Airliner elevators

[kukkudrill]

Great idea for a new forum. It's an opportunity to ask something I've been wondering about.

[photoid=491705]
The MD-90 in this photo appears to have both its elevators and the whole tailplane deflected. I know that military fast jets have all-moving tailplanes in place of conventional elevators. Do airliners use both systems in combination? If so, why not either one or the other?

[JordanD]

It's called a stabilitor (sp?). It's both an elevator and a horizontal stablizer. I know some GA planes (piper arrow) use this. Can someone tell me if I'm right or wrong?

Edit: I meant to say stabilator.

[screaming_emu]

It's called a stabilitor (sp?). It's both an elevator and a horizontal stablizer. I know some GA planes (piper arrow) use this. Can someone tell me if I'm right or wrong?

Edit: I meant to say stabilator.

You're correct Jordan, as far as I know, all piper aircraft use an all moving stabilator instead of an elevator. As far as airliners go, I can't think of one that uses a stabilator, but I believe that instead of using a conventionall trim system (or maybe in conjunction with) they use an adjustable stabilizer. You can move the entire horizontall stabilizer to relieve control pressures. You can read about it on page 4-9 (chapter 4 page 9) of the Pilots Handbook of Aeronautical Knowldege which is available online here

http://www.faa.gov/library/manuals/aviation/pilot_handbook/

you can see that there's a big jack screw that goes into the front of the horizontal stabilizer. On the large airplanes a motor will turn that screw and adjust the angle of the stabilizer through the air making it so that the pilots dont have to hold the yoke forward or back to get the airplane to do what it wants. Hope this helps.

[Star Alliance]

To my knowledge, the way Joe described the moving tailplane is correct. IIRC, there was an Alaska Airlines MD-80 where the jackscrew stripped, giving full down elevator, so it nose dived(dove?) in to the pacific ocean after taking off from Mexico City.
Hope this helps at all.
Cheers,
330

[seventwo]

yah, there an elevator for pitch control (connected to the yoke)and the entire vertical stabiliser that is "trimable". If there is more wieght in the front of a plane it will try to nose town, and the pilot can trim up and doesnt have to pull back on the stick for hours on end. There is a trim wheel near the throttles that controls the pitch of the stabiliser. The elevator on the DC-9 isnt even connected to the yoke, it connected through a little tab on the end of the elevator.

[N9103M]

In most transport category jets, you move the stabilizer for "trimming" and use the elevator for pitch change.

i.e You apply back pressure on the yoke, actuating the elevator (In the MD-80's case by moving a tab to "fly" the elevator into the desired position) then once you feel the force needed to hold a desired pitch attitude, use stabilizer trim to releive the back pressure.

Joe is excatly correct, Stabilizers move on a jackscrew. When you activate trim in an airplane like the MD-80 you inturn activate the jackscrew, moving the tailplane.

In the Alaska MD-80 accident, the jackscrew was poorly lubricated and became stripped. Essentially allowing the staibilizer to move it's full travel unrestricted with no control input from the pilots.

[ACman]

this feature is on many aircraft. It is sometimes known as "Horizontal
Trim". On the DC-9 series (includes 717), 727, Fokker rear jets, and
other T-tail planes with this feature, the pilots will set the trim
acordingly.

Most commercial airlineers have this feature of being able to set the
trim. The mechanics on the inside to make it move are simple.

The Horizontal Stabilizer has a nut in the middle where the two meet
on the inside. Through the middle of the nut feeds a giant screw which is attatched to a motor. The motor is attatched to some ribbing which is then attached to the airframe (Some times, different on T-Tails). So the motor rotates the screw one way and the HStab moves one way and visa versa. Auto Pilot uses the Trim to its best advantage.

Normal take-off requirments in airplanes are about 5-7 degrees of nose trim up. On landing, its a whole different ball game. The trim sometimes seems to be used alot. Just look.

[photoid=5623779]
[photoid=5623371]

Some airplanes, for instance and the bes example is the Lockheed L1011, have a dual feature. The pilot will pull up on the control colum and the elivator and HStab will go with it, they push down, the elivator and the HStab go with it. The L1011 also has a feature where if theres full pitch trim down, the plane can still be controlled and flown properly.

[JordanD]

I beileive with the alaska plane that the elevator was stuck in the up posistion, because the plane stalled and crashed into the water.

[N9103M]

I beileive with the alaska plane that the elevator was stuck in the up posistion, because the plane stalled and crashed into the water.

Not the case....you can read the whole NTSB report here.

http://www.ntsb.gov/events/2000/aka261/default.htm

[kukkudrill]

Thanks for the info all!

[ACman]

I beileive with the alaska plane that the elevator was stuck in the up posistion, because the plane stalled and crashed into the water.

Quite the opposite accually. The planes HStab jackscrew stripped itself and went in the AND (Airplane Nose Down) Position. The HStab just sat on the tail support, and finally broke the frame due to loads it wasnt designed to take. The HStab couldent control the aircraft anymore, and it dove the the pacific ocean.

[kukkudrill]

Interesting case, and bloody tragic. Basically lots of people lost their lives because the airline decided to skimp on maintenance with the blessing of the FAA. The last moments when the plane nose-dived must have been horrific.

If I understand right the plane was controllable notwithstanding the stabiliser being at max AND trim, until the load caused it to break its frame. What were those loads? Just the pressure of the airflow on the underside of the stabiliser? Perhaps coupled with the elevators presumably being in an up position to counter its AND effect? Would the latter explain why catastrophic failure finally occurred when the plane lowered its flaps?