The AF 447 cabin altitude change – Pt II

22/09/2009 — Leave a comment

Authors Note. Below is my follow-up post on the potential causes of the AF 447 cabin altitude advisory. What I didn’t consider was that the altitude advisory could have been triggered by correct operation of the cabin pressure control system, see  The AF 447 Cabin Vertical Speed Advisory and Pt II for more on this.   

In an earlier post I speculated about the possible link between invalid air data and the cabin vertical speed advisory recorded by AF 447’s ECAM system moments before contact was lost.

Having had a look at a copy of the A330 Simulator Flight Crew Operating Manual (FCOM)  it appears to me that invalid air data may well have been the cause. According to the FCOM (1), should altitude inputs from the FMGEC not be available the Cabin Pressurisation Control System (CPCS) will default to use of inputs from the Captains barometric altitude reference, ADR 1.

We know from the received ECAM messages that 6 seconds prior to the cabin vertical speed advisory an intermittent fault was recorded by the FMGEC 1. If as a result of the FMGEC being unable to provide valid altitude data the CPCS defaulted to ADR  1 as the source of barometric altitude any error in ADR 1 would then directly affect CPCS processing.

We also know that a fault message was received by ECAM indicating that FCPC 2 no longer considered as valid the information that was delivered to it by ADR 1. The CPCS maximum differential pressure limiter function automatically maintains the differential between cabin and external pressure at a constant value.

If an invalid (low) pressure was provided by ADR 1 that caused the pressure differential to increase to 8.42 psi this would trigger the CPCS to automatically open the outflow valves causing cabin velocity to increase, possibly to the point that the cabin vertical speed advisory was triggered

What’s interesting about this hypothesis is that the loss of the FMGEC may also have been caused by the same invalid air data that subsequently caused the CPCS to open the cabin outflow valves in error (2) (3).

References

Airbus, A330 Simulator Flight Crew Operating Manual, Revision 16, accessed from http://www.smartcockpit.com/plane/airbus/A330/, 23 September 2009.

Notes

1.  A330 Simulator FCOM Section 1.21.20 Seq 001, Rev 16, page 4.

2.  As with the loss of TCAS capability this is an easily testable and refutable hypothesis.

3.  One should however be a little cautious about drawing conclusions from operating manuals as often they do not reflect the precise behaviour of the system.

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