AF 447 cabin vertical speed Pt II

01/02/2010 — Leave a comment

Reading the 2nd BEA interim report’s analysis of ACARS message timing provides us with a further refinement of a calculation of AF 447’s terminal vertical speed (posted here) based on the cabin vertical speed advisory.

The new data

The BEA’s second interim reports discloses that given a “Maintenance Status ADR2” message was received at 2:14, an associated class 2 fault message should have subsequently been received between 2 h 15 min 00 and 2 h 15 min 14, but was not. The BEA identified a range of potential causes for the failure to transmit this message but the most credible is the impact and subsequent destruction of the aircraft between 2 h 14 min 26 and 2 h 15 min 14.

The revised analysis

So, based on a terminal airspeed derived from the cabin pressure advisory message, the time to impact (tti) can be calculated as being 57 seconds after the cabin vertical speed advisory was generated (2:14:21). This would result in a time of impact (toi) of 2:15:23.

But, these tti and toi are based upon the assumption of an average terminal vertical velocity in the last 57 seconds of flight that is the same as the average calculated for the preceding flight phase. As a still flying aircraft would have transmitted the class 2 ADR 2 fault message before impact we can use these two times to revise the time to impact (tti) and correct the terminal velocity (vterm).

Having previously calculated the altitude at which the safety valve opened (6,898.5 ft) and knowing that the valve opened at 2:14:26, 5 seconds before the advisory would have triggered, we can calculate the terminal velocity associated with both ends of the transmission window as follows.

In the case of timp = 2:15:00:

  • tti (2:15:00) = 2:14:26 – 2:15:00 = 0:00:34 seconds.

Giving an average terminal velocity of:

  • vterm (2:15:00) = 6,898.5 ft / 34 sec = 12,173.9 ft / min.

In the case of timp = 2:15:14:

  • tti (2:15:14) = 2:14:26 – 2:15:14 = 0:00:48 seconds.

Giving an average terminal velocity of:

  • vterm (2:15:00) = 6,898.5 ft / 48 sec = 8,623.1 ft / min.

The corrected results

Given the transmission window for the ADR 2 fault message the average terminal velocity of AF 447 is in the range 8,623.1 to 12,173.9 ft / min.

Conclusions

The increase in average vertical velocity from 6,718 ft/min to at a minimum 8,623.1 f/min indicates a sustained vertical acceleration of the aircraft. This terminal vertical acceleration and high terminal vertical speed when combined with the positive attitude and low bank of the aircraft is strongly indicative of the aircraft having departed into a deep stall and possibly a subsequent flat spin.

Further confidence, or refutation, of this analysis could be provided by an analysis of g loadings sustained by recovered components of AF 447 to estimate the impact velocity required to generate such loads.

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

References

BEA, Interim report no. 2, on the accident on 1st June 2009, to the Airbus A330-203, registered F-GZCP, operated by Air France flight AF 447 Rio de Janeiro – Paris, Report Number f-cp090601ae2, November 2009.

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