Air France A330 accident

[TripleBravo]

Concerning one of the last ACARS messages,

WN0906010214 213100206ADVISORY

Here's an excerpt of the A332 AMM about the Cabin Pressure System, I shortened some paragraphs:

B. Cabin Pressure Controller

(1) [...]

(2) Internal Pressure Sensor

The cabin pressure sensor within the pressure controller 311HL (312HL) is of a vibrating cylinder type. The principle is, that a physical body vibrates at its natural frequency with high stability under constant environmental conditions. The frequency of oscillation of this system is a function of materials and shape of the cylinder. The frequency depends on the environment, specifically temperature and pressure surrounding the cylinder. Therefore, the frequency can be used as a reference value of the pressure.

(3) Warnings

The cabin pressure controllers send the subsequent warnings to the System Data Acquisition Concentrators (SDACs) or the Flight Warning Computers (FWCs):
- excessive cabin altitude (EXCESS CAB ALT),
- outflow valves not open (FWD OFV NOT OPEN, AFT OFV NOT OPEN),
- low delta p (LO DIFF PR),
- safety valves open (SAFETY VALVE OPEN),
- system fault (SYS1 FAULT, SYS2 FAULT or SYS 1+2 FAULT),
- fault of the landing-field elevation selector (LDG ELEV FAULT).

(a) Excessive Cabin Altitude

If the pressure in the fuselage is less than the atmospheric pressure between 9550 ft. (2910.78 m) and 14350 ft. (4373.79 m) (depending on the airport altitude):
- the automatic circuit of the cabin pressure controllers 311HL and 312HL sends a signal to the SDACs and FWCs,
- the manual backup circuit of the cabin pressure controller 311HL also sends a signal to the SDACs and from there to the flight warning computers.

(b) Outflow Valves Not Open

[...]

(c) Low Delta P/High Descent

This shows that the aircraft will have a negative differential pressure if the descent rate is continued and that a negative pressure relief will occur. The signal is sent from the automatic part of the cabin pressure controller 311HL or 312HL 1.5 minutes before the external pressure is the same as the pressure in the fuselage.

(d) Safety Valves Open

This warning indicates that at least one of the safety valves is open. Therefore the differential pressure between cabin and atmosphere is to high and can cause damage to the aircraft structure. The cracking point of the valve (and thus the maximum differential pressure) is at approximately 610 mbar (8.84 psi) .

(e) System Fault

If there is a general fault in the CPC system, the respective warning (SYS1 FAULT, SYS2 FAULT or SYS 1+2 FAULT) comes on the ECAM.

(f) Set-LFES

[...]

The ATA2131 advisory message is not specified in detail what the problem really was. This creates some possibilities, that are (or are not) discussed up to now:

• excessive altitude of cabin pressure (lead to speculation about fuselage break up)
• excessive aircraft descent rate taking over cabin pressure (lead to speculation about deep dive)
• safety valves open (not yet discussed)
• system component fault (not yet discussed)

BUT: The measurement method to the second point, excessive aircraft descent rate, is to calculate the difference of the measured values of

• cabin pressure – measured by Cabin Pressure Sensor located in the e-bay below the cockpit
• static outside pressure – measured by the Static Ports and gathered by the ADIRUs

=> If the Air Data part of the ADIRUs is delivering erroneous data, the delta-p warning can be triggered without any physical facts behind.

=> No hard evidence of mid air break-up
=> No hard evidence of steep dive