A330 FCOM Bulletin No. 810/1 (Jun 2004)
< but still present in recent FCOM revision >
Subject: ERRONEOUS AIRSPEED/ALTITUDE INDICATIONS
Background:
Two recent fatal accidents on non-Airbus aircraft and several reported incidents attributed to unreliable speed and/or altitude indications have prompted the need to improve flight crew awarness to identify and tackle the failures described in this bulletin.
Most failure modes of the airspeed/altitude system are detected by the ADIRS and lead to the loss of the corresponding cockpit indications and the triggering of the associated ECAM drills.
However, there may be some cases where the airspeed or altitude output is erroneous without being recognized as such by the ADIRS. in this cases, the cockpit indications appear normal, but are false, and pilots must rely on their basic flying skills to identify the faulty source and take the required corrective actions. When only one source provides erroneous data, the straightforward crosscheck of the parameters provided by the 3 ADRs allows the faulty system to be identified. This identification becomes more difficult in extreme situations when two, or even all three, sources provide erroneous information.
This FCOM Bulletin provides the following information:
1. Recall of pitot/static system layout;
2. Situations which may lead to erroneous, airspeed/altitude indications;
3. Consequences of various failure cases;
4. Recall of recommended operational procedures.
DISPLAY ARCHITECTURE:
- The CAPT side pitot and static probes supply the ADIRU 1 which is normally used for display on the CAPT PFD.
- The F/O side pitot and static probes supply the ADIRU 2 which is normally used for display on the F/O PFD.
- The STBY pitot and static probes supply the ADIRU 3 which can be used for display on either PFD in case of failure. They also directly supply the standby instruments.
<read: "ISIS">
MAIN REASONS FOR ERRONEOUS AIRSPEED-ALTITUDE DATA:
The most proberly reason for erroneous airspeed and altitude information is obstructed pitot tubes or static sources. Depending on the level of obstruction, the symptoms visible to the flight crew will be different. However, in all cases, the data provided by the obstructed probe will be false. Since it is highly unlikely that the aircraft probes be obstructed at the same time, bt the same amount, and in the same way, the first indication of arroneous airspeed-altitude data available to flight crews, will most proberly be a discepancy between various sources.
CONSEQUENCES OF OBSTRUCTED PITOT TUBES OR STATIC PORTS:
All aircraft systems using anemometric data have fault accomodation logics. The fault accomodation logics are not the same for the various systems; but rely on voting priciples 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 the speed information from all ADIRUs.
It compares the 3 values
Pressure altitude information is not used by the PRIM.
Each FE (Flight Envelope Computer)
<FMGEC> receives the speed and pressure information from all ADIRUs.
For each of these two parameters, it compares the 3 values.
IF ONE ADR OUTPUT IS ERRONEOUS AND THE TWO REMAING ADRs ARE CORRECT
The PRIM and the FE
<FMGEC> eliminate it without any cockpit effect (no caution, normal operation is continued), except that one display is wrong and CAT III dual can no longer be available on the FMA.
IF TWO ADR OUTPUTS ARE ERRONEOUS, BUT DIFFERENT, AND THE REMAING ADR IS CORRECT, OR IF ALL THREE ARE ERRONEOUS BUT DIFFERENT.
The autopilot and the autothrust are disconnected by the FE<FMGEC> (whichever autopilot is engaged).
If the disagree lasts for more than 10 seconds, the PRIM triggers the ADR DISAGREE ECAM caution.
It reverts to Alternate 2 law (without high and low speed protection)
On both PFD, "SPD LIM" flag is shown, no Vls and no Vsw is displayed.
This situation is latched, untill a PRIM reset is performed on the 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 disapeared.
IF 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 System will reject the 'good' ADR and will continue to operate using the two 'bad' ADRs. This situation can be met when. for example, two or three pitot tubes are obstructed at the same time, by the same amount, and in the same way. (Flight through cloud of volcanic ash, takeoff with two pitots obstructed by foreign matter (mud, insects)).
Human beings (the pilot) tend to use the same type of 'fault accommodation' principles to detect an erroneous IAS/altitude indication. Flight crews will tend to reject the outlier information, if the other two outputs are consistent. This choice is, in the majority of cases, correct; but, all flight crews should be aware of very extreme and unlikely situations where two (or even three) speed/altitude indications can be consistent and wrong.
