Originally Posted by
Fursty Ferret
The problem with this is that all you know is that at that particular moment in time you have an altimeter that agrees with GPS altitude. Air data problems can be quite insidious in that a faulty system can provide seemingly accurate data at one moment in time, at a specific speed and altitude. This is why Airbus recommends that you switch off all three and rely on the BUSS if fitted.
Exactly, since it is completely probable, that while the data is matching at the instant, you never truly can trust it, especially under IMC/Night conditions. Essentially giving you a VERY false comfort.