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Jackscrew Mechanism On Horizontal Stabilzers  
User currently offlinec5load From United States of America, joined Sep 2008, 917 posts, RR: 0
Posted (4 years 3 months 3 weeks 5 days 16 hours ago) and read 6420 times:

When watching Air Emergency about AS 261 that crashed because of an inadequately greased jackscrew in the elevator, I wanted to know if all airplanes have that kind of jackscrew mechanism? Also, have there been any more reports of these kind of problems that we haven't heard of?


"But this airplane has 4 engines, it's an entirely different kind of flying! Altogether"
16 replies: All unread, jump to last
 
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 1, posted (4 years 3 months 3 weeks 5 days 16 hours ago) and read 6419 times:

Quoting c5load (Thread starter):
I wanted to know if all airplanes have that kind of jackscrew mechanism?

Not all, but most large commercial jets use some variant of this mechanism.

Quoting c5load (Thread starter):
Also, have there been any more reports of these kind of problems that we haven't heard of?

I'm not aware of any similar accidents on the same type. For conventional tail aircraft with a jackscrew stabilizer, the specific failure mode that brought AS261 down is usually impossible.

Tom.


User currently offlinetristarsteve From Sweden, joined Nov 2005, 3980 posts, RR: 34
Reply 2, posted (4 years 3 months 3 weeks 5 days 15 hours ago) and read 6376 times:

Quoting tdscanuck (Reply 1):
Not all, but most large commercial jets use some variant of this mechanism.

Glad you said not all, the Tristar has four enormous hydraulic jacks to move the stabiliser, one for each hyd system, and the elevator is geared to the stabiliser.

Quoting c5load (Thread starter):
inadequately greased jackscrew in the elevator,

It wasn't the elevator, it was the stabilisor jack.


User currently offlinec5load From United States of America, joined Sep 2008, 917 posts, RR: 0
Reply 3, posted (4 years 3 months 3 weeks 5 days 15 hours ago) and read 6375 times:

Quoting tdscanuck (Reply 1):
For conventional tail aircraft with a jackscrew stabilizer, the specific failure mode that brought AS261 down is usually impossible.

The way they made it sound if it wasn't greased just right, it could very easily happen. What surprised me, at least from what NatGeo said (trustworthy?), there was no kind of backup system. I thought all airplanes had to have multiple redundancies.



"But this airplane has 4 engines, it's an entirely different kind of flying! Altogether"
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 4, posted (4 years 3 months 3 weeks 5 days 13 hours ago) and read 6343 times:

Quoting c5load (Reply 3):
The way they made it sound if it wasn't greased just right, it could very easily happen.

If that's the way they made it sound, that's not accurate. It wasn't greased *enough*, for a prolonged period. Greased on the original maintenance schedule, the mechanism has no problem. No mechanism with metal-on-metal sliding contact will survive too long without lubrication. The issue was compounded by using the wrong tool to check the freeplay in the jackscrew nut.

Quoting c5load (Reply 3):
What surprised me, at least from what NatGeo said (trustworthy?), there was no kind of backup system.

The nut is actually two separate pieces, either one of which can do the job; that's where the backup was supposed to be. The problem here was that lack of lubrication caused both to wear together (a known danger of fail-safe designs when not properly maintained).

Quoting c5load (Reply 3):
I thought all airplanes had to have multiple redundancies.

That's not completely true. For example, the MLG have no redundancy (there are at least two, but you need both to land properly, so there's no redundancy). The major requirements are that no single failure can cause a catastrophic event, and any forseeable chain of failures leading to a catastrophic event must be shown to be extremely improbable. In this accident, the immediate cause was simultaneous parallel failure of both parts of the jackscrew nut.

In the horizontal stabilizer in this case, the jackscrew and nut both had redundant load paths. Unfortunately, this particular failure mechanism simultaneously degraded both load paths in the nut. A maintenance plan that should not have been approved was approved; when coupled with a slightly out-of-spec check tool, the fact that this maintenance plan was causing an issue didn't become apparent until it was too late.

