@BOAC - there's been loads. There's a good general thread here (the initial question is prosaic, but the subsequent discussion is very interesting):
http://www.pprune.org/tech-log/452973-fpv-fpv-cage.html
A brief summary by HN39 here:
http://www.pprune.org/tech-log/45283...ml#post6484413
But the short summary is that the Honeywell systems used by Airbus and Embraer use baro data to calculate the FPV - if air data is unreliable then FPV is unavailable.