MaxQ2351
Topic Author
Posts: 321
Joined: Sat Apr 01, 2006 6:41 am

MD-80 Elevator Misalignment

Tue May 16, 2006 1:58 pm

Hey everyone,

Earlier today I was flying out of DFW, and while looking at the Super 80 that I would be flying on in a short amount of time, I noticed that the two elevators were "misaligned" from each other, for lack of a better word or expression.

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After looking at several AA Super 80's, I noticed that most (if not all) of them did that. So, my question is, is that not an inherently unsafe design feature of the aircraft?? What I mean is, if the two elevators are not physically connected to each other, couldn't they, theoretically, throw in opposite directions in a hydraulic system maulfunction?? If that was possible, woud it not create very unstable, if not uncontrollable, flight for the aircraft??

Do any other aircraft have the same, or similar design incorporated into them??

-Max
The 777-200LR......2 engines 4 longer haul
 
2H4
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RE: MD-80 Elevator Misalignment

Tue May 16, 2006 2:13 pm



Here you go!

 Smile




2H4


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jeb94
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RE: MD-80 Elevator Misalignment

Tue May 16, 2006 2:18 pm

Nope. The only hydraulic system in the elevators is to assist recovering in a deep stall. The elevators are aerodynamically controlled using control tabs that use flight control cables that run all the way back up to the cockpit. This is where the tabs are connected at the yoke. In a deep stall the airflow over the engine nacelles disrupts the flow over the elevator tabs and render them useless, hence the hydraulic assist that is triggered when the yoke is pushed full forward. The British discovered this with the BAC 111 when they lost the first prototype during stall testing and passed it on to Boeing for the 727 and Douglas for the DC-9. The only hydraulic power primary control surface in all DC-9 family aircraft, including the 717, is the rudder that has a cable driven control tab for a backup. The elevator and ailerons are aerodynamic with the tabs actuated by control cables.
 
2H4
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RE: MD-80 Elevator Misalignment

Tue May 16, 2006 2:29 pm




Quoting Jeb94 (Reply 2):
The only hydraulic system in the elevators is to assist recovering in a deep stall.

On that note, have you guys ever examined the deep stall recovery aids utilized by the MD-90?

Very interesting:

Movable Control Surfaces On MD-90 Engine Pylons




2H4


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jeb94
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RE: MD-80 Elevator Misalignment

Tue May 16, 2006 2:46 pm

I never heard of that before! They didn't repeat that on the 717, maybe because its a shorter fuselage with a smaller wing. Now I forget, is it the airflow over the engines or the disturbed air from the wings that cause the problem? Time for me to research!
 
AA717driver
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RE: MD-80 Elevator Misalignment

Tue May 16, 2006 10:54 pm

Man! I love McD-D products! Big grin TC
FL450, M.85
 
MD88Captain
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RE: MD-80 Elevator Misalignment

Tue May 16, 2006 11:08 pm

Do not feel bad about not knowing how those MD80 elevators work. I've had pilots call me on the radio while taxing out to warn me about our elevator problem.
 
2H4
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RE: MD-80 Elevator Misalignment

Tue May 16, 2006 11:24 pm




Quoting MD88Captain (Reply 6):
I've had pilots call me on the radio while taxing out to warn me about our elevator problem.

It would be fun to reply that one of the elevators is MEL'd for this particular flight, or that you're confident you'll reach your destination with one inop...

 biggrin 




2H4


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havaloc
Posts: 76
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RE: MD-80 Elevator Misalignment

Wed May 17, 2006 6:51 am

DC-9s are like this too.
DC-9
 
BR715-A1-30
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RE: MD-80 Elevator Misalignment

Wed May 17, 2006 7:29 am

Quoting Jeb94 (Reply 4):
I never heard of that before! They didn't repeat that on the 717

Uh, yeah they did... Go to ATL and look at the FL 717s sitting at the gate, and you'll see the same thing.

