Originally Posted by
WideScreen
I am not sure, what you want to state with this. Of course, with cold temperature, the effective height is lower as the PA, though the (really) closer you get to the ground, the more the PA display in the aircraft starts matching the factual height, corresponding with the given airport QNH, suffering from the same cold (inversions may give surprising effects, the Theta diagrams tend to be pretty irregular). The vertical flight path to the airport is no longer a straight-in-the-vertical flight-path in GPS/ground terms, though in PA terms, it still could be, though with variable VS in the GPS/ground system. Please correct me, when I get you wrong.
If that wasn't an off-airport encounter, the off-height might have been (less than) 30 ft, 1 mb. Given the persistent rounding to whole digits, in digital calculations, just the reachable accuracy.
I wise thing to have, and it could be a NG-EGPWS, if it does have the synthetically calculated cone, to warn for being too far outside the normal (granted, ground based) approach gradient.
Yep, though, hey, when doing a PA based approach, we aren't aiming with a 3 degrees glideslope from a 10000+ ft PA, as if it would be in the GPS/ground reference system.
Wide, you are quite correct normally, a QNH given at an airport will give a value that should be correct at that altitude for the conditions. which is why this one airport was perplexing, the errors at the approach end of the runway led to a number of odd approaches. Temp correction factors should correct for colder than ISA temperatures for altitudes above the reference datum, which should be the airfield itself. Everywhere else that seems to work quite well, except for one location. That location was very terrain affected on the approach, and was non-radar, so the concerns were related to the initial approach and then the rest up to the final fix. Then came the surprise reports on late finals... Why, not resolved in my time.
