Ian
So the question is a simple one. Why, with aircraft that are capable of flying RNP LPV approaches to RNP 0.1, are pilots and operators still using NPA that are less safe?
Is there a published procedure for an GPS-RNP approach to 18? If there is not, you cannot do it. Is the company/aircraft authorised and trained for Rnav-Vnav approaches? If not, you cannot do it.
And regards why they may have descended below the nominal glideslope. One possible reason is mistaking/confusing the perspective of the approach, especially if you are not used to doing non-precision approaches.
There have been quite a number of aircraft over the years that have drifted below the glideslope, on a nice visual approach, and not realised it. Especially when making a 'black hole ' approach, where there are no ground lights below you to gain perspective from. There was an incident with a corporate jet, many years ago, on a gin-clear visual night approach to an island runway just inland from the sea in Scotland. The sea was, of course, completely pitch black. And they flew down so low, while still thinking they were on the perfect approach, that they touched down in the sea.
Silver