If this were unsafe, they would have reworked it sometime since 1965
Puhdiddle
 
md80fanatic
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RE: MD-80 Elevator Misalignment

Wed May 17, 2006 8:02 am

Jesus guys and gals.....it's that way to allow a latitudinal CoG in-flight adjustment. Remember, the MD series has a 2-3 seating configuration.

The control if I'm not mistaken, is the two little yellow sliders between the fuel control levers, center console. They are very rarely seen as being at equal throw (I think mainly due to the inherent imbalance of the 2-3 config).

If I am totally wrong here......then never mind. :P
 
N766UA
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RE: MD-80 Elevator Misalignment

Wed May 17, 2006 8:24 am

Quoting MD80fanatic (Reply 10):
If I am totally wrong here......then never mind. :P

Yeah I think we might have to take your advice on that one.  silly 
This Website Censors Me
 
md80fanatic
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RE: MD-80 Elevator Misalignment

Wed May 17, 2006 8:35 am

Fair enough.

{The message you were about to post is too short and probably not of any higher value to the topic at hand.}
 
jeb94
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RE: MD-80 Elevator Misalignment

Wed May 17, 2006 8:55 am

Um...I guess I should have been more specific BR715. I was referring to the post immediately above my last by 2H4 about the moveable pylon trailing edge of the MD-90. The 717 doesn't have a moveable pylon trailing edge. I've worked as a mechanic on blue 717s so I've had a bit of a close look at them, not to mention the 40 hour 717 maintenance general familiarization course. MD80Fanatic, those yellow sliders have nothing to do with the elevators, sorry. They are the alternate trim levers for the horizontal stab. The alternate trim motor is mounted in tandem with the primary trim motor on top of the jackscrew gearbox. Those switches have to be moved together in the same direction to do anything if they're working properly. Same thing with the suitcase handles and yoke pickle switches that operate the primary trim. The 2X3 seating is so close to the centerline that it has no effect on the balance of the aircraft. Its interchangeable in fact. Two MD88s I'm familiar with were originally delivered with a 3X2 seating configuration.
 
MD88Captain
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RE: MD-80 Elevator Misalignment

Wed May 17, 2006 9:00 am

The two little yellow controls below the throttles control the auto-pilot trim motors and work at half-rate of the normal trim motors. It allows a less abrupt longitudinal trim change than the yoke or suitcase handle trim controls.
 
2H4
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RE: MD-80 Elevator Misalignment

Wed May 17, 2006 9:01 am




Quoting Jeb94 (Reply 4):
Now I forget, is it the airflow over the engines or the disturbed air from the wings that cause the problem?

If you're referring to deep stalls, the airflow over the horizontal stab is disturbed by the wings:






2H4


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jeb94
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RE: MD-80 Elevator Misalignment

Wed May 17, 2006 9:24 am

Thank you 2H4. I figured I was getting that confused. I read a little on the BAC1-11 about this. The prototype crashed after its sixth stall became stabilized. They had no means to recover and the aircraft stayed in that condition all the way to the ground.
 
2H4
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RE: MD-80 Elevator Misalignment

Wed May 17, 2006 9:44 am




Quoting Jeb94 (Reply 16):
The prototype crashed after its sixth stall became stabilized. They had no means to recover and the aircraft stayed in that condition all the way to the ground.

{ shudder }




2H4


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comairguycvg
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RE: MD-80 Elevator Misalignment

Wed May 17, 2006 12:09 pm

Also if you notice the 2 tabs that stick out just under the cockpit windows, those help the aircraft in a deep stall situation. I asked a DL pilot while he was doing his walkaround one day in PIT and that's what he told me. Not sure how it would work...guess it would redirect the airflow to go over the elevators. This would be neat to see in action in a windtunnel.
Worked at: CV62, RJTA, KNLC, CV63, KNFL, OKAJ, KTRI, CV67, KMGE, KNQX, KVPS, KPIT, KCVG, KTYS, KATL
 
jeb94
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RE: MD-80 Elevator Misalignment

Wed May 17, 2006 12:57 pm

The strakes? No, they don't do anything in a stall. They're fixed and don't move. They help with longitudinal (pitch) stability. The larger wing has something to do with it since The DC-9s don't have them.

