HN39;
Just digging around while we anticipate further once the Ile de Sein arrives and begins work.
The following is some more information about the cabin pressure and the advisory message. Just trying to get information out there to think with:
The negative pressure relief valve (located between STA 24 & STA 25 on the port side of the fuselage), is rectangular, has 4 spring rods which force it closed until a pressure differential opens it. From what I can determine, the test equipment "vacuum" pressure for a test of the relief system is about 0.3psi. There is also a physical test...push the valve inwards for freedom of motion and re-seating after returning to the closed position. I don't know the valve's dimensions.
We know that the ADVISORY CABIN VERTICAL SPEED message was sent at 0214+.
Regarding system parameters monitoring and the ADVISORY function....Some system parameters are monitored throughout the whole flight and automatically displayed on the relevant system page when their value drifts out of normal range but well before the warning level is reached.
This is performed by the ADVISORY function of the EIS. In this case, the related key system light comes on on the ECAM control panel. The relevant page is displayed as long as the advisory is present and no other system page is called (manually or automatically).
On the right part of the lower ECAM (STS section) are displayed (in priority order):
- inoperative systems.
- maintenance items (class 2 maintenance status).
NOTE :
The class 2 maintenance statuses are not displayed in flight but they are sent in real time to the CMC and they can be transmitted to ground via the ACARS when installed.
I don't think the Advisory message is a Class 2 maintenance message - as described, an advisory is an indication of a parameter exceedence and is not advice of a failure which requires maintenance action at the destination station. But the fact that Class 2 messages are sent in real time to the Central Monitoring Computer is interesting.
The last message received was the Cabin Advisory message. We know that such an advisory indicates a system parameter exceedence, in this case, >1800fpm or <1800fpm cabin vertical rate. (The size of the negative pressure relief valve opening could be used to determine flow rates and therefore rates of descent of the cabin but the exercise may not be worth the trouble).
As discussed and posited, we have concluded that cabin pressurization would become negative below the cabin altitude, whatever that was, and we have assumed around 7500 to 8000 ft barometric altitude, perhaps slightly lower depending upon data available to the CPC, (ie, flight phase will not have changed from CRUISE to DESCENT because the FMGEC and therefore the CPC will not have been in the descent mode and may not have begun depressurizing the cabin as per the usual schedule).
Then we have this information:
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 1.5 minutes before the external pressure is the same as the pressure in the fuselage.
and,
Low Differential Pressure
If the cabin pressure controllers detect that a negative differential pressure situation may occur in 1.5 minutes: - a single chime is heard, - the amber MASTER CAUT lights come on, - on the EWD of the EIS, CAB PR LO DIFF PR and the necessary steps are shown.
Pondering the question...depending upon altitude, descent rate and the incoming data to the CPC, we might have expected this caution to trigger if the conditions which triggered the CAB VERT SPD Advisory were as we are assuming, (essentially, high aircraft descent rate "caught the cabin", as we say) a high descent rate passing through 8000', approximately. But the
CAB PR LO DIFF
PR, which, (now theorizing), according to the above description would have occurred either before (a minute and a half to be exact) or simultaneously with the Advisory, is not in the ACARS listing. We don't know if the message depends on the flight phase, (some warnings and cautions are inhibited in various flight modes).
The risk is always in over-thinking something but "obvious" is only available afterwards! These are some random thoughts which may or may not be useful in others' thinking.