Perhaps AF 447 experienced a similar AoA law event as discussed in these excerpts from the cited AAIB and it precipitated a deep stall. As best as I can determine, this AoA law is still in use today. If a similar AoA law incident occurred to AF 447 it’s severity may have been compounded by the reduced crew on the flight deck at the time. Hopefully, the AF 447 fight data will be recovered and fully disclose the cause.
My apology for the length of this post but I could not edit it more extensively without losing meaningful information. The full AAIB is available at: http://www.aaib.gov.uk/cms_resources...pdf_501275.pdf
AAIB Bulletin No: 6/2001, AIRPROX incident
Date & Time (UTC): 1422 hrs on 2 October 2000
Location: North Atlantic Track E at position 58° 28.4'North 016° 46.1'West
Aircraft Type and Registration:
i) - Airbus A330 C-GGWD
ii) - Airbus A340 TC-JDN
Background
The A340 was en-route from Istanbul to New York and the A330 was en-route from London to Ottawa. Both aircraft were assigned to North Atlantic (NAT) Track E with an entry point into Oceanic Airspace of 58°North 10°West and a next reporting point of 59°N 20°W. Both aircraft were cleared by the Scottish Oceanic Area Control Centre to cruise at Mach 0.82 with the A340 at
Flight Level (FL) 360 and the A330 1,000 feet above at FL 370. ------ At the time of the incident each aircraft was using SELCAL but on different HF radio frequencies. They were beyond radar coverage but both crews were monitoring the common VHF radio frequency used for air to air messages. Both aircraft had a limiting speed at their respective flight levels of 0.86 Mach.
Meteorological situation
A meteorological aftercast described the synoptic situation at 1400 hrs UTC as a low pressure area centered to the west and south west of Iceland with an occluding front lying through 60° North and 16° West. Visible spectrum satellite pictures taken at 1400 hrs clearly showed the position of the front. A change in the height of the tropopause was very likely at or near the incident position with
an associated change in air temperature across the frontal boundary. Moreover, NAT Track E lay some 150 nm to the north of the core of a westerly Jetstream with a speed of 170 kt at FL 310. The1200 hrs NAT Significant Weather Chart showed the position of the Jetstream and the occluded front. At the incident location, NAT track E lay within the boundary of an area marked as likely to
contain clear air turbulence between FL 200 and FL 400. This area was associated with the westerly flow of the Jetstream.
The A330 commander's report
Both aircraft were in clear air as the A330 was slowly overtaking the A340 below it. The A330 commander stated that his aircraft was slightly to the right of the A340 and almost abeam it when he saw the A340's wings start to flex. At about that time he felt a bump, which he described as similar to entering a mountain wave. Five to ten seconds later there was another bump during
which the A330's altimeter reading decreased by 200 feet. Immediately thereafter, the A330 commander heard a TCAS "climb climb" warning and he noted that the A340 TCAS symbol had changed colour to red on his navigation display. He looked out and down at the A340 which was some 200 to 300 feet to his left in a nose-up attitude and climbing steeply. The A340 passed
through the A330's level before the commander had time to react to the TCAS warning and the TCAS was still issuing a "climb" instruction for a short while after the A340 had climbed above the A330. The commander continued to monitor the A340 visually and on TCAS. It appeared to reach an apogee above FL380 although by this time it had fallen behind the A330. Nevertheless, it was still laterally quite close to the A330's track so the commander altered course to the right to make space for the A340 to descend back to FL 360. After a short discussion with the A340 crew on VHF radio, the A330 commander broadcast a warning of severe turbulence on the common VHF frequency and then reported both the turbulence and the aircraft proximity (AIRPROX) occurrence
to Shanwick on HF Radio. there were no injuries on board the A330 although there were spillages in the cabin.
The A340 commander's report
At FL 360 the A340 was 1000 feet below the maximum cruising level displayed on the Flight Management and Guidance System (FMGS). The commander was expecting a turbulence encounter around 59°N 20°W and when the aircraft first entered light turbulence he made a cabin announcement and switched on the seat belt signs. 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. The crew subsequently reported the incident to Shanwick on HF
radio and using their TCAS, they descended back to FL 360 in a safe area. At the time of the AIRPROX the commander estimated the aircraft were one mile apart laterally. After landing at New York the commander had the aircraft inspected by technical staff but no defects were found.There were no injuries on board the aircraft. The commander could not remember the sequence of warnings but he did recall being unable to reengage either autopilot which prompted him to make manual control inputs. He also remembered seeing an 'Alpha Lock' warning displayed on his Primary Flying Display.
