Microburst, is that an airbus fcom advise?
Well, I'm not
Microburst, but I may be able to help
A320 series QRH: Page 2.16
UNRELIABLE SPEED INDICATION/ADR CHECK PROC
...
- If remaining altitude indication is unreliable :
- Do not use FPV and/or V/S, which are affected.
Although the FCOM states that the FPV is an output of the IRS:
Originally Posted by FCOM 1.34.10.1000.4
The system includes :
- 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.
It appears that the vertical element is barometrically aided in order to refine the path (i.e. the IRS vertical path is subject to drift, just as the horizontal navigation is, and so isn't accurate enough over time to drive a "pure" IRS-driven vector.)