[Edited 2006-05-17 05:58:19]
 
2H4
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RE: MD-80 Elevator Misalignment

Wed May 17, 2006 1:12 pm




Quoting Jeb94 (Reply 19):
They help with longitudinal (pitch) stability

I was under the impression they were there for yaw stability. The longer the fuselage, the more of an effect "sloppy" airflow has on the aircraft, due to the longer moment arm between that airflow and the aircraft's CG. Certainly, this would affect longitudinal stability as well, but I was under the impression the elevator has more control authority to compensate for this effect than the rudder. I'll gladly stand corrected, though.




2H4


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jeb94
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RE: MD-80 Elevator Misalignment

Wed May 17, 2006 1:22 pm

The rudder on the 80 is like a barn door and is very effective. The tail was modified from the DC-9 and is a little taller. It seems shorter aircraft have greater yaw problems since the MD87 has a taller tail, the same one installed on the 717, and Airbus also put a taller tail on the A318.
 
2H4
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RE: MD-80 Elevator Misalignment

Wed May 17, 2006 1:38 pm




Quoting Jeb94 (Reply 21):
It seems shorter aircraft have greater yaw problems since the MD87 has a taller tail, the same one installed on the 717, and Airbus also put a taller tail on the A318.

I think the comparison is apples and oranges, though.

The taller tails you mention are a direct result of the shorter arm with which the tail has to work to counteract the adverse yaw produced by a (wing-mounted) engine failure.

Nose strakes, on the other hand, simply clean up and realign airflow along the front of the fuselage. A larger rudder alone could have regained control authority, but relying on that solution alone would be a band-aid, and not a cure. It's my understanding that the nose strakes are the lighter, simpler, and more elegant solution to a problem that can be traced to airflow separation.

In other words, strakes can realign airflow and increase directional stability, but strakes cannot counteract the drag and ensuing adverse yaw produced by an inoperative wing-mounted engine.

Again, though....I'll gladly stand corrected if my thinking is flawed. Where's AeroWeanie?  Smile




2H4


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Woosie
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RE: MD-80 Elevator Misalignment

Wed May 17, 2006 3:44 pm

Quoting Jeb94 (Reply 2):
Nope. The only hydraulic system in the elevators is to assist recovering in a deep stall.

This is correct - the MD-80 model had a hydraulic deep stall recovery subsystem. The MD-90 uses powered elevators

Quoting 2H4 (Reply 3):
On that note, have you guys ever examined the deep stall recovery aids utilized by the MD-90?

The MD-90 pylon are huge, aerodynamically-speaking. Due to their size, pylon flaps were added to assist deep stall recoveries.
 
Woosie
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RE: MD-80 Elevator Misalignment

Wed May 17, 2006 3:51 pm

Quoting Jeb94 (Reply 19):
The strakes? No, they don't do anything in a stall. They're fixed and don't move. They help with longitudinal (pitch) stability. The larger wing has something to do with it since The DC-9s don't have them.

Wrong Jeb - the strakes are there to provide turbulence during deep stalls, to help get air over the horizontal and provide some pitch control (or at least that's what the aero guys say). Longer versions of the DC-9 (-40's & up), the MD-80 and MD-90 need them; the shorter DC-9's and the 717 don't. The strakes don't provide pitch stability; rather, they provide drag.
 
jeb94
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RE: MD-80 Elevator Misalignment

Wed May 17, 2006 4:08 pm

I stand corrected. Here is what the MD80 maintenance manual says about the strakes:
Aerodynamic Strakes
Aerodynamic strakes are installed, one on each side of the fuselage, just below the flight compartment clearview windows and compartment floor line. The strakes are provided to improve vertical and directional stability during high angle-of-attack aircraft conditions.