss of speed and altitude information. The flight crew used backup instruments and primary data was restored in about 5 minutes. The flight landed at Sao Paulo with no further incident and there were no injuries and damage.
Then, NW incident is not exactly the same as TAM, but the root is common. As well, the same variations (speed or altitude, speed and altitude) are shown in other cases from Air France, Air Tahiti and Air Caraïbes. They are not triggering exactly the same faults depending of the conditions (exposure time is an important factor here) affecting the lines of the pitot/static system.
But the general context is almost identical (as well as the crews reaction) and it is described also fairly well in the Airbus A330/A340 Flight Crew Training Manual:
FCTM A330/A340 Non-normal Operations 8.110.3 Miscellaneous REV 1 (6 JUN 05)
UNRELIABLE AIRSPEED INDICATIONS
Unreliable airspeed indications can result from blocked or frozen lines in the
pitot/static system.
Most failure modes of the airspeed/altitude system are detected by the ADIRS
and lead to the loss of the corresponding cockpit indication(s) and the triggering
of associated ECAM procedures. The fault sensing logic relies on a voting
principle whereby if 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.
However, there may be some cases where the airspeed or altitude output is
erroneous without being recognised as such by the ADIRS. In these cases, the
cockpit indications appear normal but are actually false and pilots must rely on
their basic flying skills to identify the faulty source and take the required
corrective action. When only one source provides erroneous data, a simple
crosscheck of the parameters generated by the three ADRs allows the faulty
ADR to be identified. This identification becomes more difficult in extreme
situations when two or all three ADR sources provide erroneous information.
Normally, each PRIM receives speed information from each ADIRU and
compares the three values. Pressure altitude information is not used by the
PRIM. Each FE computer receives both speed and pressure information from
each ADIRU and compares the three values.
In a failure situation, various combinations of ADR faults may occur, each
interpreted differently by the PRIM and FE computers:
· One ADR output is erroneous and the two remaining ADRs are
correct. The PRIMs and the FEs reject the faulty ADR. On basic
A333/A343 aircraft, there is no ECAM alert, however one PFD will display
some incorrect parameters. On the A346 and enhanced A333 aircraft, if
one ADR output is erroneous, and if this ADR is used to display the speed
information on either PFD, a NAV IAS DISCREPANCY caution is
triggered. In all cases, CAT3 DUAL will be displayed as an INOP SYS on
the STATUS page.
· Two ADR outputs are erroneous but different and the remaining ADR
is correct, or if all three ADR outputs are erroneous but different. The
AP and A/THR will disconnect. If the disagreement lasts for more than 10
seconds, the PRIM triggers the NAV ADR DISAGREE ECAM caution.
Flight controls revert to ALTN 2 law. The SPD LIM flag is displayed on
both PFDs, however VLS and VSW are not displayed. This condition is
latched until a PRIM reset is performed on ground without any hydraulic
pressure. However, if the disagreement was transient, the AP and A/THR
can be re-engaged when the NAV ADR DISAGREE message 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 PRIMs and FEs will reject the “good” outlier ADR
and will continue to operate normally using the two consistent but faulty
ADRs.
Any erroneous speed/altitude indication will always be associated with one or
more of the following cues:
· Fluctuations in airspeed indications
· Abnormal correlation of basic flight parameters (IAS, pitch, attitude, thrust,
climb rate); e.g IAS increasing with large nose-up pitch attitude, IAS
decreasing with large nose down pitch attitude, IAS decreasing, with nose
down pitch attitude and aircraft descending
· Abnormal AP/FD/A/THR behavior
· Stall or overspeed warnings
· Reduction in aerodynamic noise, with increasing IAS
· Increase in aerodynamic noise, with decreasing IAS
The ADRs provide a number of outputs to many systems and a blockage of the
pitot and/or static systems may also lead to the following:
· SPD LIM flag on PFD
· Alpha floor activation (because AOA outputs from the sensors are
corrected by speed inputs)
· Wind shear warning (due to Mach input)
· Flap load-relief activation
· Flap auto-retraction from 1+F to 1
· Alpha lock on slats retraction (due to the speed logic part of the alpha lock
function)
· ALTI DISCREPANCY on ECAM
· RUD TRV LIM FAULT ON on ECAM
Always apply the ECAM procedure. If the failure is not annunciated on ECAM,
crosscheck all IAS/ALTITUDE sources (ADR 1, 2, & 3, and ISIS/STBY INST).
Early recognition of erroneous airspeed indications requires some familiarity with
the relationship between attitude, thrust setting and airspeed. If it is positively
confirmed that the outlier ADR is at fault and that the other two ADRs are
correct, select the faulty ADR OFF. This action will generate an ECAM
procedure, which should be applied in order to reconfigure the PFD to display
correct information.
However, in very extreme circumstances, two or all three ADRs may provide
identical but erroneous data. If there is any doubt, then do not instinctively reject
the outlier ADR, although the temptation may exist if the other two ADR outputs
are consistent. In most cases, this decision would be correct, but not in the case
where two speed/altitude indications are consistent but wrong. Apply the initial
actions of the UNRELIABLE SPEED INDICATION QRH procedure from memory
as they quickly provide a safe flight condition in all phases of flight and aircraft
configuration. Rely on the primary flight parameters of pitch attitude and thrust
setting.
Because the displayed information may be erroneous, the flying accuracy cannot
be assumed. Incorrect transponder altitude reporting could cause confusion.
Therefore, declare a MAYDAY to advise ATC and other aircraft of the situation.
Reference to the QRH should only be made when a safe flight path has been
established. The QRH provides pitch attitude and thrust settings for each flight
phase and for different weights.
After applying the QRH procedure, and when the aircraft flight path is stabilised,
attempt to identify the faulty ADR(s). Once the faulty ADR(s) has/have been
positively identified, it/they should be switched OFF. This will trigger the
corresponding ECAM procedure, which should be applied.
Depending of the cause of the failure, the altitude indication may also be
unreliable. However, there are a number of correct indications available to the
crew:
Unreliable Parameter: Altitude
Disregard:
.. Altimeter
.. IAS/TAS
.. Wind
.. V/S
.. FPA
Use:
... GPS altitude (on GPS monitor page)
... RA (low level)
Unreliable Parameter: Speed
Disregard:
.. IAS/TAS
.. Wind
Use:
... GPS GS (on GPS monitor page)
When flying the aircraft with unreliable speed and/or altitude indications, it is
recommended to change only one flying parameter at a time; e.g. speed, altitude
or configuration. Consequently, plan to be at VAPP by the final approach fix.
______________
[addition for people not familiar with airliners procedures: this kind of Non-Normal Operation is to be found in every aircraft manual for such an issue; this is not only related to A330/A340]