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THS Jackscrew design

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THS Jackscrew design

Old 13th Jan 2013, 19:34
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THS Jackscrew design

Reading Alaska Airlines flight 261 reports , I wonder why NTSB points out only to DC-9, MD-80/90 and Boeing 717 fleet regarding the jackscrew design considering that (as far as I know) the designs used on the 727,737,747 etc. are similar to the one used on those cited.

Could anyone with technical knowledge on the issue explain how do they differ justifying this differentiation between aircraft types in reports?

Last edited by David36; 13th Jan 2013 at 19:37.
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Old 13th Jan 2013, 19:39
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It's not the design that is the problem, it's lazy mechanics that didn't want to pull over a latter and a can of grease, just signing it off as inspected.

Same thing with the C130s cracking up doing Fires...guys weren't doing the wing spar inspections, just signing them off.
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Old 13th Jan 2013, 19:48
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I know that and it is not the point of this thread (even if the reports conclusion was that it is not appropriate to rely solely on maintenance and inspection intervention to prevent the failure from occurring).
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Old 13th Jan 2013, 20:10
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David - I get your point. but if you want to prove that this or that design is inherently flawed then you need criteria to determine 'safe' vs 'flawed' and by what universal standard.

Simply put, all sorts of products are put into service that haven't had the benefit of thousands of hours of testing. They have a general idea on how it will work, how it will hold up, and they put maintenance procedures in place to mitigate the design flaws.

Some times you can't look after and upkeep something so much, and thus get an STC or in the case of cars, a recall.

At issue is whether they purposefully put out a crappy unsafe product and try to cover it up. Sure, it happens.

I've noticed a number of aircraft that had what I would call an Achilles heel, that thing that I need to really watch...an item, flying characteristic, whatever...

If you are asking whether the jack screw is a problem, I think hundreds of thousands of hours of safe flying would say no, as long as it's looked at. Are there better systems, I don't know. Go research it.

If you want a windmill to joust about safety, push for hiring better pilots. The Alaska flight went down because the pilots hired flew a worsening flight control problem but defaulted to SOPS, calling dispatch as it got worse. They did what the company trained them to do and it got everyone killed.

Simply put you can fly any plane and for whatever reason it could have a problem, how you handle the problem is key....and it flies in the face of current theory that you design all the problems out of an aircraft so you can hire incompetents to fly that will never be faced with making a decision if something goes wrong outside of the check list.

Last edited by TheRobe; 13th Jan 2013 at 20:11.
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Old 14th Jan 2013, 07:28
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As I said, it is pretty clear for me that it's not a safety flaw considering that as far as I know it was the only one crash related to that jackscrew and the cause was gross improper maintenance.

However, maybe I'm too dumb for this, but I still don't get it. Why did they asked changes for certain aircraft while almost all other use the same system? Are there some particular differences related to design on DC-9, MD-80/90 and Boeing 717 fleet over others?
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Old 14th Jan 2013, 09:34
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Most of these things come down to money...how to make it, how to save it, how not to lose it.
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Old 14th Jan 2013, 09:39
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I seem to remember the DC-9 series was the only model to operate a single jack screw/drive mechanism. Basically the others had greater redundancy designed in so the failure mode was less likely to apply. It's been a while so I could be wrong, but I think that's the right direction.
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Old 14th Jan 2013, 09:53
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Yeah, I figure out that would be the only logical reason to make the recommandation for those aircrafts, however, as far as I know, it is not true. Most of big jets use single jackscrew design to actuate the horizontal stabilizer, so it's not the case. In fact the only one that I know they use different systems are DC-10,MD-11 (I hope my memory is not at fault) which use double jackscrew and L-1011 which has hyd actuated. The others, including Airbus, I know they have just a single jackscrew.

Maybe there are particular design features which make it different even being just one jackscrew. That's why I'm asking, I hope someone here has some detailed knowledge on these designs to clarify.

Last edited by David36; 14th Jan 2013 at 10:01.
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Old 14th Jan 2013, 12:45
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The others, including Airbus, I know they have just a single jackscrew.
While indeed havin just one threaded portion (hence a failure of the nut will result in total failure), the jackscrew is dual loadpath design with an inner pin in the screw, so fracture of the screw will not result in total failure.
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Old 14th Jan 2013, 14:35
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And that's different? I thought it's the same thing on MD types.
Edit: What I found is that Boeing uses a ballscrew design, that is on series 727,737,757,767 and maybe others.

Last edited by David36; 14th Jan 2013 at 14:40.
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Old 14th Jan 2013, 17:00
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In the wake of Alaska 261 there were cross-type and cross-manufacturer inspections and ADs mandated. No implementation (of which each manufacturer's was slightly different) was entirely free of problems in terms of maintenance or design:

Airbus : FAA mandates near-term A320 stabiliser inspections

Boeing : FAA calls for heightened 737 trim actuator vigilance

Here's a contemporary article on the subject:

Aviation Today :: Jackscrew Jugular Can Be Fixed
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Old 14th Jan 2013, 17:26
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Originally Posted by therobe
It's not the design that is the problem, it's lazy mechanics that didn't want to pull over a latter and a can of grease, just signing it off as inspected.
Ever so slightly simplistic view of the incident. There were just a few more contributing factors to that accident.
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Old 14th Jan 2013, 17:35
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What if an aircraft lost good data from sensors, and cycled elevator and THS automatically, to twelve degrees pitch down, at the same time disallowing manual override?

