Checks
The whole episode can be described as following which might answers many questions and amiguities.
The aircraft was in descent with autopilot engaged and with auto thrust probably also engaged with a speed of 270 kt and in slight turbulent weather conditions.When descending through 23,000 ft, the recorded speed from ADC1 (CAS1) drops rapidly from its current value (270 kt)
and reaches 40 kt (16 seconds after) .This reading may be erroneous in the event of probe icing and AIRBUS has said that this has happened to other operators as well operating A310 where pitot probe icing on Captain side was faulty.
In these conditions the auto thrust computer reacts to the speed decrease by ordering a thrust increase (throttle levers move from 43° TRA=IDLE to 68° TRA=CLIMB). The recorded CAS1 keeps decreasing due to probable icing conditions, the crew take over with autopilot disconnection and nose down orders (positive elevators deflection). The pitch attitude decreases from +1° nose up to -8° nose down.The crew further increase the thrust (with throttles moving to full forward stop) and still order a nose down on control column (positive elevators deflection).In these conditions, with full thrust order and nose down orders maintained, the actual aircraft speed builds up toward VMO whilst the rate of descent increases significantly up to 12,000 ft/min.Overspeed warning is triggered when crossing 20,500 ft indicating that the airspeed from ADC2 is valid at this time (overspeed warning triggers after crossing VMO=340 kt with either CAS1 or CAS2). Nose down orders are still recorded and pitch attitude further decrease to -15° nose down. In the meantime, increased vertical load factor excursions are recorded. At this time, the CAS1 recovers progressively from the probable icing conditions (CAS1 records 100 kt increasing). The crew maintain a nose down order with overspeed warning still active (and lasting 52.34 sec). The aircraft is now flying in overspeed conditions with
actual speed increasing beyond VMO and at high rate of descent (-12,000 ft/min).When reaching 16,000 ft, the CAS1 shows 471 kt (=VMO+131kt =VD+50 kt). During the event, the aircraft experiences vertical load factor excursions between -0.33g and +2.59g.As per Airbus discussions during descent,aircraft may have experienced probable icing conditions and turbulent conditions, erroneous CAS1 value (abnormally low) and increasing vertical load factor excursions. The autopilot and the auto thrust were disconnected and the aircraft was set in a descent with nose down orders and increased thrust. In these conditions, the actual aircraft speed has built up and increased beyond VMO (with associated warning triggered thanks to valid ADC2 source at that time) and eventually to 471 kt (according to recorded CAS1 after recovery). In the meantime the aircraft also encountered vertical load factor excursions between -0.33g and +2.59g.During this event the aircraft was flown (in excess of
VMO) . Therefore, the aircraft requires in depth load analysis and definition of applicable inspection program. . Airbus recommends (probe ice protection) checks, with particular attention to pt1 (Captain Pitot) probe ice protection check. No autopilot malfunction and no other system malfunction was highlighted as per preliminary analysis and AIRBUS has said that the autopilot defect does not have the relevence in the scenario.
The above is based on DFDR analysis by experts in PIA engineering and this was the reason AIRBUS was contacted and the aircraft grounded.A keen eye and a probing mind (full thrust do you read ) can conclude the issue logically .The autopilot defect does not have the relevence in the scenario.CVR transcripts have also been recorded and transcribed and is in concurrance with these facts .Probe ice checks are being done here at KHI to confirm if anti icing was ok.Poor Airbus design if icing comes out to be the reason. I would like some proffesional comments on the scenario as a food for thought.