Re: Alaska Airlines Boeing MD-83 Accident on 31 January 2000 : A McDonnell Douglas Model DC-9-83 (MD-83) airplane crashed on Jan. 31, killing all 88 aboard.
: The FAA has participated in the subsequent accident investigation to determine possible causes of the accident. One area of interest in the investigation has been the jackscrew assembly of the horizontal stabilizer.
: A Safety Board metallurgist has been examining the jackscrew recovered from the accident aircraft. The shreds of metal seen attached to that jackscrew are of the same material as the threads of the gimbel nut that would normally be attached to that jackscrew. That nut has not been recovered. There is no determination at this point when those metal transfers occurred.
: At February 11, 2000 the FAA released an Emergency Airworthiness Directive:
: "Perform a general visual inspection of the lubricating grease on the jackscrew assembly and the area directly below the jackscrew and surrounding areas for the presence of metal shavings and flakes."
: "Perform a general visual inspection of the jackscrew assembly to detect the presence of corrosion, pitting, or distress. .... If any corrosion, pitting, or distress is detected, prior to further flight, replace the jackscrew assembly with a new or serviceable assembly, in accordance with the applicable alert service bulletin."
: I was told that this visual inspection isn't a very easy procedure, nor a completely reliable.
: Does anybody has more information or any opinion.
: Rolf Diederichs
*The threads of the jackscrew, when installed on the plane, are greased to facilitate smoothe operation. In manufacture of the MD-80/83, after the installation is visually inspected and approved (the mechanics involved are meticulous about their work), there is further work being done in this area and above the jackscrew assembly. Some of it involves drilling operations, and where drilling is carried out, there are always metal shavings produced. If all the shavings are not cleaned out with pressurized air or with vacuums, then they will obviously 'drift' downward over whatever is below, in this instance the greased jackscrew assembly. This alarms me.
Inspections are done with mirror and flashlight only in some of the more difficult-to-see areas. Is a more careful inspection not being carried out because it is so difficult to have to remove the 'banana' fairing in order to see completely the jscrew assembly.
Could these shavings affect the jackscrew operation...eventually? Your opinion?
The photographs of the recovered affected parts show some very long shreds clinging to the screw. It could be from wear, defective nut, tightness, or corrosion (possible, but maybe not too likely). What bothers me the most is that it might have been caused by metal debris caught on greased threads causing grinding and shredding. I believe in normal maintenance the jackscrew is re-greased - at least, it seems to me that should be something that needs to be done periodically. Couldn't metal flakes and shavings be detected at that time?
Has it been concluded that the metal shreds prevented the normal operation of the horizontal stabilizer, keeping the pilot from recovering from that dive? Were the shreds pre or post?
Since the gimbal nut has been recovered, there should be some more findings soon, if in fact we don't have them already.