Jack screw

As previously mentioned, the horizontal stabilizer's jackscrew assembly includes an acme screw and nut, both of which have two threads. After the accident, severely worn and sheared remnants of the acme nut threads were recovered wrapped around the acme screw. The condition of the recovered acme nut thread remnants indicated that approximately 90 percent of the thread thickness had worn away before the remainder of the threads sheared off. In comparison, a jackscrew assembly with the maximum amount of wear permitted in service, as indicated by an end play measurement of 0.040 inch (that is, 0.030 to 0.037 inch of wear), would only have about 22 percent of the thread thickness worn away.