The AF 447 cabin altitude change

09/08/2009 — Leave a comment

Authors Note. Below is my original post on the potential causes of the AF 447 cabin altitude advisory, I concluded that there were a number of potential causes one of which could be an erroneous altitude input from the ADIRU. 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.

The last ACARS transmision received from AF 447 was the ECAM advisory that the cabin altitude (pressure) variation had exceeded 1,800 ft/min for greater than 5 seconds. While some commentators have taken this message to indicate that the aircraft had suffered a catastrophic structural failure, all we really know is that at that point there was a rapid change in reported cabin altitude. Given the strong indications of unreliable air data from other on-board systems, perhaps it’s worthwhile having a look for other potential causes of such rapid cabin pressure changes.

The Cabin Pressure Control (CPC) system

The Airbus cabin pressure control system controls the internal cabin altitude through-out the flight.  The system controls cabin air pressure to as close to the designated cabin altitude while not exceeding a maximum delta-p between cabin altitude and aircraft altitude (1). As an aircraft changes altitude the CPC system will vary the discharge of air from the aircraft to maintain a cabin altitude that meets these constraints. Finally the system also maintains the altitude rate of change within the comfort range of passengers. In normal operation, cabin altitude and rate of change are automatically controlled from FMGC flight plan data (2). The CPC system takes as a dynamic input actual aircraft altitude to control the cabin pressurisation de-pressurisation sequence. Aircraft altitude is derived from the ADIRU units.

Could a failure of the CPC system cause rapid pressurise changes?

In a word, yes (3). During the Korean Air Flight 691 de-pressurisation (caused by a faulty outflow valve) the cabin vertical speed reached +3,000 feet per minute (ASC 2007) so a CPC system failure is entirely capable of generating the in cabin conditions that would have caused an ECAM advisory as experienced on AF 447. As the CPC system includes a control component it is also possible that a failure of control rather than controlled hardware could cause a rapid cabin decompression or pressurisation event. In the case of the AF 447 accident the question logically arises as to whether the input of ADIRU generated erroneous altitude into the CPC systems control unit could also result in a command to open or close the valves and a rapid cabin pressure change.

Possible next steps

To answer this question would (as is also the case for the AF 447 TCAS failure) require an examination of the system’s control logic, including its behaviour in response to unexpected or out of specification values. It will be interesting to see whether the BEA investigate this issue further in the final AF 447 accident report.

This post is part of the Airbus aircraft family and system safety thread.

Notes

(1)  Intended to minimise fatigue effects on the airframe.

(2)  If the aircraft has suffered a dual FMGC failure the system reverts to a pre-programmed cabin altitude – aircraft altitude schedule.

(3) Neither can we exclude the possibility of a rapid increase in cabin pressure due to the CPC system closing the outflow valves. To eliminate this possibility would require an analysis of the pressurisation system for the Airbus A330.

References

Aviation Safety Council (ASC), Investigation Report, Korean Air Flight 691, AIRBUS A300-B4-622R, HL-7297, Loss Cabin Pressure at FL320, 30, NM South of Waypoint SALMI, On Route B-576, Taipei FIR, On May 11, 2006, ASC-AOR-07-10-001, 2007.

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