Uncontrollable. Loss of Control. Program Error.

Is that similar to Alaskan? As I recall, the MD80 had metal shards in the threads of the jackscrew, remnants of gross negligence.

What's a crew to do?

Innovate? I would say, yes.

Last edited by Lyman; 14th Jan 2013 at 17:38.
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Old 14th Jan 2013, 18:00
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As others have pointed out, there's not an airliner type in the world that cannot have its THS angle manually overridden - at least to some extent.
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Old 14th Jan 2013, 18:25
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So in the end you just don't want to hear that lazy mechanics and robots that won't get the plane down right away is the problem?

What are you digging for? Some inherent flaw that if fixed will allow the mechanics to go home and the pilots not have to think?
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Old 14th Jan 2013, 20:08
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I thought the problem with the Alaska airlines aircraft was that the jackscrew assembly failed and due to either inadequate or non existent limit stops, the horizontal tail went well past limits, and well past what the elevators could overcome. I suspect the horizontal tail would've been stalled if it had full elevator one way and full trim the other.
Hardly bad piloting.
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Old 14th Jan 2013, 20:16
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Inadequate maintenance meant that the stab jackscrew assembly had not been properly lubricated, eventually stripping the thread completely. The acme nut that was supposed to provide the stop eventually came away as there was no further resistance to it doing so. By that point the aircraft was unrecoverable, but the crew fought to recover it all the way to impact.
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Old 14th Jan 2013, 20:45
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Keep in mind they were up in the air for about an hour screwing with the trim, got harder and harder to keep the plane level.....the problem wasn't the jackscrew...any more then a vibration that got worse and worse from a bearing failure, or that oil temp that gets hotter and hotter, or that little whistle in the window that gets louder and louder, or that nut job in row 17 that keeps going to the bathroom and brings a package with him.

Things break, for whatever reason and the solution isn't always in the checklist or a call to dispatch.

The passengers had the wrong pilots that day and they paid for it with their lives.
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Old 14th Jan 2013, 21:02
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well, we really need to wait a second.

I flew the DC930 for 10 years. It is a great plane...our fleet was checked for the ''jackscrew'' thing and NONE, NONE of our planes had any problems...DC930 and MD80...NONE...

We were following the DOUGLAS manual and did everything it said.

One or two times I had problems with the trim. There is a backup trim (alt/aux) that remained in service just fine and there was a thermal cutout on the main trim motor, and when allowed to cool returned to service. But once we were on the ground, and wrote it up, it was taken into MX and checked out.

AS to the pilots...well I sure don't want to blame them on this situation. But I will say this...a sure thing is the way to go when ever there is a control problem...esp when approaches to a number of airport runways could be made while over the ocean, eliminating the hazard to innocents on the ground.

There has been a terrible philsophy promoted out there of always calling someone on the ground for advice. I've seen some pretty STUPID advice coming up from the ground.

One piece of advice was sent to a brand new 737-400 that couldn't get one of its main landing gears down. The brilliant advice was to take the crash axe, go back in the cabin, CHOP through the floor, cut the hydraulic line (under pressure) and this was SURE to get the gear down.

The pilots, smarter than average said: NUTS (paraphrasing...and offering tribute to the 82nd at Bastogne) They landed with the offending gear up. Upon investigation the wheel chocks were found in the wheel well making the gear STUCK. Mechanics left the wheel chocks in the gear well during pushback.

Call for advice any time...only take stupid advice once safely on the ground.

The screw is one of the simple machines of man...but everything needs maintenance.

in true retrospect, the pilots should have landed at any of the coastal airports along the way. Shuddering or any difficulty controling the plane is an immediate emergency, priority number one get everyone out of the way I'm coming in any runway any taxiway get the hell out of the way.

Flying the DC9 series by hand is a joy for an experienced pilot...anything that is unpleaseant in ''feel'' should be warning enough for an otherwise great flying plane.
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Old 14th Jan 2013, 21:15
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Originally Posted by TheRobe View Post
Keep in mind they were up in the air for about an hour screwing with the trim, got harder and harder to keep the plane level.....
It wasn't a gradual thing, it was stuck - literally held in place by a sliver of metal. Once that went there was nothing they could do. They kept trying the trim because experience suggested that they had a bad motor or electrical connection.

Originally Posted by sevenstrokeroll View Post
We were following the DOUGLAS manual and did everything it said.
So did Alaska, just at twice the required intervals between checks.

This was primarily an MX issue.
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