"Pitch control 101" for DC-9/MD-8x/Boeing 717.
Gen Y might wish to go straight to
bold at the end.
Trimming is via all moving horizontal stabilizer. Actuated by ONE
acme screw driven by primary AC electric motor (big) or secondary AC motor (not so big) through a planetary gearbox. Pilots actuate stab trim through 'pickle' switches on control wheel. Secondary motor is backup for primary and also used for autopilot pitch trim.
Being an all moving stabilizer, when trimmed and in un-accelerated flight (constant climb cruise or descent), MD-8x elevators 'trail', supplying no aerodynamic force. The stab does that job. Control column returns to centre when aircraft trimmed -- unlike most GA types where elevator is trimmed to provide the downforce/upforce to trim in pitch displacing the control column from central position.
WMV of MD-8x pitch trim (and failure in Alaska airlines)
http://www.ntsb.gov/Events/2000/Aka2...kscrew_261.wmv
Elevators are thus used only for momentary pitch up/pitch down manouevering.
MD-8x elevators are
not connected to any control cables, hydraulic servos, or other actuators -- except for one item that we will get to in a minute. Therefore an MD-8x elevator sits whereever it likes, when parked, and trails in the airstream if the pilot or autopilot is not pushing or pulling the control column.
Elevator control is effected by tabs. The captains control column runs a cable circuit to the left elevator servo tab. The FO control column runs a cable circuit to the right elevator servo tab. The Captain and FO columns are connected to each other via a torque tube with a breakout mechanism.
That provides for redundancy if there is a jam or failure in one elevator servo control circuit. Disconnect the columns, and use the 'free' column to control the aircraft in pitch.
Want to pitch up, like a rotation to takeoff? Captain pulls back on his column. Captains cable circuit pulls the left elevator servo down. Resulting aerodynamic force and moment arm to elevator hinge acts to deflect the left elevator up. Left elevator and moment arm to CofP acts to rotate aircraft nose up. Torque tube also makes the FO column pull back, duplicating servo tab and elevator deflections on right side. Aircraft pitches up.
So there is no direct connection between the pilots control column and the elevator in an MD-8x. No airflow, and the elevators sit where they like if they are experiencing different winds when parked. Gust dampers protect against damage due to abrupt changes.
Except for one thing. Being a T-tail aircraft where deep stall is a risk, the aircraft needs deep stall recovery capability. Low speed or turbulent airflow in stalled condition might make servo tab powered elevators unable to recover from a deep stall.
So Douglas/MD added an elevator augmentation system. Should either pilot push their control column full forward, this would be sensed and one hydraulic ram mounted on each elevator would force each elevator, not the tab, to the full down position for stall recovery.
It is a requirement to check 'flight controls full and free movement' before every takeoff. So, just before the cock-eyed MD-80 taxiies onto the runway, one pilot does a full and free check. When the stick is pushed full forward, left hydraulic system pressure activates the elevator augmentation and pushes both elevators full down. No more cockeyed elevators.