My thoughts are that there is never (a legal requirement for) any adjustment to be made to an ILS minima regardless of the QNH source used, except maybe when an area QNH is used (AIP ENR 1.5 para 5.3.3).
I'm assuming this because there is never any shading associated with an ILS minima on an Airservices approach chart.
AIP ENR 1.5 para 5.3.2 mentions that those minima which have shading are based on forecast QNH which may be lowered by 100' if accurate QNH is available. Does this imply that those without shading (all ILS approaches) are based on accurate QNH?
I have a feeling that this may be an oversight since ILS installations are typically used at controlled aerodromes with accurate QNH sources. Also, the old check height rule of adding the difference if you're indicating high would keep you safe (even though it was intended to compensate for temperature error not QNH error). What if you went through 200' high under today's "unexplained discrepancy" rule? (See ENR 1.5 para 7.3.1.)
This is a little scary I guess since with a 200' DA, a QNH error (on the high side) of just 7hPa (equivilent to 210') would have you fly straight into the 1000' markers (or thereabouts) if you hadn't got visual first. In this case the OM check height would have been cause for alarm, showing you as about 210' high. I guess despite the new rule, no harm in applying the old one (since it's more limiting).
Why did they change the rule I wonder?
VI
