How come when I notice MD and DC's taxiing they only have one side of their elevator up, usually the co-pilots side I believe. Then on take-off they even up? Control lock? Curious!

 

The hydraulic system is not powered until right before takeoff due to it not being cooled on the ground. So I've been told anyway. Could be 100% wrong.

 

Yup...100% wrong.

The elevator on the MD80 is free to blow with the wind. Often times you'll see one side up while the other side is down. With proper airflow over the tail, the elevators will come in line. The control column moves the elevator control tab, and not the elevator directly.

The elevator on the MD80 is not powered by hydraulics, however there is an hydraulically powered elevator augmenter to assist in stall recovery.

 

The elevator is a floating elevator. The aircraft is flown by tabs which in turn move the elevator. During takeoff once there is enough airflow the tabs become effective and can move the elevators. The two sides are not interconnected except through a torque tube between the control yokes. That is why when they taxi you will often see one side up and the other down, depends on the direction of the wind.

 

Are any of the flight controls hydraulically actuated?

 

Douglas did it that way to scare the crap out of the Boeing pilots...

Not really but I had to go dig out my MD-80 manual. Nice flying airplane.. anyway.

Ailerons are moved by control tabs and are cable connnected.

Pitch is controlled by two elevators with a single control tab on each elevator. "A 3000psi hydraulic power augmentor system provides elevator control for additional nose-down capability in extreme high-angle-of-attack flight conditions." This augmentor is powered when you push the control column full forward and a light illuminates "ELEVATOR PWR ON"

The rudder is powered by the right hyd systems.

As part of the BEFORE TAKEOFF CHECK, You exercise the flight controls and by going full forward on the column, you activate the powered system and that aligns the elevators. Some say the Douglas DC-9/MD-80/MD-90 family is actually fly by wire.. or more truthfully, cable. The DC stands for "direct cable" or "douglas cable".

(been a while since I drove the -80.. fun machine)

 

The rudder under normal conditions...

 

So those tabs would be called servo tabs? Used to help move the primary control device. In this case an elevator.

 

There are two sets of tabs on the ailerons, control tabs (inboard tabs) and trim tabs (outboard tabs). The ailerons are bussed to each other, the yokes are connected to their respective control tabs. So during a walk around you move an aileron only the opposite aileron moves. If you would move a control tab then the yokes would move. With hydraulics on the spoilers on the down wing will also move once approximately 5 degrees bank input into the yokes.

The elevator has control tabs (inboard) and geared tabs (outboard). The geared tabs use aerodynamics to assist in movement of the elevator. The elevator does not trim, the entire stabilizer trims. The elevators are not connected together, each yoke goes to their respective control tab. The yokes are connected via a torque tube.

The rudder has a control tab that under normal ops is locked in position, in normal ops the entire rudder moves with pilot input. With a loss of hydraulic power to the rudder the control tab unlocks and the rudder is then moved via the control tab.