Any Similarities ???
I went back to look at an old Post #825 by oldengmkr, Page 42, AF447, Thread #2. This was the A-330 / A-340 incident over the North Atlantic in October 2000. Now I do recognize that there are differences to start with, the A-340 speeds were accurate, whereas, the AF447 speeds were erroneous at the onset of the incident. However, if high speed sensing initiated the AP & AT coming off in both the A-340 and AF447, could, the events, at least initially, be similar to a degree? Just asking...
Excerpts from the A-340 incident:
Shortly before the AIRPROX event he experienced moderate turbulence and noticed outside air temperature changes. Suddenly the aircraft began to climb, the Master Warning sounded and the autopilot self-disengaged as the aircraft exceeded the speed limit of 0.86 Mach. The indicated airspeed dropped below VLS (the lowest selectable) as the aircraft climbed and the commander took manual control of the aircraft because neither autopilot would engage.
Five seconds after the autopilot disengaged, the thrust levers were closed and then the autothrust was disconnected, probably by the handling pilot in an effort to prevent another overspeed condition. Ten seconds after the autopilot disengaged, the corrected or phase-advanced angle of attack (a computed parameter which is not recorded but can be calculated by Airbus Industrie from the DFDR data) reached the 'alpha prot' value. This angle of attack excursion beyond alpha protcaused a change in the pitch flight control law from normal law (NZ law) to angle of attack protection law (AoA law). If both sidesticks are at neutral, the AoA protection law seeks to hold the angle of attack constant at alpha protuntil a sidestick pitch command is made. If the stick is pulled fully aft then the angle of attack increases to alpha max. If the sidestick is not moved aft, AoA protection law remains active until a nose-down command greater than half forward travel is made or until a nose down sidestick input has been applied for more than one second. The first recorded sidestick input was made at 14:22:08 which was some 28 seconds after the commencement of the Master Warning. For 18 seconds after the autopilot disengaged the aircraft remained within 200 feet altitude of FL 360 but once AoA law was invoked at 14:21:50 hrs, the aircraft's attitude began to pitch nose-up. The pitch-up trend continued for 17 seconds reaching a peak of 15° nose-up shortly before the first nose-down sidestick command was applied. Throughout this phase the aircraft climbed rapidly (reaching a peak rate of about 6,000 ft/min) due to the increase in lift created by the flight control system's capture of alpha prot. The aircraft reached its apogee at FL 384 at 14:22:28 hrs where the airspeed had decayed to 205 KIAS and 0.67 Mach even though full thrust had been applied. Throughout the turbulence encounter, the normal g fluctuations were between 0.5g and 1.5g. The recorded wind direction remained within 20° of the mean of 240° but the wind speed varied between 67 kt and 108 kt and the static air temperature fluctuated between -42° C and -52°C. There were 7 cycles of temperature change, the second cycle being the most severe. The mean air temperature before the AIRPROX event was -46.5° C and afterwards it was -44.5°C. The crew subsequently descended back to FL 360 and successfully re-engaged the autopilot and autothrust systems.
The DFDR recorded a change from TCAS TA to RA at 14:21:41 which was about one second after the Master Warning started. The RA persisted in the aircraft logic for 27 seconds by which time the aircraft was climbing rapidly through FL 372. The alert then changed to a TA which persisted for 8 seconds, ceasing as the aircraft climbed through FL 378.
Angle of Attack protection law
Once AoA law is active, rearward movement of the sidestick controls angle of attack between alpha prot (neutral sidestick) and alpha max (full aft sidestick). Forward movement of the sidestick disengages AoA protection law and the system reverts to normal pitch law. However, there is no aural or text message which informs a crew that AoA protection law has been invoked. If the sidestick is not moved from its neutral position, the pitch flight control system is programmed to capture alpha prot and not the airspeed that corresponds to alpha prot in 1g flight. Consequently, in turbulence the speed scale will probably be oscillating, the aircraft pitch angle could also be oscillating, and the change from normal pitch law to AoA protection law could be difficult to detect.
The commander's reported sighting of an 'Alpha Lock' message was probably an alpha floorwarning on the flight mode annunciator portion of the PFDs. Alpha floor is an autothrottle function which applies full thrust, irrespective of the position of the thrust levers, if the airspeed is likely to reduce to a value approaching alpha max. In this incident, the A340's calibrated airspeed decreased from around 270 kt before the turbulence encounter to 205 kt at the apogee of the climb.
Aircraft response to turbulence
Changes to the A340's flightpath caused by the aircraft's flight control system response to the overspeed warning and autopilot disconnect were negligible until AoA law was triggered. The fact that this law was not triggered until 10 seconds after the autopilot disconnected was a random event driven by the severity of the turbulence. Had the turbulence been more severe at the first encounter and coincident with the overspeed warning, reversion to AoA law could have been triggered as soon as the overspeed condition disconnected the autopilot. Nevertheless, it should be noted that had the autopilot remained engaged, the AoA law would not have been invoked because it is inactive except in manual control. Such was the vigour of the A340's climb in AoA law, the aircraft could well have climbed through FL 363 (thus provoking a TCAS RA with revised software version 7.0) in a very short time, even if the crew had applied nose-down sidestick as soon as they heard the (delayed) autopilot disconnect warning.