I've been reviewing the NTSB Accident Report of Alaska Airlines Flight 261 and also the NTSB animation of the failure sequence. There are many diagrams in the report with the various parts labeled. At one point in the report detailing causes of the accident are the words: "The cause of the final dive was the low cycle fatigue fracture of the torque tube . . ." [p.177]. I don't see a part called a "torque tube" in the NTSB diagrams. From the NTSB animation it looks like the torque tube is something inside the "acme screw." ???. Google searches of torque tubes have so far proven fruitless.

The cascade of failures that led to this terrible tragedy are explained in the NTSB in great detail, but I am unclear about ":the final dive was the low cycle fatigue fracture of the torque tube." Where is the torque tube? What is a low cycle fatigue fracture? Did the other failures cause or contribute to the failure of the torque tube? Was the torque tube not of a robust design or construction? "How" did the failure of this part cause the final dive?

This is not explained very much in the report. Can anyone shed light on this? I understand about the incremental and final catastrophic shearing of the threads in the acme nut [gimbal] due to lack of proper maintenance and proper inspection and tooling and I understand the failure of the restraining brackets. But the torque tube failure is something I don't understand.

Thanks in advance for any light on this subject!

Source: NTSB/AAR 02-01 PB2002-910402

Addenum. I think from the information on the diagram and description on page 15 that the torque tube is inside the acme screw. It is said to be made of titanium. The acme screw was recovered after the accident. Still not clear on how the fracture of the torque tube led to the fatal dive.

According to the NTSB Report cited: "The portion of the torque tube that extended beyond the lower end of the acme screw was fractured. The lower end of the torque tube nut washer and the torque tube nut were not recovered." [p. 61]. How and as a result of what did the torque tube fracture? Did impact with the mechanical stop cause the fracture or some kind of torsional or bending load?

Somebody told me that the Douglas DC-8 used two jack screw assemblies in order to control the horizontal stabilizer for the sake of redundancy. Why did the DC-9 family use only one? Economics?

The accident report explains how the fracture of the torque tube caused the accident.

Under paragraph 2.2.4 the second and final dive on page 133.

Thank you so much Woodreau. Somehow I missed that. I am reading that portion of the report again and again but am having a bit of trouble visualizing what happened. Does "fracture" mean that the torque tube broke into two pieces? Since the torque tube is inside the acme screw and the acme screw did not break, I am puzzled. On the NTSB video below, there is mention that. "the lower mechanical stop and torque tube separated" [6 minutes and 7 seconds into the video]. At 5 minutes and 57 seconds into the video the narrator says" "The mechanical stop and the torque tube connected to it was [sic] not designed to withstand the mechanical loads produced by the horizontal stabilizer."

https://youtu.be/Otu4sJreiS0

A footnote to the section of the Report cited by Woodreau reads: "The torque tube was subjected to a combination of bending and tension as a result of an offset loading condition [what is that?] created in the torque tube because the stop lugs on the lower mechanical stop and the acme screw prevented application of equal load around the axis of the tube. [op. cit. page 134, footnote 221].

Is this what caused the "low cycle fracture" or was the "low cycle fracture' the cause? If the torque tube had not fractured, would the accident have been avoided? I realize there was a cascade of failures, but the NTSB seems to point to the low cycle fracture as the "cause' of the final, unrecoverable high speed dive.

Would the sequence of events be (inadequate maintenance of the assembly, inadequate inspection, inadequate tool substitution for measuring end play, too much time between inspections) resulting in not enough of the proper kind of grease, excessive wear of the acme nut . . . partial stripping of the threads of the acme nut . . .near complete stripping of the acme nut threads . . .acme nut contacting the lower mechanical stop . . . failure of the torque tube due to fracture . . . failure of the upper restraining brackets as a result of excessive air loads on the horizontal stab . . .aircraft locked into a fatal and unrecoverable nose down dive. ???

convair880mfan wrote:

Addenum. I think from the information on the diagram and description on page 15 that the torque tube is inside the acme screw. It is said to be made of titanium. The acme screw was recovered after the accident. Still not clear on how the fracture of the torque tube led to the fatal dive.

