Brian,
I'm not a helicopter man, fixed wing thru and thru and the Chinook incident is a long time ago now so I may have forgotton some of the bits and pieces but........
Unless an aircraft is on a published approach to an airfield then maps and Nav aids are not to be used for terrain avoidance. That little task is the job of the Mk 1 eyeball or, if your Group Orders permit, your own on board Radar. Indeed ATC will not descend you below sector SALT using their area Radar. JSP318 used to be quite specific! I think the thread of MAC being U/S or intermittent is a Red herring, they probably wouldn't have relied on it anyway unless they were already in the **** and couldn't climb, or land, or hover, or turn around in the hover - get the drift??
BTW, was the ac equipped with the Cossor GPS? if so what software state was it? The older versions vere prone to 'running away' at a vast rate of knots - but even so - not to be used for terrain avoidance!!!
Cheers
VR