Tom.


User currently offline474218 From United States of America, joined Oct 2005, 6340 posts, RR: 9
Reply 5, posted (4 years 3 months 3 weeks 5 days 13 hours ago) and read 6326 times:

Quoting tdscanuck (Reply 4):
That's not completely true. For example, the MLG have no redundancy (there are at least two, but you need both to land properly, so there's no redundancy).

However, there are redundancies (back-ups) for landing gear extension.


User currently offline411A From United States of America, joined Nov 2001, 1826 posts, RR: 8
Reply 6, posted (4 years 3 months 3 weeks 5 days 11 hours ago) and read 6269 times:

Quoting tristarsteve (Reply 2):
Glad you said not all, the Tristar has four enormous hydraulic jacks to move the stabiliser, one for each hyd system, and the elevator is geared to the stabiliser.

Yup, simply a better design.
And, not to be unexpected, as after all, it was designed by Lockheed.

As for the landing gear, all transport types have an alternate means of extending the landing gear...sometimes, two or three.
IE, several options.


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 7, posted (4 years 3 months 3 weeks 5 days 9 hours ago) and read 6213 times:

Quoting 474218 (Reply 5):
Quoting tdscanuck (Reply 4):
That's not completely true. For example, the MLG have no redundancy (there are at least two, but you need both to land properly, so there's no redundancy).

However, there are redundancies (back-ups) for landing gear extension.
Quoting 411A (Reply 6):
As for the landing gear, all transport types have an alternate means of extending the landing gear...sometimes, two or three.

I meant the gear itself, not the extension mechanism. If a strut fails there is no redundancy. This is a big reason why the struts are safe-life.

Tom.


User currently offlinejetmech From Australia, joined Mar 2006, 2684 posts, RR: 53
Reply 8, posted (4 years 3 months 3 weeks 5 days 4 hours ago) and read 6128 times:

Quoting c5load (Reply 3):
The way they made it sound if it wasn't greased just right, it could very easily happen

Another big difference may have been due to the detail design of the screw itself. IIRC, the DC-9 stabiliser screw is an ACME thread, where the nut threads bears directly upon the screw threads, as below,



Other aircraft such as Boeings use a recirculating ball type screw as follows,

http://school.mech.uwa.edu.au/~dwright/DANotes/threads/mechanics/recircBallBIG.jpeg

Where the screw threads and nut threads are separated by ball bearings. As you can imagine, adequate lubrication is more critical for the ACME screw compared with the recirculating ball configuration.

Regards, JetMech



JetMech split the back of his pants. He can feel the wind in his hair.
User currently offlinePihero From France, joined Jan 2005, 4389 posts, RR: 76
Reply 9, posted (4 years 3 months 3 weeks 4 days 15 hours ago) and read 5962 times:
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Quoting tristarsteve (Reply 2):
Glad you said not all, the Tristar has four enormous hydraulic jacks to move the stabiliser, one for each hyd system, and the elevator is geared to the stabiliser.

Could you provide us with a tech drawing or a picture, please ? People don't believe me when I describe the system and I've lost all my old 1011 manuals...
Thanks



Contrail designer
User currently offlineavt007 From Canada, joined Jul 2000, 2132 posts, RR: 5
Reply 10, posted (4 years 3 months 3 weeks 4 days 15 hours ago) and read 5954 times:

There was nothing wrong with the design, and it wasn't simple human error. Although the report stopped short of actually saying it, basically the the actuator hadn't been greased at all for an extended period of time. The report suggests it had been "pencil whipped" up to 3 times. Added to this was the FAAs failure to properly oversee the maintenance task extension, allowing a critical system to go ungreased and uninspected for a longer interval than it should have been.
To blame the design is like blaming an engine for failing when no one ever checks the oil.