The ACME screw is the torque tube.

Imagine you had a bolt in one hand and a nut in the other. The nut is screwed onto the bolt. With one hand you hold the nut, with the other hand you turn the bolt. The nut will move up and down the bolt.
That's how the horizontal stabilizer trim generally works. The "bolt" (ACME screw) is fixed to the aircraft structure and a trim motor turns (applies torque -> torque tube) the "bolt". This moves the nut (fixed to the leading edge of the horizontal stabilizer) up or down, moving the horizontal stabilizer itself up or down.

During this accident the threads of the nut were so worn that the screw got pulled through the nut by aerodynamic force. Only the mechanical stop prevented the nut from disconnecting from the ACME screw at that point.

Low cycle fatigue means that the failure was caused by relatively few but hefty loads. Usually fatigue is caused by loads within the design limits but over a long time period and many cycles, which ultimately weakens the material and causes a failure.
The ACME nut being able to move freely up and down the ACME screw due to the threads being ripped off, lead to the screw bumping into the stops repeatedly with great force, driven by aerodynamic loads. This caused the ACME screw (torque tube) to fail. The nut moved past the 3.1° hard stop and even significantly past 14° aircraft nose down. This was not recoverable.

convair880mfan wrote:

Would the sequence of events be (inadequate maintenance of the assembly, inadequate inspection, inadequate tool substitution for measuring end play, too much time between inspections) resulting in not enough of the proper kind of grease, excessive wear of the acme nut . . . partial stripping of the threads of the acme nut . . .near complete stripping of the acme nut threads . . .acme nut contacting the lower mechanical stop . . . failure of the torque tube due to fracture . . . failure of the upper restraining brackets as a result of excessive air loads on the horizontal stab . . .aircraft locked into a fatal and unrecoverable nose down dive. ???

Your summary of the sequence of events sounds pretty much correct

convair880mfan wrote:

A footnote to the section of the Report cited by Woodreau reads: "The torque tube was subjected to a combination of bending and tension as a result of an offset loading condition [what is that?] created in the torque tube because the stop lugs on the lower mechanical stop and the acme screw prevented application of equal load around the axis of the tube. [op. cit. page 134, footnote 221].

Tension that is concentric on the member means there are no bending stresses, just axial tension. Imagine a washer that is clipped just outside of the inner hole. Loading that washer face at a uniform PSI, the equivalent point load will be some distance 'e' from the center of the rod. This causes a bending moment of P * e, it varies with dimensions and materials but usually when e = d/3 the bending stress exceeds the axial tension stress. If bending was not considered in the design it is likely the peak tension is double the design tension. Couple this with issues of the sharp notch at the bottom of threads cracking can begin. Add in the pounding noted where the nut was sliding loose on the stripped threads there is likely loadings in excess of yield. Think of a badly out of balance wheel, things come apart fast.

Thank you for that illustration, Horstroad. It was really helpful to me. I can now envision how the mechanism works and that really helps me understand it better.

What you said about the screw bumping into the stops repeatedly with great force, driven by aerodynamic loads makes sense to me too.

I am still a little confused about the ACME screw and torque tube. You say the ACME screw is the torque tube but the NTSB Report mentions them as two different things. And there is also mention of the torque tube being inside the ACME screw. Maybe they are using two names to designate the same thing, but I am not sure. One of their illustrations seems to show the torque tube inside the ACME screw [page 15 of the NTSB Report] It has an arrow pointed at the ACME SCREW and a different arrow pointed at the DRIVE TORQUE TUBE as if they were different. Confused about that.

You are right that there is a separate torque tube inside the ACME screw

Any of these commands activates one of the two
electric motors that rotate the acme screw by applying torque to the titanium torque tube
that is held fixed inside the acme screw.