AF447: Issues Clarified by the BEA Report

There are some significant issues which are clarified by the BEA’s preliminary factual report, issued at the beginning of July: specifically the uncertainties and certainties in the meaning and partial interpretation of the maintenance messages received by ACARS; the question of structural integrity; the attitude and flight path of the aircraft on impact with the ocean surface; and the weather phenomena in the vicinity of the flight at the time it was presumed to be lost. The ACARS messages indicate strongly that there was a situation with unreliable airspeed indication. Since the accident more incidents of unreliable airspeed indications at high altitude have come to light. I comment on these continuing developments in a separate post. I comment here on structural integrity and what it tells us about how the airplane may have behaved; weather and position; contacts with ATC; and the interpretation of the ACARS maintenance messages received.

The vertical tail of the aircraft was the major piece of structure found during the search. It had separated, taking some parts of some fuselage, including box-section pieces, with it at one attachment. The question arose whether it could have separated in flight. Our collaborator, the aerodynamicist Clive Leyman, showed in work in June that it would not be possible above say FL 170- FL 200 to generate enough dynamic pressure on the vertical stabiliser of the A330 to cause it to fail. And even at that general altitude, overspeed would be a necessary contributing factor to any failure. His conclusion was based on dynamic-pressure calculations, based on the datum that the A330 vertical stabiliser failed during destructive testing at 2.0 times design load. The aircraft was cleared at FL 350. So we knew here from Clive’s work that an upset would have been a necessary precursor to loss of structural integrity. The main question thus is: what would have caused an upset?

Indeed, the BEA determined from inspection of the retrieved wreckage – over 600 individual pieces – that the aircraft hit the water intact, in more or less level attitude with a high vertical rate of descent. This does not conform with the flight path of an aircraft under full control. It suggests, indeed, that the aircraft was aerodynamically stalled when it hit the ocean surface.

The BEA determined, from the loading of the aircraft on takeoff and the estimated fuel burn over the flight profile that the aircraft had an estimated weight of about 205 tonnes and CG between 37.3% and 37.8% MAC at around the time of disappearance. The half-percentage variation in CG estimate comes from the fact that fuel is pumped around between fuel tanks at cruise, to optimsie the lift-to-drag ratio of the aircraft, and there is a limit of 0.5% MAC on the CG shift allowed to occur through pumping. There has been some speculation on the Internet about the margins between stall speed and limiting Mach number at FL350 and weight of 205 tonnes. The margin is some 80-100 kts; this is large enough to allow the pilots considerable leeway in dealing with any in-flight abnormalities, such as having to fly the airplane on “pitch and power” when airspeed indications are unreliable. However, severe or extreme turbulence could make dealing with abnormalities such as unreliable airspeed a very tricky control situation indeed, at any moderately high flight level. It is plausible that an upset could thereby have occurred. The BEA report is factual and does not speculate on this.

There was considerable convective activity in the ITCZ at the time of AF447s passage. The weather, though, was pretty typical for the time of year, and had no unusual features from the point of view of meteorologists. There was a convective mass extending about 400km E-W, which the route of flight of AF447 crossed. This convective mass had formed at about 0130Z by the fusion of four powerful storm masses, deriving from convective columns (“towers” in French) , which had reached their limit and spread out horizontally as their tops reached the tropopause. The strongest of these had attained its most powerful stage many hours before. At 0200Z, the cumulonimbus clouds forming the mass had for the most part attained their mature stage. Although there may have been new columns forming between the mature columns underneath the top of the spreaded mass, there is no evidence for that in the form of a later “overshoot” into the stratosphere, which happens in the case of the most powerful columns. The temperature at the tops of the mass was by and large similar to that of the tropopause, around -80°C, as recorded by satellite 7 minutes before and after the presumed passage of AF447. The tropopause was estimated by the climate model ARPEGE to be at around FL520 at the date and time of the disappearance of the aircraft. Another aircraft participating in real-time weather data collection via AMDAR passed along the route half an hour later at FL325, then climbing to FL350 and did not record anything unusual, confirming largely what one may infer from the satellite images.

