Tim, thanks. By your silence on the subject, I presume you agree with
my first point: that moving map software using (I think) only GPS
altitude data is apt to mislead pilots into thinking they are below
(or above, depending on QNH on the day) CAS which is defined by flight
levels.

I was short cutting a bit when I said “may”, in “If CAS is defined by
height or altitude, GPS may suffice”. What I was hinting was that at
low levels (e.g. Stansted UK CTA at 1500’), and when QNH and ascents
are close to standard atmosphere, you would be unlikely to be a few
hundred feet wrong.

In principle, yes – FAI requires baro. So does the law in the UK. So
our moving map warning, re height, are undependable.


Chris N.