Alaska Flt261 and
single-Point Failure

 

ENTER the FAILSAFE JACKSCREW

If you look closely at the photos below, it's a piggy-backing "follower" (vaguely the same principle as a cam-follower - but in this case manifested as a secondary threaded nut that "shadows" the primary [see the roller-bearings upon which it "rides"] and carries the mensuration device pick-off for checking and alerting on wear tolerances of the load-carrying primary). It can also act as a backup load-carrier and take over the load in the event of a primary thread failure - via the castellation interlock. However I think you would find that its stabilizing presence actually also reduces the chance of any cross-threading thread failure. Cross-threading normally occurs because of both wear and lack of lubrication over a limited threaded mating surface. The resulting transverse forces possible with a loosened and frictional "fit" can then easily cross-thread. The backup arrangement, as far as I understand it, increases the mating threaded "run" - although the primary nut remains the sole load-carrier (short of its failure).

I think that article also said that it had organic lubrication. Metered self-lubricating would minimize cumulative wear - particularly of the type that occurred on AK261 because that one had already been found to be worn to limits two years earlier (when John Liotine said that it was worn to limits - but he was later covertly overruled by some contrived shonky testing that found it to be marginally/notionally ust "within limits"). The only added improvement would be a chip-detector. However those devices normally work only in wet-lubed systems and utilise a magnetic pickup. In a jackscrew, a heavy-lube grease would still carry metal particles or swarf into the threads, increase the wear-rate and be undetectable. Frequent wash-down, inspection and re-lube is the only answer. However a follow-up mechanism that can detect any aberration from fine tolerances is a quantum leap over the present rustic ironmongery. In service experience would enable inspection and re-lube intervals to be increased.

The problem with the T-tail jackscrewed stabilizer has always been one of nil redundancy, however theMcDD designers snuck around that by arguing that "it wasn't part of the flight controls system", it was allegedly "a structural component". That was obviously a quite specious argument and one of the primary reasons why Jim Hall announced an investigation into the FAA's airworthiness certification practises and procedures. It was a very cavalier decision by the FAA to allow McDD to get away with that misrepresentation and stick with their "convenient" design. But it is on a par with the FAA's decision to allow Boeing to keep quiet about the high aileron/rudder cross-over speeds of the 737 - on proprietary grounds. Because pilots were never aware of that being a significant adverse flight characteristic of the 737, they were poorly placed to react appropriately when they eventually encountered a rudder hard-over. It was never practiced in the 737 simulator. Nowadays 737 pilots are forced to fly higher speeds on runway approach that will give them a fighting chance (at lower altitudes) of accelerating out of that twilight zone of dubious controllability to a speed where ailerons can counter the rolling effect of uncommanded full rudder.

One of the other potent lessons of the AK261 accident is that "on-condition" lifing of mechanical (wear-prone and lube-dependent) components is open to the cunning interpretations of a maintenance management that just doesn't want to spend the money on replacement refurbished parts. However "lifing" components so that they must be removed and replaced makes much more sense. Logic dictates that a component that is found to be allegedly "within tolerances" - when 98% of similar parts are outside - must lead one to suspect that the calibration of the micrometer screw gauge used - or (more likely), wishful thinking - is "in play" also. However much "play" or tolerance is tolerated, one must suspect that a considerable amount of interpretive "freeplay" was being exercised at Alaskan. Inevitably and ultimately, they "screwed up" big-time.

IASA

the Failsafe Jackscrew pdf file (1.16mb) is now mounted at:

http://www.iasa-intl.com/pdf/Failsafe_Jackscrew.pdf

 

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