Horizontal Stabilzer Trim, How to limit the Horizontal Stabilzer Trim values mechanically. Options

[Shouvik]

Hi All,

We all remember the devastating accident of Alaska Airlines Flight 261. It was due to poor maintenance of the Horizontal Stabilizer Trim functionality. I want to know what are the current measures in the T-Tail aircrafts that prevents the Horizontal Stabilizer from overshooting the trim limits. In Flight 261 there was a single ACME nut and and titanium screw. The nut was damaged due to non-availability of grease and the thread was completely eaten up. The mechanical stops could not take the load of the stabilizer and the screw was loose. This tore the whole horizontal stabilizer off the Vertical Fin.

Can I know how are the aircrafts having this safety of flight operation implemented. I know in normal Vertical Stabilzers they still use Jackscrew mechanism but I am unaware of how do they provide limiting values via some safety mechanism. The Autopilot can limit the signal inputs but are there any Mechanical Limits that come into picture.

Kindly help me know any literature or resources that would throw some light on the above mentioned area.

Thanks and Regards
Shouvik

[Aspiring Boeing ...]

On the T-tails there is nothing other than the bolt to stop the plane's stabilizers exceeding their designed envelope. It doesn't need it if they are looked after as instructed my the manufacturer.

The autopilot can command different trim, but can't stop it if it does what it did in the Alaska 261 incident... because the pilots couldn't either.

Have you tried google'ing the Alaska 261 incident? Or looking it up on wikipedia?

regards,
Chris

[Shouvik]

Have you tried google'ing the Alaska 261 incident? Or looking it up on wikipedia?

Thanks for the reply,

Yes, I went in detail regarding the accident and have observed that apart from the Mechanical Stops there were no other safety mechanisms that come into picture. When the captain used the Suitcase handles recurring times as per the ground maintenance crew it damaged the ACME nut completely eating up the thread. So the Jackscrew was left loose. The fairy bracketting on the Vertical Stabilizer was not meant to take such heavy load of the Horizontal Stabilizer which ripped off the plane.

I therefore wanted to know if there has been any design enhancements done to the whole Trim System that would address the issue faced by Flight 261. I want an insight of the current Trimming System implementation.

Thanks and Regards
Shouvik

[Kilrah]

I would see no reason for any change to that system. Aircraft parts are designed and made to be safe in a certain operating conditions, which include periodic replacements and maintenance. These parts are certified to operate properly given those conditions.
If one fails when used correctly, then for sure a reevaluation and modification has to be conducted. BUT, in that case, it was clearly a (serious) mishandling that caused the failure.
I'd nearly say that seeing how well and long it has actually withstanded such extreme conditions, the whole thing could have maybe been actually built a bit lighter...

[Ranger]

It's my understanding that there were no design changes nor modifications to the trim system on the MD-80 series. What came out of the accident was additional maintenance and inspection requirements. But remember that I fly I don't turn wrenches.

Where's Fluff when we need him?

[The Airbuser]

It's my understanding that there were no design changes nor modifications to the trim system on the MD-80 series. What came out of the accident was additional maintenance and inspection requirements.

From the NTSB accident report about Alaska 261

"Recommendations"

Review all existing maintenance intervals for tasks that could affect critical aircraft components and identify those that have been extended without adequate engineering justification in the form of technical data and
analysis demonstrating that the extended interval will not present any increased risk and require modifications of those intervals to ensure that they (1) take into account assumptions made by the original designers, (2)
are supported by adequate technical data and analysis, and (3) include an appropriate safety margin that takes into account the possibility of missed or inadequate accomplishment of the maintenance task.

You're right Ranger, as normal

Ed

[Shouvik]

Hi there!

I am delighted to see so many replies. Thanks to all. Now that we are sure that no changes were made to the MD-80 H-Stabilizer Trim System I have one more question to ask. Will various aircrafts in the Transport segment have more or less same mechanism of Trimming. And where could I get at least a bit some insight into technical intricacies of the Trim System. I could know about the MD-80 Trim System as it was a part of NTSB Investigation report as well as a comprehensive re-enactment of the Accident along with the investigation report was aired on NGC - Aircrash Investigation.

