Dave Doe writes:

IIRC, they could have used the GPS for airspeed (IIRC, the airspeed
indicator partly worked (they got an airspeed active out of it!) - and
probably thought it was correct - during flight it was all over the
place IIRC. And the radar altimeter would have been working just fine.
Several navs would have been good, VOR, DME, ILS etc. Shaker of course
too!

The GPS would only give ground speed, not airspeed, but that would be better
than nothing. The RA would only be useful below 2500 feet AGL. But the other
stuff would be working.

Anyone remember some more details?

Aeroperu had a problem with this.