Abnormal v Alpha Prot-OEB Procedures-with Comments

8/18/2019 Abnormal v Alpha Prot-OEB Procedures-with Comments

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Reversion to alternate was due to:

- The ADR 3 rejection by the flight controls computer

AND

- The ADR 2 switch off by the flight crew

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In the first example, the AOA 3 is equal to 3 degrees(°).

The median being 1.5°, the difference between the AOA 3 (3°) and the

AOA median (1.5°) is lower than 3.6°:the AOA 3 is considered valid.

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In the second example, the AOA 3 is equal to 5.5°.

The median being 1.5°, the difference between the AOA 3 ( 5.5°) and the

AOA median (1.5°) is higher than 3.6°:the AOA 3 is considered not valid.

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In a scenario similar to the reported event, the AOA 1 and 2 are blocked toa constant value of 5°.

The median becomes in this case 5°.The median being 5°, the difference between the AOA 1 (5 °) and the AOAmedian (5°) is lower than 3,6°:

The AOA 1 is considered valid.

Idem for the AOA 3.

In this scenario, the AOA 3 is equal to 1°.

The median being 5°, the difference between the AOA 3 (1°) and the AOAmedian (5°) is higher than 3,6°:

The AOA 3 is considered not valid.

The AOA 1 and 2 are blocked to the same value but are considered validby the flight controls computers (the AOA 3 is rejected).

The flight controls computers will use the AOA 1 and 2 to compute theValphaprot and the Valpha max.

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The root cause of the in-service event is still under investigation. However,Airbus decided, as a precautionary measure, to issue temporary

procedures.Along with Flight Operations Transmissions (FOT) sent on 10th Dec 2014,temporary procedures have been released through OEBs and associatedAFM Temporary Revisions (TRs).

These temporary procedures are applicable to all A318/A319/A320/A321aircraft and to all A330/A340 aircraft.

As technical investigations are still on-going, no criteria to cancel this

temporary procedure has been defined for the moment. When aterminating action is determined to remove this temporary procedure,Airbus will issue a revision of the OEBs.

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Two main goals led the development of the temporary procedures:

• To anticipate a potential Alpha Prot activation due to AOA signals

remaining at the same constant value, by providing the flight crew withvisual effects in the cockpit to identify an issue with the AOA signals.

• To provide a simple procedure to prevent, but also to recover, from an

undue activation of the Alpha Prot.

The title refers to the most visible effect that the flight crew will see on the

PFD, if AOA values are erroneous. It consists in an Abnormal behavior ofthe V Alpha Prot, and its associated strip.

The procedure has the following steps:

- A Monitoring reminder

- A Recovery procedure

- Preventive actions

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Regarding A320 family aircraft, the Monitoring step reminds the visibleelements on the PFD Airspeed scale, that are used in the conditions of the

procedure.In this case, it is the Alpha prot strip and the Alpha max strip.

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The Recovery procedure is defined in order to recover a full authority onpitch, if the Alpha Protection erroneously activates. This is obtained by the

reversion of the Flight Control laws to Alternate law. To do so, the flighthas to switch off two ADRs.

The condition of the Recovery procedure is:

AT ANY TIME, with a speed above VLS, if the aircraft goes to a

CONTINUOUS NOSE DOWN PITCH RATE that cannot be stopped withbackward sidestick inputs, IMMEDIATELY APPLY:

ONE ADR……………..KEEP ON

TWO ADRs……………OFF

Capitals letters have been used for some words in this conditions, in order

to clearly highlight the main aspects of the procedure. These points are:

- Time related with the words AT ANY TIME

- Describing a visual and physical effect, that the flight crew will see andfeel, with the wording CONTINUOUS NOSE DOWN PITCH RATE

- Defining the required actions, which are to IMMEDIATELY APPLY:“ONE ADR…ON” and “TWO ADRs…OFF”.

Indeed, these actions must be rapidly done by memory (in less than 15

seconds).

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The flight crew must be aware of the location of the ADR pushbuttons, inorder to be able to quickly switch off two ADRs as requested.

The procedure does not define which ADR must be switched off, since theimportant step is to revert to Alternate law in order to rapidly recover a fullauthority on pitch.

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The recovery procedure must be performed in the case of an undueactivation of the Alpha Protection.

As you can see on this graph, the AOA value for the Alpha Prot and theAlpha max are both a function of the Mach number. When the Mach

number increases, the AOA value that triggers the Alpha Protection isdecreasing.

