Thanks for the update on Mahon. I had forgotten that gem. The ice cannot be hundreds of feet thick near the pointy part of Erebus; else how could it be steaming? Even if the weather radar signal penetrated the ice, rather than bouncing off it, the signal would bounce off the rocks below the ice. In fact, that's how they measure ice thickness. Also, wasn't there some visible rock on Beaufort Island?
Ice without a wet surface is indeed a poor radar reflector, but it is not radar absorbent, so it will pass the signal in both directions.
Granted, the radio altimeters are C band, 4350 MHz, while the RDR-1F is X-band, 9300 MHz, but the radio altimeters saw the 300 foot ice cliff at the water's edge, and tripped the GPWS.
BTW, weren't they flying at 700', and hit at the 1500' level?
There was no good reason for not employing the weather radar for navigation at least some time in the last half hour.
GB