Interesting discussion.
It is necessary to make a distinction between dispatch criteria and operational decisions - the concept is, generally, not well understood. Curiously enough, the principle is used in two elements of the discussion so far.
The MEL deals with dispatch criteria - once airborne, it has no relevance (incidentally, it is the opposite of your statement; if it is not in the MEL, it is needed for dispatch; if it is in the MEL, dispatch might be possible - depending upon the circumstances). Complete operational concepts (such as NVIS) might have a separate set of entries in the MEL.
Most regulations have a visibility 'criteria' for dispatch; once airborne alternative operational (and protective) strategies come into play. As has already been said, estimating visibility in flight is difficult and attempting to relate it to visual flight cueing (using external references) is problematical.
By night there is little correlation between absolute visibility and adequate references for visual flight. The provision of flight control augmentation permits more concentration on the visual aspects of the operation and less on the actual flying task. NVIS does introduce contradictions; available light sources are enhanced but visual accuity and field of view are seriously diminished (although on balance, there is a substantial improvement – particularly in the cruise).
That is why existing guidance addresses: equipment, procedures, training & experience, and recency. It is good to see that other practical elements are being added to SOPs. It might be advantageous, at some stage, to revisit the debate and adjust the ‘universal guidance’ in light of lessons learnt so far.
Jim
Last edited by JimL; 13th Jul 2009 at 18:11.