BEWARE OF INSTINCTIVELY REJECTING AN OUTLIER ADR
The following chart provides a non-exhaustive list of the various consequences of various cases of partially or totally obstructed pitot tubes and static ports on airspeed and altitude indications. It should be noted that the cases descibed 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.
FAILURE CASE:
Water accumulated due to heavy rain, Drain holes unobstructed.
CONSEQUENCES:
-Transient speed drop until water drains
-IAS fluctuations.
-IAS step drop and gradual return to normal.
FAILURE CASE:
Water accumulated due to heavy rain, Drain holes obstructed.
CONSEQUENCES:
Permanent speed drop.
FAILURE CASE:
Ice accretion due to pitot heat failure or transient blocked due to severe icing, unobstructed drain holes.
CONSEQUENCES:
-Total pressure leaks towards static pressure.
-IAS drop until obstruction cleared/fluctuation if transient erratic ATHR if transient.
FAILURE CASE:
Ice accretion due to pitot heat failure or transient blocked due to severe icing, obstructed drain holes.
CONSEQUENCES:
-Total pressure blocked
-Constant IAS in level flight until obstruction cleared.
in climb IAS increases.
in descent IAS decreases.
-Abnormal AP/FD/ATHR behaviour:
a) AP/FD pitch up in OPN CLB to hold target IAS.
b) AP/FD/ pitch down in OPN DES to hold target IAS.
FAILURE CASE:
Total obstruction of static ports on ground.
CONSEQUENCES:
- Static pressure blocked at airfield level
- normal indications during T/O Roll
- After lift off altitude remains constant
- IAS decreases after lift off
- IAS decreases when aircraft climbs
- IAS increases when aircraft descends.
Based on the information given in the preceding chart, it is clear that no single rule can be given to conclusively identify all possible cases of erroneous airspeed/altitude indications. However, any case of erroneous speed/altitude indications will always be associated to one (or more) of the following cues:
a) fluctuations of airspeed indications;
b) Abnormal correlation of the basic flight parameters (IAS, pitch, attitude, thrust, climb rate):
- IAS increasing with large nose-up pitch attitude;
- IAS decreasing with large nose down pitch attitude;
- IAS decreasing with node down pitch attitude and aircraft descending;
c) Abnormal AP/FD/ATHR behaviour;
d) Undue staal warning or overspeed warnings;
e) reduction of aerodynamic noise with increasing IAS;
f) Increase of aerodynamic noise with decreasing IAS.
RECOMMENDED POCEDURES:
GENERAL REMARKS
The procedures descriped below are intended to provide flight crews with general guidelines to be applied in case of suspected erroneous airspeed/altitude indications.
FOLLOW ECAM ACTIONS
IF FAILURE UNDETECTED:
CROSSCHECK ALL IAS/ALTITUDE SOURCES:
ADR 1, ADR 2, ADR 3 and STANDBY INSTRUMENTS
if it is obvious that the outlier is wrong, select the corresponding ADR OFF and reconfigure the PFD indications accordingly, by appling the ECAM drill which will be automatically displayed.
Flight crews should, however, be aware that in extreme circumstances, it may happen that two, or even all three ADRs may provide identical and erroneous data. Therefore,
the suspect ADR should only be switched OFF, if it is positively confirmed that the two other ADRs are correct. if in doubt:
DISCONNECT AP/FD AND ATHR
FLY TARGET PITCH ATTITUDE AND THRUSTSETTING
The initial pitch attitude and thrust values given in the QRH should be considered as 'Memory Items', since they allow 'safe flight conditions' to be rapidly established in all flight phases (takeoff, climb, cruise) and aircraft configurations (weight and slat/flaps). Once the target pitch attitude and thrust values has been stabilized. the expanded data of the QRH(Flight with unreliable speed indication) should be followed to determine the precise pitch attitude and power setting required, as a function of the aircraft's weight, configuration and desired speed.
After applying the QRH procedure, and when aircraft is stable, the flight crew should try to identify the faulty ADR(one or more). Once the dicrepant ADR has (or have) been positively identified, it (they) should be switced OFF. This will trigger the corresponding ECAM warnings and the associated drills which should be followed to adress all the consequences on the various aircraft systems.
<End of Bulletin >
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