Dingo & co.,
I'm thinking out loud somewhat....
Regardless of whether or not the flightdeck crew of 901 were or were not told the track went over Erebus, they were clearly operating this particular flight on the assumption that it didn't.
I would expect the difference in tracks to be ~5 degrees as your calculation suggests.
An error in track of ~5 degrees is arguably one that you might notice. I was mucking around today myself with my NAV display on the way home trying to figure out whether I thought I'd appreciate an error of such a magnitude myself. I fly a dual IRS, FMC equipped EFIS a/c which we use predominantly in a track up mode.
I totally agree that the heading is almost irrelevant when navigating in such a fashion but from a chart plotting point of view, assuming the wind vector was relatively constant along the track, you'd expect a particular heading to attain that track.
I've no idea whether the crew had the predicted met when they plotted their route, but nevertheless, if they did, they'd no doubt have calculated a heading to achieve that. Of course, you could ascribe any error in that heading to a different wind vector, so arguably this isn't necessarily going to alert you to an error in the flightplan.
However, if the aircraft is tracking 192 instead of 187?
I'll have to get to the bottom of that flightplan....
It is my understanding that the aircraft passed over Cape Hallet in clear air, so after a 2000 nm passage across the southern ocean, it was clear to the crew that the IRS's weren't drifting substantially.
To suggest that within the next 300 miles they suddenly started drifting to an extent such that you'd be worried about undertaking a VMC let down in an area the size of McMurdo Sound without a radio/radar fix seems a little unreasonable to me.
(And the idea that because they couldn't establish VHF comms with Ice Tower should have alerted them to the fact that there was rock between them and the antenna is completely unreasonable in my opinion. Even in Europe establishing VHF comms, where ground stations are as common as churches, can be a pain...and not because there is rock in the way.)
As for their responsibility for terrain seperation, they flew into the mountain in NAV mode.
These guys were ultimately wrong about their position but they weren't unsure about it...if thats not a contradiction. Seems to me, these guys saw that nav track as their security in unfamiliar terrain, hence their diligence in re-engaging the mode.
I'm with Brian Abraham on this. I just can't see these guys did anything unreasonable in the circumstances. They weren't randomly manoeuvering around the sky in HDG SLT. Even if they were unsure of their position whilst doing this, at the point they re-enagaged NAV mode, that ambiguity in their mind surely vanished?
But, of course, the nav track was wrong.
My previous post asked the question about what we, as pilots, are entitled to assume about our aircraft before we go flying?
A sensible flightplan has to be one of them.