Cabin vertical speed may offer clues to the last moments of AF 447
The latest revision of the BEA report on the AF 447 accident omits any discussion of the last ACARS message received, the cabin vertical speed advisory. This is unfortunate because from this advisory we can infer at least approximations of the final part of AF 447’s flight profile.
This post is part of the Airbus aircraft family and system safety thread.
The cabin pressurisation control logic
During changes in altitude the Airbus Cabin Pressure Controller (CPC) commands two Outflow Valves (OFV) to open. The system is designed such that in rapid descents it will maintain a maximum rate of change of cabin pressure at 750 ft/min. In very rapid descents from high altitude (descent velocity > 750 ft/min) this means that the positive difference between cabin pressure and external pressure will reduce to zero and then become a negative pressure differential.
To prevent this hazardous negative pressure differential occuring an independent pneumatic safety valve will open at 1 psi pressure differential to increase inflow and the rate of equalisation. This sudden change in inflow will then result in a cabin vertical speed advisory if the cabin pressure change then exceeds 1800 ft/min for 5 seconds (1). What this advisory event gives us is an ability to estimate the altitude at which the last ACARS message was initiated and from that infer AF 447’s velocity profile prior to impact.
The basic data is as follows (2):
- cabin maximum descent rate – 750 ft/min (FCOM)
- cabin cruise altitude – 8000 ft (FCOM)
- Flight level – FL 350 (35,000 ft) (BEA Report)
- safety valve diff pressure – 1 psi (2.03625437 inHg) (FCOM)
- ACARS 1st msg ‘AUTO FLT AP OFF’ time – 2:10:10 (BEA Report)
- ACARS last msg ‘CABIN VERTICAL SPEED’- 2:14:26 (BEA Report)
- Cabin press advisory delay – 5 seconds (BEA report)
From the BEA report we know that ECAM messages (such as AUTO FLT AP OFF) are transmitted in real time as soon as they are acquired while flag or advisory messages (such as cabin vertical speed) are transmitted as soon as they have been confirmed. We can therefore reasonably assume minimal time skewing of these two events due to ACARS transmission processing delays.
From the ACARS transmission times times we can calculate a time to actuation (Tact) of the safety valve of:
- Tact = 2:14:26 – 2:10:10 – 0:0:05 = 0:4:11.
Assuming that the maximum cabin altitude rate is achieved across this period, the cabin altitude at actuation (hcbn) of the safety valve is:
- hcbn = 8000 – 750 ft/min X 251/60 = 8000 ft – 3137.5 ft = 4862.5 ft
Now as a 2.03625437 inHg pressure change gives approximately a 2036 ft altitude change, and as the external pressure is 1 psi less than the cabin pressure (3), the external or aircraft altitude (hacft) is simply:
- hacft ≈ 4862.5 + 2036 ft = 6898.5 ft
Given that we know the time between first transmission and the time of safety valve actuation (251 seconds) we can determine that for an assumed a constant descent profile a velocity rate (4) of 6,718 ft/min would result in the same altitude loss (5).
If we then take that constant descent rate and determine time to impact (timp) we get:
- timp ≈ 6898.5 ft / 6,718 ft/min = 62 seconds.
As the last cabin speed advisory was generated 5 seconds after the safety valve actuation this would indicate that, again assuming a continued near constant descent rate of 6,718 ft/min, the aircraft would have impacted the ocean approximately 57 seconds after the last ACARS transmission.
The above analysis is of course a first order one that contains some significant assumptions. However it does provide a coherent explanation as to why the cabin vertical speed advisory was triggered and indicates that the impact of AF 447 with the sea likely occured seconds after transmission of the last ACAR message.
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.
Airbus, A330 Simulator Flight Crew Operating Manual, Revision 15, accessed from http://www.smartcockpit.com/plane/airbus/A330/, 23 September 2009.
1. As occured at 2:14:26 during the AF 447 flight.
2. Derived from the BEA interim report and Airbus simulator FCOM manual for the Air conditioning/pressure/ventilation system.
3. The CPC is designed to prevent the cabin/external pressure differential going negative leading to crushing of the cabin. Thus the cabin safety valve opens at at a 1 psi differential to ensure this negative differential pressure does not occur.
4. The aircraft must over the period of descent maintain a vertical speed profile that when integrated over the time equates to the loss in altitude, assuming a constant rate of descent is an approximation.
5. In comparison during the KE 691 A300 incident the aircraft maintained an average descent rate of greater than 6,000 fpm.