February 28, 2013

Something Odd About MD80 Flight Controls

One up, one down...okay for ailerons, not elevators!

One up, one down...normal for ailerons, not elevators.
It is commonly known that there is something unusual about the elevators on an MD80 that causes them to move in opposite directions while sitting on the ground. There are a lot of people out there who know just enough about aerodynamics and aircraft systems to know this isn't right, so I regularly find myself engaged in an MD80 systems lesson with a concerned passenger. What you may not know, is that the elevators are not the only unusual flight control system on the McDonnell Douglas MD80.

A small amount of aeronautical knowledge is needed to fully appreciate this post, so allow me to provide a quick lesson. I'll keep it short and simple...consider this Flight Controls 101.

figure 1

figure 2

An airplane moves on three axes, longitudinal, lateral and vertical (figure 1). Elevators, located on the trailing edge of the horizontal stabilizer, control movement on the lateral axis (pitch) as the pilot pushes or pulls on the flight controls. Pull back and the houses get smaller, push forward and the houses get bigger. Ailerons, located on the trailing edge of each wing, move opposite of each other and control the aircraft on its longitudinal axis (roll). This part is very much like driving a car. When the pilot moves the controls to the left, the aileron located on the right wing lowers while the aileron on the left wing rises. This opposite movement causes the aircraft to roll to the left. The rudder, located on the trailing edge of the vertical stabilizer, controls the aircraft on its vertical axis (yaw). Two pedals located at the pilot's feet move the rudder to the left and right in order to pivot the aircraft. Rudders serve a number of other purposes, but when airborne, rudder inputs help coordinate turns and aid in aligning the aircraft with the runway during crosswind operations (find the ailerons, elevators and rudder in figure 2).

Now for the rest of the story...

With most conventional airplanes, flight control inputs from the pilot, whether stick, control wheel or joy stick, result in immediate and direct movement of the flight control surfaces. On some planes, a combination of cables and pulleys directly manipulate flight control position, while other planes have hydraulically boosted or electrically controlled flight control surfaces that may require the involvement of on-board computer systems. Either way, and here is how the MD80 differs from the rest, the pilot usually has control over the position of all flight control surfaces, whether airborne or on the ground. I'm sure there are a few exceptions, but this is the norm.

MD80 cockpit

Rudder...

However, on the MD80, the rudder is the only primary flight control directly controlled by the pilot. Controlling the airplane on its vertical axis, rudder movement is accomplished with a two-way cable system from the rudder pedals that is mechanically controlled and hydraulically actuated in normal conditions.  When the pilot steps on the rudder pedal, the rudder moves. The elevator and ailerons aren't quite that simple.

Ailerons...

The aileron and elevator systems are a little unusual in that control inputs from the pilot move small control tabs, not the actual control surfaces.  Sitting on the ground, if the pilot of an MD80 moves the control wheel left and right, the control tabs move, not the ailerons. On most airplanes, if a pilot walked out onto the ramp and physically moved an aileron up and down, the control wheel in the cockpit would move as well.  Not so on the MD80.

When airborne, aerodynamic forces on the control tabs move, or "fly," the ailerons into the position commanded by the pilot and control the airplane along its longitudinal axis.  Cockpit control wheels are connected via pulleys and cables to the aileron control tab and are linked together by a torque tube that causes both control wheels to move together. The ailerons are also connected in such a way that each aileron must respond to movement of the opposite aileron...if one goes up, the other goes down.  So at least they look normal from a distance.  The elevators are another story.

Elevators...

Movement along the MD80's lateral axis (pitch) is controlled by of a pair of elevators attached to the trailing edge of the horizontal stabilizer. During normal flight, the elevators are controlled by an aerodynamic boost system that operates in the same way as the ailerons where a single control tab on each elevator "flies" it into the position commanded by the control wheel. As with the ailerons, when the pilot moves the control wheel forward and aft, the elevator does not move. Instead, only the control tabs are in motion.  Each control tab is driven by an independent two-way cable system from the corresponding control column in the cockpit. However, unlike the ailerons, the elevators are not connected to one another.  The elevators are weighted in such a way that they typically sit in a nose-up position, but since they are free floating and not interconnected, it is not uncommon at all to see them move in completely different directions.

MD80 control tab.


So there you have it. Much more information about the MD80 than you probably ever wanted to know. The next time you’re standing in the terminal and notice someone looking at the tail of an MD80 with a perplexed look on his or her face...clue the poor soul in and save the pilot from another systems lesson.

One up...one down.  (on the ramp at San Jose Del Cabo)


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Want to know more about the MD80/DC9 series aircraft.  Take a look at this web-site for a complete explanation of the differences.

Ultimate DC-9/MD-80/MD-90/MD-95 Guide


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And this post for a brief history of the DC9/MD80.



6 comments:

  1. I _love_ the system stuff. Back in the day when there were flight engineers, I wonder if I would have enjoyed being one more than actually flying. Thanks for the great explanation.

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  2. I was once on a MD-80 flight were the aileron was fully deflected. At the time I was quite worried and hoped it would straighten out during taxi. That didn't happen and we approached the runway. We started our takeoff roll and we were gaining speed fast and still the aileron was deflected. It wasn't until after mid-takeoff the aileron slowly went to the neutral position, and I could breathe again.

    I read on another forum that some people even got out of their seat and started flagging the flight attendants while the take-off role was happening in similar instances. Your explanation totally makes sense because on taxi I did see the pilot move the small tab on the aileron with no actual aileron movement. I know it's common to see the elevator deflected, but is it that common to see the ailerons in the same way? It would sure be kind if maintenance would just move the ailerons with a broom stick before taxiing to prevent potential heart attacks and panicked passengers banging on the cockpit door.

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  3. Hi Sam, the ailerons will blow around in the wind just like the elevator. The only difference is that the ailerons are interconnected in such a way that if one moves, the other will move in the opposite direction. So at least they still look normal. It is completely normal for one to be up like that at the beginning of the takeoff roll.

    Also, on a crosswind takeoff, the pilot will intentionally hold the controls into the wind. I don't know what the conditions were on your flight, so I can't say for sure, but if the aileron stayed up for most of the takeoff roll, then the pilot was probably doing it on purpose.

    Funny you would say that about the broom stick. I can reach the ailerons when I'm doing a walk around on the MD80...they're too high on many other airliners...and I often even them out if the wind isn't blowing too hard.

    Thanks for reading and thanks for the comment.

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  4. Haha, I forgot the MD-80 was so close to the ground. Save that broom stick trick for when you need to knock some dirt daubers off the engines.

    As for the ailerons, I'm assuming they got deflected by the last flight, because it was already like that when I first boarded. The aileron was deflected quite a lot, if not fully, so it might of needed a little more speed than usual to finally get enough airflow to push it back to neutral.

    This guy explains a similar instance which is exactly what happened in my flight. (except for the part about getting up during takeoff)
    http://www.pprune.org/archive/index.php/t-46118.html

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  5. Thanks for this post, working at an airport with a lot of MD-80 movement from your airline I always wondered why the elevators were at different angles. This was a great and informative read and I just saw your latest post so I'm going to have my other MD-80 question answered. Thanks a bunch!

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  6. The more I learn about the MD-80 the more I realize this aircraft is packed full of little quirks

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