c5load
Topic Author
Posts: 344
Joined: Tue Sep 16, 2008 9:40 pm

Jackscrew Mechanism On Horizontal Stabilzers

Sat Apr 03, 2010 4:57 pm

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"
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Jackscrew Mechanism On Horizontal Stabilzers

Sat Apr 03, 2010 5:03 pm

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.
 
Tristarsteve
Posts: 3359
Joined: Tue Nov 22, 2005 11:04 pm

RE: Jackscrew Mechanism On Horizontal Stabilzers

Sat Apr 03, 2010 5:38 pm

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.
 
c5load
Topic Author
Posts: 344
Joined: Tue Sep 16, 2008 9:40 pm

RE: Jackscrew Mechanism On Horizontal Stabilzers

Sat Apr 03, 2010 5:40 pm

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"
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Jackscrew Mechanism On Horizontal Stabilzers

Sat Apr 03, 2010 7:05 pm

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.
 
474218
Posts: 4510
Joined: Mon Oct 10, 2005 12:27 pm

RE: Jackscrew Mechanism On Horizontal Stabilzers

Sat Apr 03, 2010 7:40 pm

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.
 
411A
Posts: 1788
Joined: Mon Nov 12, 2001 10:34 am

RE: Jackscrew Mechanism On Horizontal Stabilzers

Sat Apr 03, 2010 9:26 pm

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.
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Jackscrew Mechanism On Horizontal Stabilzers

Sat Apr 03, 2010 11:15 pm

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 avatar
jetmech
Posts: 2316
Joined: Wed Mar 29, 2006 2:14 am

RE: Jackscrew Mechanism On Horizontal Stabilzers

Sun Apr 04, 2010 4:04 am

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 :shock: .
 
Pihero
Posts: 4196
Joined: Mon Jan 31, 2005 5:11 am

RE: Jackscrew Mechanism On Horizontal Stabilzers

Sun Apr 04, 2010 5:23 pm

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
 
avt007
Posts: 1989
Joined: Mon Jul 10, 2000 4:51 am

RE: Jackscrew Mechanism On Horizontal Stabilzers

Sun Apr 04, 2010 5:41 pm

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 avatar
n901wa
Posts: 406
Joined: Thu Oct 15, 2009 5:38 pm

RE: Jackscrew Mechanism On Horizontal Stabilzers

Sun Apr 04, 2010 9:14 pm

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.
 
411A
Posts: 1788
Joined: Mon Nov 12, 2001 10:34 am

RE: Jackscrew Mechanism On Horizontal Stabilzers

Mon Apr 05, 2010 6:23 am

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 avatar
HAWK21M
Posts: 29867
Joined: Fri Jan 05, 2001 10:05 pm

RE: Jackscrew Mechanism On Horizontal Stabilzers

Mon Apr 05, 2010 2:33 pm

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.
I may not win often, but I damn well never lose!!! ;)
 
474218
Posts: 4510
Joined: Mon Oct 10, 2005 12:27 pm

RE: Jackscrew Mechanism On Horizontal Stabilzers

Mon Apr 05, 2010 3:48 pm

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 avatar
n901wa
Posts: 406
Joined: Thu Oct 15, 2009 5:38 pm

RE: Jackscrew Mechanism On Horizontal Stabilzers

Mon Apr 05, 2010 4:45 pm

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.
 
AirframeAS
Posts: 9811
Joined: Thu Feb 05, 2004 3:56 pm

RE: Jackscrew Mechanism On Horizontal Stabilzers

Tue Apr 06, 2010 7:43 am

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.

Who is online

Users browsing this forum: No registered users and 24 guests