CABIN VERTICAL-SPEED ADVISORY − an Hypothesis
Quote from takata:
“I don't think they could have experienced any electrical failures before both engine possibly stopped, and if they stopped, it was after this last ACAR was sent, not before. Any relight would trigger engines/electrical fault related ACARS in priority over any cockpit advisory. Consequently, it seems hard to find any trace of such flameout during the sequence 02.10-02.14.”
Am unable to research this myself, due to the limitations of my current dial-up internet connection, but I accept that analysis.
So, returning to the cabin V/S advisory, we seem to be left with two possibilities:
(1) spurious cabin V/S signal for 5 secs or more, due perhaps to electrical transient;
(2) cabin V/S exceeding 1800 ft/min, for 5 seconds or more.
Looking at (2), and assuming that any CPC failure would trigger an ECAM warning of higher priority than this advisory, the most likely cause would be a cabin differential pressure approaching zero, in an aircraft descent of over 1800ft/min. − as HazelNuts39 proposed here on May08/11:35z. For the descending aircraft to “catch up with the cabin” − in the absence of failure of packs, bleeds or CPCs − it would be passing an altitude of about 6000ft.
So I offer the following hypothesis, based on HazelNuts39’s original idea, to be challenged:
02:10:14z
Last-known Position, apparently (?) cruising at FL350, cabin altitude ~6000ft.
02:11z
Control has been lost, for reasons unclear in the absence of the DFDR and CVR. Aircraft now descending at variable (high) rate.
With many hours to go before planned TOD, and no change of altitude selected on the FCU or FMGC, the duty CPC maintains current cabin altitude.
02:14z
Aircraft passes 6000ft in the descent, taking the cabin altitude with it (the Negative Relief Valve has opened). Cabin V/S increases rapidly through (minus) 1800ft/min.
02:14:26z
ACARS transmits Cabin V/S advisory, already shown on cockpit ECAM display.
02:15:14z
“Planned” ACARS message not transmitted, due to absence of AC power.
The latter could be due to impact with the surface, double engine-failure (extreme hail?), or AC electrical-failure (extreme lightning-strike).
Chris