Sponsor Message:
Civil Aviation Forum
My Starred Topics | Profile | New Topic | Forum Index | Help | Search 
MD-80 Elevator Misalignment  
User currently offlineMaxQ2351 From , joined Dec 1969, posts, RR:
Posted (8 years 7 months 2 weeks 1 day 4 hours ago) and read 3342 times:

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.

View Large View Medium
Click here for bigger photo!

Photo © Jorge Rocafort
View Large View Medium
Click here for bigger photo!

Photo © Matthew C. Lyons


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

25 replies: All unread, showing first 25:
 
User currently offline2H4 From United States of America, joined Oct 2004, 8956 posts, RR: 60
Reply 1, posted (8 years 7 months 2 weeks 1 day 3 hours ago) and read 3329 times:
AIRLINERS.NET CREW
DATABASE EDITOR



Here you go!

 Smile




2H4





Intentionally Left Blank
User currently offlineJeb94 From United States of America, joined Oct 2004, 608 posts, RR: 4
Reply 2, posted (8 years 7 months 2 weeks 1 day 3 hours ago) and read 3316 times:

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.

User currently offline2H4 From United States of America, joined Oct 2004, 8956 posts, RR: 60
Reply 3, posted (8 years 7 months 2 weeks 1 day 3 hours ago) and read 3293 times:
AIRLINERS.NET CREW
DATABASE EDITOR




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





Intentionally Left Blank
User currently offlineJeb94 From United States of America, joined Oct 2004, 608 posts, RR: 4
Reply 4, posted (8 years 7 months 2 weeks 1 day 3 hours ago) and read 3271 times:

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!

User currently offlineAA717driver From United States of America, joined Feb 2002, 1566 posts, RR: 13
Reply 5, posted (8 years 7 months 2 weeks 19 hours ago) and read 3084 times:

Man! I love McD-D products! Big grin TC


FL450, M.85
User currently offlineMD88Captain From United States of America, joined Nov 2001, 1338 posts, RR: 20
Reply 6, posted (8 years 7 months 2 weeks 19 hours ago) and read 3051 times:

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.

User currently offline2H4 From United States of America, joined Oct 2004, 8956 posts, RR: 60
Reply 7, posted (8 years 7 months 2 weeks 18 hours ago) and read 2993 times:
AIRLINERS.NET CREW
DATABASE EDITOR




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





Intentionally Left Blank
User currently offlineHavaloc From United States of America, joined Feb 2005, 79 posts, RR: 0
Reply 8, posted (8 years 7 months 2 weeks 11 hours ago) and read 2741 times:

DC-9s are like this too.


DC-9
User currently offlineBR715-A1-30 From , joined Dec 1969, posts, RR:
Reply 9, posted (8 years 7 months 2 weeks 10 hours ago) and read 2692 times:

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


User currently offlineMD80fanatic From United States of America, joined Apr 2004, 2661 posts, RR: 9
Reply 10, posted (8 years 7 months 2 weeks 10 hours ago) and read 2661 times:

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


User currently offlineN766UA From United States of America, joined Jul 1999, 8377 posts, RR: 23
Reply 11, posted (8 years 7 months 2 weeks 9 hours ago) and read 2634 times:

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
User currently offlineMD80fanatic From United States of America, joined Apr 2004, 2661 posts, RR: 9
Reply 12, posted (8 years 7 months 2 weeks 9 hours ago) and read 2608 times:

Fair enough.

{The message you were about to post is too short and probably not of any higher value to the topic at hand.}


User currently offlineJeb94 From United States of America, joined Oct 2004, 608 posts, RR: 4
Reply 13, posted (8 years 7 months 2 weeks 9 hours ago) and read 2573 times:

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.

User currently offlineMD88Captain From United States of America, joined Nov 2001, 1338 posts, RR: 20
Reply 14, posted (8 years 7 months 2 weeks 9 hours ago) and read 2556 times:

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.

User currently offline2H4 From United States of America, joined Oct 2004, 8956 posts, RR: 60
Reply 15, posted (8 years 7 months 2 weeks 9 hours ago) and read 2556 times:
AIRLINERS.NET CREW
DATABASE EDITOR




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





Intentionally Left Blank
User currently offlineJeb94 From United States of America, joined Oct 2004, 608 posts, RR: 4
Reply 16, posted (8 years 7 months 2 weeks 8 hours ago) and read 2513 times:

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.

User currently offline2H4 From United States of America, joined Oct 2004, 8956 posts, RR: 60
Reply 17, posted (8 years 7 months 2 weeks 8 hours ago) and read 2484 times:
AIRLINERS.NET CREW
DATABASE EDITOR




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





Intentionally Left Blank
User currently offlineComairGuyCVG From United States of America, joined Jan 2006, 337 posts, RR: 1
Reply 18, posted (8 years 7 months 2 weeks 6 hours ago) and read 2348 times:

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.

User currently offlineJeb94 From United States of America, joined Oct 2004, 608 posts, RR: 4
Reply 19, posted (8 years 7 months 2 weeks 5 hours ago) and read 2309 times:

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]

User currently offline2H4 From United States of America, joined Oct 2004, 8956 posts, RR: 60
Reply 20, posted (8 years 7 months 2 weeks 5 hours ago) and read 2279 times:
AIRLINERS.NET CREW
DATABASE EDITOR




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





Intentionally Left Blank
User currently offlineJeb94 From United States of America, joined Oct 2004, 608 posts, RR: 4
Reply 21, posted (8 years 7 months 2 weeks 4 hours ago) and read 2266 times:

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.

User currently offline2H4 From United States of America, joined Oct 2004, 8956 posts, RR: 60
Reply 22, posted (8 years 7 months 2 weeks 4 hours ago) and read 2250 times:
AIRLINERS.NET CREW
DATABASE EDITOR




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





Intentionally Left Blank
User currently offlineWoosie From United States of America, joined May 2006, 115 posts, RR: 2
Reply 23, posted (8 years 7 months 2 weeks 2 hours ago) and read 2187 times:

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.


User currently offlineWoosie From United States of America, joined May 2006, 115 posts, RR: 2
Reply 24, posted (8 years 7 months 2 weeks 2 hours ago) and read 2178 times:

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.


User currently offlineJeb94 From United States of America, joined Oct 2004, 608 posts, RR: 4
Reply 25, posted (8 years 7 months 2 weeks 2 hours ago) and read 2163 times:

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.


Top Of Page
Forum Index

This topic is archived and can not be replied to any more.

Printer friendly format

Similar topics:More similar topics...
DC-9 / MD-80 Elevator Trim posted Fri Mar 2 2001 17:09:42 by Cfalk
Longest Flight On AA MD-80? posted Wed May 9 2007 21:32:43 by Ghillier
Midwest Airlines MD-80 Question posted Mon May 7 2007 20:40:06 by Midway DC9-10
AA/TW MD-80 Question posted Mon May 7 2007 06:15:57 by Ckfred
McDonnell Douglas MD-80: A Classic posted Mon Apr 30 2007 04:57:50 by MSYtristar
Painted AA MD-80? posted Sun Apr 29 2007 05:39:52 by AirTranTUS
Alaska MD-80 - Smoke In Cabin At LAS posted Mon Apr 23 2007 23:30:11 by Vorticity
MD-80 In Iran, How Many? posted Wed Apr 4 2007 21:09:46 by 717fan
Looking For An AA MD-80 Reg - Fleet Number Is.. posted Fri Mar 23 2007 15:28:15 by Clickhappy
American Decision On MD-80 Replacement- Soon posted Fri Mar 9 2007 01:19:44 by 2wingtips