Originally Posted by
Short Approach?
Just an ATCO so excuse me if I'm asking stupid questions, BUT.
Why would you have a button which if inadvertently flicked inverts the airplane?
This button adjusts my range... this button the brightness and this one blows up the building... yeah.
The rudder trim is a round dial on the central pedestal. It is not in your direct line of sight unless you purposefully turn around and look at it.
The dial is of the continuous type. Meaning if you turn the dial to starboard the rudder will incrementally trim farther and farther into starboard.
Coincidentally the button to release the locking mechanism of the door is also a dial. It is positioned quite close to the rudder trim dial. Both dials are not in sight of a pilot who has not turned around to look at them.
In practice flight attendants will often call the cockpit to take orders for food or drinks. Sometimes people add last minute things over the flight inter-phone so the flight attendant takes a bit longer than usual to enter the cockpit. Some pilots, especially FO's are vulnerable to this, grab the wrong dial and hold it to the left. If an FO is busy reading, talking to ATC or something else he might inadvertently grab the rudder dial instead of the door dial. Because they are behind him and relatively close to his seat he won't notice this in his peripheral vision.
Holding the door dial to the left unlocks the door mechanism. You need to keep it turned to the left actively to keep the door unlocked for the flight attendant to enter the cockpit.
As you can see from the video the 737 was tipping over to port side. This coincides with my description of the operating principles above.
edit: Oddly enough: even though the dial itself is not visible to the pilot, the displacement of the yoke should be obvious almost immediately. The AP will attempt to balance the increasing out of trim position by applying opposite aileron. This becomes very clearly visible once it goes over a few degrees as the entire yoke would be turning farther to starboard as time goes by.