Originally Posted by
bearymore
The fault is in the radio altimeter which is only used below 2,500 ft. It is used by the autopilot for approach and landing and is used for the ground proximity warning system. Had the pilot not disengaged the autopilot, it would have landed the plane 1,950 ft. above the ground based on the erroneous reading. As far as inflight navigation is concerned, that is based on the normal pressure altimeter so the malfunction would have no effect.
Above 18,000 feet, altimeters reset to standard 29.96 in hg. Below 18,000 feet, the altimeters are set to the barimetric pressure at their destination airports barometric pressure as provided by ATIS recording (Airport Terminal Information System).