Thank you LeandroSecundo for having drawn our attention on this article. At the bottom it is indicated that a PDF conversion is available for download.
Eurocockpit - Archives
I have translated 2 parts, one about a "NAV TCAS FAULT" alarm message that is "not fully explained" by the BEA. This might explained to the BEA how to explain what might be the reason and, two, a rather short description of the final "VERTICAL ADVISORY CABIN SPEED" ACARS warning message sent at 0214 by the plane.
1/BEA interim report page 51 (English)
NAV TCAS FAULT (2 h 10)
- - - Flag on PFD and ND
Meaning: This message indicates that the TCAS is inoperative. At this stage of the
investigation, this message has not been fully explained.
NAV TCAS FAULT RC proposition :
NAV TCAS FAULT: the Traffic Collision Avoidance System has failed because of the dysfunction of the ADR, due to the failure of Pitot, involves in the loss of this system by a failure of the transponder 1 (if selected - because of the ADR 1 failure) or transponder 2 (if selected - because of the ADR 2 failure). The loss of the mode C leads to the loss of the TCAS function.
2/BEA interim report page 52 (English)
ADVISORY CABIN VERTICAL SPEED (2 h 14)
Symptoms: Flashing of the cabin vertical speed indicator on the SD’s PRESS page.
Meaning: This message indicates a cabin altitude variation greater, as an absolute value, than 1,800 ft/min for five seconds
RC analysis :
2:14 UTC
No more failure message will be sent by the plane. Only two alarm messages are transmitted:
MAINTENANCE STATUS ADR2: the ADR2 has its degraded operation, which is only a confirmation of the situation resulting from the initial Pitot failure.
VERTICAL ADVISORY CABIN SPEED: the rate of variation of the pressurization of the cabin is abnormal. This message indicates a variation of the altitude of the cabin higher than 1800 feet/minute.
In a simplified way, the pressurization consists in “inflating” the cabin of a plane by admitting air from outside, heated by heat exchange with certain systems of the plane, and controlling the leak flow by means of a valve located at the back of the aircrafr. By doing this, the cabin is known as “pressurized” and the air that one breathes there contains sufficient molecules of oxygen to avoid the hypoxia which one would suffer with the altitude of cruising of the airliners. Fictitious altitude inside the cabin is thus lower than that of the plane. At an altitude of 35.000 ft (feet), the pressurization makes it possible to maintain the inside altitude of the cabin at around 8.000 ft. The parameter of evolution of this fictitious altitude of cabin is measured in feet/minutes.
Contrary to what could be thought, the message of alarm is not revealing of an explosive depressurisation since it is not an alarm “excess cab alt" that would have meant an excessive cabin altitude (higher than 9550 ft). It is actually an alarm message, materialized in the cockpit by the flickering of the Cabin V/S parameter on the pressurization screen, indicating that the altitude of the cabin is in evolution according to a variation considered to be excessive.
This variation can occur while the cabin is going up or down at more than 1800 ft/mn. Several cases of figures can thus arise:
-> The failure of the 3 ADR could involve the loss of the systems controllers of pressurization and let this one evolve abnormally.
-> The possible passage in overspeed of the plane, at the time of a loss of control, could have deteriorated and even tear off certain structural elements, creating a leak of pressurization then explaining the fast variation of altitude cabin, the latter being depressurized quickly before the possible rupture of the plane and the end of the transmissions following the disintegration of the aircraft.
-> The descent of the aircraft involves a descent of the altitude of the cabin ordered by the main controller of pressurization (the other being functionally on standby). It is what occurs with each descent so that at the opening of the door, on the ground, the outside pressure is the same than the one inside the cabin. There is thus a regulation of the descent of the cabin altitude as the plane goes down. If the plane would start a very fast descent, the controller of pressurization would try to reduce very quickly the altitude of the cabin. This variation is by design limited to 750 ft/mn, which cannot thus generate an alarm corresponding to a value algebraically higher than 1800 ft/mn. If the plane was not in descent but unfortunately in a fall, it would then join the altitude of the cabin before the controller of pressurization could sufficiently reduce the cabin. In this case, the pressurization would be reversed: the pressure outside the plane would become higher than that of the cabin. To avoid the implosion, “safety valves” and “negative relief valve” open in the event of negative differential pressure (0,25 psi). In this case, the rate of variation of the cabin follows the fall of the plane to minimize the difference in pressure and the rate can thus be much higher than 1800 ft/mn.
It is what the latter message transmitted by the plane could mean.
-> The loss of ADR SIGNALS, by rejection of the ADR following a partial or total internal failure, could have also involved the appearance of this alarm.