The BEA says it is “very likely” that some of the cloud mass contained significant turbulence at FL 350. Electrical activity was also “possible” at this FL. But, crucially for those wondering whether the pitots iced up because the aircraft may have flown into heavy supercooled-rain clouds, the presence of supercooled water was said to be “not very likely” and would necessarily have been limited to very small quantities. I consider the developments with possible pitot icing in a separate article.

The last known position of AF447 was transmitted automatically over ACARS at 0210Z. This position was N2°58.800’W30°35.400′, or N2.98°W30.59° in decimal degrees. The position transmitted was that contained in the “Flight Management” data, which is partly based on the inertial reference system. It could be, said the BEA, that the GPS position differed slightly from this.

This position puts the flight in or close to the column of what had been the most powerful of the fused storms, whose column had attained its most powerful stage some many hours before and was at the time in its mature stage. The position is between ORARO and TASIL waypoints and looks to be slightly off the airway.

The last verbal contact with AF447 was by the controller of FIR ATLANTICO, in Brazil, at 0135:43Z. The controller then asked AF447 four times for his estimate at TASIL, without response. There were apparently three attempts at an ADS-C connection with DAKAR, at 0133Z, 0135Z and 0201Z. These failed with code FAK4, indicating either the absence of a flight plan, or a significant discrepancy between flight number, reported position, and planned position. Section 1.9.2 says that at 0146 the DAKAR controller asked for information about AF447 because there was no flight plan. ATLANTICO gave type (A332), airport of origin, destination airport, and SELCAL sign. DAKAR created and activated a flight plan, but there was no connection with the aircraft either on voice or ADS-C. So the first two ADS-C attempts were rejected because of, we may presume, lack of a flight plan with DAKAR at those time. The report does not determine whether the flight plan at DAKAR was activated before or after the last ADS-C connection attempt at 0201Z. Although the transcript of the exchange between ATLANTICO and DAKAR at 0135Z is included in the appendices, the later exchange is not.

As I mentioned in my note of 11 June, the order of the ACARS messages received does not necessarily reflect their order of occurrence. The reasons why are largely the reasons I gave there, with one addition. Fault messages received by the CMC are cached but not sent for a minute, to accumulate and summarise in one ACARS transmission other messages associated with that fault from other avionics devices. These associated messages are indicated by including the reporting device in the fault message compiled by the CMC (using a * for associated messages of type 2, which are not reported to crew because they have no “operational consequences”). There is prioritisation within the CMC, as well as possible race conditions from various BITE devices to the CMC, as well as prioritisation of transmission: the report explains how ACARS messages are prioritised by class. And, of course, possible delays in the transmission and processing of messages through the ACARS transmission system itself.

The interpretation of the messages is, as the BEA says, “delicate”. This is not just because of the indeterminacy of order, but also because, while a fault may be recorded, a subsequent return to normal is not reported; certain alarms such as overspeed are not registered; and although all faults (type 1) are accompanied by a cockpit effect (type 2), not all faults have their cockpit effect registered, and not all cockpit effects have the associated fault registered.

Of the type 2 effects, the BEA says it has not succeeded in explaining the meaning of the cockpit effect NAV TCAS FAULT (cockpit effect is a flag on the PFD and ND) but has explained the significance of the others.
There are five type 1 fault messages, of which the significance of two are unexplained:

the ADIRU2 fault (IR2), associated with messages from EFCS1, IR1 and IR3. The involvement of EFCS1 is a type 2 message, and it is suggested that the correlation window may have been opened by this message;
The FMGEC1 message that was the last received before the cabin pressure warning.

The BEA concludes that the type 1 and 2 messages taken together show that there had been unreliable airspeed measurements and their consequences.

That is it. Not a whole lot more than we knew in mid-June, but some of it more firmly established, especially the interpretation of the weather and the integrity of the airframe.

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