Why can't I use a METAR for MDA?
I notice that the latest amendment to AIP precludes me from using METAR QNH EVER for instrument approaches (it isn't listed in ENR 1.5 5.3.1, and it's not an approved source).
Could one of the boffins please explain the rationale behind this change?
Given that the forecast QNH (whether it be Area or TAF) can be in error by some millibars compared to a Metar, this change will have significant negative ramifications on the actual height of the aircraft at the MDA, with the aircraft possibly being much higher than it would've been had a more accurate QNH (such as a Metar) been used (I am not talking about reducing the MDA by the 100ft either).
Obviously, a Metar QNH (even given that it cannot be used for the 100ft reduction) is a far more accurate QNH than either of the Forecast QNHs, and we should be allowed to use it for Instrument Approaches. Why can't we?