FCTM UNRELIABLE AIRSPEED INDICATIONS

Question 1

The ADRs detect most of the failures affecting the airspeed or altitude indications. These failures lead to:

Answer 1

‐ Lose the associated speed or altitude indications in the cockpit

‐ Trigger the associated ECAM alerts.

Question 2

However, there may be cases where an airspeed and/or altitude output is erroneous, while the ADRs do not detect it as erroneous. In such a case:

Answer 2

No ECAM alert is triggered and the cockpit indications may appear to be normal whereas they are actually false.

Question 3

The most probable reason for erroneous airspeed and/or altitude information is:

Answer 3

An obstruction of the pitot and/or static probes.

Question 4

It is highly unlikely that the aircraft probes will be obstructed at the same time, to the same degree and in the same way. Therefore:

Answer 4

The first effect of erroneous airspeed/altitude data in the cockpit will most probably be a discrepancy between the various indications (CAPT PFD, F/O PFD and STBY instruments).

Question 5

All the aircraft systems which use anemometric data, have built-in fault accommodation logics. The fault accommodation logics rely on a voting principle:

Answer 5

When the data provided by one source diverges from the average value, the systems automatically reject this source and continue to operate normally using the remaining two sources. The flight controls system and the flight guidance system both use this voting principle.

Question 6

Each ELAC receives speed information from the:

Answer 6

three ADRs and compares the three values. The ELACs do not use the pressure altitude.

Question 7

Each FAC receives speed and pressure altitude information from the

Answer 7

three ADRs and compares the three values.

Question 8

ONE ADR OUTPUT IS ERRONEOUS AND THE TWO REMAINING ARE CORRECT

Answer 8

The ELACs and the FAC and/or FMGC eliminate the erroneous ADR. There is no cockpit effect (no caution, normal operation is continued), except that one display is wrong and the autoland capability is downgraded to CAT 3 SINGLE.

Question 9

TWO ADR OUTPUTS ARE ERRONEOUS, BUT DIFFERENT, AND THE REMAINING ADR IS CORRECT, OR IF ALL THREE ADRS ARE ERRONEOUS, BUT DIFFERENT :

Answer 9

Both the AP and A/THR disconnect. The ELACs trigger the NAV ADR DISAGREE ECAM caution.

The flight controls revert to alternate law without high and low speed protection.

On both PFDs: ‐ The SPD LIM flag appears ‐ No VLS, no VSW and no VMAX are displayed

This situation is latched for the remainder of the flight, until the ELACs are reset on ground, without any hydraulic pressure. However, if the anomaly is only transient, the AP and the A/THR can be re-engaged when the disagree disappears.

Question 10

ONE ADR IS CORRECT, BUT THE OTHER TWO ADRS PROVIDE THE SAME ERRONEOUS OUTPUT, OR IF ALL THREE ADRS PROVIDE CONSISTENT AND ERRONEOUS DATA :

Answer 10

The systems reject the correct ADR and continue to operate using the two erroneous but consistent ADRs. The flight crew can encounter such a situation 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)).

Question 11

Water accumulated due to heavy rain. Drain holes unobstructed.

Answer 11

Transient speed drop until water drains. IAS fluctuations. IAS step drop and gradual return to normal.

Question 12

Water accumulated due to heavy rain. Drain holes obstructed.

Answer 12

Permanent IAS drop.

Question 13

Ice accretion due to pitot heat failure, or transient pitot blocked due to severe icing. Unobstructed drain holes.

Answer 13

Total pressure leaks towards static pressure. IAS drop until obstruction cleared/fluctuation, if transient erratic A/THR is transient.

Question 14

Ice accretion due to pitot heat failure, or pitot obstruction due to foreign objects. Obstructed drain holes.

Answer 14

Total pressure blocked. Constant IAS in level flight, until obstruction is cleared.

In climb, IAS increases.

In descent, IAS decreases. Abnormal AP/FD and A/THR behavior :

a. AP/FD pitch up in OP CLB to hold target IAS.
b. AP/FD pitch down in OP DES to hold target IAS

Question 15

Total obstruction of static ports on ground.

Answer 15

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.

Question 16

The majority of unreliable speed events at low altitude are permanent situations, due to:

Answer 16

the obstruction of pitot probes by rain, severe icing, or foreign objects

Question 17

At high altitude, typically above FL 250, the cases of unreliable speed situation are mostly a temporary phenomenon:

Answer 17

They are usually due to contamination of the pitots, by water or ice, in particular meteorological conditions. In-service experience shows that such a contamination typically disappears after few minutes, allowing to recover normal speed indications.

Question 18

If the barometric altitude is unreliable, the:

Answer 18

Flight Path Vector (FPV) and the Vertical Speed (V/S) are affected.

In addition, the ATC transponder may transmit an incorrect altitude to ATC or to other aircraft, which can lead to confusion. Therefore, the flight crew should advise ATC of the situation without delay.

Because the barometric altitude may be erroneous, the Autopilot (AP) may not be able to maintain accurately the level flight.