AF 447 Search to resume

[Chris Scott]

Sounds good, mm43. Bon courage à tous. May all our speculations soon face the moment of truth. In the meantime…

Quote from mm43:
“If the APU had been started, would the Cabin Pressure controller have been active? I haven't seen any data on this, and have assumed that should there be no electrical supply from the prime sources that the controller would operate below 8000/7350 feet to equalize the cabin pressure.”

Don’t have an A330 Tech Manual, and no doubt the Cabin Pressure controller employs data from many sources, including the Air Data elements of the ADIRUs. As I think HazelNuts39 has said somewhere above, however, aircraft cabins are not designed to withstand a negative differential pressure, i.e., a lower pressure inside than out. In an unusually rapid descent to sea level from normal cruising altitude, the controller will do its best to keep the cabin altitude below aircraft altitude, by increasing the cabin VS; but may eventually be unsuccessful. In a rapid descent, we call that “catching up with the cabin”, don’t we? That’s why there has to be an inward relief valve, which is purely mechanical.

As for the readings of cabin altitude, I don’t see why even (improbable) icing of the static ports, as postulated by takata, should invalidate cabin altitude (or VS). If I was a passenger on the aeroplane, and was carrying on my lap a good old-fashioned altimeter and similar VSI, their readings would be little different from those used by the Cabin Pressure controller. Cabin pressure is cabin pressure, and is measured somewhere inside the cabin or pressure hull; nothing to do with static ports.

Unless I’m missing something, the readings should be valid, and their interesting implications follow.

Chris