Crash: failure of jackscrew controlling stabilizer

[baxtrom]

Yes, it's me with an airplane disaster-related question again..

 

Still recovering from shivering induced by watching a scary documentary about the Alaska airlines flight 261 crash, I ask myself why the trim tabs (is that correct terminology?) are designed in the way they are (at least on the MD-80). As far as I understand, control of the aircraft was irrevocably lost when a jammed jackscrew finally disintegrated and the control surface pitched violently up around its hinge due to the airflow. To me, it seems intuitively safer to reverse the design and have the jackscrew located behind the hinge. In case of total jackscrew failure, which I believe is less unlikely than total hinge failure, I imagine the airflow would cause the control surface to flatten out, i.e. become a passive follower instead of being pushed into an extreme position.

 

What is the reason for this design? Practical considerations?

 

(Would have posted this in airliners.net if I wasn't too cheap to pay $25 for membership..)

[TonyMcC]

Sometimes it takes an accident to highlight a weakness that in retrospect seems obvious. I remember reading about a serious air crash near my home in 1957. This was caused by a non-return valve being fitted the wrong way round. I understand that such valves now have different sized fittings each end to make it impossible to fit them the wrong way round. http://en.wikipedia.org/wiki/Sutton_Wick_air_crash

[baxtrom]

I remember reading about a serious air crash near my home in 1957.

 

Thanks for the link. A tragic example of Murphy's law. The Alaska 262 flight is, on the other hand, mostly the result of criminal negligence regarding the maintenance of the aircraft. Apparantly the company stretched the maintenance interval to save money. Still the design with the actuator in front of the hinge puzzles me. I believe the design is similar on for example the Boeing 737 and perhaps on most commercial jet aircraft. Check the photo below - if the actuator breaks the control surface is likely to snap into one of the extremes, and the plane would go either full nose down, or up.

 

 

[Enthalpy]

The design of a trimmer is difficult because they have a huge desire to oscillate.

 

Especially if you put a control surface, or a trimmer, behind its hinge, it does oscillate for sure - seen it myself. Stabilizing it requires at least a counterweight before the hinge. And with a broken jackscrew, the trimmer may well flutter to disintegration anyway.

[baxtrom]

The design of a trimmer is difficult because they have a huge desire to oscillate.

 

Especially if you put a control surface, or a trimmer, behind its hinge, it does oscillate for sure - seen it myself. Stabilizing it requires at least a counterweight before the hinge. And with a broken jackscrew, the trimmer may well flutter to disintegration anyway.

 

Good point. Didn't think about flow induced flutter. Perhaps it doesn't matter if the actuator is located in front of or behind the hinge. However, damping out flutter using added mass or dissipative dampers seems intuitively simpler than controlling an aircraft with a control surface jammed hard into an extreme position. As far as I understand, the crew on the Alaska flight tried to compensate for the jammed trim panel by pulling hard on the control column, i.e. using the elevators to pull the nose up. When the jackscrew finally disintegrated the trim panel went all the way due to the airflow pressure and the pilots lost all control of the aircraft.

 

 

Edit: what I find hard to understand is that the pilots did not immediately declare emergency and land the aircraft after the first signs of problems with one of the control surfaces. Instead they tried to fix it while airborne. Then again, it's easy to be a keyboard pilot..

Edited August 19, 2011 by baxtrom