A320 Abnormal Valpha prot

The event :

AN A321 flying at FL 250 , speed become managed at 290KT but the autopilot was unable to reach the target speed . airspeed remained around 250KT . AT FL310 , during a LEFT HAND TURN ,the crew reported PFD showing ( Valpha -prot ) rising upward current airspeed ( 245KT).

the flight crew manually disconnected the autopilot , alpha protection activated with an order of pitch down , aft stick input maintained for 55min to keep level flight at FL270

ADR 2 switched off, leading to alternate law activation and the remaining of flight performed in alternate law .

Results of airbus analysis : 

during climb the following AOA reading recorded as follow :

AOA 1 = 4.5         AOA 2 = 4.5     AOA 3 = normal behaviour

at this reading the flight controls computer rejected the AOA 3 and cause alpha-prot activated with an associated pitch down , the protection remained active until reversion to alternate law .

reversion to alternate was due to ADR 3  rejection by the flight controls computer and ADR2 switch off by the flight crew .

Two main goals led to issue 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 with visual effects in the cockpit to easy identify an issue with AOA .
• To provide a procedure to prevent ,but also to recover from an undue activation of Alpha -prot .

The visible elements on the PFD airspeed scale , that are used in the condition of the procedure which is the Alpha-prot strip and Alpha max strip .

CASE 1 :

Recovery procedure :

AT any time , with a speed above VLS , if the aircraft goes to a CONTINUOUS NOSE DOWN PITCH  , and that cannot be stopped by a full backward side stick IMMEDIATELY APPLY :

one ADR,…………….keep on

TWO ADRs …………..off

CASE 2 :

the first one is the following condition :

if the alpha max srtip (red) completely hides the alpha pro strip (black &amber )in a stabilized wings level flight without increase in load factor .

recovery procedure:

keep on one ADR

switch off two ADRs

use  the flight path vector (FPV)

case 3 : 

if the alpha prot strip ( black & amber ) rapidly moves by more than 30 kt during flight (with an increase in load factor ) with AP ON AND SPEED BRAKES  retracted

recovery procedure : 

one ADR …………..keep on

switch off two ADRs

use the flight path vector ( FPV )

LOAD FACTOR ?WHY ?

VALPAH PROT strip known by ( black &amber )

VALPHA MAX strip known by ( red ) .

per design of the A320 the vertical load factor is is equal to( 1g) with wings level , the display of valpha port strip and the alpha max strip remain limited at a value of VLS-2kt .

which mean in stabilized wings level flight , the valpha port strip and the valpha  max strip can never hide completely the VLS strip .

on the popsite , during flight maneuvers involving an increase in load factor  , such as  turns or pitch variations , there is no more limitation on the display of alpha prot strip at VLS -2kt . the alpha port and alpha max strip will increase .

if the vertical load factor returns to a value of 1g , when the turn is completed and or the pitch attitude is stabilized , the valpha prot strip decrease 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 speed brakes retracted is symptomatic of an issue with the AOA signals .