V/S geometric or pressure?

[HazelNuts39]

I'm wondering if the vertical speed output from the ADIRU, displayed on the PFD, and recorded on the DFDR represents the rate of change of geometric height or (as a barometric variometer would) of pressure altitude. I'd be grateful to anyone who can enlighten me.

[jtr]

In laymans terms (for I am a layman) the VSI indicated is an indication of barometric/pressure change which is "smoothed" by inputs from the IRU part of the ADIRU to make the I part in IVSI.

On a gusty day when you first align the ADIRUs have a look at the VSI jump around for a few minutes until the point that the ADIRUs settle down and take over the "smoothing"

All just based on observation and a limited tech knowledge.

[rudderrudderrat]

Hi HazelNuts39,

From FCOM A320:

" three identical ADIRU's (Air Data and Inertial Reference Units).
Each ADIRU is divided in two parts, either of which can work separately in case of failure in the other :
the ADR part (Air Data Reference) which supplies barometric altitude, airspeed, mach, angle of attack, temperature and overspeed warnings.

the IR part (Inertial Reference) which supplies attitude, flight path vector, track, heading, accelerations, angular rates, ground speed and aircraft position.

The displayed vertical speed information is normally based on both inertial and barometric data. If inertial data is not available, it is automatically replaced by barometric information. In this case, the window around the numerical value becomes amber."

It sounds like VS is initially sensed inertially, then corrected over the long term by barometric update.

[OPEN DES]

And is there any barometric input to the Flight Path Vector? In an orange QRH not too long ago there were cold temperature corrections for selected FPA vs. published FPA. Not to be found anywhere in FCOM and now also removed from the QRH.

??

[rudderrudderrat]

Hi OPEN DES,

FCTM AO-034 P 13/20:
"If the altitude information is unreliable, the FPV and V/S are also affected. In this case, the GPS altitude, if available, is the only means to confirm whether the aircraft is maintaining level flight. When the altitude information is reliable, use the FPV."

The FPV is inertial initially, but then modified in the longer term using Alt information.

[OPEN DES]

Ok thanks. That explains the correction table we used to have.

[HazelNuts39]

Hi guys,
Thanks for your responses.
And is there any barometric input to the Flight Path Vector? Yes, according to BEA's Interim Report no.2 on AF447, 1.6.11.2: The FPV is elaborated in the IR part of the ADIRU which, for this purpose, uses inertial parameters and also an anemometric parameter: the barometric vertical speed.

So I take it that the PFD display of V/S is barometric, and needs to be corrected for temperature difference from ISA to obtain geometric V/S.