Flight Data Recordings
The Cockpit Voice Recordings in both aircraft had over-run the event before they landed but digital flight data readouts were obtained from the aircraft by the airlines involved.
A340 Data
The A340 crew received a TCAS TA at 14:09 hrs (A340 clock time) alerting them to the proximity of the A330. At 14:20:40 hrs the aircraft entered a region of successive and increasing variations in wind and air temperature, which in turn caused fluctuations in pitch angle, normal g, altitude, calibrated airspeed, engine N1% and Mach number. One minute later in a particularly vigorous
fluctuation, the aircraft's Mach number briefly increased to 0.87. This speed excursion above the Mach 0.86 limit triggered a Master Warning at 14:21:40 and automatically disengaged the autopilot. One second later the TCAS issued an RA with a "descend, descend, descend" audio warning. In the two second period after the initial speed excursion above Mach 0.86 the Mach number decayed to 0.855 and then increased again to 0.882. It remained above 0.86 for two seconds before decreasing and remaining below 0.86 for the remainder of the turbulence encounter. 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 prot until 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 14:21:41which 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
Analysis
General
Before the incident, both flight deck crews would have been aware of the presence of the other aircraft because both had received at least one 'traffic traffic' synthetic voice message and they would have seen the intruder aircraft symbol on their navigation displays about 10 minutes before the turbulence encounter. The incident began when both aircraft deviated from their assigned flight levels whilst the lateral separation between them was less than nautical two miles triggering TCAS RA warnings in both aircraft. Initially the risk was minimal because when TCAS RAs were issued, the aircraft were about 800 feet vertically separated with transient variations in vertical speed due to the turbulence; at that stage the A340 had not begun its 'zoom climb'. The incident became serious about 10 seconds later when the A340's flight control system captured alpha prot and commenced a vigorous climb which resulted in the A340 climbing through the A330's assigned flight level whilst both aircraft were laterally separated by a few hundred feet.
A340 crew awareness
The TCAS voice message "descend descend descend" may not have been noticed by the A340 crew because of other concurrent warnings. The logic of the A340 Fault Warning Computer (FWC) prioritises warnings and requires a period of one second between successive warnings. The sequence of aural warnings commenced with a CRC (a Continuous Repetitive Chime that also
illuminated the MASTER WARNING light) which was active for two seconds, then deactivated for one second, and then active for a further two seconds. The TCAS RA aural warning should have started at about the same time as the CRC and have been active during the five second period of near continuous repetitive chime but it probably ceased before the chime stopped. Both warnings are generated through the cockpit loudspeakers and although the respective systems can generate only one aural warning and one synthetic voice message at any moment, the loudspeakers can generate simultaneously one synthetic voice and one aural tone warning. Consequently, the crew
should have heard both warnings but they may not have assimilated both. Moreover, another warning, th
e 'Cavalry Charge' alert for autopilot disconnect, was suppressed for approximately six seconds by the FWC logic and this warning would have followed the CRC and remained active until it was cancelled by one of the pilots after a further five seconds. Therefore, the succession of
aural warnings could have affected the crew's ability to assimilate a concurrent synthetic voice warning. Even though the A340 crew may not have registered the TCAS synthetic voice message, they would have had visual indications of the TCAS generated instruction not to climb. However, they were experiencing what they described as severe turbulence, which may have made reading the
displays very difficult. Also, it would have been natural for their attention to have been captured by the Master Warning triggered by the overspeed condition which took priority over the autopilot disconnect warning. That they reacted to the overspeed warning is indicated by closure of the thrust levers five seconds after the onset of the continuous repetitive chime.
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. The climb to FL 363 would have been sufficient to generate a TCAS RA in any adjacent aircraft at FL 370 but, if the intruder aircraft continues its climb, there can be no guarantee that an aircraft directly above it could respond in sufficient time to avoid a collision. Therefore, the RVSM safety case should not be driven by any assumption that a different crew might have contained the situation by making an earlier nose-down sidestick command than the A340 crew involved in this incident.