In the case of 2 blocked AOA probes, the AOA value used by the F/CTLlaw will remain constant, whatever the real aircraft AOA.

As an example, the amber dot on the graph represents the aircraft AOA

and Mach value when 2 AOA probes become blocked.

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With an increase in Mach number, the amber dot will move along thedotted line.

In this case, when the Mach number increases, the margin between themeasured AOA value and the Alpha Prot value will be smaller and smaller.

The diminution of this margin will have an effect on the display of the V

Alpha Prot strip on the PFD: it will cause the Alpha Prot strip to moveupwards on the airspeed scale, when Mach increases.

If the Mach continues to increase, there will be a point where the AOAvalue used by the F/CTL law will be equal to the value of the Alpha Prot. Atthis given Mach number, the Alpha Prot will thus activate, and order a

nose down.

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The OEB for A320 family aircraft also provides preventive actions, in orderto avoid an undue activation of the Alpha Prot.

These actions are based on two visual effects on the airspeed scale, thatare visible when there is an issue with the AOA signals.

The first one is the following condition: If the Alpha Max strip (red)completely hides the Alpha Prot strip (black and amber) in a stabilized

wings-level flight path (without an increase in load factor).

The associated actions are to:

- Keep on one ADR

- Switch off two ADRs

- Consider the use of the Flight Path Vector (FPV)

Attention is also drawn on the fact that the display of the strip of the V stallwarning (black and red) may be erroneous.

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The second visual effect on the airspeed scale is the following:

If the Alpha Prot strip (black and amber) rapidly moves by more than 30 kt

during flight maneuvers (with an increase in load factor), with AP ON andspeed brakes retracted.

Identically, the actions to be performed by the flight crew are to:

- Keep on one ADR


- Consider the use of the Flight Path Vector (FPV)

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These 2 conditions for preventive actions refer to an increase -or not- inload factor.

This is due to the fact that on A320 family aircraft, the display of theValpha Prot strip (black and amber) and the Valpha Max strip (red) islinked to the vertical load factor.

On the A320 Family, per design, when the vertical load factor is equal to1g (i.e. in stabilized wings-level flight path), the display of Valpha Prot stripand the Valpha Max strip remain limited at a value of VLS - 2kt.

This means that, in a stabilized wings-level flight path, the Valpha Protstrip and the Valpha Max strip can never hide completely the VLS strip.

On the opposite, during flight maneuvers involving an increase in load

factor, such as turns or pitch variations, there is no more limitation on thedisplay of the Alpha Prot strip at VLS - 2kt. The Alpha prot and Alpha maxstrips will increase.

If the vertical load factor returns to a value of 1g, for example when theturn is completed or the pitch attitude is stabilized, the Valpha Prot stripdecreases again towards VLS – 2kt.

Therefore, with AP ON, during flight maneuvers with an increase in load

factor, an increase in the Valpha Prot by more than 30 kt with speedbrakesretracted is symptomatic of an issue with the AOA signals.

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Regarding the temporary procedure for A330 and A340 aircraft:

The title is identical and still refers to the most visible effect for the flight

crew.The content of the procedure also details monitoring reminder, preventiveaction and recovery procedure.

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The Monitoring step reminds the visible elements on the PFD Airspeedscale, that are used in the conditions of the procedure.

In the case of A330 and A340 aircraft, the visible elements to particularlymonitor are the VLS strip and the Alpha Prot strip.

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Then, the preventive action is defined.

It is based on the following condition:

If the Alpha Prot strip (black and amber) completely and permanentlyhides the VLS strip (amber) in a stabilized wings-level flight path (withoutan increase in load factor).

In this case, the flight crew will:

- Keep on one ADR


Then it is recommended :

- Not to increase the speed

- To consider the FPV use- To reconfigure the EFIS DMC

- Not to increase the altitude, when above safety altitude.

When the altitude and the speed are not increased, the Mach number will

not increase. This limitation on Mach number will avoid the triggering ofundue stall warnings.

Attention is also drawn on the fact that:

- The display of the strip of the V stall warning (black and red) may beerroneous

- There is a risk of undue stall warning.

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Finally, the recovery procedure is provided.

This procedure is identical to the one defined for A320 family aircraft.

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Abnormal v Alpha Prot-OEB Procedures-with Comments