The T Tail and the jack screw?

Question

Just wondering if the learjet 45 has the same fault as the dc-9 with the T tail and the jack screw as there is no backup. And for that matter of fact do all T tail planes have no backup just the jack screw and the nut.

Answer 1

The MD80 is the second safest airliner flying since its inception in 1980, so I would be cautious about criticizing the safety of the design. The design of the jack screw is adequate if inspected and maintained properly... The type of aircraft was not an issue in the Alaska 261 crash.



Any airplane that is not properly maintained can fail catastrophically, hence the requirement for specified maintenance and inspections.

Answer 2

In the case of Alaska Flight 261, analysis estimated that 90% of the threads in the acme nut had been previously worn away, and that they were then completely sheared off during the accident flight. Once the threads failed, the horizontal stabilizer assembly was then subject to aerodynamic forces that it could not withstand, and ultimately failed. Based on the time since the last inspection of the jackscrew assembly, the NTSB determined that the wear had occurred at a much faster than average rate (0.012 inch per 1,000 flight hours, when the expected wear was 0.001 inch per 1,000 flight hours). The NTSB considered a number of potential reasons for this excessive wear, including the substitution by Alaska Airlines (with the approval of the aircraft manufacturer Boeing) of Aeroshell 33 grease instead of the previously approved lubricant, Mobilgrease 28. The use of Aeroshell 33 was found not to be a factor in this accident. Insufficient lubrication of the components was also considered as a reason for the wear. Examination of the jackscrew and acme nut revealed that no effective lubrication was present on these components at the time of the accident. Ultimately the lack of lubrication, and resulting excessive wear of the threads, were determined to be the direct causes of the accident.

Answer 3

As far as your statement about the "fault" of the T-tail, let me add some info to the others from the NTSB file on Alaska Airlines Flight 261, which is the one I think you are refering to.



The investigation proceeded to examine why scheduled maintenance had failed to adequately lubricate the jackscrew assembly. In interviews with the Alaska Airlines SFO mechanic who last performed the lubrication it was revealed that the task took about 1 hour, whereas the aircraft manufacturer estimated the task should take 4 hours.This and other evidence suggested to the NTSB that "the SFO mechanic who was responsible for lubricating the jackscrew assembly in September 1999 did not adequately perform the task."



Laboratory tests indicated that the excessive wear of jackscrew assembly could not have accumulated in just the 4 months period between the September 1999 maintenance and the accident flight. Therefore, the NTSB concluded that "more than just the last lubrication was missed or inadequately performed."



In order to monitor wear on the jackscrew assembly a periodic maintenance inspection called an "end play check" was used. The NTSB examined why the last end play check on the accident aircraft in September 1997 did not uncover excessive wear. The investigation found that Alaska Airlines had fabricated tools to be used in the end play check that did not meet the manufacturer's requirements. Testing revealed that the non-standard tools ("restraining fixtures") used by Alaska Airlines could result in inaccurate measurements, and that it was possible that if accurate measurements had been obtained at the time of the last inspection, these measurements would have indicated the excessive wear and the need for the replacement of the affected components.



The jackscrew assembly was designed with two independent threads, each of which was strong enough to withstand the forces placed on it. Maintenance procedures such as lubrication and end play checks were to catch any excessive wear before it progressed to a point of failure of the system. The aircraft designers assumed that at least one set of threads would always be present to carry the loads placed on it, therefore the effects of catastrophic failure of this system were not considered, and no "fail-safe" provisions were needed.

In order for this design component to be approved ("certified") by the FAA without any fail-safe provision, it had to be considered "extremely improbable".

In 2001, NASA recognized the risk to its hardware (such as the Space Shuttle) attendant upon use of similar jackscrews. An engineering fix developed by engineers of NASA and United Space Alliance promises to make progressive failures easy to see and thus complete failures of a jackscrew almost impossible.



During the course of the investigation, and later in its final report, the NTSB issued a total of 24 safety recommendations, covering maintenance, regulatory oversight, and aircraft design issues. More than half of these were directly related to jackscrew lubrication and end play measurement.



In NTSB board member John J. Goglia's statement for the final report, which was concurred with by all three other board members, he wrote:



"This is a maintenance accident. Alaska Airlines' maintenance and inspection of its horizontal stabilizer activation system was poorly conceived and woefully executed. The failure was compounded by poor oversight...Had any of the managers, mechanics, inspectors, supervisors or FAA overseers whose job it was to protect this mechanism done their job conscientiously, this accident cannot happen...NTSB has made several specific maintenance recommendations, some already accomplished, that will, if followed, prevent the recurrence of this particular accident. But maintenance, poorly done, will find a way to bite somewhere else."





So to summerize, if proper maintenance is performed (lubrication and end play check) there is no need for a "backup" (fail-safe) provisions.

Answer 4

The Alaska Airlines flight that you probably have in mind crashed because of improper and long-delayed maintenance, not because of any fundamental fault in the DC-9. The airline has obtained approval for increasingly long maintenance intervals for groups of components, which had the effect of stretching the maintenance interval for the failing part to the absolute limit. When the jack screw assembly was incorrectly lubricated after one of these very long intervals, it failed in flight. Had the maintenance interval been shorter, or had the maintenance been done correctly when it was finally done, the accident would not have happened.

Answer 5

I am not sure if the fault type is the same. I am however developing a relatively simple process that I am currently successfully using to quantify acme stem nut thread wear without removing it from the stem on valves. I would like to find a way to test this process on a aircraft jack screw stem nut but don't know who to contact that would allow me to try it.

Answer 6

The fault was failure to properly inspect and lubricate, not the design.