Originally Posted by NTSB Report

3.2 Probable Cause
The National Transportation Safety Board determines that the probable cause of
this accident was a loss of airplane pitch control resulting from the in-flight failure of the
horizontal stabilizer trim system jackscrew assembly's acme nut threads. The thread
failure was caused by excessive wear resulting from Alaska Airlines' insufficient
lubrication of the jackscrew assembly.

Contributing to the accident were Alaska Airlines' extended lubrication interval
and the Federal Aviation Administration's (FAA) approval of that extension, which
increased the likelihood that a missed or inadequate lubrication would result in excessive
wear of the acme nut threads, and Alaska Airlines' extended end play check interval and
the FAA's approval of that extension, which allowed the excessive wear of the acme nut
threads to progress to failure without the opportunity for detection. Also contributing to
the accident was the absence on the McDonnell Douglas MD-80 of a fail-safe mechanism
to prevent the catastrophic effects of total acme nut thread loss.