I was wondering if I could know the basic way of trimming and safety limit implementation in H-Stabilizers across diferent aircrafts. If anybody can help me out here it would be wonderful....

Thanks again to everybody...

Thanks and Regards
Shouvik

[The Airbuser]

http://www.amfanatl.org/Pages/11_Safety&am...NTSB8-15-02.pdf

http://www.behavioralsafetynow.com/library...%20BSN%2007.pdf (page 15)

http://www.ntsb.gov/publictn/2002/AAR0201.pdf NTSB Accident Report

Ed

[AirRabbit]

I think you'll find that most airplanes are designed to be "fool proof." However, there isn't anything man-made that is built to be "idiot-proof."

[MNT_RG_GLO]

Hi there!

I don't know very weel the MD-80 séries but i think the system was made to work and suport more severous phases of flight forces and have made your work good and safety.

If necessary write new project the manufacturer will do it and inform all operators. The mechanism is very safety and strong but over correct maintenance intervals nothing more others accidents will occur.

Rgds

[Shouvik]

Hi There,

Definitely agree with all of you but was still trying to find out if at all some changes were made to the trim system which became a reason for loss of life and property even though the initiation was human. Was there any design modifications carried out? As per NTSB report no. They just ordered the Airline to adhere to a stricter maintainance schedule.

But then there is always a scope for new things isn't it

Regards
Shouvik

[Kilrah]

Like you said, no, no changes. But again, WHY would there be any if the fault is not due to it?

You usually want to correct the problem, which was bad maintenance - so you call for correct maintenance. You don't randomly start changing a design that actually works beyond expectations.

The reason for the loss of life was bad maintenance, not the trim system.

[Shouvik]

HI Guys,

I found this. Very simple and interesting
Failsafe Jack Screw

Thanks and Regards
Shouvik

[Aspiring Boeing ...]

I found this. Very simple and interesting
Failsafe Jack Screw

Quite interesting. I will save that to my favorites as I don't have much time to read it today.

Regards,
Chris

[ChillSpiller]

What a coinsidence, we just talked about this very incident in a Human Factors trainings course.
I completely agree to what Kilrah stated above. The one and only reason for this accident was human failure as in poor maintenance. As propably evryone here knows by now the jackscrew hadn't been lubricated properly. Neither with the right grease nor within the correct inspection intervalls.
As usual it was a chain of events that made this tragedy happen. If I still remember correctly AirCanada had filed a proposal to the MOT long before the actual accident happened. The proposal suggested to expand maintenance/lubrication intervalls of the horizontal trim jackscrew from 12months or 20 cycles up to 12 months only (not sure if I still got the numbers right). Since the Maintenance Manuals left room for some interpretation wether the cycle or the time limit was crucial or even both, the MOT agreed. However usually it is understood that whatever occurs first initiates an inspection on the componets. Never the less this request was granted - the airline saved maintenance time. Secondly a bad and dangerous habbit comes into play: Complacency. The jackscrew had been looked after numerous times. It was never found to be in a "dangerous" shape nor without sufficient grease. Intervalls had been extended, the screw begann to wear off. The third and saddest part would be "Lack of resources". The mechanics had found out that the screw was out of limits but no spare part was in stock. "Pressure" and "Lack of assertiveness" propably made them reinstall the used nut, which they had remeasured untill the numbers were right. Now it was only a matter of time for the threads to entirely wear off.
After this incident the maintenance Intervalls had been set back to the original MM figures. It had also been observed that other airlines encountered the same problems in the past.

As Kilrah said, it is amazing that the nut actually did withstand that long. There is nothing to be to improve with this jack screw. If it had been maintained properly the airplane would still be flying. The article "Failsafe jacksrew" that Shouvik had set a link to is interesting but imho not truly suitable for aircraft. Its extra systems, extra weight, and extra maintenance componets that don't have to be installed if maintained properly.