Originally Posted by
Old Boeing Driver
@ FDR Thanks for your response.
Here is the "proposed AD
" ....in 2000"On January 31, 2000, there was an accident involving a McDonnell Douglas Model DC-9-83 (MD-83) airplane. The National Transportation Safety Board (NTSB) determined that the probable cause of this accident was a loss of airplane pitch control resulting from the in-flight failure of the acme nut threads of the jackscrew assembly of the horizontal stabilizer trim system.
The NTSB concluded that the thread failure was caused by excessive wear, resulting from insufficient lubrication of the jackscrew assembly.Start Printed Page 58621The drive mechanism of the horizontal stabilizer on McDonnell Douglas Model DC-9-83 (MD-83) airplanes has a jackscrew assembly with an acme screw.
The drive mechanism of the horizontal stabilizer on Boeing Model 767 airplanes uses a ballscrew. Acme screws and ballscrews have some differences in design, but perform similar functions and have the same airplane level effect following failure. The manufacturer's safety analysis of the 767 drive mechanism found no safety problems with the configuration of the drive mechanism, but showed that changes to the maintenance procedures and maintenance intervals are required to keep the drive mechanism properly maintained and operating as designed.
We have received a report indicating that the ballscrew in the drive mechanism of the horizontal stabilizer on a Boeing Model 757 series airplane showed extensive corrosion, which could lead to excessive wear. The ballscrew on Boeing Model 757 airplanes is similar to that on Boeing Model 767 airplanes that are the subject of this proposed AD. Therefore, both of these airplane models could have the same unsafe condition.
We are considering separate action for the Boeing Model 757 series airplanes and other similar Boeing airplanes.Extensive corrosion of the ballscrew in the drive mechanism of the horizontal stabilizer, if not corrected, could cause an undetected failure of the primary load path for the ballscrew and subsequent wear and failure of the secondary load path, which could lead to loss of control of the horizontal stabilizer and consequent loss of control of the airplane."
I am a commercial pilot and A&P and was an inspector at a major repair station with a B.S. degree in Aeronautical Science. We did freighter conversions on Boeing 727s that had been pulled out of storage. Many of the 727s had pitting corrosion on the jack screws and had to be replaced. It would be interesting to know where the Atlas aircraft had been stored as the environment of storage to determine the probability and speed with which corrosion could occur. As an inspector, I had rejected the lubrication of jackscrew work numerous times. Proper lubrication involves pumping grease into the ball nut until clean lubricant comes out, cleaning the screw, moving the jackscrew, recleaning the screw, applying oil to the screw, and re-greasing the nut. This is a timely and very messy operation if performed correctly. After the Alaska Air disaster our MRO initiated the procedure that only specific individual mechanics would perform lubrication procedures and that
all lubrication work cards would be inspected by an inspector. Your theory of a jack screw failure seems the most likely scenario to me as I could see the aircraft rolling in a vertical dive on the video. As a pilot, I think that if I experienced a sever unintentional dive without elevator response, I would try to manipulate any other controls available to avoid imact. It would be interesting to know if the pilots of the Alaska Air crash migt have used roll control to avoid their demise.