[Edited 2010-04-04 10:43:29]

User currently offlinen901wa From United States of America, joined Oct 2009, 453 posts, RR: 0
Reply 11, posted (4 years 3 months 3 weeks 4 days 11 hours ago) and read 5877 times:

Here the best I got from home. The Actuators are item 17. I think I still my training manuals at work,but the Component access guide is a sweet book. HTH . I once was back there while we ran a check to check travel, and it was impressive watching the whole hoz stab. move that fast. I think Delta had one stab lock lock up after takeoff out of SAN before I got hired, and I think the crew got a award for bringing back the 1011 safely. But I gotta look that one up.


User currently offline411A From United States of America, joined Nov 2001, 1826 posts, RR: 8
Reply 12, posted (4 years 3 months 3 weeks 4 days 2 hours ago) and read 5740 times:

Quoting n901wa (Reply 11):
I think Delta had one stab lock lock up after takeoff out of SAN before I got hired, and I think the crew got a award for bringing back the 1011 safely. But I gotta look that one up

Actually, it was a jammed elevator.
Alternate pitch procedure in this case, is to split the spoilers, for pitch control.
Works good, very effective.


User currently offlineHAWK21M From India, joined Jan 2001, 31667 posts, RR: 56
Reply 13, posted (4 years 3 months 3 weeks 3 days 18 hours ago) and read 5650 times:

The Importance of Lubrication added by regular Scheduled Mx checks could have saved this situation.
Jetmech.....Great Description on the Crewjack mechanism there.......Hows it on the B737s.

regds
MEL.



Think of the brighter side!
User currently offline474218 From United States of America, joined Oct 2005, 6340 posts, RR: 9
Reply 14, posted (4 years 3 months 3 weeks 3 days 17 hours ago) and read 5624 times:

Quoting n901wa (Reply 11):
I think Delta had one stab lock lock up after takeoff out of SAN before I got hired, and I think the crew got a award for bringing back the 1011 safely. But I gotta look that one up.
Quoting 411A (Reply 12):
Actually, it was a jammed elevator.

411A is correct it was a jammed elevator. The elevator jam was a caused by a bearing in the elevator pushrod that failed during the controls check prior to departure. The failed bearing allow the pushrod to tilt jamming the elevators in the full up position. The pushrod is about three feet long weighs about 30 pounds it positions the elevators opposite of the stabilizer movement.

At the time there was no elevator jam indication, so when the stabilizer was returned to its trimmed position the crew and no idea the elevators were in the full up position. After the incident an elevator jam indicator was added.


User currently offlinen901wa From United States of America, joined Oct 2009, 453 posts, RR: 0
Reply 15, posted (4 years 3 months 3 weeks 3 days 16 hours ago) and read 5598 times:

Hey 411A and 474218, thanks for the info on the Elevator jam. I always wondered what really happened. I only got the scuttlebutt from stories at work.

User currently offlineAirframeAS From United States of America, joined Feb 2004, 14150 posts, RR: 24
Reply 16, posted (4 years 3 months 3 weeks 3 days 1 hour ago) and read 5416 times:

Quoting tdscanuck (Reply 4):
The issue was compounded by using the wrong tool to check the freeplay in the jackscrew nut.

   The jackscrew in the tail on N963AS was supposed to be replaced. It was also a paperwork snafu as well which created a "war" between mechanics, leads and supervisors in OAK. A lot of people got fired over the situation after teh crash, mostly OAK mechanics and supervisors and my former C-check manager was involved as well, also got fired, too.

Keep in mind this was all focused on the OAK MX Base. OAK did the 15K and 30K's on the M80's. We did the 737's in SEA, both C & D's. I left AS shortly after the accident.

Quoting HAWK21M (Reply 13):
Hows it on the B737s.

Similar to what the M80's have, but not exactly the same. I'm strictly talking about the 732 and 734.



A Safe Flight Begins With Quality Maintenance On The Ground.
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